Physical Characteristics of Commercial Sheet Muscovite in the Southeastern United States

GEOLOGICAL SURVEY PROFESSIONAL PAPER 225

Prepared under a cooperative agreement with the State of North Carolina and the Tennessee Valley Authority Physical Characteristics of Commercial Sheet Muscovite in the Southeastern United States

By RICHARD H. JAHNS and FORREST W. LANCASTER

GEOLOGICAL SURVEY PROFESSIONAL PAPER 225

Prepared under a cooperative agreement with the State of North Carolina and the Tennessee Falley Authority

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1950 UNITED STATES DEPARTMENT OF THE INTERIOR Oscar L. Chapman, Secretary

GEOLOGICAL SURVEY W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - Price 31.25 (paper cover) CONTENTS

Page Page Abstract.-_.____— ——— __ —. —. — ----- — ._-__ — __ 1 Sheet muscovite in the Southeastern States.______28 Introduction. _._—______-__--____--_--_-_-__-___--_.-— 2 Distribution of deposits———————————_———_—_ 28 General statement___--__------_-----_------__--__- 2 History and production——_—_———______. _ 29 Field and laboratory work.-_.___-._.__.__.__.___. 3 Occurrence- .___—__—______———__—______30 Acknowledgments.-.--. __-._-__-.______4 General geologic features—————__—_———___ 30 Commercial sheet muscovite_----_-_--_-__-__---__-_-_____ 4 Pegmatites———————————————————————— 31 General properties.______4 Mica deposits. ___——_—————_—___—_____ 33 Properties affecting value------_--__----___-____- 6 Tests of sheet muscovite from the Southeastern States————_ 34 Cleavage...__„______.____-_-_.--_-___ 6 General statement...————————————————————_ 34 Hardness, flexibility, and elasticity.______» 7 Color tests.—————————————————————————— 42 Structural imperfections.__—_-______--_-____--__ 8 Methods of testing——————————————————.._ 42 Color..______-._-—-- — „.__ 11 Results...,———————————————————————— 42 Staining, intergrowths, and inclusions______12 Petrographic examination.._—______—_———....._, 43 Primary impurities-... ——______12 ___ Staining and inclusions-...___-._-__.__--__..... 43 Secondary impurities.______.______-.-_.__._ 15 Pinholes and hair cracks____—____.._._._.____ 44 Electrical properties_-__-____-______-_--___--_.-_- 16 Electrical tests___-_-_——————————————.——... 45 Uses— ___--__- — __.-__----_--_--___---____--__-, 16 Apparatus and methods of testing._ _——————__. 45 Grading and classification_---_--_-__---__-----__-_-_-_ 17 General tests..--..—_—_—______45 Visual methods.._..__-___.______—______17 Electrical methods.______. 22 Tests of visually qualified mica..———_ ——— ______97 Combination method of the American Society for Test­ Summary and conclusions.———————— ——— ____-_-__-.__ 103 ing Materials.... ______24 List of references..—„__-__—_.__—__-______.--_-____._ 106 Preparation and marketing----___--______-_-_-_--_---_ 26 Index______----___-.-_---____.__-__.--_ 109

ILLUSTRATIONS

Page PLATE 1. Books and cleavage pieces of muscovite__-____--_--_-__-_____--_----_------_------_---_---_ 8 2. Reeve structures in muscovite-______.______-__-__---_-____-___----__.__ 9 3. Wedge structure and deformation in muscovite____-____--_--_--_--___-____----_-----_---_------__------__ 10 4. Ruling in cleavage pieces of muscovite__.______-_____-__-_--_-_----__---____-___--___-__ 11 5. Primary stain in muscovite______-__-_---______.______-_-______-___--_----__-_-_ 14 6. Primary hematite stain in muscovite---....-.. ______--___----____-__-----_--__---_---__ 15 7. Color variations and primary stain in muscovite______-_-______---_-----___-_-_-----_---_---_.----_--_-_.-. 16 8. Mineral inclusions and clay stain in muscovite—_____...__-__-___—-___ ———— __ — _ —— -_-___-__ — __-__ 17 9. Test sets for electrical classification of sheet mica______—___.__---__---.-_ ——— _____--__--___--__--__ 24 FIGUKE 1. Typical single and twinned crystals of muscovite______.._————_-.__--_--.-__ —— — -___-_--__--_----._-__ 5 2. Orientation of pressure and percussion figures in muscovite—- ———— ._—______———— ___._ — —— __-__-_-____ 6 3. Inclusions of muscovite, quartz, and apatite in muscovite..-._-_.____-__-_-_---_——_-_-______-_-____-_.____ 7 4. Relation of reeves to crenulations and discontinuities in muscovite laminae_____-_-___--_----_---_ —------._---_ 8 5. Relation of reeves to crystal directions in muscovite-.______..____.______-______---_.-_--_-__-___--____-_ 9 6. Deformation of muscovite laminae_--____-____._.______-__- — _-__--__--__---__--_____-_ 10 7. Types of muscovite fragments formed by breaking along parting or ruling planes—______-__-__---____-_--_---____ 10 8. Biotite-muscovite intergrowths______—_____——______15 9. Size-grading chart for domestic block mica______--_-_-___ — ——— _-_--__-_-___---___-____-_ 19 10. Chart for grading mica according to Indian standard sizes___.___---_.. ———— ———— __-_.__ — _.__-_____—— 20 11. Simplified circuit of spark-coil set for testing sheet muscovite——___———— ——————— —— _- —. — — ... — -. 23 12. Simplified circuits of equipment for determining power factor of sheet muscovite—————— _____ — ______— __._ 24 13. Meter scale of the Bell Telephone Laboratories' Q-test set______-_--_-—_--_---—--—------.------—__- 24 14. Outline of operations involved in the extraction and preparation of sheet, punch, and scrap mica——____-___ —______26 15. Distribution of principal pegmatite areas and districts in the southeastern United States.__ —— —————— .-_-._____ 29 16. Idealized plans of a pegmatite body, showing typical distribution of pegmatite units———_---_-__,-__——._--__-._ 32

III IV

TABLES

1. Theoretical composition of muscovite, compared with composition of specimens from several localities.______5 2. Domestic and Indian size groups for sheet mica, including punch and sheets larger than punch______18 3. Electrical, physical, and visual quality requirements of natural block mica and mica films for use in capacitors..______25 4. Q and power-factor values for electrical-quality groups E-l, E-2, and E-3____-_-_---_-____--_-_-----___-__-__-__ 26 5. Price ranges of clear and stained sheet mica in the Southeastern States during the period 1910-40.-_-__.-______27 6. Price schedules for domestic clear sheet mica during the period December 1941-February 1945—— ______28 7. Summary data on the number of mica deposits in the southeastern United States._.______-_-__-____-_-___-_ 30 8. Production of sheet and punch mica, 1912-14, in North Carolina, the Southeastern States, and the entire United States. _ 30 9. Occurrence of stain in 2,148 test lots of muscovite from the Southeastern States______44 10. General test data for mica specimens, arranged by lot and by mine or prospect-_-___--_-___-_-.___---___-_____--_ 47 11. Results of electrical tests of mica samples, from specific mines or deposits, that include material of E-2 or E-3 quality. _ 96 12. Test data for mica, taken from the production line of the Colonial Mica Corporation in 1945, previously qualified by visual means______— ______98 13. Test data for mica (not identified by mine), taken from the production line of the Colonial Mica Corporation in 1943 and placed in storage, qualified by visual means in 1943______-______--_-______--_-__--_-___-_-______102 14. Results of electrical tests of mica samples, taken from the production line of the Colonial Mica Corporation in 1943, that include material of E-2 or E-3 quality______--__-___---__-_------_---__---_--__---__---_____-- 103 PHYSICAL CHARACTERISTICS OF COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

By RICHARD H. JAHNS and FORREST W. LANCASTER

ABSTRACT Most raw muscovite is sold according to requirements speci­ yielded much sheet material, but the bulk of production is de­ fied by the purchaser, and for sheet material emphasis gen­ rived from deposits in western North Carolina. During the erally is placed on features subject to grading by careful visual period 1912-44 the Southeastern States accounted for examination. Specifications vary with individual end uses, and 25,028,404 pounds, or about 54 percent of the total United the chief users of high-quality sheet muscovite ordinarily set States production of sheet and punch mica, and its value of very exacting requirements. Correlations between end uses $9,720,273 amounted to 62 percent of the total. The average and specifications have not been fully developed for some phys­ value of the output to January 1945 was about 39 cents per ical properties of mica, particularly color and clarity. The pound, in contrast to the value of 89 cents per pound during investigations described in this report were undertaken to the wartime period 1942-44. provide further basic data that bear on this problem, chiefly The mica-bearing pegmatites range from lenses and by correlation of significant electrical properties of sheet stringers less than an inch thick to dikes, sills, and irregular muscovite from the Southeastern States with geographic oc­ masses several hundred feet long and 200 feet or more thick. currence, geologic occurrence, and physical characteristics of In general, the distribution of book mica within a given de­ direct commercial application. posit reflects the shape of the containing pegmatite body, but The present project, carried out jointly in 1945 and 1946 by there appears to be little correlation between the size of the the United States Geological Survey, the State of North body and the quantity and coarseness of the mica that it Carolina, and the Tennessee Valley Authority, involved the contains. Most of the mica concentrations are confined to preparation, electrical testing, color identification, and petro- specific parts, or zones, of the enclosing pegmatite bodies and graphic examination of 2,507 lots containing 237,764 pieces of occur within many of these zones like the shoots of ore mica. The test specimens were obtained from at least 850 de­ minerals in metalliferous deposits. The position and distribu­ posits in the States of Alabama, Georgia, North Carolina, South tion of such shoots commonly can be correlated with the over­ Carolina, and Virginia. This report includes a general dis­ all shape of the containing zones or with rolls, bends, bulges, cussion of commercial muscovite in the Southeastern States, constrictions, or other irregularities in these zones. together with compilations, descriptions, and discussions of The results of the color tests of block mica show that ma­ the tests and examinations. terial from the Southeastern States covers essentially the The uses of sheet mica are based upon its perfect cleavage, entire range known for commercial muscovite. Drab, buff, and exceptionally low conductivity of heat and electricity, high brownish ("ruby") mica is locally abundant, but in general dielectric constant and dielectric strength, heat resistance, the bulk of the sheet material produced from the entire region noninflammability, low dielectric loss, mechanical strength, is green, yellowish olive, or brownish olive. Systematic color flexibility, elasticity, transparency, and ease with which it can variations occur within some mica shoots, within single peg­ be worked into final form. A very high proportion is used as matite bodies, and within large groups of pegmatite bodies. electrical insulating material, and during the recent wartime Variations in individual mica books and within single mica period exceptionally heavy demands for sheet material of su­ shoots are most pronounced where the muscovite is green or perior quality were based upon uses in radio and electronic yellowish olive. On a larger scale, brownish books generally equipment, aircraft generators and spark plugs, and other are nearer the walls of a given pegmatite body than green electrical apparatus. Such mica must have a power factor (a ones, and few brownish books occur in those pegmatites with measure of the loss of electrical energy in a condenser in which green mica near the walls. Green books are rare in pegmatites the mica is the dielectric) of less than 0.04 percent at a fre­ with interior concentrations of brownish mica. quency of 1 megacycle. A substantial proportion of Southeastern muscovite is The preliminary preparation of sheet mica involves separa­ mineral-stained, and little of this stain can be effectively tion from other minerals, rough splitting, rough trimming with the fingers, further splitting, and extensive trimming removed by even the most careful splitting. In contrast, with a knife or other blade. This yields block mica, which is thicker and larger euhedral inclusions of biotite, apatite, gar­ then classified according to size, degree of trimming, and net, pyrite and other sulfides, quartz, tourmaline, zircon, quality. Quality can be classified by visual or electrical means, zoisite, and other minerals can be removed with little difficulty, or by a combination thereof; specifications and methods of test leaving holes or distinct depressions in the host muscovite. have been published by the American Society for Testing Ma­ Various types of curdy greenish or brownish stains and super- terials. Block mica generally is further prepared by additional gene clay, iron, and manganese stains are locally abundant, splitting, trimming, and punching or stamping. but air-stained books are very rare. The Southeastern States constitute the chief mica-producing Iron oxide stains are common in green, yellowish-olive, and area in the United States. Fourteen mining districts have brownish-olive micas but are rare in those that are buff and COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES brown. Thin shreds and wisps of biotite are common in mus- on. On the other hand, improvements in the design of the covite of nearly all colors. In general, mineral-stained mica electrical testing equipment are to be expected, and these is confined to the outer parts of those pegmatite bodies that devices probably will be adapted to continuous semiautomatic also contain clear green books and to the inner parts of those or automatic operation. that also contain clear buff or brown books. "Pinholes" are common in most districts but are abundant in the mica from only a few mines. Most of them occur in INTRODUCTION the brown and buff, or ruby, micas in which zircon and apatite GENERAL STATEMENT inclusions are most numerous. Hair cracks, said to develop in part after rifting and trimming of sheet mica from certain deposits, are much more abundant in green micas than in the Sheet muscovite is used mainly for special electrical brown and buff varieties. installations and equipment, both as simple pieces The results of the electrical tests indicate a general correla­ split from individual crystals and as composite pieces tion between power factor and physical appearance, but built up from many thin splittings. Most raw mica is evidently there are numerous exceptions, particularly among graded and classified according to quality, size, and stained micas. Nearly all these exceptions are consistent, however, in that the power factor of such mica is lower than thickness and is sold according to requirements speci­ that expectable on the basis of visual appearance alone. There fied by the purchaser. Quality classification is based appears to be a partial correlation between power factor and upon such features as hardness, ease and perfection of the amount of hematite-magnetite stain in sheet mica. On the splitting, flexibility and elasticity, color, reaction to other hand, many pieces of heavily stained mica evidently heating, electrical properties, and amount and distri­ possess excellent electrical characteristics, whereas some per­ fectly clear pieces are inferior in this respect. As far as bution of structural imperfections, stain, and inclu­ electrical characteristics are concerned, the best quality of sions. Emphasis generally is placed upon features stained mica cannot be distinguished from the poorest quality subject to qualification by careful visual examination. by ordinary visual means, but much muscovite that contains Classification according to size and thickness is based substantial quantities of iron—either as inclusions, as ex- upon the dimensions of usable material within a given solved intergrowths, or in solid solution—has distinctly less desirable electrical properties than the iron-poor varieties. piece of mica. Green and brown mottling, "vegetable stain," wispy to Specifications vary with individual end uses. It is platy inclusions of green and brown biotite, and some clay only natural that fabricators and other purchasers of stains that do not transgress the laminae of the host mica raw mica should set high quality requirements for appear to have little adverse effect upon its power factor. specific uses, thereby reducing the proportion of ma­ Neither the color nor the depth of color in clear muscovite appears to have any definite relation to its power factor, and terial that conceivably might cause failure in finished clear green, yellowish-olive, and brownish-olive micas selected equipment. Correlations between end uses and speci­ by visual means should prove as satisfactory for condenser fications have been developed for some physical prop­ use as clear buff and light-brown micas. The electrical tests erties of muscovite through long periods of trial and have shown that there is no intrinsic difference, grade for careful study, and tolerances are recognized and un­ grade, between clear ruby and clear nonruby micas from the derstood within narrow limits. Little basic or em­ Southeastern States and little discernible difference between clear domestic micas and clear material from India. pirical information is at hand for other properties, The use of electrical testing and adoption of the combined however, and specifications concerning them appear visual-electrical system of mica classification proposed by the to be founded upon less certain ground. The setting American Society for Testing Materials should result in up­ and maintenance of high acceptance standards for ward qualification of much sheet material from the South­ the less fully understood properties of commercial eastern States and might well go even farther toward improving the currently inferior position of the abundant sheet muscovite have made it difficult to determine greenish varieties in the trade. Electrical testing might also whether or not so-called inferior types of material are force rejection of some sheets of apparently high quality satisfactory for a given use. The effects of color, stain, material, owing to pinholes or other defects difficult to recog­ and inclusions on the electrical properties of musco­ nize. Determination of power factor alone cannot suffice for vite are perhaps least known, and there are serious the proper classification of sheet mica; hence it should be gaps in the correlation of these with other properties. combined with the results of spark testing and careful visual In the absence of data that would permit positive appraisal. Electrical testing of much visually classified mica, especially assignment of a certain physical feature in a given mineral-stained sheets, should result in considerable con­ lot of mica as the cause of an undesirable proportion servation of condenser-grade material. This may well amount of failure in finished electrical equipment, any re­ to 50 percent or more. Although such a procedure might be luctance to lower consumers' specifications is easily very important and even vital during periods of unusual understood. On the other hand, the steadily growing demand or restricted supply, the increased cost of piece-by- demand for muscovite of superior grade, especially piece or even lot-by-lot electrical testing might not permit during the past two decades, has forced some use of competition with other methods of classification during ordi­ lower-quality mica and even the substitution of other nary times. Competition with foreign sources of supply, with ceramic, glass, treated paper, and plastic substitutes, and materials. Such conservation measures became a even with synthetic mica probably will increase as time goes practical necessity because of the unprecedented de- INTRODUCTION mand during the recent wartime period, when the studies by Sterrett 1 and in part to provide new and entire problem was thrown into sharp relief. The more detailed coverage of structural, mineralogic, best qualities of mica were allocated only for the man­ and economic features. The work involved some areal ufacture of articles in which no other material was geologic mapping and the examination of 1,644 de­ judged usable, and supplies were further conserved posits, more than 400 of which were mapped in detail. by requiring the use of poorer qualities wherever pos­ Samples of muscovite were collected from stock piles sible, by the use of the smallest possible sizes, by the or lots of mine-run material at operating mines, from reduction of waste and the reclamation of partly the walls and backs of mine workings, and from spoiled material, and by the substitution of ceram­ dumps at mines no longer accessible. Most of the mica ics, treated paper, or other insulating substances described in this report was obtained by J. C. Olson wherever practicable. during the early stages of the work and, during the Most of the recent wartime substitution was later stages, by W. R. Griffitts, E. W. Heinrich, J. E. ordered in great haste and under the stress of ex­ Husted, W. P. Irwin, R. H. Jahns, P. W. Lancaster, pediency, so that much of it was necessarily based R. W. Lemke, J. M. Parker III, and R. A. Swanson. upon incomplete and empirical information. Never­ Where different types of muscovite were recognized theless, the program of conservation was markedly in the same pegmatite body, an attempt was made to successful in terms of equipment performance. With determine their respective spatial and genetic rela­ a return of peacetime conditions, however, there has tions. been a partial return of higher specifications for raw Mica collected from 124 mines in 1939 and 1940 was sheet muscovite. The degree to which this return be­ electrically tested by the National Bureau of Stand­ comes complete may well depend upon the results of ards, and several samples were tested in greater de­ extensive fundamental tests on physical properties, tail by two large fabricators of electrical equipment.2 methods of preparation, and methods of grading, Early in 1942 Jasper L. Stuckey, State Geologist of classification, and testing carried out by several or­ North Carolina, suggested more extensive and system­ ganizations during the war. Many of these tests were atic investigations that might further define the po­ parts of longer research programs that are currently tential usefulness of Southeastern sheet muscovite active, but some of the results already have been re­ as a critical wartime material. Plans were made for leased in preliminary form. electrical testing of samples from many mines, but it The question of the suitability of a given piece of was not possible to begin the work at that time. muscovite for a given use involves so many variables Late in 1944 the War Production Board issued a that no single series of tests seems capable of provid­ directive that only ruby sheet mica should be pur­ ing a general answer, but the investigations described chased from domestic producers by the Federal Gov­ in this report were designed to bring together as many ernment after January 1, 1945. The directive re­ types of data as possible. Some significant electrical flected the increasingly favorable position of the properties of sheet muscovite from Virginia, North United States in terms of sheet-mica stocks, but it Carolina, South Carolina, Georgia, and Alabama were raised several serious problems. In many instances it determined and then were correlated and compared proved exceedingly difficult to distinguish ruby from with geographic occurrence, geologic occurrence, and nonruby micas with assurance, and it was some time physical characteristics of direct commercial applica­ before a usable definition for "ruby" mica and a pro­ tion. The report contains a general discussion of com­ cedure for color grading of mica were developed.3 mercial muscovite in the Southeastern States, de­ Moreover, a real need for discrimination between scriptions of tests made on thousands of specimens, types of commercial sheet muscovite on the basis of and a discussion of the results of these tests. In the color, as implied by the directive of the War Produc­ compilation and integration of data on physical prop­ tion Board, does not yet appear to have been satis­ erties, emphasis was placed upon factors of economic factorily demonstrated. rather than theoretical significance. The question raised concerning the quality of so- called nonruby mica, which constitutes a large part FIELD AND LABORATORY WORK of the output from the Southeastern States, focused During the period 1939-46 the United States Geo­ attention more strongly than ever on the need for re-

logical Survey carried on extensive field investiga­ 1 Sterrett, D. B., Mica deposits of western North Carolina: U. S. Geol. tions of mica deposits in the Southeastern States. The Survey Bull. 315-M, pp. 400-422, 1907; Mica deposits of North Carolina: U. S. Geol. Survey Bull. 430-J, pp. 593-638, 1910; Mica deposits of the North Carolina Department of Conservation and De­ United States: U. S. Geol. Survey Bull. 740, 1923. velopment cooperated in this work from July 1941 2 Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, through the year 1946. The investigations were in­ N. C.: U. S. Geol. Survey Bull. 936-A, pp. 18-30, 1942. * Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards Jour. tended in part to supplement the results of earlier Research, vol. 35, pp. 253-256, 1945. COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES liable correlation between physical properties and Edward Ellingwood III, V. C. Fryklund, P. W. Gates, commercial classification of sheet material. Plans for L. Goldthwait, W. R. Griffitts, J. B. Hadley, E. W. a program of mica testing by electrical means were Heinrich, F. W. Hinrichs, J. R. Husted, W. P. Irwin, revived and greatly expanded in March 1945, when T. L. Kesler, M. R. Klepper, D. M. Larrabee, R. W. Jasper L. Stuckey arranged a conference with H. S. Lemke, Roswell Miller III, J. J. Norton, J. C. Olson, Rankin and Charles E. Hunter, of the Tennessee J. J. Page, J. M. Parker III, L. C. Pray, L. W. Seegers, Valley Authority, and R. A. Laurence and R. H. Jahns, R. L. Smith, J. H. Stillwell, W. C. Stoll, R. A. Swan- of the United States Geological Survey. It was de­ son, and J. R. Wolfe, Jr., in this connection. W. J. cided at this time that electrical testing of numerous Alexander, southern district manager for the Colo­ samples of mica representing many different deposits nial Mica Corporation, drew generously on his broad might well establish within reasonable limits the in­ experience in giving general advice and in comment­ fluence of color, staining, and other properties on the ing upon the results of the tests. He also provided electrical behavior of sheet mica. facilities and equipment on several occasions and ex­ G. A. Rosselot, of the State Engineering Experi­ pedited the investigations in many other ways. Help­ ment Station, Georgia School of Technology, was em­ ful suggestions and comments were made from time ployed as a Consultant to investigate previous work to time by Bradley Johnson, Adrian Newhouse, and of this general nature; he made valuable recom­ L. A. Norman, Jr., of the Colonial Mica Corporation. mendations based on visits to the Bell Telephone Lab­ The chief organizers of the project, J. L. Stuckey oratories in New York City and the laboratories of and H. S. Rankin, retained an active interest from its the Battelle Memorial Institute in Columbus, Ohio, inception to its close and were primarily responsible where he discussed the proposed studies with other for the planning, financing, and successful completion investigators in the field. Helpful advice and sug­ of most phases of the work. C. E. Hunter, of the gestions were also contributed by W. J. Alexander, Tennessee Valley Authority, contributed many help­ L. A. Norman, Jr., and Bradley Johnson, of the Colo­ ful suggestions; Jane Anderson, of the North Caro­ nial Mica Corporation. The North Carolina State lina State Department of Conservation and Develop­ College made available the services of F. W. Lan­ ment, compiled most of the electrical-testing data; caster for conducting the electrical testing. and L. L. McMurray, superintendent of the Asheville The project was carried out jointly by the State of laboratories, North Carolina State College, aided North Carolina, the Tennessee Valley Authority, and greatly in compiling and checking the electrical and the United States Geological Survey during the period other data for table 10. Frances H. Jahns assisted July 1945-April 1946. All samples collected by the in the sorting and classification of samples, in the Geological Survey were studied, and several lots of determination of several physical properties of the prepared and qualified mica loaned by the Colonial muscovite, and in the final preparation of the manu­ Mica Corporation also were investigated in detail. script. The technical assistance of Ida M. Morgan, The spark-coil test set and the rapid, direct-reading Madeline F. Harding, Joan T. Rounds, and Louis Q-meter developed by the Bell Telephone Labora­ Reeder also is gratefully acknowledged. The report tories were used for the electrical testing, which was was critically reviewed by H. S. Rankin and J. L. done in Asheville, N. C., during the summer of 1945 Stuckey and by E. N. Gamer on and L. R. Page, of the by F. W. Lancaster. Careful color determinations United States Geological Survey. were later made by Frances H. Jahns in Asheville Mine owners and operators were uniformly cordial and Spruce Pine, N. C., and other physical character­ and cooperative throughout the investigations. Per­ istics were studied by R. H. Jahns. These investiga­ mission to collect samples for purposes of examination tions were completed in the laboratories of the Cali­ and testing was granted freely and consistently. H. fornia Institute of Technology, Pasadena, Calif. A A. Knight, of High Point, N. C., was particularly total of 2,502 lots containing 237,764 pieces of mica helpful in lending nearly 200 pounds of prepared elec­ was examined and tested. This material was obtained tric (stained sheet) mica for detailed testing, and from at least 850 deposits. he generously permitted the retention of a portion of this material as a reference sample. Smaller lots of ACKNOWLEDGMENTS commercial muscovite were loaned or donated by Many members of the United States Geological other mine operators as well. Survey participated in the collection of samples and COMMERCIAL SHEET MUSCOVITE the accumulation of basic geologic data on mica de­ posits, and without their help it would have been im­ GENERAL PROPERTIES possible to obtain a reasonably complete background Minerals of the mica group are complex aluminum for the testing program. It is a pleasure to acknowl­ silicates whose outstanding characteristic is an al­ edge the contributions of W. B. Alien, H. K. Dupree, most perfect basal cleavage. In the true micas this COMMERCIAL SHEET MUSCOVITE

TABLE 1.—Theoretical composition of muscovite, compared with com­ Muscovite crystallizes in the monoclinic system but position of specimens from several localities is nearly hexagonal in symmetry. Much commercial mica occurs in rough crystals or "books," some of Composition of specimens which are partly or completely bounded by poorly Hiddenite Harding developed faces. The books generally are tabular to mine, Miask, mine, Theoretical Alexander Ural Taos equant, with their shortest dimension perpendicular com­ County, Mtns., Bengal , Auburn, County, position N. C.' Siberia 2 India' Maine 4 N. Mex. * to the cleavage direction. Some are elongated normal to the cleavage direction and are characteristically SiOj..... 45.2 45.40 44.17 45.57 44.48 44.80 TiO2._... 1.10 tapered; many of these are distorted by slippage along AbOs---. 38.5 33.66 37.35 36.72 35.70 37.72 Fe2O3 __ 2.36 1.29 .95 1.09 .67 the cleavage planes and are known as "step crystals." MmOa... N.d. N.d. N.d. N.d. .21 FeO--.._ .20 1.28 1.07 N.d. Most well-developed crystals are hexagonal or rhom­ MnO.... .10 N.d. N.d. MgO_._. 1.86 .38 Trace None bic in outline, with six or more faces forming the CaO.---. .21 .10 K2O.-.. 11.8 8.33 10.00 8.81 9.77 10.66 margins of cleavage pieces (pi. 1). The simplest forms Na*O._.. 1.41 1.14 .62 2.41 1.40 LisO._... Trace .19 Trace Trace comprise basal, prismatic, and clinopinacoidal faces H2O— ._ 4.5 5.46 5.73 5.05 5.50 4.52 F ___ ... .69 .90 .15 .72 .20 (fig. 1), but modifying faces are common. As ob- Total 100.0 100.27 100.88 99.93 100.84 100.18 O correc- .29 .37 .08 Correct- ed total- 99.98 100.51 — — — - 100.10

1 Clarke, F. W., Analyses of rocks and minerals from the laboratory of the United States Geological Survey: U. S. Geol. Survey Bull. 591, p. 330, analysis D, M 1915. 2 Idem, analysis E. 3 Dana, E. S., The system of mineralogy of James Dwight Dana, p. 617, analy­ sis 1, 1909. 4 Idem, analysis 6. 5 Schaller, W. T., and Henderson, E. P., Purple muscovite from New Mexico: Am. Mineralogist, vol. 11, p. 12, 1926. cleavage permits splitting into thin sheets, laminae, or films that are tough, flexible, elastic, and extremely low in electrical and heat conductivity. In contrast, the clintonites (or "brittle" micas) and the chlorites yield cleavage sheets that are brittle and inelastic. The true mica group includes several species, one of M which is muscovite, or potassium mica. Biotite con­ tains iron and magnesium, and phlogopite contains magnesium and a little fluorine. Among the other species are lepidolite, a lithium mica; zinnwaldite, a B lithium-iron mica; and roscoelite, a vanadium-bear­ ing iron-magnesium mica. Sodium, barium, manga­ nese, chromium, titanium, and other elements are minor, generally rare constituents in the group. All micas yield water upon ignition; ordinarily this amounts to 4 or 5 percent of the molecule. Muscovite is the only true mica that is sufficiently M abundant in the Southeastern States to be of present commercial importance in sheet form, and the discus­ M sions of mica in this report are therefore devoted to this species alone. In general it is an orthosilicate of aluminum, potassium, and hydrogen, with the theo­ retical composition shown in table 1. Analyses of specimens from five localities are included in the table for comparison. Muscovite is chemically very stable and is little decomposed by weathering or by ordinary I acids in the laboratory. It is said to yield water, how­ ever, when heated to temperatures above 400° C.,4 and in some varieties this change is accompanied by swelling and loss of elasticity and transparency. D FIGURE 1.—Typical crystals of muscovite, showing the most common faces: 4 Horton, F. W.. Mica: U. S. Bur. Mines Inf. Circ. 6822, p. 6, 1935 (re­ c, hase ; M, prism; h, clinopinacoid; N, prism. A, Simple crystal; B, C, vised 1941). and D, Twinned crystals. 6 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES served on a cleavage surface or basal face of a crystal, hence perpendicular to the principal direction of the the traces of the prismatic and clinopinacoidal faces percussion figure (fig. 2). The others are perpendicu­ meet at angles of almost exactly 120°. Twinning is lar to the prism faces and therefore do not bisect the common, chiefly with the crystals united along the angles between the rays of the percussion figure. The base or along irregular surfaces nearly perpendicular cracks of the pressure figure coincide with distinct to the base (fig. 1). glide or parting planes in the crystal and meet the The symmetry of muscovite crystals is clearly cleavage planes at an angle of about 67°. Both per­ shown by percussion figures. If a cleavage plate of cussion and pressure figures are very useful in de­ sheet mica is struck sharply by a blunt punch or large termining the crystallographic orientation of mica needle with a dulled point, a partial or complete six- books, especially where no crystal faces are present. rayed pattern of cracks is developed. Individual rays In this report the orientations of other features are of this percussion figure extend outward from the generally described in terms of percussion- and pres­ point of impact, and their orientation is constant re­ sure-figure directions, which serve as convenient ref­ gardless of where the mica piece is struck (pi. 1). erences. Two of the cracks, generally deeper and longer than Muscovite is transparent and nearly colorless when the other four, intersect the cleavage surface to form split into thin sheets, but most thick plates are dis­ a single line parallel to the trace of the clinopinacoidal tinctly colored in shades of green, brown, yellow, face (fig. 2). The others form two lines that meet orange, and pink. Phlogopite, the other mica com­ monly used as an electrical insulator, is yellow, golden brown, dark brown, or black; much of it has a coppery, translucent appearance, and in general it is less trans­ parent than muscovite. Freshly cleaved fragments of muscovite have a hard, brilliant luster, in contrast to the dull, rough surfaces of most books. The mineral ranges in hardness from 2.0 to 2.5 and is easily cut by a knife. In general the hardest books are the least flexible; commonly they can be split only with diffi­ culty. The specific gravity of muscovite ranges from 2.76 to 3.00. Many muscovite books are intergrown with biotite or enclose smaller biotite crystals, and some musco­ vite occurs as well-crystallized inclusions in books of biotite. The cleavages of the two micas generally are parallel. Inclusions of magnetite, hematite, and goethite occur in the muscovite of some deposits, com­ monly as latticelike crystals or crystal aggregates that are distributed in accordance with the crystal directions of the host mineral. Such inclusions are so FIGURE 2.—Pressure and percussion figures in muscovite, showing orienta­ abundant in some books that thin cleavage pieces of tion with respect to the prismatic (M) and clinopinacoidal (b) faces. the mica are opaque or nearly so. Apatite, garnet, quartz, zircon, tourmaline, and many other minerals at angles ranging from less than 53° to nearly 56° and also occur as inclusions within or between the mica are nearly parallel with the trace of the prismatic laminae. faces.5 A second type of figure can be developed by firmly PROPERTIES AFFECTING VALUE pressing a dulled point against the piece of mica, CLEAVAGE particularly if the mica rests on a flat but elastic sur­ The edges of most crystals or books of muscovite face. Like the percussion figure, this pressure figure are twisted, crushed, tangled, and irregularly inter- is six-rayed wherever complete, but only two or three grown with other minerals and hence must be cut rays are commonly developed. Some pressure figures away before the remaining material can be split into are formed simultaneously with percussion figures, films or thin sheets. Owing to their elasticity, strength, and the two have common centers. One set of cracks and almost perfect basal cleavage, undistorted books is perpendicular to the trace of the clinopinacoid and or parts of books readily yield films less than one- thousandth of an inch (1 mil) thick. Films that are 6 Walker, T. L., Observation's on percussion figures on cleavage plates of uniform in thickness and have fiat or very nearly flat mica: Am. Jour. Sci., 4th ser., vol. 2, pp. 6-7, 1896; Sterrett, D. B., Mica deposits of North Carolina: U. S. Geol. Survey Bull. 740, p. 12, 1923. surfaces are commercially the most desirable. Their COMMERCIAL SHEET MUSCOVITE value increases with their size, and the rate of in­ or by tapping them against a thick piece of wood or a crease is greatest in the largest sizes. Book mica that knuckle of the hand. Pieces of very hard mica sound yields such films is said to be free splitting. The films like glass when shaken together. can be easily tested for constancy of thickness by Flexibility and elasticity are important properties, means of machinist's or dial micrometers or by plac­ especially in mica that must be bent sharply without ing them between crossed polaroids and observing breaking or must be exposed to unusual jarring or vi­ whether uniform interference colors are obtained. bration. To meet most commercial specifications a Some mica does not split uniformly, but tears into sheet of mica 0.005 inch (5 mils) thick must return irregular partial films. Such material may split evenly and easily in some places but very imperfectly in others. It is known in the Southeastern States as 'locky," "gummy," "tangled," "tanglesheet,' 'tangle­ foot," or "tacky." The designation "tanglesheet" is also applied to coarse aggregates of irregularly inter- grown books, which commonly form masses of "bull" mica several feet in maximum dimension. Discon­ tinuity of cleavage, or lockiness, generally is caused by a partial intergrowth of books or of laminae in a single book (fig. 3), by internal distortion of the book, by finely divided inclusions, or—rarely—by twinning with composition planes nearly normal to the base (fig. 1). Many locky books do not differ markedly in appearance from those that split freely and evenly. The hardest varieties of muscovite commonly are more difficult to split than softer varieties, but numer­ ous exceptions are known. Many very dark varieties also are not free splitting, but others yield large, uni­ form sheets. There appears to be some correlation between the lockiness of Southeastern micas and the presence of disseminated flakes, shreds, and very thin plates of biotite and chlorite—especially in the FIGURE 3.—Inclusions in muscovite. A, Intergrowth of muscovite in musco­ vite. Orientations of sheets are shown by percussion figures. B, Inclusions hard brown, brownish-olive, or buff micas—but more of apatite (ap) and quartz (qtz) in muscovite. C, Locking inclusions of detailed microscopic study is needed to demonstrate muscovite in muscovite. The cleavage planes of the inclusions are not parallel with that of the host crystal. the mechanism and consistency of this relation. The effects of larger inclusions are clearer. Prismatic promptly to its normal planar condition after being crystals of apatite and quartz, for example, commonly wrapped around an ordinary lead pencil and then re­ lie nearly normal to the mica cleavage and hence are leased. Flexibility and elasticity are most seriously like nails driven through a series of thin boards (fig. affected by cracks, holes, and other structural defects. 3). Such mica is said to be "tied," "nailed," or "nail- Most of these are easily recognized, but some ex­ locked." Similar tying is accomplished by small elon­ tremely thin and fine cracks commonly escape detec­ gate crystals of muscovite that are oriented obliquely tion. These are known as "hair cracks" or "hair to the host crystal (fig. 3). lines." Films of mica in which some laminae are hair-

HARDNESS, FLEXIBILITY, AND ELASTICITY cracked generally fail when bent, especially if the flaws are abundant or if they extend through con­ The hardness of Southeastern mica varies from one siderable thicknesses of the films. Such mica is termed deposit to another, less commonly from one book to "brittle." another, and in some instances within a single book. Where exposed to the weather for long periods, In general the brown, cinnamon-brown, and buff muscovite loses its luster and gradually becomes soft, micas are harder than the greenish-olive and green pliable, and "punky." This weathering is chiefly a varieties in the Southeastern States. Where other mechanical process involving the separation and factors are equal, the hardest varieties are the least splitting off of cleavage laminae by moisture, temper­ flexible and consequently are among the most difficult ature changes, and vegetation, acting either singly or to split. Relative degrees of hardness are easily de­ in combination. Some chemical attack by organic termined by judging the ease with which sheets of acids and possibly by other acids as well reduces many known thickness can be cut, by bending them slightly, of the separate laminae to lusterless, opaque, crumbly 8 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES flakes. Any inclusions of magnetite or hematite that ward from below. Such reeves are thus a combina­ are present in such mica generally are altered to hy­ tion of corrugations and the edges of incomplete lami­ drous iron oxides. nae (fig. 4). Their depth is a function of the number of missing laminae, and their spacing is a function of STRUCTURAL IMPERFECTIONS the distribution of discontinuities in the laminae. Perfection of cleavage is of fundamental impor­ The edges of some incomplete sheets are flanked by tance in governing the usefulness of sheet muscovite, series of narrow ribbons or strips, and the reeves in but the size and flatness of sheets obtainable from such mica are very closely spaced (fig. 4). mica books are limited by several other structural Reeves are specifically oriented with respect to features as well. Chief among these are "reeves," crystallographic directions in the mica. They are per­ "wedging," "warping," and "ruling." pendicular to the trace of the prismatic and clino- "Reeves," or "cross grains," are lines, striations, pinacoidal faces and hence are parallel to the rays of shallow corrugations, or small, narrow folds that lie the pressure figure. "A" ("housetop," "fishtail," "V- in the plane of cleavage. Some reeves are simple, ridge," or "spearhead" ) mica is distinguished by two closely spaced flexures or crenulations, presumably series of reeves that intersect at an angle of about 60°. caused by stress during or after crystallization of the The third series necessary to complete the "A" is not mica. Others, however, are formed by discontinuities present, so that the structure actually resembles the in incomplete sheets or laminae. As traced across a letter "V." Typically a single pair of "A" reeve mica book, such laminae die out abruptly along groups extends across an entire mica book, with the straight lines but commonly reappear along parallel point of the "A" very near one edge (pi. 2). Such straight lines beyond. Where the distance between books rarely show well-developed crystal faces. lines is very small and the space from which the lami­ Three directions of reeves are present in "double- nae are missing is accordingly narrow, the reeves ap­ A" mica, and their pattern suggests two pairs of "A" pear only as fine lines, but where laminae are missing reeve groups with a common point and a common side over greater distances the spaces are occupied by ad­ (pi. 2, no. 6). The relation of "A" structure to crystal jacent laminae warped downward from above and up­ directions is best shown in books with reeves that ex­ tend in six directions from a common center. In de­ scribing such a book from the Marie mine, Spruce Pine district, N. C., Kesler and Olson 6 state that "a complete six-sided crystal, 2.5 inches in diameter, was divided at intervals of 60° by typical 'A' reeves into six parts like the pieces of a pie, the reeves diverging from the center, and each set bisecting the face of the mica crystal." Books with only one pair of "A" reeve groups thus can be considered as highly distorted crystals representing little more than one-sixth of a perfect crystal of flat mica (fig. 5). Some "A" books in which the imperfections are shallow, widely spaced, or otherwise not seriously de­ veloped can be split into sheets of commercial value. "A" structure is developed only near the edges of other books (pi. 2, no. 3), and flat sheets of good quality can be split from their inner parts after the reeved ma­ terial is trimmed away. Very large trimmed sheets have been obtained from such "flat-A" mica in many Southeastern mines. In still other mica the reeves are confined to certain sheets or groups of sheets, so that

FIGURE 4.—Diagrammatic sections, showing the relation of distribution and 6 Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, depth of reeves to crenulations and discontinuities in muscovite laminae. N. C.: U. S. Geol. Survey Bull. 936-A, p. 14, 1942.

EXPLANATION OF PLATE 1 1. Cluster of books with poorly developed crystal faces. The muscovite is 4. Books with somewhat irregular crystal faces. One-inch square gives locally intergrown with white albite. scale. 2. Parallel growth of small, well-developed crystals. 5. Percussion figures in trimmed sheet. One-half natural size. 3. Single wedged book and associated irregular books of much smaller size 6. Books with smooth to very rough crystal faces. One-half natural in plagioclase^quartz pegmatite. One-fourth natural size. size. OEOIXXMCAI. Sl'HVEY PROFESSIONAL PAPER 22.5 PLATE 1

BOOKS AM) CLEAVAGE PIKCES OF MUSCOMTE

GEOLOGICAL STRVEY PROFESSIONAL PAPER 22.1 PLATE 2

REEVE STRUCTLBES liN MUSCOVITE COMMERCIAL SHEET MUSCOVITE 9 imperfect material can be split out and good sheets commonly contain a single set of reeves. If the reeves obtained from the remainder of the book. Much "A" are thin and fine, the material is sometimes referred mica, however, is so seriously marred that it must be to as "hair-lined." Such reeves are not to be confused, classed as scrap. however, with the true hair lines and hair cracks pre­ "Herringbone" ("fishbone," "fishback," "feather," viously discussed. or "horsetail") mica is marked by reeves and hence is "Wedge" structure, or "wedging," is caused by the somewhat similar in appearance to "A" mica; how- interlayering of sheets of unequal size. Some extend entirely across the book, whereas others taper out at intermediate points. Books in which incomplete lami­ nae extend inward from all edges can be externally regular in shape, but owing to their complex internal wedging they yield no sheet mica. Books in which a preponderance of incomplete laminae extends in­ ward from one edge are markedly thicker on one side than on the other (pi. 3). Wedge structure is common in herringbone and "A" micas, and the term "wedge- A" is used in contradistinction to "flat-A." Herring­ bone and most wedge-A books consist almost wholly of scrap, whereas many flat-A books contain appreci­ able quantities of sheet material. Not all wedged mica is marked by reeves, and Sterrett 7 states that in plate mica the difference in thickness on opposite edges may be greater than half an inch in a crystal 3 inches in diameter. The amount of wedging generally is greater in reeved than in un- reeved mica, however, and wedge angles of 25° or more are common in "A" books. Small, thickly wedged "A" books are known as "chub-A" (pi. 3, no. 5). "Ribbed," "rippled," "ridged," or "creped" mica is marked by waves or ridges, generally shallow, that are not assignable to "A" structure or to other reeve FIGURE 5.—Relation of reeves to crystal directions in muscovite. Orientations groups. Unlike reeving, most ribbing and rippling are shown by percussion figures. A, Complete directional development of "A" reeves in crystal; B, Typical development of "A" reeves; C, Typical appear to be the result of deformation of the mica herringbone reeves. after, rather than during, crystallization. Some rip­ ples are traceable along their strike into broad warps, ever, the angle of intersection of the reeve groups is some may be traced into fractures or partings, and about 120°, rather than 60°. The reeves characteris­ others die out abruptly (pi. 3). Still others extend tically flank a central line or strip of reeves to form a across entire cleavage surfaces without marked pattern resembling that of a feather or the skeleton change in size. In general these minor warps or of a fish (pi. 2, no. 7; fig. 5). The central line or strip crenulations are spaced much farther apart than generally is perpendicular to the trace of the clino- typical reeves, and good sheet material can be recov­ pinacoidal crystal face. Few herringbone books from ered from parts of many rippled books. Sheets that deposits in the Southeastern States contain flat ma­ are only slightly affected are termed "wavy," and terial; hence nearly all are processed as scrap. mica that is bent on a broad scale is said to be "buck­ Combinations of "A" and herringbone structures led," "warped," or "cupped." Many types of deforma­ occur in some books, particularly those of the flat-A tion occur, and all gradations are known between flat type. The herringbone reeves generally are discon­ tinuous and irregularly distributed in such mica. 7 Sterret, D. B., Mica deposits of the United States: U. S. Geol. Survey Bull. Books not marred by "A" or herringbone structure 740, p. 16, 1923.

EXPLANATION OF PLATE 2 1. Flat-A mica. One-fourth natural size. 6. Partial intergrowth of two books of "A" mica. One-half natural size. 2. Severely reeved mica, split from same book. 6. Double-A structure, with flat material between reeve groups. One-half 3. Flat-A or broad-A mica, with reeves near edges only. One-fourth natural natural size. size. 7. Herringbone structure in somewhat clay-stained mica. Two-thirds natural 4. "A" mica with widespread development of reeves. One-fourth natural size. size. 10 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES and severely buckled mica (fig. 6). An extreme ex* tire thickness of severely ruled books, but in others it ample of deformation is the chevronlike folding of is confined to certain layers in which it may extend some mica books in the Meadow deposit, Spruce Pine partly or entirely across the cleavage faces (pi. 4). district, N. C. (pi. 3, no. 10). Another type of folding, Where one set of ruling planes is well developed, the known as "cleavage stepping," comprises small, sharp mica is thereby separated into strips or ribbons that monoclinal flexures. These are subpiarallel, and they commonly are less than an inch wide. The ruling in typically distort the cleavage faces into series of some mica is so closely spaced that individual ribbons broad, low steps (pi. 3). are sliver like or hair like (pi. 4, no. 3). Accumulations of such slivers are termed "hair mica." Ribbons in some large books, on the other hand, are as much as 4 or 5 inches wide and hence yield satisfactory sheets if free from other defects. Ruling appears in much "A" and herringbone mica, where it either coincides in di­ rection with the striations and corrugations or forms the cross bar of the "A" (pi. 4, no. 4). It is more com­ mon, however, in unreeved books, where its distribu­ tion is of prime importance in determining the sizes of sheets that can be trimmed out. Ruling, warping, buckling, and rippling are most intense in books that occur near faults or slip joints and plainly are the result of distortion from post- crystallization movements. Both percussion figures and ruling are strongly developed in some books that lie near blast holes, where they appear to have been formed by the shock of the blasting. Such ruling gen­ erally occurs in small hexagonal or rhombic patterns (fig. 7). D

E FIGURE 6.—Diagrammatic sections, showing deformation of Muscovite laminae. A, Broadly -warped, or -wavy, sheets; B, Warped or rippled sheets ; C, Gradation of -warped mica into buckled or folded mica; D, Mica cut by parting planes (ruling); E, Cleavage-stepped mica.

One of the commonest structures in mica books is "ruling," or "secondary cleavage," which occurs as regular, sharply defined parting planes that intersect the basal cleavage plane at an angle of nearly 67°. They are parallel to the pressure figure. Only one set of these partings is present in many books, but two or all three sets occur in others. Their traces on cleav­ age surfaces intersect at angles of about 60°, and where all three sets are present they commonly sepa­ rate the sheets into triangular or hexagonal frag­ ments (fig. 7). Many sheets ruled in two directions are similarly separated into rhombic or diamond- shaped fragments or into strips and laths. FIGURE 7.—Types of fragments formed by breaking along parting or ruling planes in a muscovite crystal. Orientation is shown -with respect to per­ The structure generally continues through the en­ cussion-figure directions and crystal faces.

EXPLANATION OF PLATE 3

1. Wedge-A book. One-half natural size. 5. Chub-A book. Approximately natural size. 2. Edgewise view of same book. Approximately three-fifths natural size. 6. 7. Ribbed, or cleavage-stepped, mica. Two-fifths natural size. 8. Curved -wedge-A book. One-half natural size. 8, 9. Buckled, or cupped, mica. Two-fifths natural size. 4. Edgewise view of same book. Approximately three-fifths natural size. 10. Severely folded mica. One-half natural size. GEOLOGICAL STRVEY PROFKSSIONAL PAPER 225 PLATE 3

WEDGE STRUCTURE AM) DEFORMATION 11N MUSCOVITE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 225 PLATE 4

RILING J]\ CLEAVAGE PIECES OF MUSCOVITE COMMERCIAL SHEET MUSCOVITE 11

COLOR color differences in the so-called ruby and rum vari­ Most mica is distinctly colored, especially in sheets eties. The colors of smooth-faced crystals, viewed one-sixteenth of an inch or more thick. Thick sheets parallel to their cleavage, generally are pale tints of and plates of commercial muscovite have been vari­ yellow, yellowish green, green, or yellowish brown, ously described as "red," "ruby," "rum," "red rum," whereas the colors as seen perpendicular to the cleav­ "green," "amber," "yellow," "gray," "white," "water- age direction are darker and commonly are more colored," and "black," with or without modifying brownish or reddish. Muscovite is much more trans­ terms indicating tints, shades, or intermediate hues. parent to light rays parallel to the cleavage than to Much confusion has arisen from duplication and in­ those normal to it; thus some books of cinnamon- consistencies in the usage of these terms. "Amber" brown mica 3 inches in diameter and a quarter of an mica, for example, is the common trade name for inch thick are fully as transparent when viewed edge­ phlogopite; therefore the term "amber" is not wholly wise as when viewed from top or bottom. desirable for muscovite. Deep-brownish to greenish- Few mica books are colored uniformly throughout. olive mica is termed "water-colored" in North Caro­ Some are crystallographically zoned and yield cleav­ lina, whereas the same designation is applied to very age plates with well-defined color bands that are called light green mica in northern New Mexico. "White" "mine markings" in several Southeastern districts. and "gray" are used either to describe mica that is The bands are parallel to crystal outlines and hence very pale in color or to distinguish any thinly split are generally hexagonal or rhombic in plan (pi. 7). muscovite from the amber phlogopite. Some micas Alternating narrow bands of slightly but distinctly with many air bubbles or clouds of minute light- differing shades of brown or green characterize the colored inclusions also are referred to as "gray." books of some deposits. They are most abundant near "Black" is applied to muscovite with abundant inclu­ the rims of most zoned books but are concentrated sions of magnetite or hematite and also to sheets of near the centers or are scattered throughout others. dark-brown, greenish-brown, or black biotite ("black­ Broader color bands are known from many deposits jack"). "Red" mica either contains numerous inclu­ but in general are not so common. Color differences sions of brightly colored goethite and turgite or is in- between adjoining bands generally are greater in terlayered with iron-stained clay. broadly zoned books than in those with more abundant In general the color of commercial muscovite, as but narrower bands. Crystallographic zoning in viewed by light transmitted through cleavage pieces, many books appears only as narrow rims or as small ranges from drab and pinkish buff through reddish centers or cores. Some pinkish-buff and cinnamon- brown and shades of brown and green to pale yellow­ brown micas are marked by distinctive color patterns ish green and yellow. The color of muscovite is chem- of a grating or gridiron type (pi. 7). The distribution ico-compositional,8 as it depends upon composition for of pattern areas in otherwise uniformly colored sheets pieces of a given thickness. Studies in the biotite is related to the crystal directions of the mica. group have indicated that iron causes a green color, The margins of nearly all books are distinctly titanium produces brown and red, and magnesium lighter in color than their inner parts. This appears dilutes or masks the effects of titanium,9 but no such to be a bleaching effect and is recognizably later studies appear to have been made in the muscovite than true Crystallographic color zoning. Similar group. Systematic and detailed tests for color-compo­ bleached areas flank cracks and surround holes and sition relations in several muscovites from the South­ many inclusions. The boundaries between the bleached eastern States are being made at the present time, and and the unaffected parts of the books are gradational those thus far completed indicate that variations in and irregular. Some mica is mottled, with very ir­ iron content are accompanied by appreciable varia­ regular splotches of one color in a background of a tions in color. slightly different color. Most of this mica is clear, Muscovite is distinctly pleochroic, with the greatest transparent, and otherwise of sound appearance. Color has affected the merchantability of musco­ * Kennard, T. 6., and Howell, D. H., Types of coloring in minerals: Am. vite since the days when its chief uses were based Mineralogist, vol. 26, p. 418, 1941. largely upon transparency. The light-brown and 9 Hall, A. J., The relation between colour and chemical composition in the biotites: Am. Mineralogist, vol. 26, p. 29, 1941. light-green micas, which yield more transparent

EXPLANATION OF PLATE 4 1. Part of thick book, bounded by two well-developed ruling planes. Ap­ piece at top. Hair mica, developed by very closely spaced ruling along proximately natural size. edge of broken fragment, appears in two pieces at bottom. One-half 2. Deeply ruled pieces from large, flat book, with typical strip mica, or natural size. ribbon mica, at bottom. One-third natural size. 3. Deeply ruled and cracked pieces from slightly warped book. Ruling inter­ 4. Large piece of flat-A mica, showing relation of ruling to reeve directions. rupted by inclusion of quartz elongated parallel to the ruling plane in Approximately one-half natural size. 12 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES sheets of a given thickness, commanded a higher Iron stain. price than the darker-colored micas. This was carried Manganese stain. over into periods of increasing electrical uses, the Vegetable stain (organic). dark-brown and brownish-green micas being "classed The primary stains were formed during or soon after as 'No. 2,' even when flawless and clear." 10 Later, crystallization of the mica and hence are distributed when the scrap-mica industry first expanded into without regard to the position of the deposit with re­ prominence, it was found that the lighter-colored spect to the present land surface. These blemishes micas yield a whiter ground product; hence such are as likely to increase with depth as to decrease. varieties of scrap were sold for higher prices. Secondary stains, in contrast, occur only at or near In selecting sheet mica for electrical uses during the surface and characteristically are absent from recent years, purchasers have placed varying degrees those parts of the deposit beneath the oxidized zone. of emphasis upon the desirability of the so-called ruby micas as contrasted with the green and es­ PRIMARY IMPURITIES pecially the dark-green varieties. It has been stated "Air-stained" mica contains flattened pockets, tiny repeatedly that dark or green muscovites "usually bubbles, or groups of closely spaced gas bubbles (pi. are poorer dielectrics," but the basis for such state­ 5, no. 1). Clusters of very small bubbles are known ments is not entirely clear. At best, they appear to as "silver spots" in the Southeastern States. Other be generalizations extrapolated from scattered and types of air stain include grayish to silvery spots, unsystematic data. That they are open to serious specks, and lines, many of which are so closely spaced question has been plainly indicated by several series that they destroy all transparency of the mica. Where of tests made during recent years, and their accept­ they are confined to certain sheets they can be re­ ance therefore might well be deferred until the effects moved by careful splitting, but they are scattered of color have been more fully investigated. through all parts of many books. In general the ef­ fects of air inclusions on the splitting and electrical STAINING, INTERGROWTHS, AND INCLUSIONS qualities of muscovite are not so serious as the effects of most mineral stains, and moderately to heavily The presence of visible mineral or vegetable im­ air-stained mica is unsatisfactory only for certain purities in book mica generally reduces its value. types of specialized electrical equipment. Some impurities occur as crystals or crystal groups Some mica is marked by a pale-green, yellowish, that extend through considerable thicknesses of the or greenish-brown discoloration that is termed mica; they must be removed and the immediately "vegetable stain." Where essentially primary, such surrounding mica trimmed away before the remain­ stain consists of minute scales or finely divided ag­ der of the book can be split into sheets. Others are gregates of chlorite, biotite, or material rich in fer­ finely divided or thinly flattened between the mica rous iron. Individual crystals and mineral masses laminae, and the part of the book in which they occur cannot be recognized megascopically or even with low generally is trimmed away and either discarded as magnifications under the microscope. Most vegetable scrap or prepared as sheet stock of inferior grade. stain is evenly distributed as extremely thin, curdy The proportion, distribution, and type of inclusions, aggregates. Similar material that occurs as separate intergrowths, and stain lead to such designations as clumps is generally referred to as "mottling." Specks, "mottled," "specked," "spotted," "freckled," "lined," spots, and lines of such stain also are known (pi. 5). "black," "black-stained," "black-spotted," "lightly Vegetable stain and mottling rarely are so dense that stained," "heavily stained," "dotted," "powder- they seriously affect the transparency of mica that specked," "blotched," and "curdy." otherwise is of good quality. Like air stain, they Two general types of impurities occur in com­ generally are significant defects in terms of the most mercial muscovite. These are most simply desig­ exacting end uses only. nated as primary and secondary and can be subdi­ Deep grassy-green specks, spots, and lines are vided as follows: sparsely scattered through much book mica. These Primary impurities: are like typical vegetable stain in general curdy ap­ Air stain. pearance, but individual masses are darker and more Mottling and "vegetable stain" (inorganic). clearly defined and appear to be somewhat denser Mineral intergrowths and inclusions. than the scales and aggregates that compose most Secondary impurities: vegetable stain. Some mica contains scattered green, Air creep. brown, or reddish-brown spots and "bursts" of curdy Clay stain. stain. Where they are a quarter of an inch or more in diameter, the brown spots are commonly termed 10 Sterrett, D. B., Mica deposits of the United States: U. S. Geol. Survey Bull. 740, p. 17. 1923. "cigarette burns." In some districts the occurrence COMMERCIAL SHEET MUSCOVITE 13 of brown and green spots with fuzzy edges and dark, of tiny inclusions lead to recognition of phantom well-defined centers leads to the designation "frog- mica crystals that probably are analogous to the well- eye" mica. Some spots and bursts are haloes of dis­ known phantom quartz crystals. The thinnest in­ coloration that surround tiny inclusions of zircon clusions of magnetite do not affect the general split­ and allanite, but others contain no recognizable cores ting quality of the muscovite, and sheets and films of foreign material. that enclose such plates, laths, and needles are easily Mineral stain, which comprises intergrowths and split. The thicker inclusions, in contrast, tie the mica inclusions of recognizable crystals, is the most seri­ on a small scale and thus seriously impair its filming ous of the primary impurities. Among the minerals properties. Such mica is sometimes referred to as that occur within books of muscovite are actinolite, "spot-welded," "spot-locked," or "black-pitted." albite, allanite, apatite, beryl, biotite, brookite, chlor- Hematite, the most abundant and widespread in­ ite, columbite, dumortierite, epidote, fluorite, garnet, clusion mineral in commercial muscovite, occurs as hematite, kyanite, magnetite, manganese oxides, flattened skeletal crystals with a hexagonal outline, marcasite, microcline, pyrite, pyrrhotite, quartz, laths and flattened needles, simple and complex den­ rutile, sillimanite, sphene, staurolite, thulite, topaz, dritic forms, and latticelike forms that are extreme tourmaline, vermiculite, zircon, and zoisite. The dis­ developments of skeletal crystals (pi. 6). The lat­ tribution and shape of many of these minerals are tices are characteristically triangular, with the three influenced or controlled by crystal directions in the elements parallel to rays of the percussion figure or, host mica. This is especially clear in the case of mag­ much less commonly, to the rays of the pressure fig­ netite, hematite, and some manganese oxides but can ure in the enclosing mica. Other elements lie per­ be demonstrated for other minerals by means of pendicular to the three principal directions of some statistical studies only.11 Probably few of the included lattices, and the symmetry of the whole is hexagonal. minerals are unoriented in the strictest sense. The hematite inclusions are black, dark brown, red­ Magnetite and hematite are the most common in­ dish, smoky brown, and buff (pi. 7), and none show clusion minerals. Magnetite occurs as laths, needles, the bluish shades of most very thin magnetite inclu­ skeletal forms, and flattened crystals that are six- sions. In general they are more transparent than the sided in plan (pi. 5). In general, these last are of an magnetite crystals. octahedral-dodecahedral combination form 12 and are The dendritic and latticelike growths have been characterized by prominent octahedral parting identified as magnetite by many investigators 14 and cracks. Some of the inclusions, gray to bluish gray or as hematite by others.15 It can be demonstrated that violet, are so thin that they are transparent. Crystal those in micas from the southeastern and southwest­ outlines and parting directions are oriented in ac­ ern United States are hematite, and those in micas cordance with the pressure and percussion figures. from other localities probably are hematite as well. The six-sided crystals are about 9 millimeters in Frondel and Ashby 16 have summarized the principal maximum diameter, with an average of less than 1 differences between magnetite and hematite inclu­ millimeter. The markedly elongated crystals are at sions in muscovite, chiefly on the basis of detailed least 4 millimeters in average length, with maximum studies of collections from the northeastern United recorded lengths of 10 centimeters.13 Most magne­ States. tite inclusions are less than 0.01 millimeter thick, Some hematite laths and plates are bounded by but some are 0.1 millimeter and a few nearly 1 milli­ smooth and regular crystal faces, but the edges of meter thick. most are so irregular that they create a feathered or The inclusions in much of the so-called specked dendritic appearance. All are extremely thin, espe­ and lightly specked muscovite are magnetite, char­ cially as compared with their areal extent, and they acteristically scattered through the books. In some are not separable from the enclosing mica by ordi­ books, however, they are confined to certain sheets nary mechanical means. Their maximum thickness is or groups of sheets, and in others they occur in well- considerably less than 0.01 millimeter, whereas most defined belts parallel to "A" reeves or in concentric occupy areas of several square centimeters. Individ­ zones parallel to crystal faces of the host mica. The ual lattices and skeletal crystals more than 3 feet zonal arrangements appear to reflect the stages of long occur in some large books. Where the hematite mica growth, and some very narrow individual zones 14 Dana, J. D., and Brush, G. J., On the magnetite in the mica of Penns- 11 Frondel", Clifford, Oriented- inclusions of tourmaline in muscovite: Am. bury, Pa.: Am. Jour. Sci., 2d ser., vol. 48, pp. 360-362, 1869; Sterrett, D. B., Mineralogist, vol. 21, pp. 777-799, 1936; Oriented inclusions of staurolite, zir­ Mica deposits of the United States: U. S. Geol. Survey Bull. 740, pp. 17-18, con, and garnet in muscovite; skating crystals and their significance: Am. 1923. Mineralogist, vol. 25, pp. 69-87, 1940. 15 Rose, G., Aaterism: Preuss. Akad. Wiss. Monatsber., pp. 614 et seq., 12 Frondel, Clifford, Oriented inclusions of tourmaline in muscovite: Am. 1862; Frondel, -Clifford, and Ashby, G. E., Oriented inclusions of magnetite Mineralogist, vol. 21, p. 107, 1936. and hematite in muscovite: Am. Mineralogist, vol. 22, pp. 104-121, 1937. 18 Idem. 16 Frondel, Clifford, and Ashby, 6. E., op. cit. 862911°—60—2 14 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES stain is dense, it consists of many superposed crystals pseudomorphs of other iron oxide minerals (pi. 7). that are separated by thin films of mica, and where Most appear to have been formed by alteration of it is less dense, correspondingly fewer crystals or hematite and magnetite inclusions, but some of the lattices are present. The stained parts of some books most finely divided scales may well have been de­ can be split out, leaving clear sheets of high quality. veloped directly by precipitation from solid solution The stain is confined to the centers or to one or more in the mica. sides of other books, in which the hematite-bearing Quartz and albite are interlayered with some mus­ mica can be trimmed away from the clear material. covite plates to form composite masses of little or no The outer parts of hematite-stained books are char­ economic value. The edges of other books are inter- acteristically free from inclusions, even where the re­ grown with these minerals. Books in which quartz, mainder of the mica is very heavily stained. Clear apatite, or tourmaline spindles are present are known mica commonly flanks cracks and parting planes and as "gritty," "sandy," "stony," or "sand-pitted." surrounds holes in books that elsewhere contain hema­ Where the axes of such spindles are oblique or per­ tite inclusions. The distribution of such stains also is pendicular to the cleavage, the impurities effectively influenced by some warping, rippling, and other sec­ tie the books. Actinolite, allanite, beryl, kyanite, ru- ondary structures in the mica. Zonal concentrations tile, tourmaline, zoisite, and other species of elongate of hematite comparable to those of magnetite have habit commonly occur as individual crystals, bundles, not been observed, and in general the hematite ap­ and parallel groups of crystals, sprays, and rosettes pears to have developed after crystallization of the (pi. 8). Although they lie parallel to the cleavage muscovite. On the other hand, its distribution within surfaces, they generally penetrate enough laminae of individual unaltered mica books and its occurrence in the mica to affect its splitting properties seriously. deposits at considerable depths beneath the oxidized Many of the crystals are oriented parallel to rays of zone preclude its explanation as an ordinary altera­ the pressure or percussion figures. Others appear to tion product of the mica or as material introduced be distributed at random, although preferred orienta­ from sources outside the mica books. It appears rather tions for several minerals have been demonstrated by to have crystallized from solid solution in the musco­ statistical studies.18 vite, as suggested by Frondel and Ashby,17 and as such Fluorite, garnet, pyrite, and other minerals occur is here considered "primary" in the same sense as as inclusions that are much flattened parallel to the analogous exsolution growths in metallic ore minerals. plane of cleavage in the mica. They are characteris­ Iron oxide films and spots, presumably of exsolution tically equant in that plane. The flattening is extreme origin, can be developed experimentally by heating in many books, but in others the inclusions are much some varieties of clear muscovite to temperatures of thicker and they tie considerable numbers of mica 550° C. or more, but typical hematite dendrites and laminae. The average diameter of garnet inclusions triangular lattices are not so simply reproduced. probably is less than 5 millimeters, although flattened Inclusions of hematite appear to have little effect tablets an inch or more in diameter are known. Most upon the splitting qualities of muscovite, at least as are less than 0.3 millimeter in thickness, and many far as its commercial preparation is concerned. Ordi­ are as thin as 0.05 millimeter. These dimensions also narily the stained parts of books with hematite lat­ apply to other inclusion minerals of similar habit. tices or spots split as easily and uniformly as the un­ The prismatic and other elongated inclusions are of stained parts, and there is no perceptible tying of comparable thickness but commonly reach lengths of sheets. Both hematite and magnetite, however, seri­ several inches. ously increase the conductance of the mica in which Biotite is intergrown with muscovite in many de­ they occur and hence lower its value. The amount of posits. Inclusions of biotite in muscovite are common lowering depends upon the thickness, abundance, and (pi. 8), but inclusions of muscovite in biotite are distribution of the inclusions. sparse or rare. In some districts the muscovite is in- Goethite occurs in sheet mica as yellow, orange, 18 Frondel, Clifford, Oriented inclusions of tourmaline in muscovite: Am. red, reddish-brown, or brown scales, stains, and Mineralogist, vol. 21, pp. 777-799, 1936; Oriented inclusions of staurolite, zircon, and garnet in muscovite; skating crystals and their significance: Am. 17 Frondel, Clifford, and Ashby, G. E., op. cit. Mineralogist, vol. 25. pp. 69-87, 1940.

EXPLANATION OF PLATE 5 [Figures approximately three-quarters natural size] 1. Air stain. 6. Flattened crystals of magnetite elongated parallel to pressure-figure direc- 2. Dense green mottling and vegetable stain. tions. Note thin, fine color strips and air creep, which appears as dark- 3. Grassy-green stain in form of rectangular lattice. gt&y areaa fa ^ mica shee^ 4. Curdy green stain with little systematic arrangement. 5. Dense lattice of green stain and abundant vegetable stain, with black spots of hematite. 7. Scattered specks and small spots of hematite. Widespread air creep. GEOLOGICAL STRVEY PROFESSIONAL PAPER 225 PLATE 5

PRIMARY STAIN J1N MUSCOVITE

GEOLOGICAL PURVEY PROFESSIONAL PAPER 225 PLATE 6

PRIMARY HEMATITE STA^ IN MUSCOVITE COMMERCIAL SHEET MUSCOVITE 15 tergrown with vermiculite, which presumably was with muscovite inclusions or of muscovite cores with derived from biotite by alteration. Where biotite and successive rims of muscovite. These are difficult to muscovite are intergrown, the cleavages of the two recognize but probably are not common. minerals generally are parallel, but the pressure and percussion figures of the inclusions are commonly SECONDAKY IMPUEITIES oriented perpendicular to those of the host (fig. 8). A few inclusions of biotite are elongated normal to their "Air creep," a secondary stain, is similar in appear­ cleavage direction and lie oblique to the enclosing ance to some types of air stain. It is merely air that muscovite, thus tying the sheets of the host book. Most enters the mica sheets through their edges and pene­ of the inclusions are bounded by well-formed prism trates them along cleavage planes (pi. 5). In general and pinacoid faces, and the contacts between the two it is caused by rough handling during preparation of micas are tight. Other biotite crystals, however, are the mica, especially by trimming with shears or a markedly pyramidal in habit, and their contacts with dull knife. This type of stain is easily distinguished the muscovite intersect the mutual cleavage surface from primary air stain, which consists of many small at low to moderate angles. Such inclusions are less bubbles or larger air pockets that are fully and firmly firmly joined to the host mineral and tend to pop out enclosed. The creep pockets, in contrast, are either when the mica book is split. connected with the trimmed or natural edge of the sheet or can be so connected by pressing the mica be­ tween the thumb and forefinger. They do not consti­ tute a significant defect unless they occupy so many cleavage openings that the transparency of the mica is materially reduced. Most books that have been exposed to weathering and the action of downward-percolating surface waters are coated with calcite, chalcedony, clay min­ erals, hydrous iron oxides, manganese oxides, or other secondary minerals. Where these have been deposited by waters that penetrated between the laminae of the mica, they must be removed by careful splitting and trimming (pi. 8). The value of the books is thus ma­ terially reduced. Books marred chiefly by silica, cal­ cite, and clay minerals are termed "clay-stained," whereas those that are strongly colored yellowish, reddish, or brownish by iron oxides are referred to as "iron-stained." The term "manganese-stained" has a similar basis. The organic type of vegetable stain, a truly second­ ary feature characteristic of the weathered zone in FIGURE 8.—Biotite-muscovite intergrowths. A, Biotite in muscovite, with mica deposits, consists of plant material that coats orientation shown by percussion figures; B, Muscovite in an inclusion of biotite. the outer surfaces and some of the cleavage laminae of mica books. Some of the material is carried into the A few inclusions of biotite in muscovite themselves mica by waters that penetrate fractures and cleavage contain inclusions of muscovite (fig. 8), and the re­ cracks, and some forces .its way into the books lations of some of these "multiple crystals" are some­ through the action of growing plants. The near-sur­ what similar to those of crystal zoning. The biotite face mica of many deposits is veined by the roots of inclusions in some books are merely rinds around grasses, bushes, and even trees. Most organic vege­ smaller muscovite crystals, and in others they sur­ table stain is accompanied by heavy clay and iron round muscovite that forms the core of the book. staining, and books so affected yield little usable sheet Other composite books are composed of muscovite material. On the other hand, appreciable quantities

EXPLANATION OF PLATE 6 [Figures approximately three-fifths natural size] 1. Dense aggregates of spots and triangular lattices confined to sectors of 4. Lathlike intergrowths, showing relation to crystal faces. Note absence a book. of stain near edges of book. 2. Coarse spots and thin-bladed, latticelike crystals. 5. Thin triangular lattices, with simpler rows of spots at top. 3. Blotches, small spots, and pale-green vegetable stain. 6. Coarse blades in lattice arrangement, with rows of dendrite spots. 16 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES of clear sheets can be obtained from many near-sur­ others are so small or inconspicuous that they are face books that are marred by only one of these stains. easily overlooked. High dielectric strength and heat resistance are particularly desirable in mica used for ELECTRICAL- PROPERTIES electrical generating equipment. A very significant electrical property of mica used The extremely low electrical conductivity of musco­ in condensers is its power factor (PF), which is a vite is a very important property as far as most of its measure (expressed in percent) of the loss of elec­ commercial uses are concerned. Unstained mica is trical energy in a condenser in which the mica is the the least conductive and is therefore suited for elec­ dielectric. Such loss is due chiefly to dielectric absorp­ trical equipment of the best quality. Many mineral tion, which produces a component of the applied volt­ inclusions, particularly magnetite and hematite, in­ age in phase with the current as though an additional crease the conductivity; hence stained mica generally resistance were placed in series with the condenser. is used in articles that do not require the most effec­ Excessive overheating and damage result from high tive insulation. Conducting impurities in sheet mus­ power losses; hence good condenser mica must have a covite can be detected by means of a high-voltage power factor of less than 0.04 percent at a frequency spark, which causes glowing or small-scale arcing as of 1 megacycle. The Q value, a factor more commonly it passes through the mica at or near the inclusions. used in recent years, is the reciprocal of the power The dielectric constant (K), or specific inductive factor, so that Q = 1/PF. The Q value of good con­ capacity, of muscovite is the ratio of the capacitance denser mica therefore should be at least 2,500. of a condenser in which the muscovite is the noncon­ ducting substance to the capacitance of a condenser USES in which air (or, more exactly, a vacuum) is the di­ electric. Values for this property generally are given The uses of mica are based upon its perfect cleav­ in terms of specific conditions of applied voltage, age, exceptionally low conductivity of heat and elec­ frequency of applied current, and temperature, mois­ tricity, high dielectric constant and dielectric ture content, and other features of the mica in ques­ strength, low dielectric loss, heat resistance, nonin- tion. K for sheet mica ranges from 2.0 to about 8.5 but flammability, mechanical strength, flexibility, elas­ generally is more than 6.5, with an average of about ticity, transparency, luster, and lubricating proper­ 7.2. It is a property of great significance for many ties and the ease with which it can be worked into electrical uses but is so uniformly satisfactory in final form. The degree of emphasis placed upon given micas that are otherwise of good quality that limiting properties by the purchaser depends upon the specific values are rarely specified by purchasers. end use involved.19 Flexibility is particularly impor­ The dielectric strength of muscovite is its ability to tant, for example, in the "cigarette" mica used in resist break-down or rupture under conditions of high spark plugs for aircraft engines. This material, in voltage or electric field strength. It is defined in terms films 0.0012 inch or less thick, is wrapped around rod- of the maximum potential gradient that material of a like spindles a little more than an eighth of an inch in given thickness can withstand and generally is ex­ diameter. In condenser mica dielectric properties are pressed in kilovolts per centimeter or volts per mil most desirable, and—in contrast—the use of mica for of thickness. It is tested by means of the high-voltage windows in furnace walls and doors is founded upon spark (concurrently with testing for conducting im­ its transparency, heat resistance, and mechanical purities in the mica) or—more commonly—by ap­ strength. plying high voltages through spherical or platelike A very high proportion of all sheet mica is used as contact electrodes. Dielectric weakness is caused by electrical insulating material. Washers, disks, and pinholes, cracks, tears, and other inhomogeneities or discontinuities in the mica sheets. Many of these are 19 Wierum, H. F., and others, The mica industry: U. S. TarifE Gomm. Rept. 130, 2d ser., pp. 11-26, 1938 ; Spence, H. S., Mica: Canada Dept. Mines, recognizable in ordinary visual examination, but Mines Branch, Pub. 701, pp. 102-120, 1929.

EXPLANATION OF PLATE 7 Crystallographic color zoning. One-third natural size. 9, 10. Hematite specks, spots, and blotches In yellowish-olive to yellowish- 3. Major color change, with green-brown boundary parallel to crystal green mica, with local vegetable stain. Two-fifths natural size. faces. One-half natural size. 11, 12, 13. Lattices and spots of hematite in yellowish-olive to brownish- Rude crystallographic control of hematite stain. One-half natural size. olive mica. Two-fifths natural size. Alined spots and specks of iron oxides, with faint vegetable stain. One- 14. Rectangular lattices of curdy brown stain in yellowish-olive mica. Two- half natural size. fifths natural size. Relations of iron oxides and other stains to crystal directions in a color- zoned sheet. One-half natural size. 15. Hematite spots, blotches, and trigonal lattices in yellowish-olive mica. Color zoning apparently caused by marginal bleaching. One-half natural Two-fifths natural size. size. 16. 17. Hematite specks and spots, with pale vegetable stain, in yellowish- Greenish stripes and gridiron color patterns. One-half natural size. olive mica. Two-fifths natural size. flKOLOCICAI. SIKVKY 1'KOKKSSJONAl, 1'AI'KR 223 IM.ATK 7

17

OOI.OK \AKIVno\S \M) I'HIMAin S'I'MIS LN MVSCOMTK

GEOLOGICAL SURVEY PROFKSSIONAL PAPER 225 PLATE 8

MINERAL INCLUSIONS AND CL4Y STAIN IIN MUSCOVITE

COMMERCIAL SHEET MUSCOVITE 17 other small trimmed or stamped forms are not only mica therefore was sharply increased, and as a result employed as such, but can be built up into rods, tubes, many problems were raised in connection with in­ or other articles that are bonded with a suitable ce­ creasing its supply.21 To stimulate domestic produc­ menting material. Simple and composite pieces are tion of such mica, the Metals Reserve Company, a used, for example, as tubes, sleeves, studs, washers, subsidiary of the Reconstruction Finance Corpora­ bushings, laminations, and thin perforated plates in tion, designated the Colonial Mica Corporation as its condensers, transformers, small heating elements, agent, with authority to purchase mica of certain rheostats, fuses, incandescent bulbs, radio and elec­ types and to assist the operators of mines with equip­ tronic tubes, various types of coils, and acoustic, X- ment leases, development loans, and consulting serv­ ray, and other specialized equipment. Thin splittings ices on problems of mica mining and preparation. are built up into mica board or are applied as facing on paper, cloth, and other materials used in the manu­ GRADING AND CLASSIFICATION facture of heater elements; commutators; boards, VISUAL, METHODS panels, and other mounting forms; parts of condens­ As taken from the mine, the mica crystals or books ers; and many other electrical devices. In addition, are designated as "mine-run," "run-of-mine," "book," coarsely and finely ground micas have a wide variety or "block" mica. The term "block" mica is more com­ of uses. monly used for partially prepared stock that will yield The unusual conditions of wartime periods create sheet material and hence might well be dropped in heavy demands for so-called "strategic" mica, which favor of "book" mica. Commercial muscovite is is used as splittings in the form of built-up mica com­ broadly classified by visual methods as "sheet," mutator segments and coil insulation in motors and "punch," "circle," "washer," or "scrap," depending generators and in transformers, switchboards, blast­ upon the type of material obtained from the mine-run ing apparatus, and aircraft generators and spark books. "Scrap" comprises books, flakes, and fragments plugs.20 Some of the greatest demands and most exact­ that are too small or too marred by inclusions, cracks, ing requirements during World War II were out­ holes, or other imperfections to yield acceptable sheet growths of developments in the field of military radio or punch stock, as well as the waste from those books and electronic equipment. When it became evident that that do yield punchings and trimmed sheets. The ma­ the need for dielectric materials of high quality would terial removed from the mine-run mica within or near increase without concomitant assurance of adequate the mine portal is known as "rough" or "cobbed" supplies of sheet muscovite from India, the electrical scrap, or more commonly as "mine" scrap. Ordinarily industry was strongly encouraged to develop satis­ it never reaches the shops where the better mica is factory substitutes for mica. Plain or treated papers, split and trimmed. "Bench" scrap is the mica obtained manufactured to meet precise standards of homo­ as discarded splittings and trimmings in the prepara­ geneity, thickness, porosity, strength, chemical com­ tion of sheet and punch goods and in general is of position, and freedom from conducting particles, better quality than mine scrap. Bench scrap generally proved satisfactory for many condensers, particu­ amounts to more than 90 percent of the mica that is larly those not designed to operate at the highest volt­ not classified as scrap at the mine. ages. Some ceramic dielectrics also gave excellent re­ "Sheet" muscovite, in the broadest sense, is any sults when used in capacitors, even at very high volt­ material other than scrap. It is flat, or nearly so, and ages. is sufficiently free from structural defects to be manu­ Substitution of other materials, though successful factured into products of certain shapes that are used for some types of condensers, was by no means satis­ factory for general application. Mica remained the 21 Billings, M. H., and Montague, S. A., The wartime problem of mica sup­ ply: Eng. and Min. Jour., vol. 145, no. 8, pp. 92-95, 1944; Burgess, B. C., only suitable dielectric for many varieties of capaci­ Mica mining and preparation cost: Am. Inst. Min. Met. Eng., preprint of tors, particularly those for military use at radio fre­ paper presented at New York meetings, February 1944, pp. 1-17, 1944; Lintner, E. J., Mica, a war essential mineral: Rock Products, vol. 47, no. 5, quencies. The consumption of condenser-quality pp. 48-50, 92-93; no. 6, pp. 74-76, 114-116, 1944 ; Wayland, R. G., Mica in war: Am. Inst. Min. Met. Eng. Tech. Pub. 1749, 7 pp., 1944 ; Spence, H. S., 20 Wayland, R. G., Mica in war: Am. Inst. Min. Met. Eng. Tech. Pub. 1749, Mica as a critical war mineral: Canadian Min. Jour., vol. 67, pp. 611-617, pp. 1-2, 1944. 710-717, 1946; War Production Board, Mica releases 1 and 2, 1942.

EXPLANATION OF PLATE 8 1, 2, 3. Single crystals and sprays of black tourmaline. Three-fifths natural mately one-half natural size. Clay stain, as viewed here in transmitted size. light, appears dark. 4. Flattened crystals of pyrite. Three-fifths natural size. 10. Flattened garnet crystals in hematite-stained mica. Three-fifths natural 5. Rosettes of zoisite in clay-stained mica. One-half natural size. size. Note absence of iron oxide spots from immediate vicinity of gar­ 6. 7, 8. Euhedral crystals of biotite. Three-fifths natural size. net inclusions. 9. Magnetite specks and two brown "cigarette burns" in clay-stained mica, 11. 12. Color zoning partly obscured by clay stains. Approximately one-half with thin color stripes parallel to pressure-figure directions. Approxi­ natural size. 18 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES in electrical equipment, stoves, lamps, and other ap­ The size groups for domestic and Indian sheet pliances. "Uncut" sheet mica is partly prepared stock mica are summarized in table 2, and a chart for grad­ that has been freed of obvious scrap, split or "rifted" ing mica according to Indian standard sizes is shown into plates three-eighths of an inch or less thick, and in figure 10. A discussion of size grading and simpli­ trimmed by any of several methods. If it will yield fied charts for grading are included in a report issued sheets or "patterns" of regular shape that are li/_» by by the United States Bureau of Mines.24 2 inches in minimum size, it is specifically known as Thickness of mica is determined by means of a "sheet" or "pattern" mica in the New England and machinist's micrometer or a rapid-reading dial gage, Southeastern States and as "plate" mica in the South­ and specifications for standard methods of determi­ western States. Gwinn 22 has pointed out that the gen­ nation have been outlined by the American Society eral term "block" mica is most suitable for such ma­ for Testing Materials.25 The minimum acceptable terial and has defined it as prepared stock of "random thickness for sheet muscovite is 0.007 inch, but thin­ thickness i/8 inch to less than 1/100 inch (125 to 10 ner material is sometimes sold under specific agree­ mils), which contains a usable area of l1/^ by 2 inches ments. Uniformity of thickness, a function of split­ minimum." ting quality, is judged by viewing the split-out films "Punch" mica is difficult to define rigorously, owing between crossed polaroid sheets. chiefly to inconsistencies in the usage of the term. In general, however, it is uncut material capable of yield­ TABLE 2.—Domestic and Indian size groups for sheet mica, including ing punched or trimmed sheets that are free of cracks punch and sheets larger than punch and other defects, are at least l1/^ inches in diameter, ^Adapted from charts issued by the Colonial Mica Corporation. Applies to sheets not less than 0.007 inch in thickness] and are less than I1/! inches wide and 2 inches long. "Circle" mica will yield prepared sheets 2 inches in Usable area in single rectangle, diameter, and "small punch," "washer," or "washer- Usual in square inches Minimum Standard domestic dimension India punch" mica will yield 1-inch sheets. Some washer grades of one side, grades Mini­ Maxi­ in inches mica is little more than scrap, and many users do mum mum not recognize it as a separate class, especially during periods of low prices. "Punch" mica is sometimes Small punch '_... 1 6 small. 6. used as a general term, including circle, punch (in the 3 1 1 Vz by 2 inches _ 3 4 strict sense), and some small punch or washer. The 2 by 2 inches _ .. 4 6 5. 2 by 3 inches _____ 6 10 4. terms "uncut punch" and "uncut circle" are synony­ 3 by 3 inches _____ 10 12 3. 3 by 4 inches __ . 12 15 } 2 mous with "punch" and "circle" as generally used but 3 by 5 inches. _ . 15 24 2 2. f 24 36 3 1. may be helpful in distinguishing such material from 4 by 6 inches _____ \ 36 48 4 A-l (Special). 6 by 8 inches __ _ 48 60 4 Extra special. the prepared sheets or punchings. "Trimmed punch," 8 by 8 inches _____ 60 80 4 Extra extra special. 8 by 10 inches ____ 80 100 4 Over extra extra special. or small sheet material, is prepared from ordinary Larger than 8 by 100+ 4 Over over extra extra punch stock, generally by knife trimming. 10 inches. special. Size grading of sheet mica is based upon the area 1 Ordinarily included under general designation of "punch," which applies to mica yielding usable sheets less than 1 J_ inches wide and 2 inches long but not less and minimum width of the largest rectangle of a than 1 square inch in area. NOTE.—In a new list of tentative specifications, proposed for adoption by the given quality that can be obtained from-the block. In A. S. T. M. in 1949, the limiting dimensions for grade 5}^ are altered_sligntly, general, the rectangle must meet the quality specifi­ grade 6 small is changed to grade 6, and the old grade 6 is eliminated. cations set by the purchaser. A size-grading chart of the type adopted by the American Society for Testing Quality designations for sheet mica vary according Materials for domestic block mica and mica splittings to the visual classification used, and a combined visual is reproduced in figure 9. The method outlined by the and electrical classification recently adopted by the Society 23 is as follows : American Society for Testing Materials yields still another set of terms. Interpretations of visual stand­ * * * The specimen to be graded shall be laid upon the chart ards differ from one observer to another, but attempts so that it covers point O and has its maximum and minimum dimensions extending along and covering the lines AO and OB, have been made to define the standards and to describe respectively. The specimen should be shifted until its usable the methods of determination in such exact terms that area completely covers the largest rectangle determined by a inconsistencies are reduced to a minimum. The Indian diagonal extending from point O to or beyond a point on any standard groups, beginning with the best quality, are of the curves, Nos. 6, 5%, 5, 4, 3, 2, 1, or A-l (Special). The clear, clear and slightly stained, slightly stained, fair number of the curve at greatest distance from O cut by the diagonal of the rectangle designates the grade of the specimen. stained, good stained, stained, heavy-stained, light- dotted, black-spotted, and black-stained. In 1938 the 22 Gwinn, G. R., Strategic mica: U. S. Bur. Mines Inf. Circ. 7258, p. 18, 1943. 24 Gwinn, G. R., op. cit., pp. 7-10. 28 American Society for Testing Materials, Standard methods of testing, 25 American Society for Testing Materials, Standard methods of test for grading, and classifying natural mica, A. S. T. M. Designation: D 351-46: thickness of solid electrical insulation, A. S. T. M. Designation: D 874-42: 1946 Book of A. S. T. M. Standards, pt. 3-B, p. 144, 1947. 1946 Book of A. S. T. M. Standards, pt. 3-B, pp. 47-51, 1947. COMMERCIAL SHEET MUSCOVITE 19

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< COMMERCIAL SHEET MUSCOVITE 21 American Society for Testing Materials 26 set up Quality classification of sheet muscovite—Continued A.S. T. M. A.S.T.M. Supplementary Indian grading as the American standard and desig­ designation description description 1 nated the seven principal qualities here listed. Several Heavy-stained. _. Free of mineral in­ May be soft or hard (ap - clusions, but con­ proximately 50 percent of these groups are sometimes subdivided into more tains more clay hard and 50 percent specific categories. and vegetable soft) and may be wavy stains than stained and slightly buckled Quality classification of sheet muscovite quality and is dis­ (about 10 percent may A.S.T.M. A. S. T. M. Supplementary tinctly inferior as be buckled). The designation description description ! regards rigidity largest rectangle may Clear...___ Free of all mineral Hard and substantially and toughness. contain dense air stains, and vegetable in­ flat. Ten percent of and two-thirds of the clusions, stains, air largest rectangle may area may contain dense inclusions, waves, contain small air spots, vegetable stains, spots, or buckles. Hard, and the remainder may or lines. transparent contain a few small air sheets. specks. Black-stained Apt to contain some and spotted. mineral inclusions Clear and slightly Free of all mineral Hard, but may be slightly consisting of mag- stained. and vegetable in­ wavy. The largest rec­ netite (black), clusions, cracks, tangle may contain specularite (red), waves, and buck­ small air specks, and and hydrous iron les, but may con­ one-third of it may oxide (yellow). tain slight stains contain small air spots. and air inclusions. One-tenth of the rec­ tangle may contain 1 Specific quality-classification data were kindly supplied by Bradley Johnson shop_ superintendent for the Colonial Mica Corporation, Asheville, N. C. The small, light air stains, specifications were applied to sheet muscovite purchased by the Colonial Mica light vegetable stains, Corporation and in general are slightly less rigorous than those set up by the or a few mineral specks. American Society for Testing Materials. Terms of degree are based on the follow­ ing descriptions: Fair stained. Free of mineral and Hard, but may be slightly Small specks: Size of small pin or needle holes. vegetable inclu­ wavy. One-fourth of Specks: Twice the size of small specks or about the size of the periods made by a typewriter. sions and cracks. the largest rectangle Small spots: About three times the size of small specks or about one-sixteenth of an Hard. Contains must be free of all inch in diameter. slight air inclu­ stains except small air Spots: Any spots larger than small spots. Lines: Series of specks or small specks, forming continuous rows. Individual sions and is slight­ specks, and three- specks may be in contact or slightly apart. ly wavy. fourths may contain Light stains: Stains not sufficiently dense to impair the transparency of block mica small air spots. One- when ordinary print is read through the sheet. Dense stains: Stains so dense that they distinctly impair the transparency of the fourth of the rectangle mica, as roughly determined by the reading test. may contain light air Degree of flatness is determined by observation of light reflected from the mica and light vegetable sheet as the sheet is rotated back and forth: stains or small mineral Flat: No wave nor ripple of light. specks. Substantially flat: Only a slight wave or ripple of light. Slightly wavy: Light appears to rise and fall evenly, without jerkiness, like waves Good stained..._ Free of mineral in- Hard, but may be wavy on a pool of water. Slightly buckled: Light appears to rise and fall jerkily, but the rise and fall from clusions and though not buckled. different parts of the sheet do not overlap. cracks, but con­ The largest rectangle Buckled: Light appears to rise and fall jerkily, with overlap and confusion of tains air inclusions may contain small air motion. and some vege­ specks and small air table inclusions spots over the entire and may be some­ area. Half of the rec­ Much mica was purchased in this country accord­ what wavy. tangle may contain ing to the so-called domestic classification during light air stains and light vegetable stains, World War II. This fourfold classification and its but not more than one- correlation with the A. S. T. M. categories are shown tenth may contain dense air stains or in the following tabulation. Mica sold for use in stove mineral spots and lines. manufacture is generally graded as A-No. 1, No. 1, Stained. Free of mineral inclu­ May be soft or hard (ap­ A-No. 2, and No. 2, in order of decreasing quality. sions and cracks, proximately 75 per­ but many contain cent hard and 25 per­ However, a general twofold classification is most com­ considerable clay cent medium soft) and monly used for all domestic sheet mica: Black-stained and vegetable may be slightly buck­ stains and may be led. The entire area and spotted material is referred to as "stained" or more wavy and of the largest rectangle "electric," and the other types are grouped under the softer than the may contain air specks better qualities. and stains, but not so general term "clear." Clear micas, according to this densely concentrated as usage, include the No. 1, No. 2, and No. 2 inferior to give a silvery ap­ pearance to more than categories listed below: one-third of the rec­ tangle. Light vege­ Domestic quality table stains or some classification A. S. T. M. designation small mineral specks No: 1_____._____--__ 20 percent clear and slightly stained; may be present over 80 percent fair stained. the entire area, but No. 2______Good stained. not more than one- No. 2 inferior.______50 percent stained; 50 percent heavy- third of the rectangle stained. may contain dense veg­ No. 3______Black-stained and spotted. etable stains, lines, or __ __ spots. During the recent wartime period, particular at­ 24 American Society for Testing Materials, Standard methods of testing, tention was given to "strategic" mica, which was de­ grading, and classifying natural mica, A. S. T. M. Designation: D 351-46: 1946 Book of A. S. T. M. Standards, pt. 3-B, p. 145, 1947. fined in War Production Board Conservation Order 22 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES M-101 as reasonably flat block and sheet mica of approximating those under which the particular lot of heavy-stained quality or better that is free from mica is to be ultimately used. Similarly, the mica is cracks and comparable imperfections. Excluded from tested in air or oil, depending upon the type of im­ this category are scrap mica, block mica that will trim mersion to be employed in the finished electrical to a size less than 1 by 1 inch, splittings used in mak­ equipment. ing built-up mica, and the so-called "electric" mica. The magnitude of the break-down or rupture volt­ The definition of strategic mica, however, is by no age ordinarily varies with the period of time during means fixed, and the meaning of the term varies with which the voltage is applied, and most electrical in­ changes in military needs, anticipated future require­ sulating materials are more resistant to voltages ap­ ments, and conditions of supply. Most strategic mica plied briefly than to comparable voltages applied over is simply sheet material of superior quality. Gwinn 27 longer periods. For this reason two types of test are suggests the alternative term "mica of military commonly used. For short-time determination of grade," which would include all sizes and qualities of dielectric strength, the voltage is increased from zero mica used in the manufacture of equipment for the to the break-down value at a uniform rate, generally armed forces. He further suggests that "strategic" 0.5 or 1.0 kilovolt per second.30 In the long-time or mica then would be the quality or sizes in short supply step-by-step test, the initial applied voltage is equal at a particular time. In 1944 the Army and Navy to about half the break-down voltage as determined Munitions Board approved the definition of strategic by the short-time test. This voltage is subsequently materials (including mica) as those "required for increased by equal increments, the duration of each essential uses in a war emergency, the procurement "voltage step" being determined from specifications of which in adequate quantities, quality, and time is for the material involved. sufficiently uncertain for any reason to require prior The break-down voltage of an insulating material provision for the supply thereof." decreases in value with increase in frequency of the applied current, so that the dielectric strength gen­ EIjECTRICAL METHODS erally is lower at radio frequencies than at audio fre­ Owing to increasing demands for high-quality mica quencies. At high frequencies the currents flowing in in electrical equipment, attention has been focused a dielectric material produce heat, corona, flash-over, during recent years upon grading and classification blistering, or other deterioration, with a subsequent of sheet material in terms of electrical properties. It change in values of current and voltage as application has been noted that the dielectric constant of musco- continues. Application by means of different types of vite that is otherwise of good quality is uniformly electrodes, as well as other variations in test pro­ within the required limits prescribed for condenser cedure, are briefly discussed by Horton.31 use and that consequently there is little need for further systematic determination of this property. Mica sheets or films can be tested for electrically However, such other properties as dielectric strength, conducting impurities according to the standard electrical conductivity, and power factor are of con­ methods of test, established by the American Society siderable significance in predicting how well given for Testing Materials, for conducting paths in elec­ sheets or lots of mica will serve in finished condensers. trical slate.32 The mica is laid on a metal plate, and its The American Society for Testing Materials 28 has upper surface is then explored by means of a pointed, adopted standard methods of test for dielectric probelike electrode. The electrode and plate are con­ strength of electrical insulating materials at com­ nected to the output terminals of a vibration-type mercial power frequencies. In the testing of musco- spark coil operating on an input voltage of 6. The vite, high voltages are applied to cleavage plates of secondary windings should be capable of forming a measured thickness by means of metal electrodes. The %- to 1-inch spark between two needle points. A electrode contact surfaces can be either rounded or battery-operated spark test set, developed during the flat, but one type or the other should be used in any recent wartime period by the Bell Telephone Labora­ tests where direct comparison of results is desired. tories, is a compact instrument that permits rapid The A. S. T. M. standards ^ specify metal disks 1 inch thick and 2 inches in diameter, with edges rounded, to »° American Society for Testing Materials, Standard methods of test for dielectric strength of electrical insulating materials at commercial power a radius of a quarter of an inch. The tests are made frequencies, A. S. T. M. Designation: D 149-44: 1946 Book of A. S. T. M. under conditions of temperature and humidity closely Standards, pt. 3-B, p. 129, 1947. ' » Horton, F. W., Mica: U. S. Bur. Mines Inf. Circ. 6822, pp. 32-33, 1935 (revised 1941). 37 Gwinn, G. R., Strategic mica: U. S. Bur. Mines Inf. Circ. 7258, p. 3, 1943. 82 American Society for Testing Materials, Standard methods of test for 28 American Society for Testing Materials, Standard methods of test for conducting paths in electrical slate, A. S. T. M. Designation: D 273-40: 1946 dielectric strength of electrical insulating materials at commercial power Book of A. S. T. M. Standards, pt. 3-B, pp. 131-134, 1947; Tentative speci­ frequencies, A. S. T. M. Designation: D 149-44: 1946 Book of A. S. T. M. fications for natural block mica and mica films suitable for use in fixed mica- Standards, pt. 3-B. pp. 124-130, 1947. dielectric capacitors, A. S. T. M. Designation: D 748-46T: 1946 Book of 89 Idem, p. 126. A. S. T. M. Standards, pt. 3-B, pp. 705-717, 1947. COMMERCIAL SHEET MUSCOVITE 23 exploration of block and film mica for conducting im­ ment of the power factor of the specimen by means of purities (pi. 9; fig. 11). a power-factor dial on the bridge. Each test specimen Such mechanical imperfections as pinholes, hair is made into a simple condenser by means of metal- cracks, and tears in the mica permit ready passage of foil electrodes fastened with a little petroleum jelly. the spark from electrode to plate during a test, and Full descriptions of this method have been made by those that do not extend entirely through the sheet Hall.35 generally cause puncture or rupture. Similar failure A method of test tentatively adopted by the Ameri­ is not ordinarily caused by conducting impurities, can Society for Testing Materials 36 is similar to the however. These are revealed instead by a distinct above method but specifies cleaning, heating, and sparking or glowing in the mica, observation of which drying of the samples before testing, use of steel or is best made in a room with subdued lighting. mercury electrodes arranged in such a way that two pieces of mica are required for each test, and enough determinations of power factor for different electrode pressures to obtain a pressure-power factor curve. The Bureau of Standards tests ordinarily are made on single pieces of mica, without any special pro­ TO CASE cedure in pretreatment of the samples and without any attempt to determine changes in power factor with changes in electrode pressure.37 The time required for an ordinary power-factor determination of mica has been reduced during re­ cent years to about 3 minutes, but prospects for fur­ ther quickening of the usual method do not appear to be good. Thus it remains too slow for commercial use, as far as its inclusion in any combined visual-elec­ trical testing procedure is concerned. More rapid techniques for estimating power loss in mica have TO CASE been described,38 and a very useful instrument was FIGURE 11.—Simplified circuit of spark-coil set for testing sheet muscovite. Courtesy of the Bell Telephone Laboratories, Inc. developed by the Bell Telephone Laboratories in re­ sponse to an appeal from the War Production Board Determination of power loss of muscovite is of in 1942. This instrument, a resonant-circuit test set great value in estimating the quality of condenser- known as a Q-meter (pi. 9), is used for direct deter­ grade material, particularly if it is to be used in high- mination of Q value, or reciprocal of the power factor. frequency circuits. Such loss is best expressed in As shown in figure 12, it consists essentially of a high- terms of power factor, Q value (reciprocal of power frequency oscillator, a vacuum-tube voltmeter, an factor), or heat equivalent. Systematic measurements inductance with a Q value of at least 300 at 1 mega­ of power factor have been made in steadily increas­ cycle, an adjustable capacitor with a maximum of ing numbers during recent years and have yielded about 10 micromicrofarads, and a fixed low-loss air much useful information. The National Bureau of capacitor of about 20 micromicrofarads. When used, Standards has been particularly active in this con­ the circuit is first tuned to the applied frequency, and nection. a full-scale voltmeter reading is obtained across the A resistance-variation method was used in a series air condenser by appropriate adjustment of the input. of power-factor tests by E. L. Hall and others 33 on When the test piece of mica is inserted in series with mica specimens submitted by the Federal Bureau of the fixed air condenser, the adjustable condenser is Mines, and the results of these tests were first re­ 85 Hall, E. L., Report on measurement of power factor in muscovite, in ported in 1931.34 Subsequent tests in the laboratories Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, N. C.: of the Bureau of Standards were made by a much U. S. Geol. Survey Bull. 936-A, pp. 18-24, 1942 ; Equipment and method for measurement of power factor of mica: Inst. Radio Eng. Proc., vol. 32, pp. more rapid bridge method, in which a preliminary 394-395, 1944; Power factors, in Hidnert, Peter, and Dickson, George, Some capacitance measurement of a given specimen is made physical properties of mica: Nat. Bur. Standards Jour. Research, vol. 36, pp. 342-344, 1945. with a radio-frequency bridge, followed by (1) an ad­ 84 American Society for Testing Materials, Tentative methods of test for justment of the bridge power-factor dial to zero with power factor and dielectric constant of electrical insulating materials, A. S. respect to a standard air condenser and (2) measure­ T. M. Designation: D 150-46T: 1946 Book of A. S. T. M. Standards, pt. 3-B, pp. 647-681, 1947. 87 Hall, E. L., Equipment and method for measurement of power factor of 83 Horton, F. W., op. cit., pp. 41-46. mica: Inst. Radio Eng. Proc., vol. 32, p. 395, 1944. 34 Lewis, A. A., Hall, E. L., and Caldwell, F. R., Some electrical properties 38 Barber, A. W., Simplified dielectric loss measurements: Inst. Radio Eng. of foreign and domestic micas and the effect of elevated temperatures on Proc., vol. 17, p. 26, 1937; Snow, H. A., Losses in mica and simple test pro­ micas: Nat. Bur. Standards Jour. Research, vol. 7, research paper 347, 1931. cedure: Inst. Radio Eng. Proc., vol. 17, p. 19, 1937. 24 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES used to reestablish resonance, and the voltmeter read­ more as a fast and simple means for electrical grading ing is then compared with the original full-scale read­ of mica than as a precision instrument; nevertheless, ing. The series resistance causing the voltage drop is it yields Q values that agree closely with determina­ the measure of the power loss in the mica. More de­ tions made by means of other, more exact, methods. tailed descriptions and analyses of Q-meter opera­ tions appear in the published record.39 COMBINATION METHOD OF THE AMERICAN SOCIETY FOR TESTING MATERIALS Tests aimed at correlation of electrical properties of sheet muscovite with its behavior under actual use TO OSCILLATOR CIRCUIT in condensers have been made from time to time by manufacturers of electrical equipment. Most of these tests have been used to solve specific problems en­ I countered during production, rather than to estab­ lish basic generalizations; hence few have provided comprehensive information or have involved material of specifically known source. In response to a grow­ ing need for more systematic data, the United States Geological Survey, the National Bureau of Standards, the R. C. A. Manufacturing Co., and the Scintilla Magneto Division of the Bendix Aviation Corp. de­ termined the relations between power factor and visual characteristics and between these properties and utility in built-up condensers for several samples of muscovite from mines in the southeastern United States. This work was done in 1940 and 1941, and the results were summarized and discussed by Kesler and Olson in 1942.42 The investigations were limited in scope by the small quantities of raw mica available FIGURE 12.—Simplified circuits of equipment for determining power factor for testing, but both the manufacturing organizations of sheet muscovite. Courtesy of the Bell Telephone Laboratories, Inc. A, Circuit indicating general method of testing: L, inductance; Cv, adjustable concluded that most of this mica compared favorably capacitance; Ca, fixed air condenser; Cs, condenser with test piece of mica with India mica in dielectric strength and power as the dielectric; V, voltmeter. B, Simplified circuit of rapid-reading Q- meter: Vlf tube of oscillator circuit; Ct, adjustable capacitance; CT, ad­ factor. It was pointed out, however, that the propor­ justable capacitance ; PJt P2, variable resistances; V, voltmeter; V2, tube tionate yield of finished films was inferior to that of vacuum-tube voltmeter; M, meter of vacuum-tube voltmeter; Ca, fixed air condenser. from India mica, owing mainly to relatively poor preparation of the domestic material.

The Q-meter of the Bell Telephone Laboratories 40 was first tested for calibration on small lots of sheet mica, and since that time it has been employed in 20 80 numerous types of testing programs. It has been shown that its usefulness can be extended, through 100- proper calibration, to permit direct reading of capaci­ tance, dielectric constant, and absolute value of Q or power factor.40 The thickness of each test specimen must be dealt with in this connection (fig. 13), al­ FIGURE 13.—Meter scale of the Bell Telephone Laboratories' Q-test set, show­ ing the relation between power-factor groups (E—1, E-2, and E-3) and though the calibration for Q value is based funda­ thickness of test pieces (0.010-, 0.020-, and 0.030-inch controls). mentally on the probable Q values of capacitors made from splittings of the mica.41 The meter was designed A much more extensive series of investigations was begun in 1942 by the Bell Telephone Laboratories at 89 Coutlee, K. G., Saving mica by testing: Bell Lab. Rec., vol. 22, pp. 509- the request of the War Production Board. Block mica 513, 1944; Progress report on A. S. T. M. round robin power factor tests of block mica, Am. Soc. for Testing Materials, sec. 3, subcommittee 9, and from seven domestic mines was classified according Bell Telephone Lab., Inc., New York, 8 pp., 1945; Judging mica quality to Q value by the rapid method and was tested for con­ electrically: Am. Inst. Electrical Eng. Trans., vol. 64, pp. 1-7, 1945. 40 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical ducting impurities with the spark set. This electri­ Eng. Trans., vol. 64, pp. 2-3, 1945. cally selected material, much of which was marred by 41 American Society for Testing Materials, Tentative specifications for natural block mica and mica films suitable for use in fixed mica-dielectric capacitors, A. S. T. M. Designation: D 748-45T: 1946 Book of A. S. T. M. 42 Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, Standards, pt. 3-B, pp. 705-717, 1947. N. C.: U. S. Geol. Survey Bull. 936-A, pp. 18-30, 1942. GEOLOGICAL SURVEY PROFESSIONAL PAPER 225 PLATE 9

A. SPARK-COIL SET FOR RECOGNITION OF PFNHOLES. CRACKS, AND CONDUCTING IMPURITIES

B. RAPID, DIRECT-RE DING Q-METER TESTS SETS FOR ELECTRICAL CLASSIFICATION OF SHEET MICA Courtesy of the Bell Telephone Laboratories, Inc.

COMMERCIAL SHEET MUSCOVITE 25 air stain and inclusions of iron oxide, was then split, Appreciable amounts of air inclusions and waviness also are punched, and built up into molded 1,000- and 4,000- permitted in all electrical quality classes providing the mica micromicrofarad capacitors by the Western Electric meets specific electrical and physical requirements. However, Co. In subsequent performance tests these capacitors, such mica is not considered generally as desirable as mica having lesser amounts of such defects. Mica capable of meet­ as noted by Coutlee,43 ing specific requirements also may be of any basic color such satisfactorily met Q, insulation-resistance, dielectric-strength, as white, ruby, light green, dark green, brownish green, rum, corona-voltage and life requirements. ... As a result of these or other colors derived from any source. tests, it was concluded that block mica could be selected de­ pendably for use in capacitors, some for the better grades and In this latest A. S. T. M. method of classification the rest for less critical types, and that the test methods ap­ the mica is graded for size according to the so-called peared entirely adequate for the electric control of capacitor India scale (figs. 9, 10) and is qualified according to mica. Even the two lots of capacitors made with block mica in certain electrical, visual, and other physical features which conducting inclusions were detected by the spark test by means of carefully specified methods. Division and low Q by the block mica Q tests gave excellent dielectric strength and life performance. into three general classes is accomplished by evalua­ tion of several properties as shown in table 3. Stand­ A new set of specifications for condenser-quality ard procedures are recommended for determination mica, based upon electrical, physical, and visual re­ of visual properties, dielectric strength, and other quirements, was then developed in terms of all avail­ characteristics, and two methods for determining Q able testing data, and checking of these specifications value or power factor are recognized as satisfactory. on a commercial scale was started immediately there­ after. This involved electrical classification of more TABLE 3.—Electrical, physical, and visual quality requirements of than a ton and a half of block mica from Brazil, natural block mica and mica films for use in capacitors Canada, India, and the United States. Twenty-two [Adapted from A.S.T.M. specifications] different types of material were included. All the mica C-l C-2 C-3 had been previously qualified, by visual means alone, as stained or poorer according to the India scale or as Electrical properties: Q value or power No. 2, No. 2 inferior, or No. 3 according to the do­ factor at 1 iE-1.... ___ ... iE-2— ------"E-3. mestic scale. The blocks were split, punched, and Dielectric strength (volts per mil made up into more than 47,000 capacitors of various at 60 cycles per second), mini­ types by eight manufacturers. Results of perform­ mum: 1,000...... 1,000 .._.._...._. 1,000. ance tests on these capacitors, which have been re­ Single. __-- . 850 ___ ...... 850 ____ ...... 850. Dielectric con- corded and discussed in detail elsewhere,44 led to the (') (2) « Physical properties: Weight loss on conclusion that electrical-visual methods of selection heating (5 min- utesat600°C.)t can be satisfactorily applied to capacitor-grade mica maximum per- 0.2 ______. ... 0.2 __ . _ . ___ required for given end uses. Thickness uni­ formity (mica A set of specifications for commercial sheet mica, films)3 Best...... Best... _ ..._.. Temperature co­ based upon a combination of visual and electrical efficient of ca­ pacitance and tests, was subsequently proposed by the American (4) (4) («> Visual qualities: Society for Testing Materials.45 The system of classifi­ Air inclusions:5 A __ ...... None to slight _ _ None to slight .... None to slight. cation based upon these specifications has been tested B_. --.....- C.-.-. _ .. commercially and "found to be both practicable and Waves: • A..... — .- Flat to slight .... reliable." It B _ ...... C— — .... does not discriminate against the presence of spots or stains Cracks __ ------None ———— ..... None _ ------None in even first quality electrically selected mica providing the Pinholes. ------None —— -____-_- None _ ------... None Stones __ ------None __ ...... None —————— ... None mica conforms to specific electrical and physical requirements. Buckles— —— -_ None —— ———— . None...... _..__. None

48 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical 1 See table 4 for corresponding values of Q and power factor. Eng. Trans., vol. 64, p. 6, 1945. 2 No specified requirement is needed. ** Townsend, J. R., Final report on commercialization of mica testing 3 Until definite values can be specified, the permissible amounts of such defects shall be agreed upon by the purchaser and the manufacturer. equipment: War Production Board, War Metallurgy Comm. Rept. W-1B1, 4 It has been found that the temperature coefficient of capacitance and retrace of 1944 ; Townsend, J. R., Addendum to War Metallurgy Cbmm. Rept. W-1B1: capacitors made with classes C-l, C-2, and C-3 are more dependent on such War Production Board Rept. W-170, 1944; Coutlee, K. G., Saving mica by factors as electrical and mechanical design and manufacturing technique than any differences that may be attributed to the mica itself. testing: Bell Lab. Rec., vol. 22, pp. 509-513, 1944; Coutlee, K. G., Progress 6 The amount of air inclusions shall not exceed the specified limits for each report on A. S. T. M. round robin power factor tests of block mica. Am. subclass as judged by the photographic reference standards given in figures 1-3 on pages 708-713 of the 1946 Book of A. S. T. M. Standards. The permissible Soc. for Testing Materials, sec. 3, subcommittee 9, and Bell Telephone Lab., amount of air inclusions shall be stated by suffixing the letter A, B, or C, as the Inc., New York, 8 pp., 1945 ; Coutlee, K. G., Judging mica quality electrically: case may be, to the required electrical-quality class. Am. Inst. Electrical Eng. Trans., vol. 64, pp. 1-7, 1945. 8 Until definite values can be specified, the permissible amount of waves, buckles, and ridges shall be agreed upon by the purchaser and the manufacturer. 45 American Society for Testing Materials, Tentative specifications for The permissible amount of waves shall be stated by suffixing the letter A, B, or C, natural block mica and mica films suitable for use in fixed mica-dielectric as the case may be, to the letter denoting the amount of permissible air inclusions. capacitors, A. S. T. M. Designation: D 748-45T: 1946 Book of A. S. T. M. For example, class C-l B A block mica films are of the best electrical quality with a medium amount of air inclusions and an absence, or only a slight amount, of Standards, pt. 3-B, pp. 705-717, 1947. waves. 26 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES On the basis of its Q value, mica can be divided into mate equivalents between electrical quality and visual the three groups shown in table 4. These are desig­ quality based on an A. S. T. M. table: nated E-1, E-2, and E-3. E-1 mica that is flat to only Electrical quality Visual quality slightly wavy, contains no conducting impurities and (A. S. T. M. Specifications D 748) (A. S. T. M. Methods D SS1) little or no air stain, and is free from cracks, tears, Block mica and mica films Block mica Mica films1 pinholes, stones, buckles, and ripples corresponds to Class C-l AA_____ Fair stained (min.) First quality Class C-l BB _____ Good stained Second quality fair stained or better material in the older, strictly Class C-l CC_ _ _ Stained Third quality visual, classifications and yields films of top quality, as i Mica-film quality is not specified in A. S. T. M. Methods D 351. .The qualities may be seen in the following tabulation of approxi­ given are in commercial use, and the equivalent block-mica quality is assumed.

TABLE 4.—Q and power-factor values for electricaJrquality groups E-1, E-2, and E-S [Adapted from A. 8. T. M. specifications] rn Rapid-method meter reading Q or power-factor (thickness of block mica or stacked mica films, in inches) group Form of mica Q value Power factor 0.010 (0.007-0.015) 0.020 (0.015-0.025) 0.030 (0.025-0.035)

E-1. __._._„__ /Block... .._.--- 2500 (min.) » ______95-100 95-100 95-100 \Films_.--._: _ 2500 (min.) * ______E-2. _ ...... /Block..... _ .. 350-2500 i— _„ __._ 0.00285-0.0004.... .___.. ———————— .- 87-95 77-95 71-95 1500 (min.) *______0.00066 (max.)--- ———————-- E-3 __ ...... /Block.. ______50-3501 ______0.02-0.00285.....-.- ——————————— - 50-87 32-77 24-71 200-15002 ______0.005-0.00066——-- — ------——————

i Extensive commercial tests have verified the validity of the Q values of capacitors made with group E-1 block mica to a satisfactory degree. However, the ranges for groups E-2 and E-3 are tentative and subject to further verification. •>•*» <* (*»»i«a.^-»_», •<•_»• * Minimum Q values for material purchased as films and tested in stacks of films 0.010 to 0.030 in. in thickness and probable minimunvQ values of.molded-type, 1,000-mmf., stacked-foil, and silvered capacitors. These will apply when all factors that would adversely influence the Q value are under control.j______^. These equivalents are valid only if the Q value of means of a blade into plates that generally are less the mica is E-1. Such mica "is suitable for use in all than three-sixteenths of an inch thick. Through skilled sizes and types of silver and foil electrode molded and handling of the rifting knife, defective laminae are clamped unit capacitors, including the most critical types, for use in high stability tuned circuits, as well as high current radio frequency capacitors used in radio transmitter circuits." 46 Mica that is graded as E-2 and has the other characteristics of class C-2 is also suitable for such condensers, although "a cer­ tain percentage of capacitors made with class C-2 block mica and films may show a somewhat higher Rough-cobbed of temperature rise in transmitter types than capacitors selected mine-run mica (incl thumb-fi- made with class C-l block mica or mica films."47 Mica orid rou materia of class C-3 (E-3 in terms of Q value), though not of sheds) top quality, is none the less suitable for use in con­ densers where high Q value, high stability, and low temperature coefficient are not required.

PREPARATION AND MARKETING The first rough separation of mica generally is made at the mine—at the face or portal, in a nearby shed, or on the dump at a later time (fig. 14). Obvious mine scrap is separated from the better books, from which adhering fragments of quartz, feldspar, and other foreign material are then cobbed. Some of this Coarse-and fine- Cut,punched,and stamped Mica plate, built- ground mica sheets and splittings up mico board, rough-cobbed or selected mine-run mica is sold as and other compos­ such to jobbers or manufacturers, but at many mines ite forms FIGURE; 14.—Outline of operations involved in the extraction and preparation it is prepared further. The books are split or rifted by of sheet, punch, and scrap mica.

*• American Society for Testing Materials, Tentative specifications for natural block mica and mica films suitable for use in fixed mica-dielectric removed with a minimum waste of higher-quality ma­ capacitors, A. S. T. M. Designation: D 74&-46T: 1946 Book of A. S. T. M. terial, and block mica, punch and washer stock, and Standards, pt. 3-B, p. 706, 1947. * Idem, p. 707. bench or shop scrap are thereby obtained. In some COMMERCIAL SHEET MUSCOVITE 27 districts the cobbed mica is commonly split into plates also, from thin films or skimmings that are a byprod­ more than three-sixteenths of an inch thick, but both uct of the rifting of larger sheet material. Splittings these and the thinner riftings are generally known as are used in the manufacture of built-up mica board "plate" mica. As such, they constitute a specially se­ and other forms of electrical insulation. Although lected form of mine-run material. many mechanical devices have been tested for the After rifting, the ragged and broken edges of many preparation of these films, practically all are still split plates are removed with the fingers, a process known outside the United States by hand methods, generally as "thumb trimming." This is an especially common in places where labor costs are very low. practice in districts where much of the mica is se­ The cut mica blocks that represent punch, circle, verely ruled or marked by "A" structure. Some and larger sheet stock are processed into disks, wash­ thumb-trimmed material may be sold to manufac­ ers, and thin plates of various sizes and1 shapes. This turers as such, or it may be further trimmed with a generally involves additional splitting, followed by knife and its value thereby increased. Some mica is trimming, cutting, or stamping into more or less trimmed with shears, but most is cut by a sickle or standardized patterns. Most of this material is then hand knife. As pointed out by Spence,48 material from cut to final form, if necessary, by the manufacturers which waste has been removed by a mechanical knife, of the devices in which the mica is to be used. Com­ guillotine, or shears is known in the trade as "knife- posite forms can be built up to any desired thickness trimmed" or "shear-trimmed." It has unbeveled edges by the cementing of individual pieces with shellac, that generally are straight or nearly straight. Mica glyptol, or a similar bonding medium. In general only trimmed by hand, with beveled and less regular edges, a small proportion of the prepared block material is is termed "sickle-trimmed." This distinction is not represented in the finished product. The bulk of such often made, however, in the preparation of domestic material is skimmed or cut away as waste, which is mica, and material trimmed with a hand knife is com­ marketed as scrap of superior grade. monly referred to as "knife-trimmed." During recent Most scrap mica, including material derived from years attempts have been made to employ several nonpegmatitic sources, is processed by grinding. It is forms of blades and saws for mica trimming in the classified on the basis of its freedom from quartz, United States, but without much success. feldspar, and other gritty impurities and on the basis Most India mica is knife-trimmed or sickle- of its color when ground. trimmed free of cracks and flaws, but domestic pro­ cedure is somewhat different. Half-trimmed mica, for Prices for sheet mica not only fluctuate widely in example, is cut on two adjacent sides with no cracks, response to variations in demand, but vary at any reeves, cross grains, nor ribs extending from those given time according to the size and quality of the sides. Three-quarter-trimmed mica is cut on all sides, material. The general ranges for clear and stained with no cracks nor comparable flaws extending from trimmed sheet mica in the Southeastern States two adjacent sides or into the final pattern area. Only during a 30-year period are shown in table 5. The full-trimmed mica is comparable to India-trimmed value of punch or untrimmed small sheet mica ranged material in that it is cut on all sides and contains none from 21/2 cents to about 15 cents per pound during of the flaws noted above. Moreover, upper limits gen­ the same period. Trimmed electric (stained sheet) erally are set on the number and size of "V" or figure mica is sold according to a sliding price scale, with cuts on any one piece of mica, as well as on the pro­ values consistently lower than those for clear ma­ portion of pieces with such cuts in a given lot of mica. terial of comparable size. Electric mica also is sold Careful rifting and trimming are very important as thumb-trimmed block, generally at prices that vary in the preparation of sheet mica, not only because according to the estimated proportion of waste. Many waste is thereby held to a minimum, but because well- prepared mica yields a higher proportion of usable TABLE 5.—Price ranges of dear and stained sheet mica in the Sovth- material in the later stages of manufacture. The mica k eastern States during the period 1910-JfO may be rifted, or it may be trimmed to the required Range in price per pound (dollars and cents) degree of quality, and the process adopted depends Size (inches) chiefly upon the kind and distribution of impurities Clear Stained and structural imperfections that the mica contains. 1H by 2-_. — . ————— ------——— . ———— . 0.12-0.60 0) A large proportion of sheet mica is consumed in the 2 by 2 _____ „— — . ——— . —————————— .22-1.05 0.06-0.40 2 by 3 _____ „ — . ——————————————— .38-1.45 .10- .60 form of splittings. These are films 0.0007 to 0.001 3 by 3 ___ -. ————— - ———— - ——————— - .58-2.00 .15-1.25 3 by 4 __ —————— -. ————————— - ——— — .78-2.30 .30-1.50 inch thick that generally are cleaved from punch and 3 by 5--_ —————— ----- ———— . ————— — .95-2.70 .48-1.75 4 by 6 ___ —— . — - ——————————————— 1.75-3.65 1.70-2.25 the smaller sizes of sheet stock. Some are derived, 6 by 8 __ ————— - ——— - —————————— -- 2.25-7.25 1.25-2.50 8 by 10_ —————— . ——— . ———— - —————— 3.50-11.50 2.00-3.00

48 Spence, H. S., Mica: Canada Dept. Mines, Mines Branch, Pub. 701, p; 1 Under ordinary conditions the smallest size of stained mica purchased as sheet 54. 1929. material in the Southeastern States is 2 by 2 inches. 28 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES jobbers purchase selected mine-run material (either strategic-quality mica was purchased by private indi­ clear or stained), rift and trim it, and sell the pre­ viduals and organizations and by the Colonial Mica pared mica to manufacturers. Others purchase punch Corporation as well, but during most of the wartime and washer stock from which they recover and trim period purchases were made solely by the Colonial sheet material, and still others have recovered sheet Mica Corporation. With the end of hostilities and the and punch mica from mine scrap. Buyers of mica are trend toward peacetime economy, most of the sub­ listed in United States Bureau of Mines Information sidies for mica production were removed and the Circular 7258. prices for sheet material dropped sharply. The at­ During war periods, when the demand for sheet tendant closing of many mines and general diminu­ mica is greatly increased and the problems of supply tion of sheet-mica production curtailed the supply of are often complex, prices reach very high levels. The punch and washer material, so that prices for such rising trend during the period December 1941-De- mica actually remained at wartime levels or even cember 1944 and the subsequent sharp drop in prices rose. During 1945, for example, some lots of punch are shown in table 6. Through parts of 1942 and 1945 mica were sold at prices of 45 to 50 cents per pound.

TABLE 6.—Price schedules for domestic clear sheet mica during the period December 1941—February 1945 [Adapted in part from Billings and Montague, The wartime problem of mica supply: Eng. and Min. Jour., vol. 145, no. 8, p. 94, 1944]

Price per pound, in dollars and cents

Private purchasers Colonial Mica Corporation Size Private February 1945 « purchasers, ^December April-May June November May February August February February 1944 * 3 1945' 1941 1942 19421 1942 i 1943 « 1944* 1945* No. 1 No. 2 No. 2 inf. quality quality quality

0.10-0.15 0.12-0.16 0.22 0.30 1.70-3.50 1.25-2.50 0.50-1.10 0.08-0.15 IJiby 2 inches. .45- .65 .50- .65 1.10 2.40 5.00 3.50 1.60 1.00 2 by 2 inches . .60- .85 .95-1.10 1.75 3.52 1.40 2 by 3 inches. 1.30-1.50 1.50-1.85 2.75 4.64 7.80 5.50 2.60 2.00 3 by 3 inches . 1.90-2.05 2.00-2.35 3.50 5.12 5.00 6.00 6.00 & 8.00 2.25 9.10 6.50 3.40 2.55 3 by 4 inches. 2.15-2.25 2.25-2.60 4.25 6.08 3.00 3 by 5 inches. 2.60-2.75 2.75-3.00 5.00 7.04 10.50 7.10 4.65 3.45 4 by 6 inches. 3.60-3.70 3.75-4.00 6.25 8.00 13.30 8.30 6.20 4.30 6 by 8 inches. 5.25-5.50 5.50-6.00 8.00 9.12 20.10 12.70 9.20 6.50

1 Punch, material required to yield 20 percent or more of trimmed pieces 1 by 1 4 Premium price of 88 paid for full-trimmed sheet mica 2 by 2 inches in size i nch or larger; price scale for larger mica based on No. 1 quality and half trim, with and larger; $6 for full-trimmed punch material and full-trimmed sheet mica smaller maximum bonuses of 30 and 40 percent for three-quarter trim and full trim, re­ than 2 by 2 inches and for three-quarter-trimmed sheet in the larger sizes. spectively. 6 Blanket price for full-trimmed sheet and punch mica regardless of size or quality ^ Uniform price per pound established regardless of size or quality within within strategic range; drop in price reflects general subsidy-removing policy with strategic range. Punch material required to be full-trimmed; sheet mica, three- respect to production of strategic mica. quarter-trimmed . • Alternative price schedule based on size and quality of three-quarter-trimmed 3 Includes $1 bonus. mica. 7Price based on untrimmed punch and three-quarter-trimmed sheet mica.

SHEET MUSCOVITE IN THE SOUTHEASTERN STATES ginia; the Ridgeway-Sandy Ridge district of Virginia DISTRIBUTION OF DEPOSITS and North Carolina; the Shelby-Hickory district of The Southeastern States constitute the chief mica- North Carolina; the Hartwell district of Georgia and producing area in the United States. The bulk of this South Carolina; the Thomaston-Barnesville district production is obtained from mines in western North of Georgia; and three districts in east-central Ala­ Carolina, but important deposits also are worked in bama. Many small deposits are scattered through the parts of Virginia, South Carolina, Georgia, and Ala­ areas between and around individual districts. bama. Fourteen well-defined mica-mining districts The Spruce Pine district, which occupies parts of occupy a belt nearly 600 miles long and about 60 miles Avery, Mitchell, and Yancey Counties in the Blue in average width. It extends from east-central Vir­ Ridge province of western North Carolina, is the ginia southwestward through the western Carolinas largest in North America, both in annual and total and north Georgia to central Alabama (fig. 15). This production of mica and in the number of mines and general mica-producing region can be divided into prospects that lie within its limits. Since 1900 the two smaller belts: the Blue Ridge belt of North Caro­ output from at least 800 mines has been marketed lina and Georgia on the northwest and the longer and and hundreds of other deposits have been prospected. broader Piedmont belt on the southeast. Within the The number of mica deposits that have been mined or Blue Ridge belt are the Jefferson-Boone, Wilkes, prospected throughout the entire southeastern part of Spruce Pine, and Buncombe districts of North Caro­ the United States probably amounts to 4,900 or more. lina; the Franklin-Sylva district of North Carolina Table 7 is a tabulation, by districts, of the number of and Georgia; and the North Georgia district. The deposits from which strategic-quality mica was ob­ Piedmont belt comprises the Amelia district of Vir- tained during the period of World War II, the number SHEET MUSCOVITE IN THE SOUTHEASTERN STATES 29 of deposits studied in detail by the United States Geo­ extent (fig. 15). The Spruce Pine district, for ex­ logical Survey during the period 1939-46, and the ample, occupies an area of 250 square miles or less, estimated minimum number of deposits worked since whereas the much more extensive Buncombe district 1880. There is little correlation between the number to the south probably contains less than one-twentieth of mica mines and prospects in a district and its areal as many deposits.

89° 87

33 33

31

' Pegmatite area or district

87

FIGURE 15.—Distribution of principal pegmatite areas and districts in the southeastern United States.

HISTORY AND PRODUCTION Mining operations for sheet mica date back at least The pegmatites of the Southeastern States have to the fourteenth century, and the remnants of many yielded large quantities of sheet and scrap mica, feld­ ancient trenches, pits, and cuts are recorded from spar, quartz, and kaolin, as well as some beryl, cas- several districts.49 Evidently these were made by siterite, garnet, monazite, specimen material, spodu- Indians, who probably used the mica for ornamental mene, tantalum-columbium minerals, topaz, uranium purposes. Their work was confined to the soft, minerals, vermiculite, and zircon. Feldspar has been kaolinized parts of the deposits, but a little of the mining was done underground. Modern mining was an important byproduct of some of the mica mining, and much mica has been produced as a result of wide­ 49 Kerr, W. C., The mica veins of North Carolina: Am. Inst. Min. Eng. Trans., vol. 8, pp. 457-462, 1880 ; Phillips, W. B., Mica mining in North Caro­ spread operations for feldspar and kaolin. Many of lina: Eng. and Min. Jour., vol. 45, pp. 286, 306-307, 322, 324, 382-883, 398. the minor minerals have been produced solely as by­ 418, 436, 1888; Smith, C. D., Ancient mica mine in North Carolina: Smith- sonian Inst. Kept., pp. 441-443, 1876; Sterrett, D. B., Mica deposits of the products. United States: U. S. Geol. Survey Bull. 740, pp. 28, 167, 224-225, 1923. 852911°—50—3 30 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES started shortly after the Civil War, chiefly in the Vir­ States production, of sheet and punch mica, and its ginia Piedmont and in the Shelby-Hickory, Spruce value of $9,720,273 has amounted to 62 percent of the Pine, and Franklin-Sylva districts of North Carolina. total. In general these proportions have been rising Many mines were opened after 1875, and large quan­ slightly during recent years. The average value of the tities of stove mica were produced from 1885 to 1900. output from the Southeastern States has been about Increasing demands from the electrical industry 39 cents per pound. In contrast, it was 50 cents per stimulated later production, especially after 1910, and pound during the wartime period 1917-19 and 89 the favorable price schedules of World War I brought cents per pound during the period 1942-44. mining and prospecting activities to the highest levels known up to that time. A severe but brief postwar TABLE 8.—Production of sheet and punch mica, 1912-44, in North Carolina, the Southeastern States, and the entire United States slump was followed by a 9-year period of prosperity in [Based on data from U.S. Geological Survey and U.S. Bureau of Mines, Mineral the industry, during which the average annual pro­ Resources of the United States and Minerals Yearbook, 1912-44] duction of sheet and punch mica in the Southeastern Southeastern States North Carolina (Va., N. C., S. C., United States States was about 760,000 pounds. The output fell Ga., and Ala.) (all) sharply in 1931 and 1932 but later slowly rose until it Year Amount Value Amount Value Amount Value reached the million-pound-per-year level. The un­ (pounds) (dollars) (pounds) (dollars) (pounds) (dollars) precedented demands during World War II led to 1912 __ . 489,599 219,874 ?> • 0) 845,483 282,823 1913 ..... 803,462 230,674 0) 0) 1,700,677 353,715 even greater production, and in 1943 a total of 2,132,- 1914 ..... 274,121 171,370 308,121 175,704 566,933 278,540 1915 ..... 281,074 266,660 294,376 272,830 563,821 378,259 826 pounds of sheet and punch mica was obtained. 1916 __ . 546,553 380,700 616,700 406,000 865,863 524,485 1917 ..... 643,476 543,207 761,044 581,707 1,276,533 753,874 This figure is all the more remarkable, as compared 1918 ..... 941,200 460,450 1,239,700 587,100 1,644,200 731,810 1919 __ . 1,021,306 331,498 1,092,152 385,312 1,545,709 483,567 with previous records, when the superior degree of 1920..... 1,084,946 405,654 1,395,838 461,936 1,683,480 546,972 1921 ..... 230,632 51,851 260,084 55,245 741,845 118,513 preparation of the recent output is taken into account. 1922 ..... 544,495 119,767 598,321 130,601 1,077,968 194,301 1923 ..... 1,130,283 188,317 1,194,628 194,652 2,063,179 311,180 Much mica that would ordinarily have been sold as 1924 _ .. 597,385 108,656 682,961 116,475 1,460,897 212,035 1925 ..... 592,478 105,376 622,421 111,328 1,793,865 321,962 half-trim or punch was prepared as large full- and 1926 ..... 700,313 150,362 742,345 157,576 2,172,159 400,184 1927 __ . 665,360 114,514 713,554 127,755 1,512,492 212,482 three-quarter-trimmed sheets and as small full- 1928.-— 777,395 129,706 804,457 133,492 1,681,777 230,956 1929 ..... 894,200 150,293 936,524 155,649 2,035,128 286,321 trimmed sheets; hence bulk was materially reduced. 1930 ..... 749,074 112,451 776,075 117,573 1,465,485 177,307 1931 ..... 389,426 51,657 405,234 53,475 962,953 111,830 1932 ..... 127,696 18,322 139,863 19,554 338,997 45,882 1933 ..... 162,672 21,107 162,731 21,113 364,540 53,179 TABLE 7.—Summary data on the number of mica deposits in the south­ 1934 __ . 293,381 38,671 295,994 38,788 583,528 90,268 eastern United States 1935 ..... 512,590 77,598 517,157 78,052 936,633 191,160 1936 ..... 730,446 119,653 783,768 124,115 1,319,233 203,879. 1937 ..... 1,044,328 218,176 1,057,320 219,247 1,694,538 285,244 Number of de­ Number of 'de­ 1938 __ . 632,646 87,879 655,866 89,706 939,507 139,333 posits yield­ posits studied Estimated mini­ 1939..— 401,170 69,344 419,210 71,317 813,708 138,963 District or area ing strategic in detail by mum number of 1940——— 1,002,646 218,154 1,046,535 223,817 1,625,437 291,685 mica during Survey during mines and pros­ 1941 » .... 1,614,863 318,783 1,707,488 332,050 2,666,453 566,858 World War II 1939-46 pects 1942 » _ . 1,654,895 505,634 1,783,646 555,475 2,761,844 725,030 1943 » .... 1,901,120 1,772,324 2,132,826 2,007,583 3,448,199 3,228,742 1944 » __ 814,874 1,530,625 891,465 1,715,046 1,523,313 3,262,711 28 12 en Wilkes.. ___ . _____ . *>*> 7 50 Total. 24,250,005 9,286,297 25,028,404 9,720,273 46,656,377 16,134,040 708 594 2,000 47 21 sn 411 433 onn 1 No complete data available. Outlying North Carolina * Includes splittings. 19 7 iwi 14 3 Rfl OCCURRENCE 13 83 120 Ridgeway-Sandy Ridge.. 34 30 80 Outlying Virginia _ . 24 29 200 GENERAL. GEOLOGIC FEATURES ShelBy-Hickory. — _ . _ 263 168 360 Outlying North Carolina 37 6 100 Most of the mica-bearing pegmatites occur in Outlying South Carolina. 27 13 60 Hartwefl ______76 33 150 metamorphic rocks, chiefly mica schist and gneiss, Thomaston-Baraesville _ _ 59 69 160 34 11 170 impure quartzite, and hornblende schist and gneiss, Alabama „_ 41 126 250 with minor interlayered kyanite gneiss, sillimanite Total 1,847 1,645 A Q7O gneiss, graphitic schist, recrystallized limestone and dolomite, and various types of chloritic rocks. The The yearly production of sheet and punch mica foliated micaceous rocks have been included in the from North Carolina, from the Southeastern States, Wissahickon schist of Virginia, the Carolina gneiss and from the entire United States during the period of Virginia, Georgia, and the Carolinas, and the Ash­ 1912-44 is shown in table 8. Nearly 97 percent of the land mica schist of Alabama. They form the bulk of Southeastern production has been derived from de­ the country rock in most of the pegmatite districts. posits in North Carolina, and this mica represents The hornblendic rocks, which generally are inter- about 95 percent of the total value. Most of the re­ layered with the micaceous types, have been grouped maining 3 percent of the production has been obtained with the Roan gneiss in Georgia and the Carolinas from Georgia and Virginia. Through the same period and have been termed metagabbro in parts of Vir­ the Southeastern States have accounted for 25,028,- ginia. They are abundant in the Jefferson-Boone, 404 pounds, or about 54 percent of the total United Spruce Pine, Franklin-Sylva, Shelby-Hickory, and SHEET MUSCOVITE IN THE SOUTHEASTERN STATES 31 Ridgeway-Sandy Ridge districts but are not common, the pegmatite districts is available in the maps and for example, hr the Amelia, Hartwell, and Alabama reports of Stose,62 Stose, Jonas and others,63 Keith,84 districts. In general these hornblendic rocks appear La Forge and Phalen,65 Stose and Smith,66 Keith and to be metamorphosed sills, dikes, and flows of inter­ Sterrett,67 and others. In addition the literature con­ mediate and basic composition; probably they are ap­ tains many papers and reports of less general scope preciably younger than the micaceous schists and or less immediate application. They are referred to in gneisses. Both rock types may be pre-Cambrian in this report in specific connections only. age. Large masses of silicic intrusive rocks, probably PEGMATITES late Paleozoic in age, are exposed in many districts. The pegmatites of the Southeastern States consist They are fine to very coarse grained and range in com­ chiefly of plagioclase and quartz, with subordinate position from quartz monzonite to quartz diorite. perthite, museovite, and biotite and accessory garnet, Some, like those in the Spruce Pine district, are peg- apatite, beryl, columbium-tantalum minerals, ura­ matitic in texture and are so leucocratic that they nium minerals, sulfides, and other species. They are have been termed alaskite.50 Others contain more granodioritic rather than truly granitic in composi­ biotite or other mafic minerals and have a typical tion. Muscovite is present in some deposits as dis­ "salt-and-pepper" texture. Associated with most of seminated flakes, foils, and tiny books, but in others it these intrusive masses are finer-grained satellitic sills occurs as very large books, some of which are 2 feet and dikes, as well as many types of hybrid or migma- or more in diameter and weigh several hundred titic rocks. pounds or more. All variations between these ex­ Most of the commercial mica-bearing pegmatites tremes are known, and many pegmatites contain book occur in metamorphic rocks. In general they are similar in composition to nearby intrusive masses and museovite in a wide variety of sizes and forms. In in some districts are demonstrably related to them. general the distribution of mica within a given de­ Some pegmatites in Spruce Pine and other districts posit reflects the shape of the containing pegmatite occur within, rather than adjacent to, stocks and body, but there appears to be little correlation be­ large sill-like bodies of quartz diorite and granodio- tween the size of the body and the quantity of mica rite, but these are in the minority and account for only that it contains. a small proportion of the mica produced from these Both concordant and discordant pegmatites are districts. Many geologic features of mica-bearing common. Dikes, sills, discoidal lenses, pods, tongues, pegmatites in the Southeastern States have been de­ and cigar-shaped bodies are most widespread, and scribed by Sterrett.51 Those in Virginia are discussed archlike or troughlike bodies and more irregular by Pegau,52 C. B. Brown,58 and W. R. Brown;54 those masses are locally abundant. The bodies range from in North Carolina by Sterrett,55 Maurice,56 Kesler and lenses and stringers less than an inch thick to dikes Olson,57 and Olson; 58 those in Georgia by Galpin 59 and sill-like masses several hundred feet long and 200 and by Furcron and Teague; 60 and those in Alabama feet or more thick. The emplacement of these pegma­ by Clark.61 General geologic information concerning tites appears to have been controlled by primary layering and other planar structures in the igneous M Hunter, G. E., Residual alaskite kaolin deposits of North Carolina: Am. Ceramic Soc. Bull., vol. 19, p. 98, 1940. rocks, by bedding, foliation, and schistosity in meta­ 61 Sterrett, D. B.. Mica deposits of the United States: U. S. Geol. Survey morphic rocks, and to a considerable degree by frac­ Bull. 740, pp. 28-30, 70-71, 281-282, 307-308, 1923. tures in both rock types. Numerous pegmatite M Pegau, A. A., The pegmatites of the Amelia, Goochland, and Ridgeway areas, Va.: Am. Jour. Sci., 5th ser., vol. 17, pp. 543-547, 1929; Pegmatite de­ masses, especially those that are highly irregular, posits of Virginia: Virginia Geol. Survey Bull. 33, 1932. probably owe their form to combinations of these fea­ 52 Brown, C. B., Outline of the geology and mineral resources of Gooch­ land County, Va.: Virginia Geol. Survey Bull. 48, 1937. tures. The orientation of many pegmatites—and of "Brown, W. R., Mica deposits of Virginia: Virginia Geol. Survey Bull, minor structures within them—can be correlated (in preparation). w Sterrett, D. B., Mica deposits of western North Carolina: U. S. Geol. Survey Bull. 315, pp. 400-422, 1907; Mica deposits of North Carolina: U. S. 42 Stose, G. W., Geologic map of Alabama, Alabama Geol. Survey and U. S. Geol. Survey Bull. 340, pp. 593-638, 1910. Geol. Survey, 1926. M Maurice, C. S., The pegmatites of the Spruce Pine district, N. C.: Econ. ** Stose, G. W., Jonas, A. I., and others, Geologic map of Virginia, Virginia Geology, vol. 35, pp. 49-78, 158-187, 1940. Geol. Survey and U. S. Geol. Survey, 1928. "Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, w Keith, Arthur, U. S. Geol. Survey Geol. Atlas, Cranberry folio (no. 90), N. C.: U. S. Geol. Survey Bull. 936-A, 1942. 1903; Asheville folio (no. 116). 1904; Mount Mitchell folio (no. 124), 1905; 58 Olson, J. C., Economic geology of the Spruce Pine pegmatite district, Cowee folio (unpublished manuscript), 1906; Nantahala folio (no. 143), N. C.: North Carolina Dept. Cons, and Devel., Div. Min. Resources, Bull. 1907; Pisgah folio (no. 147). 1907; Roan Mountain folio (no. 151), 1907; 43, 1944. Lincolnton folio (unpublished manuscript), 1911. 62 Galpin, S. L., Feldspar and mica deposits of Georgia: Georgia Geol. m La Forge, Laurence, and Phalen, W. C., U. S. Geol. Survey Geol. Atlas, Survey Bull. 30, 1915. Ellijay folio (no. 187), 1913. 80 Furcron, A. S., and Teague, K. H., Mica-bearing pegmatites of Georgia: w Stose, G. W., and Smith, R. W., Geologic map of Georgia, Georgia Div. Georgia Geol. Survey Bull. 48, 1943. Mines, Mining and Geol. and U. S. Geol. Survey, 1939. 81 Clark. G. H., Mica deposits of Alabama: Alabama Geol. Survey Bull. 24, w Keith, Arthur, and Sterrett, D. B., U. S. Geol. Survey Geol. Atlas, Gaff- 1921. ney-Kings Mountain folio (no. 222), 1931. COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES With the orientation of major and minor structures The idealized diagram in figure 16 shows the distri­ in the adjacent country rock. bution of zones and later units in a typical mica-bear­ A general systematic arrangement of mineralogic ing pegmatite body. and lithologic units in many pegmatites has long been recognized, and references to "shoots," "ribs," "pipes," "barrels," "columns," "veins," "zones," "pods," "streaks," and "layers" are common among miners and in geologic literature. Recent detailed ex­ amination and mapping of pegmatites in many parts of the United States have fully confirmed these earlier observations and have added abundant detail and new information as well. As suggested by members of the United States Geological Survey,68 the units of pegmatites can be divided into three groups: zones, fracture fillings, and replacement bodies. Zones, where fully developed, are successive shells concentric about an innermost zone or core. In a gen­ eral way they reflect the shape of the pegmatite body. Where incomplete or discontinuous they commonly form curving lenses, layers, pods, and hoodlike shells. Fracture fillings are bodies that fill fractures in previ­ ously consolidated pegmatite; generally they are dis­ tinctly tabular. Replacement bodies are units formed by replacement of preexisting pegmatite, with or without obvious structural control, and are not to be confused with the relatively simple pegmatite formed by replacement of country rock. They generally occur along boundaries between zones, between zones and fracture fillings, between pegmatite and wall rock, or along any other structural element within the pegma­ tite. Both fracture fillings and replacement bodies form structural patterns that are superimposed on the concentric or quasi concentric patterns of the earlier-formed zones; hence a division of pegmatite 40 Feet units into two general age groups is easily made. FIGURE 16.—Idealized plans of a pegmatite body, showing typical distribu­ Such a broad division is perhaps in part compatible tion of pegmatite units. A, Zones, comprising a core of massive quartz sur­ rounded successively by an inner intermediate zone of coarse, blocky with the findings and conclusions of Schaller,69 perthite, a middle intermediate zone of perthite-plagioclase-quartz-mus- Landes,70 Hess,71 and many other students of pegma­ covite pegmatite, an outer intermediate zone of perthite-plagioclase- tite geology, although markedly differing interpreta­ quartz pegmatite, a wall zone of plagioclase-quartz-muscovite pegmatite, and a border zone of fine-grained plagioclase-quartz-muscovite pegmatite; tions of these age groups have been advanced from B, Same zones, with fracture fillings and replacement bodies of albite- time to time. muscovite pegmatite (solid black) formed in them or at their expense. The following classification of pegmatite zones has been proposed: 72 Most border zones are fine-grained selvages that are not more than a few inches thick. Others, par­ 1. Border, or outermost, zones. ticularly those that have formed wholly or in part 2. Wall zones. through replacement of country rock, are much 3. Intermediate zones. thicker, more irregular, and not sharply defined. Wall 4. Cores, or innermost zones. zones, which are next inside the border zones, are 88 Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., The in­ characteristically coarser and much thicker. Al­ ternal structure of granitic pegmatites: Econ. Geol., Econ. Mon. 2, pp. 13-16, 1949. though they are actually the zones second from the 69 Schaller, W. T., The genesis of lithium pegmatites: Am. Jour. Sci., 6th margins of pegmatite bodies, the designation "wall ser., vol. 10, pp. 269-279, 1925. 70 Landes, K. K., The paragenesis of the granite pegmatites of central zone" has been retained in recognition of a termi­ Maine: Am. Mineralogist, vol. 10, pp. 355-411, 1925. nology firmly established among pegmatite miners 71 Hess, F. L., The natural history of the pegmatites: Eng. and Min. Jour., vol. 120, pp. 289-298, 1925. and mine operators. Most border zones are of little 72 Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., The in­ significance in the mining of pegmatites; hence in the ternal structure of granitic pegmatites: Econ. Geol., Econ. Mon. 2, pp. 20-24, 1949. industry they have not been distinguished from ad- SHEET MUSCOVITE IN THE SOUTHEASTERN STATES 33 joining zones. As border zones rarely are economic Dakota, Colorado, Wyoming, and the Southeastern or mappable units, the term "wall zone," though States are currently in preparation by members of strictly a misnomer, is nevertheless useful and the United States Geological Survey. widely understood. The core, or innermost zone, generally occurs at or MICA DEPOSITS near the center of the pegmatite body. Any zone be­ Some deposits of book muscovite in the Southeast­ tween the core and the wall zone is an intermediate ern States occur in pegmatites that are not clearly zone. There is no theoretical limit to the number of zoned, but most are in well-defined units that in gen­ intermediate zones in a single pegmatite body, but eral are quite distinct from adjacent barren units, and few contain more than three such units. The inner­ their distribution is clearly governed by zonal struc­ most zone of a pegmatite cannot always be recognized tures. Entire zones in some pegmatites are sufficiently as such, for not every core can be seen or even pre­ rich in mica to be mined, but most deposits of com­ dicted with reasonable assurance. Thus a zone identi­ mercial interest are confined to certain portions of fied as a core at one level may prove to be an inter­ zones and hence occur as shoots not unlike the shoots mediate zone when the top or edge of the true lenslike of ore minerals in metalliferous deposits. The position core is exposed by subsequent mining.73 The cores, and distribution of such shoots commonly can be cor­ and possibly other zones as well, are not exposed in related with the over-all shape of the containing zone many pegmatites that have been only slightly cut by or with rolls, bends, bulges, constrictions, protuber- erosion. "ances, or other irregularities in the zone. Little salable Numerous pegmatite bodies contain one or more material is associated with fracture fillings or re- podlike concentrations of minerals. These pods gen­ .placement bodies, particularly in the pegmatites erally can be recognized as segments of discontinuous yielding the greatest production. Such late-stage mica cores where they are clearly distributed midway be­ is common only in a few pegmatites in the Amelia dis­ tween the walls of the pegmatites, but elsewhere their trict of Virginia and in some larger, feldspar-rich, distribution is not so easily interpreted. In most de­ pegmatites in other districts. posits, however, they can be considered as parts of The classification of zones provides a ready means discontinuous zones. of naming many mica deposits. Such designations as The zones in most pegmatite bodies coarsen pro­ "wall-zone deposit," "intermediate-zone deposit," gressively inward from the walls. Moreover, many "footwall deposit," "pod deposit," or "core-margin zones are transected by offshoots or apophyses from deposit" require a minimum of explanation. If two or other zones that lie nearer the center of the pegmatite, more intermediate zones are present, they can be but the reverse relation has not been observed. On the distinguished by means of letters, numbers, or such basis of widespread observation it seems clear that position modifiers as "outer," "inner," "lower," and zones in the mica-bearing pegmatites of the South­ the like. eastern States were developed successively from the Wall-zone deposits yield most of the sheet musco­ walls inward. The internal structure of many peg­ vite produced in the Southeastern States. Substantial matites in New England has been described in detail quantities also are obtained from disseminated de­ by Cameron and others.74 Pegmatites in western posits in poorly zoned pegmatites and from core- North Carolina have been reported on by Olson 75 and margin or other intermediate-zone concentrations. those in northern New Mexico by Jahns.76 In addi­ Detailed field studies have demonstrated that the mica tion, other reports on mica deposits in Idaho, South within a given zone is rather constant in color, clear­ ness, type and distribution of structural defects, and other physical properties, whereas the books from 78 Bannerman, H. M., and Cameron, E. N., The New England mica indus­ try: Am. Inst. Min. Met. Eng. Tech. Pub. 2024, 1946 ; Jahns, R. H., Strategic different zones within the same pegmatite commonly pegmatite minerals from the Southwestern and Southeastern States during differ very strikingly. Green "A" mica, for example, World War II: Am. Inst. Min. Met. Eng. Tech. Pub. (in press, 1949) ; Cam­ eron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., The internal struc­ is especially abundant along the edges of quartz cores ture of granitic pegmatites: Econ. Geol., Econ. Mon. 2, pp. 21-24, 1949. in many pegmatites, whereas the mica in the earlier- 74 Cameron, E. N., Larrabee, D. M., McNair, A. H., Page, J. J., Shainin, V. E., and Stewart, G. W., Structural and economic characteristics of New formed wall zones of the same pegmatites is cinnamon England mica deposits: Econ. Geology, vol. 40, pp. 369-393, 1945. brown, brown, or brownish olive and is relatively free 75 Olson, J. C., Economic geology of the Spruce Pine pegmatite district, N. C.: North Carolina Dept. Cons, and Devel., Div. Min. Resources, Bull. from reeves. Strongly reeved mica is most abundant 43, 1944 ; Mica deposits of the Franklin-Sylva district, N. C.: North Caro­ in the pegmatites and pegmatite zones that are rich lina Dept. Cons, and Devel., Div. Min. Resources, Bull. 49, 1946. in potash feldspar, and it is later than the flat mica 7* Jahns, R. H., Mica deposits of the Petaca district, Rio Arriba County, N. Mex.: New Mexico School of Mines, State Bur. Mines and Min. Resources, in those pegmatites that contain both kinds of books. Bull. 25, 1946; Strategic pegmatite minerals from the Southwestern and Systematic variations in other properties are dis­ Southeastern States during World War II: Am. Inst. Min. Met. Eng. Tech. Pub. (in press, 1949). cussed in the following section of this report. 34 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES TESTS OF SHEET MUSCOVITE FROM THE SOUTH. of this mica was specifically identified as to source, EASTERN STATES and in many instances its position within a given GENERAL STATEMENT mine and with respect to the walls, crest, or keel Of Electrical tests of sheet mica from numerous spe­ the containing pegmatite was known. cific deposits in the southeastern United States were All raw mica was cobbed free of quartz, feldspar, first made in 1940 and 1941, when material collected and other waste when necessary and was then rifted from 124 mines by the Federal Geological Survey was and trimmed before testing. Trimming was done by studied by the National Bureau of Standards.77 As means of small, hand-operated, sickle-bladed knives, previously pointed out, several of these samples were with formation of beveled edges on the finished ma­ subsequently treated in greater detail by two large terial. Most sheets were full-trimmed, whether or not fabricators of electrical equipment. More extensive they were marred by reeves, ribs, clay stains, or investigations, later made by the War Production mineral stains, and many of them could be classified Board in Asheville, N. C., involved the testing of sev­ visually as little better than scrap grade. Some sam­ eral thousand pieces of block mica supplied by the ples consisted of such poor material that they were Colonial Mica Corporation. This material represented rifted but not trimmed, thereby yielding pieces that most of the Southeastern mines operated in 1942 and were loosely classed as "washer stock." Others, how­ 1943. It was graded and classified visually and by ever, contained clear sheets of apparently excellent means of the Q-meter and spark-coil test set, but the quality. During preparation of the mica considerable results of this work have not yet been published. quantities of very thin laminae or "skimmings" were The studies described in this report were made produced. These were later stacked to thicknesses of under the joint auspices of the Geological Survey, the 30 mils (0.030 inch), and the stacks were tested for State of North Carolina, and the Tennessee Valley power factor and conducting impurities. Authority during the period July 1945-April 1946. A Mines and prospects from which test specimens total of 2,502 lots containing 237,764 pieces of mica were obtained are: was examined petrographically and tested electri­ cally. This material, obtained from at least 850 de­ Alabama: Clay County: posits in the states of Alabama, Georgia, North Caro­ M and G mine. lina, South Carolina, and Virginia, was broadly clas­ Eandolph County: sified as follows: Arnott mine. Tallapoosa County: 839 lots: Material previously collected, labeled, and Berry mine. stored by the Geological Survey. Collum (McCray) mine. 474 lots: Special material collected by the Geological Collum "Quartz-Blowout" deposit. Survey, including large lots of mica from the Kidd mine. Southeastern States and some mica from New Eng­ McCray. See Collum mine. land, South Dakota, and New Mexico. Mica Hill mine. Georgia: 493 lots: Material collected by the Geological Survey Elbert County: and the State of North Carolina during the period Cooley mine. of the mica-testing program. Crawford-Daniel mine. 243 lots: Stored material loaned by the Colonial Mica Gaines, C. U., prospect. Corporation and previously graded by visual Gaines, M. L., mine. New Bethel M. E. Church prospect. means. Skelton, J. M., prospect. 41 lots: New material prepared and graded by visual Turner prospect. means and loaned by the Colonial Mica Corpora­ Hall County: tion. Merck (Old Hope) mine. 78 lots: Heavily stained electric mica, prepared and Old Hope. See Merck mine. loaned by H. A. Knight of High Point, N. C. Hart County: Alien, Lon. See Wood mine. 334 lots: Miscellaneous material furnished or loaned Bailey mine. by mine operators, manufacturers, and various or­ Carter mine. ganizations. Garner mine. The lots furnished by the Geological Survey com­ Gully. See Wood mine. prised raw mica obtained from mines, mine dumps Harper-Pierman mine. and waste piles, and rifting shops, as well as some Horsehead mine. prepared stock from shops and storage sheds. Most Jones, Ruth, mine. Scrap mine. n Kesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, Waterhole mine. N. C.: U. S. Geol. Survey Bull. 936-A, pp. 18-30, 1942. Wood (Gully, Lon Alien) mine. 35

Georgia—Continued North Carolina—Continued Lamar County: Avery County—Continued Coggins prospect. Elk (Milton English) mine. Vaughn, Early, mine. Emmons Knob mine. Monroe County: English, Milton. See Elk mine. Battles mine. Ewing mine. Babun County: Four Foot Square mine. Kell mine. Franklin mine. Norton mine. Green, Euben, mine. Tunnel mine. Gusher Knob mine. Troup County: Hoppey mine. Hogg mine. Houston Eock mine. Smith Store prospect. Johnson mine (on Plumtree Creek). Upson County: Johnson, I. M., mine. Adams mine. Justice mine. Barron (Bennie Barren, Walker Wakefield) mine. Lincoln Eock mine. Barren, Bennie. See Barron mine. Lincoln Eock No. 2 mine. Boyt mine. Madam Bank mine. Brown (Parrish) mine. Meadow mine. Carter mine. Mill Eace mine. Corley mine. Moulton (Big Meadow) mine. Gibson, B. S., prospects. Old Eock mine. Herron mine. Ollis, Jake, mine. Pancake mine. Johnson mine. Plumtree (Race) mine. Marshall. See Stevens Eock mine. Powdermill Eough mine. Mauldin mine. Pyatte, Zeb, mine. Mauldin Eoad prospect. Eace. See Plumtree mine. McKinney. See Stevens Eock mine. Eed mine. Mitchell Creek mine. Eed prospect. Parrish. See Brown mine. Slippery Elm mine. Stevens Eock (Marshall, Sullivan, McKinney) mine. Taylor, Fate, mine. Sullivan. See Stevens Eock mine. Vance, Freel, (Jose Comina) mine. Wakefield, Walker. See Barron mine. Westphalen. See Bug Eock mine. North Carolina: White Eock mine. Alexander County: Wiseman, Honey Waits, mine. Dagenhart mine. Wolf Eidge mine. Gwaltney prospect. Buncombe County: Patterson mine. Abernathy Watershed. See New Balsam Gap mine. Ashe County: Burco mine. Buck Mountain mine. Corner Eock mine. Hodson prospect. ' Mitzes mine. Avery County: New Balsam Gap (Abernathy Watershed) mine. "A" mine (on Brushy Creek). "A" mine (on Little Elk Ridge). Smith, Beede. See Swannanoa mine. Aldrich mine. Swannanoa (Beede Smith) mine. Alfred mine. Tipton mine. Benfield mine. Burke County: Benfield, Byard, mine. Brittan, Floyd, mine. Big Meadow. See Moulton mine. Hudson prospect. Bluff mine. Eeed mine. Branch mine. Young, Noah, mine. Brushy Creek East mine. Caldwell County: Buchanan, Aaron, mine. Butler mine. Buchanan, Franklin, (Lee Cook) mine. Daniels, Cling, mine. Buck Hill Eock mine. Caswell County: Bug Eock (Westphalen) mine. Old Milton. See Yarboro No. 1 mine. Burleson, Charley, mine. Slaughter, J,, G., prospect. Carpenter, Tom, mine. Yarboro No. 1 (Old Milton) mine. Charlies Ridge mine. Catawba County: Comina, Jose. See Vance, Freel, mine. Abernathy Long Cut (Hickory) mine. Cook, Lee. See Buchanan, Franklin, mine. Abernathy Water mine. Corn mine. Drum mine. Cow Camp South mine. Hickory. See Abernathy Long Cut mine. Doublehead mine. ' Moose mine. Dugger mine. Sigmon mine. Eli Eock mine. Tallant prospect. COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

North Carolina—Continued North Carolina—Continued Cleveland County: Cleveland County—Continued Anthony prospects. Powell (Sugar Barrel) mine. Archie mine. Putnam mines. Big Hill. See Bonnett Split mine. Randall mine. Blanton prospect. Rice mine. Blanton, C. Robert, mine. Seism prospect. Blanton, Cliff, mine. Spangler, D. H., prospects. Blanton, Coleman, mine. Spangler, Ruben. See Spangler, T. N., prospects. Bonnett Split (Big Hill) mine. Spangler, T. N., (Ruben Spangler) prospects. Bowen mine. Stroud, T. C., prospects. Bridges, Pleaz, mine. Sugar Barrel. See Powell mine. Bumgarner mine. Thompson, W. H. See Norman, Archie, mines. Cabaniss prospect. Thompson, W. H. See Norman-Thompson mine. Cabaniss, Alma, mine. Warlick, Clyde, mine. Cabaniss, Tom, mine. Weathers mine. Campbell mine. Webb mine. Carpenter mine. Williamson mine. Chrysolite mine. Wright prospect. Cooke mine. Gaston County: Cornwall, Frank, (Old J. S. Blanton) mine. Beam, Claude, prospect. Cornwell, Charles, prospect. Big Bess mine. Davis, Walter, mine. Huskins mine. Elliot, L. R., mine. Old Neale. See Self, E. R., mine. Fortenberry, W. H., prospect. Self, E. R., (Old Neale) mine. Foster, J. L., prospect. Haywood County: Gantt, B. T., prospect. Arrowood. See Little East Fork mine. Gantt, M. H., mine. Big Ridge mine. Gettys No. 1 mine. Champion Fibre Co. prospect. Gettys No. 2 mine. Gray. See Putman mine. Glover, Eli, mine. Little East Fork (Arrowood) mine. Gold, Mary, mine. Medford, Frank, prospect. Green, J. F., mine. Putman (Gray) mine. Griggs mine. Shiney mine. Harris mine. Spruce Ridge mine. Herndon mine. Stringfield mine. Homestead. See Mauney, S. S., mine. Wilson, W. T., prospect. Hoyle, A. F., mine. Jackson County: Humphries, Joe E., mine. Abbs Creek mine. Humphries, W. H., prospect. Ashe, Bob, mine. Hunt mine. Bald Ridge mine. Indian Graveyard mine. Bearwallow Fork. See Piney Mountain Creek mine. Indiantown (Mull) mine. Betts Gap mine. Jimmy mine. Bettys Creek No.. 1 mine. Jones mine. Big East Fork mine. King, Marvin, prospects. Big Flint (Grassy Ridge, Glassey Rock) mine, Lail prospect. Big Terrapin mine. Lattimore mine. Blackjack mine. Ledford mine. Blanton prospect. Martin mine. Brown's Coward Mountain. See Wood mine. Martin, J. T., prospect. Bryson mine. Mauney, Bailey, mine. Bryson, C. D., mine. Mauney, M. M. See Mauney, S. S., mine. Bryson, C. V., mine. Mauney, S. S., (M. M. Mauney, Homestead) mine. Bryson, Diller, mine. McGinnis, F. G., mine. Buchanan, B. C., prospects. McSwain, G. B., mine. Buchanan, Henry, prospects. Metcalf mine. Buchanan, Marsden, prospect. Mill Race mine. Buchanan, Pole, mine. Moose mine. Buchanan, Ramsey, mine. Mull. See Indiantown mine. Buchanan's Old. See Gradin mine. Niagara mine. Buckeye Gap mine. Norman, Archie, (W. H. Thompson) mines. Buck Knob mine. Norman-Thompson (W. H. Thompson) mine. Bumgarner mine. Old Carroll. See Patterson, Bun, mine. Buzzard Roost mine. Old J. S. Blanton. See Cornwall, Frank, mine. "C" (Puncheon Camp) mine. Patterson, Bun, (Old Carroll) mine. Cabe mine. Peeler No. 1 mine. Cedar Cliff mine. TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 37

North Carolina—Continued North Carolina—Continued Jackson County—Continued Jackson County—Continued Chastine Creek. See Woods mine. Radeker, J., mine. Choga (Chogey) mine. Reed, Sally, mine. Chogey. See Choga mine. Rhoda mine. Clark, Hardy, prospect. Rice mine. Clouse mine. Richland Balsam prospect. Collins mine. Ridge mine. Cox (Cox and Davies) mine. Roaringhole (Sheep Knob) mine. Cox and Davies. See Cox mine. Rock mine. Cox Murray Mountain. See Engle Cope mine. Rocky Branch (McKay) mine. Dave Ridge mine. Rogers mine. Davis prospect. Sheep Knob. See Roaringhole mine. Dead Timber Ridge mine. Sheep Mountain mine. Deets, Early, mine. Shell Ridge mine. Dietz, Jeanie, mine. Spence mine. Dillard, W. G., prospect. Stephens, Duke. See Hooper, L. E., mine. Double Gap mine. Stillwell mine. East Fork prospect. Sugarloaf Creek mine. East Laport mine. Sugarloaf Mountain prospects. Engle Cope (Cox Murray Mountain) mine. Tater Cove. See Potato Cove mine. Far Top Field prospect. Tennessee Creek mine. Ferguson, Judge, mine. Tilley mine. Flukens. See Long, John, No. 1 mine. Toy (Tustin, Moody) mine. Frady mine. Tustin. See Toy mine. Frady Creek. See Woods mine. Tustin mine. Glassey Rock. See Big Flint mine. Upper Sugar Creek Gap prospect. Gradin (L. C. Presley, Buchanan's Old) mine. Wayehutta Clay (Weary Hut, Worry Hut) mine. Grassy Ridge. See Big Flint mine. Weary Hut. See Wayehutta Clay mine. Gregory mine. Wetmore prospect. Higdon mine. Wilkes mine. Hooper, Aaron, mine. Williams, Bud, mine. Island Ford mine. Wilson prospect (in Cashiers Valley). Jasper mine. Wilson, Cleve, mine. Kolb mine. Wilson, Shirley, mine. Long, John, No. 1 (Flukens) mine. Wolff. See New Wolff mine. Long, John, No. 2 mine. Wood (Brown's Coward Mountain) mine. Long Branch mine. Woods (Chastine Creek, Frady Creek) mine. McCall, Lin, mine. Woodward, Tyler, mine. McKay. See Rocky Branch mine. Worry Hut. See Wayehutta Clay mine. Mills, Luce, prospect. Lincoln County: Moody. See Toy mine. Baxter, Jack, (Tom Baxter) mine. Moss mine. Baxter, Jack, prospect. Murray Cove. See Murray Mountain mine. Baxter, Tom. See Baxter, Jack, mine. Murray Mountain (Murray Cove) mine. Beam prospect. New Wolff (New Wolff No. 1, Wolff, Old Wolff) mine. Bess mine. New Wolff No. 1. See New Wolff mine. Biggerstaff (Deadman) mine. Nichols, Doc, mine. Deadman. See Biggerstaff mine. Nicholson, Andy, mine. Eaker, Doris, mine. Old Wolff. See New Wolff mine. Foster No. 1 (W. A. Thompson No. 1) mine. Painter mine. Foster No. 2 (W. A. Thompson No. 2) mine. Parker, Jerome, mine. Foster, W. L. C., prospect. Parker Knob mine. Foster, W. T., (W. A. Thompson) mine group. Pine Ridge mine. Hallman mine. Piney Mountain Creek (Bearwallow Fork) mine. Houser, Plato, mine. Pinhook Creek mine. Houser, Plato, No. 2 mine. Pinhook Gap mine. King (Norman and Cecil) mine. Potato Cove (Tater Cove) mine. Leatherman, Pink, mine. Presley, Bud, mine. Norman and Cecil. See King mine. Presley, L. C. See Gradin mine. Thompson, W. A. See Foster, W. T., mine group. Presley, Mack, mine. Thompson, W. A., No. 1. See Foster No. 1 mine. Price, J. B., mine. Thompson, W. A., No. 2. See Foster No. 2 mine. Prince, J. S., (Ben Queen) prospect. Macon County: Puncheon Camp. See "C" mine. "A" mine. Queen, Ben. See Prince, J. S., prospect. Allman Cove mine. Queen, Oscar, mine. Ammons (Henry) mine. Raco mine. Anderson mine. 38 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

North Carolina—Continued North Carolina—Continued Macon County—Continued Macon County—Continued Angel, George, (Cabe, Moore) mine. Kiser mine. Angel, Mel, mine. Leatherman, S. C., prospect. Annie Laurie (Wade Moody) mine. Ledford Cove mine. Arnold, Fred, prospect. Lenoir prospect. Arrowwood mine. Lin Cove prospect. Bailey mine. Little Spring mine. Baird (Smith) mines. Littlefield mine. Barnard. See Burke, John, mine. Locust Tree prospect. Bearpen mine. Lower Mack mine. Beasley No. 1 (Bradley Butt) mine. Lucas, Doc, mine. Beasley No. 2 mine. Lyle, Doc, mine. Berry. See Deerlick Knob mine. Lyle Cut mine. Berry mine. Lyle Knob mine. Boyd Knob. See Shepherd Knob mine. Malonee mine. Bradley Butt. See Beasley No. 1 mine. Mashburn, Norman. See Roaring Fork mine. Bradley-Campbell (Campbell, Higdon) mine. Mason, Lee, mine. • Brawley mine. May mine. Bryson. See Burr Knob mine. McCrary mine. Bryson prospect. Mill Knob mine. Bryson, Terrell, (Old Bryson) mine. Miller (Sanders) mine. Buoy No. 1 mine. Moody mine. Buoy No. 2 mine. Moody, Wade. See Annie Laurie mine. Burke, John, (Barnard) mine. Moore. See Angel, George, mine. Burleson, John, mine. Moore, A. J., mine. Burningtown. See Poll Miller mine. Moore, Jim, mine. Burr Knob (Bryson) mine. Morgan, Denver, prospect. Buttermilk mine. Moss Knob mine. Cabe. See Angel, George, mine. Mount Hope Church prospects. Camp Branch prospect. Mud Hole mine. Campbell. See Bradley-Campbell mine. Mud Hole prospect. Chalk Hill mine. Old Bryson. See Bryson, Terrell, mine. Corbin Knob mine. Passamore. See Turkey Knob mine. Corbin Knob prospects. Pendergrass prospect. Crawf ord, Lester, prospect. Pine Knob. See Kelly, Lassie, mine. Cunningham mine. Poll Miller (Burningtown) mine. Dalton mine. Poplar Cove prospect. Deerlick Knob (Berry) mine. Quizzenberry mine. Dills mine. Raby (Grady Duvall) mine. Dobson mine. Ray, Lissie, prospects. Downs, Charlie, prospect. Ray, Tom, prospect. Duvall, Grady. See Raby mine. Ray Cove prospect. Elmore mine. Rickman mine. Elmore Branch prospect. Roaring Fork (Norman Mashburn, Smith) mine. Evans, A. J., prospect. Rocky Face. See Henry, Jake, mine. Fox mine. Rocky Face mine. Gibson mine. Roper, Bud, mine. Gibson prospect. Rough Fork mine. Gum Gap mine. Russell prospect. Gurney mine. Sanders. See Miller mine. Hall prospect. Sanders, Doc, mine. Hall-Burra mine. Shepherd Knob (Boyd Knob) mine. Harris mine. Shotgun mine. Henry. See Ammons mine. Siphon Hole mine. Henry, Jacob W. See Henry, Jake, mine. Slagle mine. Henry, Jake, (Jacob W. Henry, Rocky Face) mine. Slagle-Drake mine. Higdon. See Bradley-Campbell mine. Smith. See Baird mines. Higdon prospect. Smith. See Roaring Fork mine. Holbrook. See Kasson mine. Stamey mine. lotla. See lotla-Bradley mine. Swell prospect. lotla-Bradley (lotla, lotla Bridge) mine. Taylor, Fred, prospect. lotla Bridge. See lotla-Bradley mine. Thorne Mountain Northeast mine. Jack Knob mine. Thorne Mountain Southwest mine. Kasson (Holbrook) mine. Turkey Knob (Passamore) mine. Kelly, Lassie, (Pine Knob) mine. Turkey Nest mine. Kinsland prospect. Upper Raby mine. TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 39

North Carolina—Continued North Carolina—Continued Macon County—Continued Mitchell County—Continued Verdell mine. Deer Park No, 5 mine. Welch mine. Devils Looking Glass mine. Wildes, Jud, prospect. Dog Pond mine. Winding Stair mine. Drawbar mine. Winecoff mine. Duck Branch mine. Zachary, Hal, mine. Eagles Nest mine. Zoom mine. English Knob mine. McDowell County: Estatoe Northeast (Mossy Bock) prospect. Thunder Knob mine. Estatoe Southeast prospect. Mitchell County: Field mine. A. S. O. mine. Field prospect. Abernathy mine. Flat Rock mine. Adams mine. Flukens Hill mine. Adams prospect. Glenn mine. Autrey, Al, mine. Gopher mine. Bardon (Bordon) mine. Gouge, Slim, prospect. Bear Creek prospect. Gouge Falls prospect. Bearden mine. Graveyard prospect. Birch mine. Gudger mine. Birdeye. See Phillips mine. Guy mine. Bloodworth mine. Hall and Boone mine. Bordon. See Bardon mine. Harlan mine. Branch. See Young, Zack, mine. Harlan prospect. Buchanan, Adam, mine. Haw Flat mine. Buchanan, Erby. See Buchanan, J. K., mine. Hawk mine. Buchanan, J. K., (Milt Wilson, Erby Buchanan) mine. Hawkins mine. Buchanan, Jim, mine. Hootowl mine. Buchanan, Jim, prospect. Hootowl prospect. Buckeye mine. Hoppus prospect. Bunker Hill mine. Hoppus Northeast mine. Burleson, W. C., (Joe Stevenson) mine. Horton Rock mine. Byrd. See Byrd, Jeff, mine. Howell, George, (Waterhole) mine. Byrd, Jeff, (Byrd) mine. Howell, Jeff, mine. Carolina China Clay Co. mine. Jack Eock mine. Carolina Mineral Co. No. 3 mine. Jase mine. Carolina Mineral Co. No. 6 mine. Jase prospect. Carolina Mineral Co. No. 12 mine. Jeff. See Jeff Cut mine. Carolina Mineral Co. No. 12 (White) prospect. Jeff Cut (Jeff) mine. Carolina Mineral Co. No. 29 mine. Jimmy Cut mine. Carters Eidge prospect. Johnson prospect. Case mine. Johnson, Ernest, mine. Chalk Mountain mine. Johnson, Theo, mine. Chalk Mountain prospect. Landers mine. Chalk Mountain Northeast prospect. Liberty Hill prospect. Chestnut Flat mine. Lick Eidge mine. Chestnut Flat prospect. Little Hawk Eidge mine. Chestnut Flats mine. Long Cut mine. Clarissa mine. Lower Sugar Tree Cove mine. Cloudland (Pizzel) mine. Lower Sugar Tree Cove prospect. Cook. See Sparks, Eube, mine. Miller mine (north of Penland). Cook mine. Miller mine (west of Spruce Pine). Cook prospect. Mossy Eock. See Estatoe Northeast prospect. Cox prospect. Mountain Top mine. Cox, Ben, mine. Murphy Eock mine (on Crabtree Creek). Cox, Wood, mine. Murphy Eock mine (on Eebels Creek). Cox, Wood, prospect. McBee, Ed, mine. Crabtree No. 1 (Wildcat) mine. McChone prospect. Crabtree Northwest prospect. McKinney mine. 7 Crabtree Summit prospect. McKinney prospect. Dake. See Deake mine. New. See Wisemari'No. 2 mine. Davis mine. Pannell mine. Deake (Dake) mine. Pegram mine. Deer Park No. 1 mine. Penland Southeast prospect. Deer Park No. 2 mine. Pepper Pot (Perrin) mine. Deer Park No. 3 mine. Perrin. See Pepper Pot mine. 40 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

North Carolina—Continued North Carolina—Continued Mitchell County—Continued Stokes County—Continued Phillips (Birdeye) mine. Spencer mine. Pine Mountain mine. Steele mine. Pizzel. See Cloudland mine. Transylvania County: Potato Hill mine. Bee Tree No. 1 (Bee Tree Fork) mine. Poteat mine. Bee Tree Fork. See Bee Tree No. 1 mine. Poteat prospect. Farlow Gap. See Furlow Gap mine. Poteat Southwest prospect. Furlough Gap. See Furlow Gap mine. Poteat West prospect. Furlow Gap (Farlow Gap, Furlough Gap) mine. Putman mine. Reid mine. Putman, Marsh, mine. Wagon Road Gap prospect. Queen mine. Wilkes County: Randall mine. Creek. See Shell, Zolly, mine. Raven Cliff mine. Ferguson mine. Renfro mine. Hall mine. Richards prospect. Proffitt, W. A., mines. Ridgecrest prospect. Shell, Zolly, (Creek) mine. Self mine. Yancey County: Silvers Ridge mine. Aley Creek. See Ayles Creek mine. Silvers Ridge prospect. Alford mine. Sinkhole mine. Alien, Henry, mine. Smith, Calhoun, mine. Allis Creek. See Ayles Creek mine. Smith, Lissie, mine. Anglin mine. Soft Ridge mine. Autrey (Linzie Autrey) mine. Sparks, Ken, mine. Autrey, J. W., mine. Sparks, Rube, (Cook) mine. Autrey, Linzie. See Autrey mine. Stevenson, Joe. See Burleson, W. C., mine. Ayles Creek (Allis Creek, Aley Creek) mine. Summer mine. Bailey Mountain mine group. Twiggs mine. Balsam mine. Upper Crabtree prospect. Barger (Spruce Pine Mica Co. No. 21) mine. Vern Muse prospect. Barger prospect. Waterhole. See Howell, George, mine. Bee Ridge mine. White. See Carolina Mineral Co. No. 12 prospect. Bennett. See Red mine.. Wildcat. See Crabtree No. 1 mine. Bittner and Beech mine. Willis prospect. Black Brothers. See McKinney, Chet, mine. Wilson, Milt. See Buchanan, J. K., mine. Black Dixie mine. Wiseman No. 2 (New) mine. Blake (Boomer Tom Young) mine. Wiseman, W. W., mine. Bland. See Old Gibbs mine. Wolfden prospect. Blevins, D. 0.,-Wes Thomas mine. Wolfden North prospect. Boomer Tom Young. See Blake mine. Young prospect. Boone, Nelson, mine. Young, George, mine. Boone, W. K., prospect. Young, S. S., mine. Branch mine. Young, Zack, (Branch) mine. Brown Creek mine. Rockingham County: Buckeye mine (on Crabtree Creek). Evans, Rosa, mine. Buckeye mine (west of Boonford). Holland mine. Carolina Mineral Co. No. 20 mine. Knight mine. Carson Rock mine. Smith, Ben. See Smith, Short Tom, mine. Cattail (Cattail Creek, Ison) mine. Smith, Ernest, mine. Cattail Creek. See Cattail mine. Smith, Short Tom, (Ben Smith) mine. Celia. See Cilley mine group. Rutherford County: Chestnut Branch mine. Dycus mine. Cilley (Silly, Celia) mine group. Isinglass Hill mine. Commissary Ridge mine. Kay mine. Cora mine. Maurice mine. Creson mine. Stroud, Lax, mine. Eagle Bluff mine. Stokes County: Edge mine (near Fawn Mountain). Brown mine. Edge, John, mine. Hawkins (Joe Hawkins) mine. Edwards mine. Hawkins, J. C. See Ruby King mine. Edwards, J. W., mine. Hawkins, Joe. See Hawkins mine. Flukens Ridge mine (head of Blue Rock Branch). Hole (Jack Hole) mine. Flukens Ridge mine group (at Newdale). Jack Hole. See Hole mine. Georges Fork mine. Ruby King (J. C. Hawkins) mine. Gibbs mine. Shelton, G. R., mine. Gibbs, Elmyra, mine. TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 41

North Carolina—Continued North Carolina—Continued Yancey County—Continued Yancey County—Continued Gibbs Clay. See Old Gibbs mine. Willis Shanty mine. Gibbs Spar mine. Wray. See Ray mine. Gimbel prospect. Young mine (near Harding mine). Googrock mine. Young, Charley. See Harding mine. Gouge. See Shehan, John, mine. Young, Charley. See Hensley-Laurel mine. Gouge,, Fannie, (Spruce Pine Mica Co. No. 10) mine. Young, Josh, mine. Gouge, Jim, mine. Young, Wilt, mine. Green Mountain mine. Young, Zeph, mine group. Griffin. See Griffith, Mills, mine. Young, Zeph and Alien, mines. Griffith, Mills, (Griffin) mine. South Carolina: Hall, Andy» mine. Anderson County: Hall, Cleveland, mine. Burgess (L. E. Hunter) mine. Harding (Charley Young) mine. Gaillard mine. Harp and Blevins prospect. Hunter, L. E. See Burgess mine. Heaton, Sport. See McKinney, W. A., mine. Martin, Ben, mine. Hector mine. Cherokee County: Hensley-Laurel (Charley Young) mine. Blanton, Troy, mine. Higgins mine. Pickens County: Hilliard mine. Bolding mine. Irby Cut mine. Fowler, Will, prospect. Ison. See Cattail mine. Spartanburg County: James mine. Cowpens mine. Laurel Branch mine. Virginia: Laws mine. Amelia County: Ledford mine. Berry mine. Letterman, M. P., mine. Bland. See Champion mine. Little Zeph mine. Champion (Jefferson No. 4, Bland) mine. Locust Rough (B. T. Snopp) mines. Dobbin prospect. Lower Crabtree prospect. Jefferson No. 4. See Champion mine. Mathis, Wyman, prospect. Jefferson No. 6 mine. McDowell, Gaston, mine. Ligon mines. McKinney, Chet, (Black Brothers) mine. Maria (Old Pinchbeck, Smith) mine. McKinney, W. A., (Sport Heaton) mine. Mays mine. McPeters, Flem, mine. McCraw No. 1 (Old Pinchbeck No. 2) mine. Middle Ridge mine. McCraw No. 2 (Old Pinchbeck No. 3) mine. Old Gibbs (Washout, Bland, Gibbs Clay) mine. McCraw No. 3 (Old Pinchbeck No. 1) mine. Old Owlie mine. Morefield mine. Poll Hill mine. Old Pinchbeck. See Maria mine. Presley. See Presnell mine. Old Pinchbeck No. 1. See McCraw No. 3 mine. Presley, S. W., mine. Old Pinchbeck No. 2. See McCraw No. 1 mine. Presnell (Presley) mine. Old Pinchbeck No. 3. See McCraw No. 2 mine. Ray (Wray) mine. Rutherford mines. Red (Bennett) mine. Rutherford No. 2 mine. Riddle, Jim, mine. Smith. See Maria mine. Robinson, Charles, mine. Vaughn mine. Robinson, Corb, mine. Wingo mine. Rock mine. Bedford County: Sally Knob mine. Young mine. Shakerig mine. Charlotte County: Shehan, John, (Gouge) mine. Crews No. 1 prospect. Silly. See Cilley mine group. Crews No. 2 prospect. Silvers mine. Howell prospect. Sleepy Hollow mine. Goochland County: Snake Den mine. Amber Queen mine. Snopp, B. T. See Locust Rough mines. Monteiro (Monteiro Tract) mine. Spruce Pine Mica Co. No. 10. See Gouge, Fannie, mine. Monteiro Tract. See Monteiro mine. Spruce Pine Mica Co. No. 21. See Barger mine. Hanover County: Tantrough mine. Saunders No. 2 mine. Tolley Bend. See Tolley Bent mine. Henry County: Tolley Bent (Tolley Bend) mine. Coleman No. 2 mine. Washout. See Old Gibbs mine. DeShazo mine. Westall mine (on Colberts Ridge). Eanes No. 2 mine. 42 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES Virginia—Continued inclining more toward red as the thickness of the specimen Henry County—Continued viewed is increased; so the inspector must take into account Garrett mine. the thickness of the specimen in order to arrive at a judgment Greer and Merriman mines. characteristic of the mica independent of its thickness. This Jones No. 1 mine. he does by viewing the mica in several ways; single specimens Jones No. 2 mine. of various thicknesses viewed against the sky, a handful of Taylor, Nettie, mine. specimens viewed against the sky, single specimens of various Morrison, C. R., (Oak Level) mine. thicknesses placed on a white surface and viewed by light that Oak Level. See Morrison, C. R., mine. passes through each specimen twice, once to* illuminate the Price mine. white surface, once to reach the eye of the observer, and Ridgeway mine. finally directly in the stock box or barrel in an opaque layer by Pittsylvania County: light reflected from the nearly parallel faces of the various Broomfield prospect. specimens in random position at various depths in the pile. Roach. See Sycamore mine. His judgment is complicated by the fact that mica from a single mine to be classified exhibits more or less variation in Sycamore (Roach) mine. color from specimen to specimen and from spot to spot within Powhatan County: the same specimen. Dolphin, Clinton, mine. Herbb No. 1 mine. It is true that an experienced inspector learns to apply Herbb No. 2 mine. mental evaluations and comparisons in achieving re­ Miller. See White Peak No. 1 mine. markably good color indemnifications for block mica, Purcell. See White Peak No. 1 mine. but his results cannot be as accurate nor as consistent White Peak No. 1 (Purcell, Miller) mine. as those based upon direct comparison with material COLOR TESTS standards. Such standards may themselves change METHODS OP TESTING through normal deterioration or through careless use, All lots of mica were classified according to color, but they can be checked from time to time against so that systematic comparisons could be made be­ more absolute standards. tween this property and the results of electrical and All samples used for color classification in the pres­ petrographic tests. Three general methods of classi­ ent tests comprised pieces of mica at least 1^4 inches fication were considered. Simple inspection, long and long and half an inch wide. These were cleaved to a uniform thickness of 15 mils (0.015 inch) and were successfully used by experienced men in the industry, then correlated with elements of the filter-type color is rapid but is subject to human errors. Determina­ chart prepared by Ridgway.81 Direct comparisons tion of color by means of a chromaticity-difference were made by means of a white sheet of cardboard, colorimeter 78 and photometer is an accurate and each test being placed upon the sheet beside a rec­ readily reproducible method and was used by Judd 79 tangular hole slightly smaller than one of the colored in establishing a color standard for ruby mica. It is elements of the chart. The cardboard was moved until based upon the derivation of indices for lightness and one of the standard colors, as seen through the hole, hue and gives results in fundamental terms. Unfor­ could be matched with the mica under test. All com­ tunately, however, this method requires time and parisons were made under illumination from two equipment not available to the writers when the Ashe- General Electric 17-inch 3,500° white fluorescent ville tests were made. This was true, also, of several lamp tubes placed 15 inches above the working sur­ other methods that were suggested. Direct visual face. No difficulty was experienced in satisfactorily comparison of the mica with color standards was the matching the test pieces with the standard colors of scheme finally adopted, as it yields rapid and repro­ the chart, although great care was necessary in the ducible results that are accurate within the limits re­ observation of samples heavily stained by clay or quired in the investigations. other minerals. The difficulties of judging color by ordinary inspec­ RESULTS tion are clearly summarized by Judd,80 who notes that More than 75 different colors were identified among the distinction between ruby and nonruby micas the micas tested. No samples were found to match "often depends upon color differences so small as to be pure spectrum colors or intermediate hues; instead, scarcely perceptible." He goes on to state that the identifications reflect variations in lightness, or a given borderline mica will exhibit a progression of colors tone, and variations in brightness and dullness. In from nearly colorless through yellowish gray to dark brown, general the colors range from buff and drab through

78 Judd, D. B.. Specification of uniform color tolerances for textiles: Tex­ shades of brown and green to pale yellowish green.82 tile Research, vol. 9, p. 258, 1939. ™ Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards Jour. "Ridgway, Robert, Color standards and color nomenclature, 43 pp., 63 Research, vol. 36, pp. 246-256. 1945. color pis., Washington, D. C., 1912. 80 Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards Jour. w Jahns, R. H., Color characteristics of sheet muscovite in the Southeastern Research, vol. 36, pp. 245-246. 1945. States (abstract): Geol. Soc. America Bull., vol. 66, p. 1170, 1945. TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 43 They are readily grouped into seven main categories, are known from many pegmatites, but they typically as follows: occur in separate concentrations within those pegma­

Main color categories General terms of the trad tites. Where green and brown muscovite occur in the Pinkish buff and drab same pegmatite body, the brown is near the walls. Those pegmatites with green book muscovite near Cinnamon brown Ruby their walls contain no brown books and generally con­ tain no biotite other than that possibly derived Brown Rum through reaction with wall-rock material. Biotite is Brownish olive characteristically associated with buff, drab, and I I brown muscovite. Yellowish olive I I Most of the book muscovite in the Piedmont prov­ Green ince is brown and pinkish, in marked contrast to that Yellowish green in the Blue Ridge province. Green mica in the Pied­ Green mont province is common only in the Ridgeway- Sandy Ridge district and in numerous outlying areas None of the colored groups is sharply bounded, and in Virginia and North Carolina. Pinkish and brown all gradations between groups adjacent on the list are micas in the Blue Ridge province are common only in known. Each is easily subdivided on the basis of light the Franklin-Sylva district, in outlying parts of the tints and dark shades, as well as in terms of relative Spruce Pine district, and in areas farther north. The brightness or dullness. The darkest shades are browns outer parts of many color-zoned books in the Alabama and brownish olives; the lightest tints, those of yel­ deposits are green. lowish green, buff, and drab. The results of the color Broad and systematic color variations in the mica tests are included in table 10. of some areas appear to be related to nearby masses In general the buff, drab, and cinnamon-brown of intrusive rock that probably are genetically re­ micas are the "ruby" micas of the trade, and the lated to the pegmatites. Such variations are particu­ brown and brownish-olive varieties are the so-called larly clear in the Spruce Pine, Franklin-Sylva, Hart- "rum" micas. The trade terms are inexact and are well, and Ridgeway-Sandy Ridge districts. The not everywhere consistently used, so that only a gen­ pegmatites in and near the intrusive masses generally eralized correlation between them and the more ac­ contain muscovite of dominantly green color, whereas curate color designations has been attempted. In those farther from such masses are more likely to many instances the trade terms are misnomers. As contain buff, drab, or brown muscovite. pointed out by Judd,83 most specimens that are near the ruby-nonruby border line actually are weak PETRO GRAPHIC EXAMINATION yellow, very pale brown, or pale brown according to STAINING AND INCLUSIONS more appropriate designations.84 It has been indicated in previous sections of this Nearly all lots of mica were examined megascopi- report that few mica books are colored uniformly cally for staining and inclusions and then were fur­ throughout. Color zoning, with gridiron or chess­ ther studied under the petrographic microscope. Many board patterns, concentric color bands parallel to of the test pieces contain mineral stains in the form crystal outlines, stripes parallel to pressure-figure di­ of intergrown hematite, magnetite, and biotite, either rections, and faint, irregular mottling are wide­ singly or in combination, but only 286 lots—13.3 per­ spread. Further, nearly all books contain pale rims cent of the total—consist of such stained pieces to that tend to be more greenish than the interior. Or­ the extent of 25 percent or more (table 9). The dinarily, however, it is possible to assign a single amount of stain varies greatly from one lot to another general color to a given sheet of mica, and it is only and even from one piece of mica to another. The bulk in exceptional instances that two or more distinct of material in 96 lots—4.5 percent of the total—is colors merit individual note in this connection. (See marred by moderate to large quantities of such im­ table 10.) purities, but numerous other samples are entirely Although variations within individual books are clear, and still others contain biotite or iron oxides in common, the color of muscovite in the Southeastern such minor quantities or in such tiny specks that deposits is fairly constant within a single shoot or they are virtually clear. even within a single zone in the pegmatite body in The proportion of material with mineral stain and which it occurs. Micas of more than one general color the type and distribution of stain in each lot are in­ ** Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards Jour. dicated in columns 6 and 7 of table 10. It should be Research, vol. 35, p. 248, 1945. pointed out that these data apply only to the mica that M Judd. D. B., and Kelly, K. L., Method of designating colors: Nat. Bur. Standards Jour. Research, voL 23, p. 355, 1939. was tested and hence may or may not represent the 44 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES average of all material obtainable from any given inorganic type, and several other forms of pale, curdy mine. stain mar some or all test pieces of 192 lots, or nearly 9 percent of all lots examined (table 9). Much of this TABLE 9.—Occurrence of stain in 2,148 test lots of muscovite from the Southeastern States thin, finely divided foreign material occurs in specks, spots, lines, rows, clumps, clusters, or more irregular Proportion of Type of stain Number of total lots aggregates. "Bursts" and "cigarette burns," large lots (percent) spots with dark, opaque centers and pale, irregular Hematite and /or magnetite and /or biotite inter- margins, are abundant in some of the lots (table 10). 550 25.6 Clay stains are widespread, and nearly a fourth of Mineral stain in 25 percent or more of test pieces.. 286 13.3 the test lots include some pieces of mica marred by Moderate to large quantities of mineral stain in 96 4.5 this defect (table 9). Secondary iron oxide stains Vegetable stain and /or green mottling and /or are almost as abundant, but manganese oxide stains brown mottling present in one or more test pieces. 192 8.9 are common in only a few lots. None of the pieces of Clay stain and /or iron stain and /or manganese 514 23.9 Southeastern mica is appreciably air-stained. Hematite is the most abundant staining mineral in Much of the included hematite and magnetite oc­ the entire mass of tested mica, but inclusions of this curs as specks (less than 0.5 millimeter or 0.02 inch mineral are rare in the buff or brown micas. Nor is in diameter), small spots (0.5 to 1.5 millimeters or magnetite abundant in such material. In contrast, 0.02 to 0.06 inch in diameter), or large spots (more shredlike intergrowths of biotite are common in mus­ than 1.5 millimeters or 0.06 inch in diameter). Nearly covite of nearly all colors. Ordinarily the biotite in all the spots and specks of magnetite have rather green muscovite is green, whereas that in brown and smooth and regular edges, but a substantial propor­ buff muscovite is deep brown. The coarse, euhedral tion of the hematite occurs in simple or complex den­ type of biotite generally is intergrown with reddish dritic forms. The designations "dendrite specks" and muscovite, rarely with green. It is very rare in ma­ "dendrite spots" are appropriate for these irregular terial that is marred by hematite or magnetite stains. particles. Numerous large inclusions are most easily Garnet appears to be a characteristic associate of referred to as "plates," "laths," or "blotches." green muscovite; apatite, of the buff and brown "Blotches" are irregular masses or groups of masses varieties. whose diameters generally are greater than 3 milli­ In ge\ieral stained mica is most abundant in the meters. The arrangement and distribution of stain outer parts of the containing pegmatite body and also and inclusions are readily described by means of such is concentrated along the margins of wall-rock septa terms as "rows," "belts," "triangular lattices," "rec­ and inclusions. Pegmatites with core-margin or other tangular lattices," "clusters," "sprays," "clouds," centrally distributed concentrations of stained mica "scattered," "widespread," "local," and "isolated." generally contain few clear green books, and those Biotite that is intimately intergrown with mus­ with wall-zone concentrations of clear green mica covite as very thin films without good crystal form rarely contain many stained books. Stained mica is is considered a type of mineral stain in this report. abundant in some pegmatites with wall-zone concen­ Individual films are platy, wispy, or shredlike and trations of buff or brown books but is rare in those commonly cause the host muscovite to appear green­ with centrally located concentrations of such books. ish or brownish and distinctly curdy. The biotite staining is concentrated near the centers of some PINHOUES AND HAIR CRACKS muscovite books, and locally, where it forms triangu­ Pinholes are tiny flaws that generally extend lar latticelike patterns, it closely resembles some of through only a few laminae of the host mica book. the lighter-colored types of hematite intergrowths. Many are extremely difficult to detect without the aid In contrast to the "mineral-stain" type of biotite are of a microscope or suitable spark-test apparatus, and larger, euhedral inclusions of brown or black biotite, a substantial proportion of condenser failures may some of which are an eighth of an inch or more in well be attributable to the presence of such imperfec­ thickness. These seem best classed with inclusions tions in visually qualified mica. Hair cracks (or hair of such minerals as apatite, garnet, pyrite and other lines), the extremely thin, fine cracks that cause fail­ sulfide minerals, quartz, tourmaline, zircon, and ure of laminae during filming, are somewhat more zoisite. When removed, inclusions of this type leave easily recognized by careful inspection, although holes or distinct depressions in the host muscovite. many of them do not become apparent until the mica Their occurrence and distribution in the tested ma­ is split into very thin sheets. terial are summarized in table 10, column headed In the present investigation no attempt was made "Remarks." to determine the proportion of pinholed sheets in each Green and brown mottling, vegetable stain of the lot of mica by means of petrographic examination TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 45 alone, as such quantitative data were much more operated throughout each 8-hour work day for the readily obtained during the electrical testing. On the duration of the program. other hand, it was possible to demonstrate under Three women, selected on the basis of their knowl­ the microscope that most pinholes are formed by edge and experience in the rifting, trimming, and the "popping out" of tiny inclusions of zircon, apatite, visual inspection of sheet mica, were contracted to garnet, and other "stony" minerals. Not only are prepare the samples and operate the test sets. Each such inclusions similar in size and shape to the pin- was trained to adjust, calibrate, and manipulate the holes occurring in the same lot of mica, but the gen­ electrical equipment. All work was done in a non- eral proportion of these inclusions in block mica can air-conditioned ground-floor room, 25 feet wide and be correlated with the proportion of pinholed films 40 feet long. The relative humidity varied from 45 obtained from the blocks. Moreover, it has been long percent to 98 percent, and the temperature from 68° recognized by experienced mica inspectors that the to 97° F. The spark-test machine was used in a proportion of pinholes in a given lot of mica increases darkened room, so that the operator could readily during its preparation and especially during filming, distinguish any conducting material or structural presumably as a result of separation of the stony in­ flaws in the samples. clusions from the mica laminae. These imperfections The entire program was organized on a continuous are much more abundant in the brown and buff, or production-line basis. The mica books were rifted by ruby, micas than in the green ones, probably because one operator, placed in labeled boxes, trimmed by a of the greater numbers of zircon and apatite inclu­ second operator to sizes recommended by the Ameri­ sions in the brown and buff varieties. can Society for Testing Materials, and then tested in Some hair cracks are irregular, with trends that the Q-meter by the third operator. Job assignments do not form a recognizable pattern within the host were rotated each day, in order to avoid monotony book of mica. Most, however, are straight and sys­ and increase the efficiency of the work. In only a few tematically disposed. Many of these appear to be days the women acquired sufficient technique and ruling developed on a very small scale, whereas skill to test adequately 15 to 25 mica specimens per others are parallel to rays of the percussion figure. minute on the Q-meter. The best operator was able According to some mica buyers, hair cracks com­ to test 20 specimens per minute for periods of 15 monly develop after the rifting and trimming of sheet minutes or more. Even so, the time and precision re­ mica from certain deposits, particularly if the books quired for the Q-meter work made it the slowest part are not thoroughly dried before preparation. If such of the procedure, and some balancing of operations block mica is filmed promptly, however, relatively was necessary. This was done by assigning either few hair cracks are said to form. This may well be of the workers engaged in preparing the mica to op­ true, but it also is known that hair cracks are much eration of the spark-test set for appropriate periods. more readily recognizable in mica films than in book In the Q-value or power-factor test, the pieces of or block material. In general these tiny imperfections mica were designated as E-l, E-2, or E-3, and the are more abundant in green micas than in the brown results of all 30-mil stack tests were recorded. After and buff, or ruby, varieties. They are especially com­ each lot of mica was tested, it was placed in paper mon in the yellowish-olive books from several mines envelopes according to its electrical classification. in the Spruce Pine district. Washer material was kept in separate bags. The lots were then counted, labeled, and checked, and the ELECTRICAL TESTS numbered envelopes were boxed in groups of 50 and APPARATUS AND METHODS OF TESTING taken to the spark-test bench. In the spark testing The Bell Telephone Laboratories' resonance-cir­ a record was made of all mica with conducting im­ cuit vacuum-tube-voltmeter portable set D-167113 purities, hair cracks, and pinholes, and the general was used to determine the Q value of all samples of proportion of pieces with conducting material or block mica, and the spark-coil test set D-l 67407 was other defects was noted for each lot (table 10). The used to establish the presence of pinholes, cracks, and samples also were inspected visually, and a record conducting impurities. The circuits and operation of was made of color, transparency, and any stain or these instruments have been described in a foregoing large cracks that were present. section of this report. Before the testing began, the Q-meter was overhauled, cleaned, and checked for GENERAL TESTS satisfactory tube characteristics, battery voltages, The results of the general electrical tests are listed and calibrations. The meter was again checked by by sample lot and by mine in table 10, where they are the supervisor of the electrical work before actual correlated with descriptions of color, transparency, test runs were begun, and it was rechecked after each and other properties of the mica. Of the 2,502 lots 2-hour period to insure continued accuracy. It was tested with the Q-meter 2,441, or 97.6 percent, con- 862911°—BO—4 46 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES tained only mica that lay within the 95-100 range There is little correlation between power factor and on the dial and hence qualified as E-l material. All color of the test pieces, or between power factor the mica in 16 lots was classed as E-2 in quality, and and amount of shredlike biotite inclusions, mottling, all of that in two other lots as E-3. Of the remaining vegetable stain, large biotite inclusions, or brown samples 21 contained E-l and E-2 material, 19 con­ bursts in the test pieces. The spark tests verified the tained E-l, E-2, and E-3, and 3 consisted of E-2 and presence of cracks and holes in much of the trimmed E-3 pieces only* The results of electrical tests of the sheet material, and a. few of the hematite and mag­ samples that include material of other than E-l qual­ netite inclusions in some pieces reacted as conducting ity are listed separately in tables 11 and 14. substances (table 10). Pinholes and hair cracks were In terms of individual test pieces, 229,781 of the found to be much more common than they appeared total 237,764 specimens—more than 96.6 percent— to be on the basis of careful visual examination and qualified as E-l; 4,747 specimens were classed as E-2 were present in some of the test pieces that ordinarily and 3,236 as E-3. Most of the E-2 and E-3 mica was would have qualified as No. 1. slightly to heavily stained with magnetite or hema­ The testing program included the study of 550 lots tite, and approximately half of it was so badly clay- that contained a total of more than 39,000 pieces of stained, iron-stained, rippled, reeved, cracked, or very slightly to densely mineral-stained mica. Nearly otherwise blemished that it could be classed only as half of these pieces were typical No. 3 or electric washer stock. Some electrically inferior sheets, on stock, with scattered to dense specks, spots, blotches, the other hand, were perfectly clear and otherwise laths, and lattices of iron oxides. Most of the mica was found to be E-l, and less than a tenth of it could of excellent appearance. be classed as E-3. A 15,987-piece sample of half- and Of the 61 lots containing mica of E-2 or E-3 qual­ three-quarter-trimmed No. 3 material from the ity, 29 were obtained from mines in the Spruce Pine Knight mine, Rockingham County, N. C., tested 61 district, N. C.; 5 from the Shelby-Hickory district, percent E-l, 23 percent E-2, and 16 percent E-3. Only N. C.; 11 from the Franklin-Sylva district, N. C. and 3 percent of the mica contained conducting impur­ Ga.; 2 from Charlotte County, Va.; 4 from Troup ities. Most of the specimens were heavily stained County, Ga.; and 1 from Tallapoosa County, Ala. with brown to black hematite, and the entire lot was Nine lots were not specifically identified as to source. a representative sample of the lowest-quality sheet This distribution by no means reflects the general mica from the mine. There appears to be a partial quality characteristics of sheet mica in one area or correlation between power factor and the amount of another, as no attempt was made to obtain the orig­ hematite-magnetite stain in the mica, in that most of inal samples in any systematic manner with respect the E-2 and E-3 pieces were heavily stained, but to such factors as the relative production of mica and evidently there are numerous exceptions. Some pieces the relative quantities of mineral-stained mica in each that qualified as E-l, for example, were more densely area. stained than many with a higher power factor. GENERAL TEST DATA FOR MICA SPECIMENS 48 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

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TABLEGeneral10.dataformicaspecimens,arrangedbytestlotmineandContinuedprospector— — Wl "O as • ' ! >> muchgreen mottling; smalldark- den­green •tTS'S ^c ^ 2-2 i g»a05 scS "Os -i_i . Clay-stained; dritespots. -do_.—-..- 3 > •§ •§ - • -a |1 £ 1 .Sffl » as c •*=c3 o1 jj>. fljSec i—••S'O'P -a^3-§ -a 1 1 J *0 ffl 02 j&Sl 1 ;Sl ^J'll D h"d OQ O O [02 o W o tf >

1°SM §«1- 83 Sparsehematite andraremagne­ dendritespecks andshredsspots Scatteredsparse andofspots .-.-dO.-——...... •sa ^^ -----dO-.------|1 S-JS |S 1 -S 10 . „ 0 titespots. hematite. I* SI18 :§ -a «H 2^« n ^ *> p g •=> oa g"^ -§11 ^^-g^ «» • 'C fl-§ ,; •au'-iS'u i S as o> >-i '« D, £~OJ3 83 O K S|| c^ll^si'll o tf PS JW OQ » Q P3 P3

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1 1 jj ||| i 4) _0 1 j ? rf? S C O *t O C o "S "£ e3 c e e c e j c e c TI t? ^* "O'S. % 3 ^ Tit Q iC 1•£ 1C ^ s •£ g gj g "§ Q. Pn W fe ;OQ ;'o o QQ 02 OQ oT "S £P oo 08 ^ r2 e>'3 || .|? ||| ct | o o . fe s o C5 •» ^ QQ ^ N^N iil 60 X || 1 h | |l OHOH O" OH o^ X >H t Young, Zeph, ... ..do...... Yancey, N. C... 5, 6...... Yellowish to Trace Rare magnetite Locally abun­ 81 100 0 E-l (1)... 0 0 5 mine group. brownish olive. specks and spots. dant vegeta­ ble stain and green biotite shreds; gar­ net inclu­ sions. Do—.- — ---..do.--..---. -—— dO-_ —— —— ---.-do.----.... Trace -—do—.. — — .--.-do.----. 12 100 0 ..---do.. . ------—— -dO—— —— —— 7 _ — -.do— — — 20 Scattered magne­ Sparse green 137 100 0 0 0 0 Alien, mines tite and hematite shreds of (in Zeph spots. biotite. Young group). 7----.... <5 Rare dendrite Clay-stained; 40 100 0 0 0 0 mine. some pale specks and spots curdy brown brownish olive. of hematite. stain at cen­ ters of some books, Do...... __---do.-.- — .. ... -do— ... — ....-do...... <5 .....do...... -.---do...... 100 0 --——dO———- —— - -. ---dO-_ ------—do——— 0 100 0 0 0 100 iron-, and manganese- stained.

SPECIMENS FROM SOURCES OUTSIDE THE SOUTHEASTERN STATES

1, 2, 3, 4, <5 2 35 100 0 0 0 0 0 N. Mex. 5. green and yel- drite specks and lowish to triangular lat­ brownish olive. tices. -—do—;.— ———— dO—— —— —— 4, 5, 6 _ . Trace 1 17 100 0 0 0 0 0 green. tite and hema­ tite specks, Do.————— -----dO-. ------L-do— — — — .-do- — —— 30 .-..do- — ...... 1 9 100 0 0 ----dO------i-.do-..- — - 3. ... — -- 0 1 4 100 0 0 6 0 0 green to yel­ lowish olive. ' 4 — 100 1 20 0 85 15 5 0 5: mica (localities blotches of hem­ green mot­ unknown) . atite with heavy tling. triangular lat­ tices. 0= ---do—— ----- 5—-—- _ 0 1 211 100 0 0 0 0 3, 4— — . -——dO- —— —— . S ^ f;n 100 0 0 0 10 15 mica (localities specka and spots. some green unknown) . mottling.

en- 96 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

TABLE 11.—Results of electrical tests of mica samples, from specific mines or deposits, that include material of E-% or E-S quality

Proportion Proportion of tested of defects (to nearest 5 percent) material Number Name of mine County and Type of with of Percent Percent or prospect District State material mineral pieces E-2 E-3 Material stain tested with Material Material (to nearest conducting with with 5 percent) substances cracks pinholes

Mitchell, N. C- _ .. Washer _ 20 100 100 0 0 25 -_.--dO—— 0 21 8 0 0 0 Black Dixie mine (in —— -_dO—— -- —— _-___ _.__ _do— 0 12 42 0 0 0 Bailey Mountain group) . Frariklin-Sylva__ Jackson, N. C-__--_ ------dO--- 100 8 100 0 0 0 Shelby-Hickory. Cleveland, N. C._- . — - -do... 35 81 6 0 0 0 Boone, Nelson, mine..._ Spruce Pine ______Yancey, N. C __ --- __— do... 50 7 100 0 0 0 Buchanan, Henry, pros­ --—— do—— 35 17 29 18 0 0 pects . Buchanan, Jim, pros­ Spruce Pine ______Mitchell, N. C ______——— do— 25 9 100 0 0 10 pects . Campbell mine. ______Cleveland, N. C __ _ 6— _ —— 20 89 60 0 0 0 0 Do _ _ ..do - Washer... 100 7 100 0 0 0 Carolina Mineral Co. Spruce Pine ______. Mitchell, N. C .-_-_. —— .do— 100 18 0 100 0 0 No. 6 mine...... do _ -_— __ . . _ _do __ . ______.do— 100 16 44 0 0 Trace Chalk Mountain pros­ ..—do.——————— .__„ do _ _.______.....do... 0 11 55 0 0 0 pect...... do _ ..._....__ 3-7 ______65 842 0 2 0 0 0 Cox, Wood, mine__-____ .....do _ ._.-...-._ Mitchell, N. C___ Washer _ 100 8 100 0 0 0 Crab tree No. 1 (Wildcat) ——do————__— __ .do ______———do— 5 63 19 0 0 0 mine. Crabtree Summit pros­ — _.do_— __.—__. __ _do._ _ . ______5, 6_ __ _ 100 6 67 33 100 0 0 pect. Crews No. 1 prospect.. ._ Outlying Virginia- — Charlotte, Va _.____. 5-7———— 100 185 3 11 65 0 0 Emmons Knob mine_ _ _ Spruce Pine__ _ _ _ Avery, N. C— _____ 5__------5 48 2 0 0 0 0 Estatoe Northeast Mitchell, N. C——— _ Washer .__ 60 5 100 0 0 0 (Mossy Rock) pros­ pect. Furlow Gap (Farlow Franklin-Sylva. — Transylvania, N. C__ 5-6. — -- 100 184 35 3 0 0 0 Gap) mine. Spruce Pine______Yancey, N. C— .___ Washer ___ 65 20 35 0 0 0 _---dO-______Mitchell, N. C.------do... 100 3 100 0 0 0 Hogg mine (wall-zone Outlying Georgia—.. 5-7.- __ 100 410 41 9 100 <5 <5 mica). Spruce Pine. _____ Mitchell, N. C--._- 4-6-__-_. 100 248 2 0 45 0 0 Hoppus Northeast mine...... do... ____._.___ Washer _ 5 8 100 0 0 0 Irby Cut mine ___ __.. —..do... ______Yancey, N. C_— ... ----_do___ 0 7 100 0 0 0 Outlying North Car­ Rutherford, N. C_-_ fi, 7— -__ 100 94 13 0 100 0 5 olina Piedmont. Franklin-Sylva ___ Jackson, N. C_____- S-7---.-- Trace 1,112 4 0 0 10 <5 Jimmy mine (in Archie Cleveland, N. C____. Washer _ Trace 7 100 0 0 50 Norman group) . Kell mine _ ... __ _ . Franklin-Sylva _ ... ————dO—— 100 30 50 0 0 0 Ridge way-Sandy Rookingham, N. C-_ 3-7 ______90 15,987 23 16 5 10 15 Ridge. Washer . . 0 7 100 0 0 0 Locust Rough (B. T. 5, 6 __ -. 100 85 7 5 0 5 0 Snopp) mines. -_-_dO-_ _--_-_-____ Mitchell, N. C ___ _ Washer - _ 100 12 0 25 0 100 5-7 _ -_. 65 849 6 0 5 <5 <5 New Wolff (Wolff, Old 5-7- ——— 10 113 0 3 5 0 0 Wolff) mine. _——— do———————— —— __ 3-7— — 85 1,269 2 14 80 Trace <5 Ollis, Jake, mine __ _ Spruce Pine __ __.._ Avery, N. C _ - __ 5,6 _ --. 70 150 6 3 10 0 Trace Mitchell, N. C__ _ _ 10 15 47 0 0 0 ———do—— —— — .. 6 _ -__-. 95 200 4 3 100 <5 <5 Renfro mine ______.....do.. .._.____.__ Mitchell, N. C ___ Washer __ 50 15 100 0 0 0 Rickman mine __ -_-___ Franklin-Sylva- Macon, N. C_--_.-_ .---.do— 0 15 100 0 0 0 Outlying Georgia.... Troup, Ga ____ ... 6 100 83 14 0 65 0 0 Do ______-_--do__-_ — -__-._ Washer . _ 100 100 0 0 0 Smith, Calhoun, mine Spruce Pine ______Mitchell, N. C— .„ .--.-do— 30 11 18 82 0 0 Smith, Ernest, mine _____ Ridge way-Sandy Rockingham, N. C._ 6, 7— — 95 175 6 0 5 0 <5 Ridge. Mitchell, N. C——— _ 3,4———- 100 3 0 100 100 0 0 6,7——.. 60 174 9 2 75 0 0 Tunnel mine____.___ _ Franklin-Sylva------Rabun, Ga_----_--_ 4-6 ___.._ 100 1,216 20 3 40 0 5 Outlying Virginia- _„- 5-7 ___ . 75 128 4 1 50 0 25 5-6- — -- 15 114 4 1 10 , o 15 Shelby-Hickory- . _ Cleveland, N. C __... Washer . . 60 15 100 0 0 0 Wiseman, W. W., mine. Spruce Pine __ ----- MitcheU, N. C ___ 100 28 4 0 100 0 0 Young, George, mine ____ —— -do.. ___ ------—— - -dO——- —— ... _.- Washer . . <5 30 37 0 0 20

SAMPLES FROM SOURCES OUTSIDE THE SOUTHEASTERN STATES

New Hampshire mica -—— — - ——— New Hampshire _ __ 4 ______100 20 85 15 5 0 5 (localities unknown) . TESTS OF SHEET MUSCOVITE FROM THE SOUTHEASTERN STATES 97 TESTS OF VISUALLY QUALIFIED MICA material and 1 of E-3, and 72 lots of buff and brown Through the courtesy of W. J. Alexander, Southern mica yielded only 68 specimens of E-2 and 13 of E-3. District Manager, 354 lots of prepared and qualified Yet 92 of the lots had been graded as No. 2; 5 as No. 1 mica were loaned by the Colonial Mica Corporation and No. 2; and 14 as No. 2, No. 2 inferior, and No. 3. for electrical testing. This material had been proc­ Most of the down grading apparently had been done essed and qualified by visual methods in Asheville, on the basis of waviness and inclusions, but the spark N. C. The test data for 243 lots taken from the pro­ testing showed that pinholes were present in 33 per­ duction line of the Colonial Mica Corporation shops cent of the lots, hair cracks in 8 percent, and conduct­ in 1945 are shown in table 12. Twelve of these had ing impurities in more than 6 percent of the lots. The been qualified by careful inspection as No. 1; 19 as proportion of pieces in each lot so marred varied No. 2; 208 as No. 2 inferior; 2 as No. 3; and 2 as No. 1 from 2 percent to 100 percent. The results of elec­ and No. 2. trical tests of the sample lots that contain material of The results of electrical testing were in marked other than E-l quality are listed separately in contrast to this grouping. All pieces of mica in all table 14. the lots registered E-l on the Q-meter dial, and nearly The results of these comparisons fully support the all were in the uppermost part of the E-l range. On conclusions of Coutlee, Townsend, and others who the other hand, the spark testing revealed pinholed have pointed out that much mica ordinarily classed material in 64 percent of the lots, hair cracks in as No. 2 or No. 2 inferior by visual appraisal actually nearly 7 percent of the lots, and conducting inclusions has excellent dielectric properties, as determined by in one lot. Pinholes and cracks were present in 12.6 means of the Q-meter. Moreover, the spark-coil test percent of the test pieces, yet none of these flaws was set provides a means for identification of conducting readily observable with the naked eye. This high regions and tiny structural flaws that may well be proportion of blemished sheets evidently is unusual, more effective than even the most critical visual scru­ as only 3.35 percent of the 237,790 pieces tested dur­ tiny. In its present form the Q-meter permits the ing the entire program contained pinholes or hair testing of 15 to 20 pieces of mica per minute by a cracks. reasonably competent operator, the average rate de­ Another series of mica samples had been prepared, pending in part upon the type of material under test qualified, taken from the production line, and placed and in part upon the period of time between rest in storage by the Colonial Mica Corporation in 1943. pauses for the operator. The spark-coil test set per­ These samples were retested by the electrical meth­ mits much more rapid manipulations. Adaptation of ods, and the results of the tests are compared with the Q-meter to semi-automatic or even fully automatic those of the earlier grading in table 13. Thirty-nine operation appears entirely feasible and would ap­ lots of green mica yielded only 10 specimens of E-2 preciably lessen the cost of electrical testing. 98 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES

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Proportion Listed Proportion of tested defects of material quality, (to nearest 5 percent) Name of mine Type of with mineral deter­ 30-mil or prospect District County and State material Color stain mined Total stack (to nearest by visual number Percent Percent Percent (number Material o 5 percent) inspection of pieces E-1 E-2 E-3 of stacks with con­ Material Material tested in paren­ ducting with with o theses) sub­ cracks pinholes stances

Alabama. _..._.._ Randolph, Ala...... 5,6 ——— do—— — — — 0 2 inf... _ . 367 100 0 0 E-1 (!)___ 0 0 20 Do ...... ——— do— ______. 5 ——— do— — — —— 10 2 inf—— „ 317 100 0 0 E-1 (!).__ 0 0 0 o Do——— _ ...... do...... ——— do— —— . _____ 6 —— -do— ... __ — - 25 3— ______169 100 0 0 E-1 (!)___ 0 0 10 Do— — —— — __ do...... _____ ——— do— ———— — . _ .do...... ----- 50 3...... 133 100 0 0 E-1 (!)___ 25 0 0 t-p1 Do— — —— ——— do— ————— __ .do.. — — .-_..- — ..do— .. ------20 2 inf ___ .. 412 100 0 0 E-l (1)_- 0 0 10 Do. _ . _ - . _ .do.. — ...... —— .do— — ------...do— .-.-. — - 20 2inf— — . 439 100 0 0 E-1 (!)___ 0 0 5 CQ Do.. — _ ._ — ——dO———— ..... — —do——— — ——— -—do—— — — — 0 2 inf ___ ._ 300 100 0 0 E-1 (1)___ 0 0 20 "T1 HH Do- ————— dO——————— ——— —— do— — ——— . 5>_!6 — ..do— ..... ----- 20 2 inf ______425 100 0 0 E-1 (!)_._ 0 0 20 Do————— ——— -dO—— ...... ———do—— —— — 5>_. 6 ———do———— — — 20 2 inf...... 400 100 0 0 E-1 (!)___ 0 0 5 Do——._...... do— _._... „ —— .do. — — —— - .....do——— —— — - 20 2 inf__ — .. 425 100 0 0 E-1 (1) __ 0 0 20 Do __ —— .do———.. __ ---. do.- — . — — ..—do— ... — —— 20 2 inf ______400 100 0 0 E-1 (I)... 0 0 5 Do—————— -----do __ ---___. ..—do-—————- 5)1! 6 —— do—— —— — 0 2 inf.. _ „ 425 100 0 0 E-1 (!)___ 0 0 10 Do. —— _...... __ do—— _. _____ —— .do— —— ...... 5)_, 6 _____ do—— — — — 0 2 inf... ____ 410 100 0 0 E-1 (!)_._ 0 0 20 Do... ______. ———do—— ———— --.do———. _ —— 5)_, 6 . — ..do— ------0 2 inf..——— 405 100 0 0 E-1 (1)- — 0 0 10 Do. _ ...... __ do—— ——— . ——— do. —— ——— .. 5)_, 6 -— do——— — ----- 0 2 inf...... 400 100 0 0 E-1 (1)... 0 0 5 Do—— — — ———do— __ . _ ———do———— _____ — _ __ .dO—— -. ------0 2 inf ..._.__ 310 100 0 0 E-1 (I)... 0 0 20 3 Do.—— _ ...... do...... „ — _.do_—— ——— — 5j|!6 ——— do. — —— ——— 0 2 _._._____. 235 100 0 0 E-l (!)-__ 0 0 0 O Do——————• .--..do—— ______— 5)3,6 ... _.do——— — ——— 0 2— ...... 300 100 0 0 E-1 (1)... 0 0 0 O Do— _ .... —— .do _ —— .... ---dO—— —— — __ -- 5)_,6 .....do—— _ 0 2 ...... __ 300 100 0 0 E-1 (1)... 0 0 0 Do. _ ...... —— -dO—— —— ————— — -do— ——— ——— 5)3,6 0 2 _ _. _____ 300 100 0 0 E-1 (!)._. 0 0 0 Do...... _ ——— -dO.-.——— ... — ..do— — — —— 5)_,6 ——— do— ——— —— 0 1 ______265 100 0 0 E-1 (!)_.. 0 0 0 Miller Douglas — _ Shelby-Hickory . . . Caldwell, N. C.__... 6 -.... do—— ------0 2 inf ____... 385 100 0 0 0 0 0 Do——— _ .. ._.__ do— -_.____._ ———do— ——— — - 0 2 inf.._ .... 188 100 0 0 0 0 0 Do..___..._. - —— -dO...... ——...... do—— ———— ...... do— ——— _-___ 0 2 inf ______193 100 0 0 0 0 0 Do—— ______. __ do———— _ .. --.do— — _ — - 5)3,6 —— _do___ --.-. — - 0 2 _..._._.__ 145 100 0 0 0 0 0 Do. _ ...... ___ dO- — -- __ .. -———dO——— ————— ——- 5 ..... do—— —— —— 0 2 _..._.____ 128 100 0 0 0 0 0 Do. __ ...... —— .do.... _ .... . — .do. — _ -...-- 5,6 ———do—— ——— —— 0 1 ______133 100 0 0 0 0 0 Do- .....do — ... _ „ 5,6 __... do— —— . —— _ 0 1 ______142 100 0 0 0 0 0 Do—————— ___ do.... _ .... . _ .do— — ———— 6 ———do————— —— - 0 2 ______. 303 100 0 0 0 0 0 Do...... _ do- ___ ...... -dO—— -_.- —— .-- 6 _——do— — — ——— 0 1 ______. 365 100 0 0 0 0 0 HartweU..— . _ . Hart, Ga _ . _ .... 5, 5)_,6 ——— do— — ______. 0 2 inf ___ . 200 100 0 0 E-1 ( '.)"'. 0 0 5 .....do ______— —do— ______5, 5)_,6 ... -do. — ... — — 0 2 inf...... 200 100 0 0 E-1 ( 0 0 5 Do—— .... __ ... -do...... ——— .dO--- —— ——— . — 5, 5)_,6 —— .do— ... -- — 0 2 inf.. —— . 200 100 0 0 E-1 ( 0 0 20 Do- —— .do.. ______——— do———— ——— 5, 5)_,6 ———do——— - — — 0 2 inf—— _.- 200 100 0 0 E-1 ( I)... 0 0 0 Do. _ ._.— — _._do..._ _ .„- — -.do— ——— — - 5, 5)_, 6 .....do ______0 2 inf...... 200 100 0 0 E-1 ( .)--- 0 0 0 Do— ———— - ___ dO-- __ -_.. ---..do— — ______5, 5)_,6 0 2 inf ..__.__ 200 100 0 0 E-1 ( '.)'.".' 0 0 10 Do. _ ... _ -----dO——— ------5, 5)_,6 .....do ______0 2 inf __._.__ 200 100 0 0 E-1 ( 0 0 20 Do. _ --..do _ ...... 5, 5)_, 6 — __do ____ . — ._ 0 2 inf———— 200 100 0 0 E-1 ( 0 0 0 Do—— ...... do.. ______- — .do————— —— 4, 5,6 .——do _ . __ ——— 0 2 inf .....__ 200 100 0 0 E-1 ( .)'.'.'. 0 0 5 Do— ———— .. _ do. ___ ..... ——— do.'—— — _ — 4,5,6 0 2 inf _ ,-.- 200 100 0 0 E-1 ( 0 0 0 Do. _ ...... — ..do _ . _ .... ——— do— ...... 4,5,6 _ ..do _____ ..... 0 2 inf ______200 100 0 0 E-1 ( 0 0 5 Do. ______— _ do— __ . _ ——— do—— ————— 4,5,6 .....do _____ ..... 0 2 inf...... 200 100 0 0 E-1 ( I)... 0 0 0 Do— _..._ __ . __ do ____._..._. ——— do— — —— — 4,5,6 .....do ...... 0 2 inf———— 200 100 0 0 E-1 ( 0 0 5 Do.__ ___ ._ .....do ___ .. — do——— ——— — 4,5,6 .....do ...... 0 2 inf...... 200 100 0 0 E-1 ( 11::: 0 0 5 Do ____ .... — —do _ ...... __ -.— do— --..-. -. 4, 5,6 __...do _____ ..... 0 2inf—— — 200 100 0 0 E-1 ( ,)... 0 0 5 Do...... —— -dO. — . __ ——— ——— do... ——— — __ 4,5,6 . __ do.. ___ ..... 0 2 inf ____ . 200 100 0 0 E-1 ( .)'.'.'. 0 0 5 Do———— .... —— ..dO—— —— __ — _.__ .do— ——— _____ 4,5,6 .. _ do _____ — ... 0 2inf___ ... _ 200 100 0 0 E-1 ( 0 0 5 Do— ...... ——do _ .. _ do ____ ...... 0 2 inf———— 200 100 0 0 E-1 0 0 10 —— .do- —— ______. 4,5,6 E-1 Do- ——do.. ———— — ---.dO—— ______-- — 4,5, 6 0 2 inf ___ .. 200 100 0 0 0 0 5 Do—————— .... -do_—__ ——— --.do-.- — _ .--. 4,5,6 .....do _____ ..... 0 2 inf...... 200 100 0 0 E-1 I III 0 0 0 Do. _ . _ .. — __.do_. __ __.__ ———do— — ——. 4,5,6 .....do— — _...... 0 2 inf ___ .. 200 100 0 0 E-1 I ... 0 0 20 Do. _ ...... -—do ___ .-..do— — ._____. 4, 5,6 0 2 inf... .___ 200 100 0 0 E-1 I ... 0 0 50 Do—————— . ..-.do... __ .... — -do— ——— — . 4,5,6 0 2inf———— . 200 100 0 0 E-1 ( 1 ... 0 0 5 Do..... _ „ - __ .dO——.._ ——— _ —— -dO.-.--.. —— - 4,5,6 -...do—..— — ._ 0 2inf______200 100 0 0 E-1 ( I ___ 0 0 5 Do————— .. ———do—— ______———do—— — — — 4,5,6 ———do——— _____ 0 2 inf.. _ ... 200 100 0 0 E-1 ( I ... 0 0 0 Do...... —— .do——— ______. . do 4,5,6 ——— do— ——— — - 0 2 inf.. _____ 200 100 0 0 E-1 ( 1 ___ 0 0 10 Do- _ — .do...... — -do——— ———— 4,5,6 ..... do. — ...... — 0 2 inf ______200 100 0 0 E-1 0 0 5 Do——— _ .. . _ .do...... ______.do— ______. 4,5,6 .....do—— —— ——— 0 2 inf ______200 100 0 0 E-1 0 0 0 Do. _ . _ .. — ...do—— __ ... 4,5,6 ..... do——— ....—— 0 2 inf..... „ 200 100 0 0 E-1 '.)'" 0 0 75 ——— do__. ______... E-1 Do ____ . ... —— -do———— .... ——— .dO.----. —— 4,5,6 —— .do——— — ——— 0 2 inf. .__ — 200 100 0 0 0 0 25 Do————— „ ---dO—— ———————— - .—.do——— — ——— 0 2inf— ___ _ 200 100 0 0 E-1 l)._. 0 0 5 ———do——— ...... 4,5,6 E-1 Do- — __ dO—— --..._. —— do—— ———— 4,5,6 __ .do— ___ ...... 0 2 inf———— 200 100 0 0 !)._- 0 0 5 Do——— _ .. ——— do—— ———— — -do— ----- 4,5,6 ———do——— ———— 0 2 inf. ______200 100 0 0 E-1 ( .)--- 0 0 5 Do...... T ... .do. ——— .... — -do——— ———— _ 4,5,6 do _ -_. . 0 2 inf ______200 100 0 0 E-1 ( 0 0 5 Do... _ .... ——— do—— ————— 4,5,6 .....do—.——- _ 0 2 inf ______200 100 0 0 E-1 ( oIII 0 0 5 Do. _ ...... -- —— dO—— ...... _ -—do——.———— 4,5,6 __ .do—— —— ——— 0 2 inf... _ . 200 100 0 0 E-1 ( L)-.. 0 0 0 Do____..__ __ ... ..do——— ______. _ .do———— ..... 4,5,6 __._.do———— _. — . 0 2 inf .____.. 200 100 0 0 E-1 ( I)... 0 0 10 Do—— .... __ ..... do————— _ ———— dO——— ——— —— — 4,5,6 ..—do— ------0 2 inf ______200 100 0 0 E-1 ( L)-.. 0 0 5 TESTS OP SHEET MUSCOVITE PROM THE SOUTHEASTERN STATES 101

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TABLE 13.—Test data for mica (not identified by mine}, taken from the production line of the Colonial Mica Corporation in 1943 and placed in storage, qualified by visual means in 1943

Electrical tests Proportion Listed of material quality, Proportion of tested defects Lot Type of with mineral determined 30-mil number material stain by, Total stack (to nearest visual number Percent Percent Percent (number of Material 5 percent) inspection of pieces E-l E-2 E-3 stacks in with Material Material tested parentheses) conducting with with substances cracks pinholes

1 1 3 PL 6 0 2 _____ . ____ 204 100 0 0 0 0 0 1 1 37 6,7 0 2 ______115 100 0 0 0 0 SO 1 1 3ft 6 0 2 ______109 100 0 0 0 0 0 1139 5, 6J_, 6 0 2 ______- — 100 100 0 0 0 0 0 1140 6, 7 0 2 _____ -. —— 108 99 1 0 0 0 0 1141_,— __-_- 6, 7 0 2 ______104 100 0 0 0 0 0 1 -I AO 6, 7 0 1, 2 ____ — 101 100 0 0 0 0 so 1 1,13 6,7 0 2— ._ __ _._ 110 100 0 0 0 0 0 1144 _ . ___ 6,7 0 2. __- — —— - — 102 100 0 0 0 0 0 1145 ______6, 7 0 2 ____ —— _ —— _ 195 100 0 0 0 50 0 1146 _____ - 5, 5^_ 0 2 ______100 100 0 0 0 0 0 i 1 ei 4 0 1 ______.-. 100 100 0 0 0 0 0 1155 6 0 100 100 0 0 0 0 0 1156 __.__ — _ 7 0 1. _____ --. ' 100 100 0 0 0 50 0 1158______5 0 199 100 0 0 0 5 0 1159——— — - 5*. 0 1. ______100 100 0 0 0 0 3 1165 _____ . 5, 6 0 o 140 100 0 0 0 0 30 1214 ___.___-_ 6 0 2 ______—— 104 100 0 0 0 0 5 191 1 6, 7 0 2- ______106 100 0 0 E-l (l)_.-_- 0 0 2 1216 ____ _ 6 0 2___ _ —— .____ 114 100 0 0 E-l (l)-_- — 0 0 2 1 9O 1 6 0 2.. ______—— 100 100 0 0 0 0 2 1931 6 0 2. __„_____„ 100 100 0 0 25 0 0 1 939 6 0 2—. __ . _____ 105 100 0 0 0 0 0 1234 _ . ___ 6 0 2 105 100 0 0 E-l (!)---__- 50 0 0 1 93 PL 6 0 2. ______106 100 0 0 0 0 0 1 93ft 5, 6 0 2. ______112 100 0 0 E-l (!)__.___ 0 0 0 1937 6 0 2______106 100 0 0 0 0 25 19Rn 6, 7 0 2 ______100 100 0 0 0 0 0 1261 ____._ — . 5, 6 0 9 100 100 0 0 0 0 5 1262 ___ . __ 5A,1 6 0 2 ______.. 105 100 0 0 E-l (1) ______0 0 0 1 9ft3 5, 6 0 2 ______105 97 1 2 E-l (!)__„__ 0 0 2 1264 ______5}_, 6 20 2 ______100 100 0 0 0 0 0 1265— _--- 6 0 2 109 99 1 0 0 0 0 1266 ______5J_, 6 10 2 ______120 100 0 0 E-l(l)-.- — 0 0 5 1270 _ . ___ 6 0 2. _____ ._ . 101 100 0 0 0 0 0 1 971 5, 6 0 2 ______110 100 0 0 0 0 5 1 979 5, 6 25 2. ______102 100 0 0 0 0 5 1973 5}., 6 0 2 ______195 100 0 0 0 5 0 197,1 6, 7 0 2 ______. 105 100 0 0 0 0 5 1276_.—._ —— - 6 5 2 ______110 100 0 0 E-l (!)______0 0 2 1280 6,7 0 1, 2 ____ -_ 128 100 0 0 E-l (!)__— _ 0 0 0 •J OftO 6 0 2. ______105 100 0 0 0 0 2 1285 _ __-... 6 0 2 ______103 100 0 0 0 0 0 1 9ftfi 6 0 2 ______104 100 0 0 5 0 0 1 9S7 6,7 0 2______106 100 0 0 E-l (1).——— 0 0 0 1288. _____ 5, 5}_, 6 O 2 ______105 100 0 0 0 0 2 1 9ftQ 6 0 2 ______121 100 0 0 E-l (1)__ —— 0 0 0 1294— ----- 5, 6 0 2 ______192 100 0 0 0 0 0 1305 ______5, 5^_ 0 1, 2, 2inf____ 199 100 0 0 0 0 2 1306 ___ __._ KV^ 0 2 ______105 100 0 0 E-l (!)______0 0 2 1307 ______6H, 6 O 2______106 100 0 0 E-l (!)__--_- 0 0 0 1 3flft 6 0 2 ______105 100 0 0 E-l (!)______0 0 2 1 3flQ 6, 7 0 2 ______103 100 0 0 E-l (!)______0 0 0 1310 ___ . _ 6 O 2. __ . ___ __ 110 100 0 0 0 0 . 10 1311 5, 5V_, 6 0 2. ______131 100 0 0 E-l (!)______0 0 0 1319 5}_, 6 0 2. ______116 100 0 0 E-l <-)___.__ 10 0 0 1313 5, 6 0 2______133 100 0 0 E-l (1)_.____ 0 0 0 1314 ______6 0 1, 2, 2inf.___ 106 100 0 0 20 0 0 1^1 1 5, 6 0 1, 2, 2inf____ 106 100'100 0 0 50 0 so 1316 ______6 0 2 ______._ 113 0 0 0 0 0 1317 ______3,4, 5 0 1,2 _ ___ _ . 150 100 0 0 E-l (!)_.—— 0 0 0 1 31 ft 5 0 2 ______102 100 0 0 0 0 0 1319. ______5H. 6 0 1, 2 ____ ... 100 100 0 0 0 0 60 1320 ______6 0 2 ______106 100 0 0 E-l (!)___ — 0 0 0 1 391 6 0 2______198 100 0 0 0 0 0 1 399 5, 6H. 6 25 2 ______104 100 0 0 E-l (!)__„__ 0 0 2 1323 _ _ _ __ 5}_, 6 0 2 ______105 100 0 0 0 0 5 1324 __ _ _ _ 6 0 2 ______114 100 0 0 0 0 0 1 39 PL S, 5M, 6 25 1, 2, 2inf._._ 145 99 1 0 E-l (!)______0 5 0 1326 ______6, 7 0 2 ______107 100 0 0 E-l (!)______0 0 6 1327 ______5, fij_, 6 0 2 ______108 100 0 0 E-l <_)______0 0 6 1 39Q 6 0 2 ______130 100 0 0 E-l (!)__—_ 0 0 0 1337 6,7 0 2— ______112 99 0 1 E-l <_)_.„„ 0 0 0 1338 ______6 0 2 ______101 100 0 0 0 0 0 1^4.1 6 0 2 ______104 100 0 0 E-l (!)_——_ 0 0 0 1343 ______6 0 2 ______105 100 0 0 E-l (!)_._„_ 0 0 10 1345 ______6 0 2 ______100 100 0 0 0 0 0 1346 ___.__— 6 0 2 ______118 100 0 0 E-l (!)____„ 0 0 6 1347 ______6 0 2 ______100 100 0 0 0 0 0 1348 ___ _ 6 0 2 ______106 100 0 0 0 0 6 1349 ____ .. 4, 5 0 1,2—— —— ._ 144 100 0 0 E-l (1 ______0 0 25 1369 ______5H, 6 0 2 ______109 98 2 0 E-l (1 ______0 0 0 1371 ______6 0 2, 2inf ______102 100 0 0 E-l (1 ______60 0 0 1376 ___ . _ 6 0 1, 2, 2inf____ 104 100 0 0 E-l (1 ______0 0 50 1377 ______5, SH, 6 0 2-_.______113 100 0 0 E-l (1 ______0 20 0 1378 ______6 0 2. ______184 100 0 0 E-l (1 ______0 0 0 1379 _ _ _ _. 6 0 2 ______104 100 0 0 E-l (1)______0 0 5 1381. __ -___ 6 0 2______115 100 0 0 E-l (!)___„_ 0 0 0 1385 ____ _. 6 0 2______107 100 0 0 E-l (!)______0 20 0 1386 ______5J..6 0 2-__ _ _.__. 107 100 0 0 E-l (1) ______0 0 0 1387 ______6 0 2— ______108 100 0 0 E-l (!)__ _ _ 0 0 0 1388 _ . _____ 6,7 0 2______105 100 0 0 E-l (!)___ _ 0 0 0 1389 ______6 0 2_ ___ --_.__ 106 100 0 0 E-l (!)__ __ 0 0 10 1390 ______6,7 0 2 ______106 100 0 0 E-l 1) ______0 0 0 1391——- —— 6,7 0 2 ______100 100 0 0 0 0 0 SUMMARY AND CONCLUSIONS 103

TABLE 13.—Test data for mica (not identified by mine), taken from the production line of the Colonial Mica Corporation in 1943 and placed in storage, qualified by visual means in 1943—Continued

Electrical tests Proportion Listed of material quality, Proportion of tested defects Lot Type of with mineral determined 30-mil number material stain >y, Total stack (to nearest visual number Percent Percent Percent (number of Material 5 percent) inspection of pieces E-l E-2 E-3 stacks in with Material Material tested parentheses) conducting with with substances cracks pinholes

1392 ______6,7 0 2 ______.. 100 100 0 0 0 0 0 1393 ______5H, 6 0 2 ______.. 109 100 0 0 &-!(!)-- — . 0 0 0 1400 ______6,7 0 2--. ------100 100 0 0 0 0 0 1404 ______4, 5, 6 0 2, 2 inf., 3 __ 108 100 0 0 E-l (1)__ —— 0 0 0 1405 ____ -. 5,6 0 2 101 100 0 0 0 0 0 1406 ______5, 6 0 2. ____ — -__. 100 100 0 0 0 20 0 1407———-.-- 5, 6 0 2 _____ ....- 103 100 0 0 E-l (!)__. ___ 0 0 0 1408— — -.- 4, 5, 6 0 2 116 100 0 0 E-l (!)_____- 0 0 20 1409 ______5,6 0 2 102 100 0 0 E-l (!)___.__ 0 0 0 1410 ____ .- 5,6 0 2 ____ ----- 108 100 0 0 E-l (1) ______0 20 0 1411 ______5, 6 0 2 100 100 0 0 0 0 0 1412—, — -- 5, 6 0 2 105 100 0 0 E-l (I)-.--. 0 0 0 1413 ______5,6 0 2_ — _ . _ 107 100 0 0 E-l (!)_-— _ 0 0 0 1414 _ . _ .. 5,6 0 2.-- ___ . _ 110 100 0 0 E-l (!)__,„_ 0 0 5 1415 ______5, 6 0 2 113 100 0 0 E-l (!)___.__ 0 0 0 1416 ______5, 6 0 2 ______... 101 100 0 0 0 0 0

TABLE 14.—Results of electrical tests of mica samples, taken from the production line of the Colonial Mica Corporation in 1943, that include material of E-2 or E-S quality

Proportion of tested defects Proportion of Listed material with quality, Number Material Material Material Mine lot No. Type of mineral stain determined of pieces Percent Percent with con­ with with material (to nearest by visual tested E-2 E-3 ducting cracks pinholes 5 percent) inspection substances

1263 ______5,6 0 2___ _ .... _____ 105 1 2 0 0 2 1265 .______—„ 6 0 2 _ . 109 1 0 0 0 0 1325-.--- ______5-6 25 I,2,and2inf___ 145 1 0 0 5 0 1337.. ______6,7 0 2 ______112 0 1 0 0 0 1369 ______6J_, 6 0 2 _____ 109 2 0 0 0 0

SUMMARY AND CONCLUSIONS A substantial proportion of Southeastern musco­ The results of the color tests show that Southeast­ vite is mineral-stained; that is, it contains specks, ern muscovite ranges from buff and drab through spots, or broader masses of hematite, magnetite, or shades of brown and green to pale yellowish green. biotite. Much hematite also occurs as latticelike in- Most books are not colored uniformly throughout, tergrowths of triangular or rectangular pattern. and both irregular and several types of regular varia­ Little mineral stain can be effectively removed by tions are known. Buff and brownish ("ruby") mica careful splitting, owing to the extreme thinness of the is most abundant in the Piedmont province and in foreign masses. In marked contrast are thicker and large parts of the Franklin-Sylva, Spruce Pine, and larger inclusions of euhedral biotite and such "stony" Jefferson-Boone districts of the Blue Ridge province, minerals as apatite, garnet, pyrite and other sulfides, but in general the bulk of the sheet material produced quartz, tourmaline, zircon, and zoisite. Such inclu­ from the entire Southeast region is yellowish or sions can be removed with little difficulty; when this brownish olive. is done, holes or distinct depressions are left in the Systematic color variations occur within some mica host muscovite. Other types of foreign matter, main­ shoots, within single pegmatite bodies, and within ly mottled to curdy pale-greenish or brownish stain, large groups of pegmatite bodies. Variations in indi­ are widespread and locally very abundant. Clay vidual mica books and within single mica shoots are stains and supergene iron and manganese stains are most pronounced where the muscovite is green and very abundant also, but air-stained mica is rare in yellowish olive. On a larger scale, brownish books the Southeastern deposits. generally are nearer the walls of a given pegmatite Iron oxide mineral stains are common in brownish body than green ones, and no brownish books occur and yellowish-olive micas but are rare in those that in those pegmatites with green wall-zone mica. Green are buff or brown. Thin shreds and wisps of biotite books are rare in pegmatites with core-margin or are common in muscovite of nearly all colors, whereas other interior concentrations of brownish mica. the coarse, euhedral type of biotite occurs almost 104 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES wholly in buff and brown muscovite. Mottling, "vege­ Green and brown mottling, "vegetable stain," wispy table stain," and similar impurities are present in to platy inclusions of green and brown biotite, and nearly all types of muscovite. In general, mineral- some -clay stains that do not transgress the laminae stained mica is confined to the outer parts of those of the host mica appear to have little adverse effect pegmatite bodies that also contain clear greenish upon its power factor. Air stain also was found to books and to the inner parts of those that also contain have no effect in the few test pieces with this feature, clear buff or brown books. although appreciable increase in power factor due to Pinholes appear to be developed in Southeastern air bubbles in mica has been demonstrated by Hall 88 muscovite by the "popping out" of tiny inclusions of at a frequency of 1,000 kilocycles. zircon, apatite, garnet, and other "stony" minerals Evidently neither the color nor the depth of color in during splitting of the host books. They are not un­ clear muscovite has any definite relation to its power common in most districts but are abundant in the factor. This is in full agreement with the findings of mica from only a few mines. Most occur in brown Hall,89 Coutlee,90 and others 91 and is contrary to the and buff, or ruby, micas, in which zircon and apatite oft-repeated assertion that green and dark-colored inclusions are most abundant. Hair cracks, said to varieties have basically inferior electrical character­ develop in part after rifting and trimming of sheet istics.92 The clear green, yellowish-olive, and brown­ mica from certain deposits, are more numerous in ish-olive micas, both light and dark, were found to lie greenish micas than in the brown and buff varieties. within essentially the same power-factor range as the They are especially common in the yellowish-olive pinkish-buff and light-brown, or ruby, varieties. If, books from several mines in the Spruce Pine district. however, all greenish muscovite is considered without The results of the electrical tests are similar in regard to whether or not it is mineral-stained, its gen­ many respects to those of several earlier testing pro­ eral dielectric properties are distinctly inferior to grams,85 and the detailed study of 237,764 pieces those of the other types, simply because damaging specifically identified with at least 850 deposits gives mineral stain is much more common in such mica. them additional statistical value. The correlation be­ This very important point is summarized by the tween power factor and physical appearance sug­ American Society for Testing Materials 93 as follows: gested by Kesler and Olson 86 holds in a general way, Experience has shown that the Q value range of ruby and but evidently there are numerous exceptions, par­ white types of block mica, regardless of source, is 80 to 95 per ticularly among stained micas. Nearly all these ex­ cent E-l, whereas, the Q value range of light green, dark green, ceptions lie in one direction, however, in that the greenish brown, and rum-colored block mica is 45 to 90 per power factor of such mica is lower than that expect­ cent E-l. It is important to bear in mind that it is permissible for all qualities of mica prescribed in A. S. T. M. Specifications able on the basis of visual appearance alone. D 748, to contain spots and stains, providing the mica meets all There appears to be a partial correlation between electrical and physical requirements, whereas, no spots or power factor and the amount of hematite-magnetite stains (other than air stains) are permitted in the * * * visual stain in sheet mica, in that most test pieces of E-2 and quality groups per A. S. T. M. Methods D 351. E-3 electrical quality were heavily stained. On the It should be recognized that power factor alone does other hand, many other pieces of heavily stained mica not determine the applicability of a given piece of tested E-l, and some perfectly clear pieces tested E-2 mica for condenser or other high-grade uses. The and E-3.87 Careful inspection has demonstrated that mica must meet other physical requirements, hence E-l stained mica cannot be distinguished from E-3 must also be tested visually and with the spark-coil stained mica by ordinary visual means. In general, set. Tightly intergrown plates and wisps of biotite, however, much muscovite that contains substantial for example, have little effect upon the power factor quantities of iron—either as inclusions, as exsolved of block muscovite, yet they may well interfere seri- intergrowths, or in solid solution—has distinctly less 88 Olson, J. C., Mica-bearing pegmatites of New Hampshire: U. S. Geol. desirable electrical properties than iron-poor varie­ Survey Bull. 941-P, p. 382, 1942. ties. 89 Hall, E. L., Equipment and method for measurement of power factor of mica: Inst. Radio Eng. Proc., vol. 82, p. 896, 1944. 80 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical 85 Eesler, T. L., and Olson, J. C., Muscovite in the Spruce Pine district, Eng. Trans., vol. 64, pp. 1, 7, 1945. N. C.: U. S. Geol. Survey Bull. 936-A, pp. 18-30, 1942; Horton, F. W., Mica: 91 American Society for Testing Materials, Tentative specifications for U. S. Bur. Mines Inf. Circ. 6822, pp. 41-46, 54-55, 1935 (revised 1941) ; Hall, natural block mica and mica films suitable for use in fixed mica-dielectric E. L., Equipment and method for measurement of power factor of mica: capacitors, A. S. T. M. Designation: D 748-45T: 1946 Book of A. S. T. M. Inst. Radio Eng. Proc., vol. 32, p. 396, 1944 ; Townsend, J. R., Final report on Standards, pt. 3-B, p. 705, 1947. commercialization of mica testing equipment: War Metallurgy Comm., War "Horton, F. W., Mica: U. S. Bur. Mines Inf. Circ. 6822, p. 22, 1935 (re­ Production Board Rept. W-151, pp. 21-25, 1944; Coutlee, K. G., Judging vised 1941) ; Gwinn, G. R., Strategic mica: U. S. Bur. Mines Inf. Circ. 7258, mica quality electrically: Am. Inst. Electrical Eng. Trans., vol. 64, pp. 5-7, p. 8, 1943; Judd, D. B., Color standard for ruby mica: Nat. Bur. Standards 1945. Jour. Research, vol. 85, p. 245, 1945. 88 Eesler, T. L., and Olson, J. C., op. cit., p. 18. "American Society for Testing Materials, Tentative specifications for 87 Hall, E. L., Equipment and method for measurement of power factor of natural block mica and mica films suitable for use in fixed mica-dielectric mica: Inst. Radio Eng. Proc., vol. 82, p. 396, 1944; Coutlee, K. G., Saving capacitors, A. S. T. M. Designation: D 748-45T: 1946 Book of A. S. T. M. mica by testing: Bell Lab. Rec., vol. 22, p. 513, 1944. Standards, p. 3-B, p. 717, 1947. SUMMARY AND CONCLUSIONS 105 ously with its filming properties or lead to develop­ tuted to eliminate the excessive handling and prepara­ ment of cracks and holes in finished films. Pinholes, tion costs entailed in preparing green- and black- hair cracks, and conducting impurities are particu­ stained material." Although perfectly accurate with larly objectionable. Reeves, warping, rippling, or respect to black-stained mica, this statement—like other structural imperfections may lead to undesir­ many another in the published record—is not at all ably high proportions of waste during the final stages clear with respect to green material. The use of a of mica preparation, even though the electrical prop­ hyphen after the word "green" implies that mica with erties of the mica are perfectly satisfactory for con­ green stain is intended, whereas the decision of the denser use.94 War Production Board applied to all green mica (or, It was demonstrated more than a decade ago that more exactly, to all nonruby mica), whether stained domestic muscovite compares favorably with India or clear. There appears to be no good evidence that muscovite, quality for quality, in dielectric strength, clear green muscovite is any more difficult or expen­ changes on heating, power factor, flexibility, and sive to handle and prepare, grade for grade, than hardness,95 and it generally has been assumed that clear muscovite of any other color. most consumers' preference for India mica is based Some of the economic factors involved in the use of upon its more careful preparation and better grading. electrical testing for the upgrading of visually low- Through the efforts of the Colonial Mica Corporation quality mica have been summarized by Watts,97 who during the period of World War II, however, the prep­ points out that the cost will be increased by (1) the aration of domestic mica was standardized and expense of selecting the mica; (2) losses involved in greatly improved, and consumers were able to use the rejection of E-2 or E-3 mica or of mica with con­ large quantities of such material without further test­ ducting impurities; (3) additional rejections in the ing. Moreover, the results of the present investiga­ voltage testing of films for large-capacity or potted- tions show that there is no intrinsic difference, quality type condensers; (4) difficulty in splitting and punch­ for quality, between clear ruby and clear nonruby ing some types of mica; (5) the additional inspection micas from the Southeastern States. Clear green, and care required in manufacture; and (6) the yellowish-olive, and brownish-olive micas, selected by greater number of rejected capacitors. He adds that visual means, should prove just as satisfactory for electrically selected low-quality (visually deter­ condenser use as clear buff and light-brown micas, mined) ruby or green mica can be used to produce and several manufacturers have indeed found this to satisfactory capacitors and that if the industry were be the case. In addition, the use of electrical testing "to use the lower qualities when sufficient higher and adoption of the combined visual-electrical system qualities were obtainable, the anomalous situation of of mica classification should result in upward grading greater cost for capacitors made with lower quality of much sheet material from the Southeastern States mica than higher quality mica would result." and might well go even farther toward improving the It has been established that much mineral-stained currently inferior position of the abundant greenish mica is not as readily split and filmed as the clear varieties in the trade. Electrical testing might also varieties,98 hence that its preparation is more time force rejection of some sheets and lots of apparently consuming and involves somewhat greater losses of high-quality material, owing to pinholes or other de­ material. No such difference, however, has been dem­ fects difficult to recognize. onstrated between clear micas of different colors. Although possibilities for the use of visually in­ The grading of mica according to the combined ferior grades of mica in condensers have been pointed visual-electrical method proposed by the American out, the requisite electrical testing and increasing Society for Testing Materials should result in a sub­ costs of preparation might well exceed the potential stantial conservation of sheet material of condenser gains from such a procedure, especially under peace­ quality. This saving, which might well amount to as time conditions. In discussing the mica situation in much as 60 percent,99 would be particularly impor­ 1944, Gwinn and Tucker 96 noted that "by the end of the third quarter the supply and stock of strategic tant during periods of short supply, especially with mica, especially the smaller sizes, reached such a point respect to clear sheet stock. At other times, however, that the War Production Board felt it no longer neces­ competition with foreign sources of supply, with sary to purchase either domestic or imported green- ceramic, glass, treated paper, and plastic substitutes, and black-stained mica. This curtailment was insti­ OT "Watts, F. F., Notes on NRC-587, in Townsend, J. R., Addendum to "War Metallurgy Comm. Kept. W-151: War Production Board Kept. W-170, pp. 94 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical 1-2, 1944. Eng. Trans., vol. 64, p. 7, 1945. 88 Coutlee, K. G., Judging mica quality electrically: Am. Inst. Electrical ^Horton, F. W., Mica: TJ. S. Bur. Mines Inf. Circ. 6822, p. 54, 1985 (re­ Eng. Trans., vol. 64, pp. 6-7, 1945; "Watts, F. .F., Notes on NRC-587, in vised 1941). Townsend, J. R., Addendum to "War Metallurgy Comm. Rept. "W-151: "War 98 Gwinn, G. R., and Tucker, E. M., Mica, in Minerals Yearbook for 1944, Production Board Rept. W-170, p. 8, 1944. p. 1470, U. S. Bur. Mines, 1946. M Cottttee, K. G., Saving mica by testing: Bell Lab. Rec., vol. 22, p. 518, 1944. 106 COMMERCIAL SHEET MUSCOVITE IN THE SOUTHEASTERN UNITED STATES and even with synthetic mica 10° might well be so great Dana, E. S., 1909, The system of mineralogy of James Dwight that piece-by-piece electrical testing would not be Dana, pp. 611-622, New York. economically feasible as a means for bringing visually Dana, J. D., and Brush, G. J., 1869, On the magnetite in the mica of Pennsbury, Pa.: Am. Jour. Sci., 2d ser., vol. 48, inferior grades of domestic mica into the field of con­ pp. 360-362. denser use. As pointed out by Watts,101 though, "the de Schmid, H. S., 1912, Mica, its occurrences, exploitation, and use of the instruments by industry need not be con­ uses: Canada' Dept. Mines, Mines Branch, Pub. 118. fined to selecting the better micas from the lower Dellinger, J. H., and Preston, J. L., 1923, Methods of measure­ qualities. They are being used to check shipments, ment of properties of electrical insulating materials: Nat. Bur. Standards Sci. Papers, vol. 19, pp. 39-72. evaluate mica from untried sources and for selection Fillon, S. 0., 1913, Mica manufacturing and marketing: of the highest electrical quality from high-quality Canadian Min. Inst. Trans., March 5, 1913. mica. In the event that the industry adopts A.S.T.M.- Frondel, Clifford, 1936, Oriented inclusions of tourmaline in D748-43T, their use would become mandatory." More­ muscovite: Am. Mineralogist, vol. 21, pp. 777-799. over, improvements in the design of the Q-meter and ————, 1940, Oriented inclusions of staurolite, zircon, and garnet in muscovite; skating crystals and their signifi­ spark-coil test set are to be expected, and these de­ cance: Am. Mineralogist, vol. 25, pp. 69-87. vices probably will be adapted to continuous semi­ Frondel, Clifford, and Ashby, G. E., 1937, Oriented inclusions automatic or fully automatic operation, either for of magnetite and hematite in muscovite: Am. Mineralo­ piece-by-piece or for sample-lot testing. gist, vol. 22, pp. 104-121. Fuller, M. L., 1899, The occurrence and uses of mica: Stone, LIST OF REFERENCES vol. 19, pp. 530-532. American Society for Testing Materials, 1947, 1946 Book of Furcron, A. S., and Teague, K. H., 1943, Mica-bearing pegma­ A. S. T. M. Standards. tites of Georgia: Georgia Geol. Survey Bull. 48. Bannerman, H. M., and Cameron, E. N., 1946, The New Eng­ Galpin, S. L., 1915, Feldspar and mica deposits of Georgia: land mica industry: Am. Inst. Min. Met. Eng. Tech. Pub. Georgia Geol. Survey Bull. 30. 2024. Given, F. J., 1943, Mica for war purposes: Bell Lab. Rec., vol. Barber, A. W., 1937, Simplified dielectric loss measurements: 22, pp. 60-63. Inst. Radio Eng. Proc., vol. 17, p. 26. Gwinn, G. R., 1943, Strategic mica: U. S. Bur. Mines Inf. Circ. Billings, M. H., and Montague, S. A., 1944, The wartime prob­ 7258. lem of mica supply: Eng. and Min. Jour., vol. 145, no. 8, Gwinn, G. R., and Tucker, E. M., 1946, Mica, in U. S. Bur. pp. 92-95. Mines, Minerals Yearbook for 1944, pp. 1470-1475. Bradley, Milton, 1895, Elementary color, 128 pp., Springfield, Hackett, Winifred, and Thomas, A. M., 1941, The electric Mass., Milton Bradley & Co. strength of mica and its variations with temperature: Brown, C. B., 1937, Outline of the geology and mineral re­ Jour. Inst. Electrical Eng., vol. 88, pp. 295 et seq. sources of Goochland County, Va.: Virginia Geol. Survey Hall, A. J., 1941, The relation between colour and chemical Bull. 48. composition in the biotites: Am. Mineralogist, vol. 26, pp. Brown, W. R., Mica deposits of Virginia: Virginia Geol. Sur­ 29-33. vey Bulletin (in preparation). Hall, E. L., 1942, Report on measurement of power factor in Burgess, B. C., 1944, Mica mining and preparation cost: Am. muscovite, in Kesler, T. L., and Olson, J. C., Muscovite in Inst. Min. Met. Eng., preprint of paper presented at New the Spruce Pine district, N. C.: U. S. Geol. Survey Bull. York meetings, February 1944, pp. 1-17. 936-A, pp. 18-24. Cameron, E. N., Jahns, R. H., McNair, A. H., and Page, L. R., ————, 1944, Equipment and method for measurement of 1949, The internal structure of granitic pegmatites: Econ. power factor of mica: Inst. Radio Eng. Proc., vol. 32, pp. Geol., Econ. Mon. 2, 115 pp. 393-396. Cameron, E. N., Larrabee, D. M., McNair, A. H., Page, J. J., -, 1945, Power factors, in Hidnert, Peter, and Dickson, Shainin, V. E., and Stewart, G. W., 1945, Structural and George, Some physical properties of mica: Nat. Bur. economic characteristics of New England mica deposits: Standards Jour. Research, vol. 36, pp. 342-344. Econ. Geology, vol. 40, pp. 369-393. Hall, G. M., 1933, Flattened garnets in mica at Spruce Pine, Clark, G. H., 1921, Mica deposits of Alabama: Alabama Geol. N. C.: Tennessee Acad. Sci. Jour., vol. 8, pp. 268-272. 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S. -, 1945, Judging mica quality electrically: Am. Inst. Geol. Survey 20th Ann. Rept., pt. 6, pp. 691-707. Electrical Eng. Trans., vol. 64, pp. 1-7. Horton, F. W., 1935, Mica: U. S. Bur. Mines Inf. Circ. 6822 (revised 1941). 100 Spence, H. S., Mica as a critical war mineral: Canadian Min. Jour., vol. 67, pp. 4-5, 1946. Houk, L. G., 1942, Marketing strategic mica: U. S. Bur. Mines 101 Watts, F. F., Notes on' NRC-537, in Townsend, J, E., Addendum to War Inf. Circ. 7219. Metallurgy Comm. Kept. W-151: War Production Board Kept. W-170, p. 2, Hunter, C. E., 1940, Residual alaskite kaolin deposits of North 1944. Carolina: Am. Ceramic Soc. Bull., vol. 19, pp. 98-103. LIST OF REFERENCES 107 Jahns, R. H., 1945, Color characteristics of sheet muscovite in ————, 1946, Mica deposits of the Franklin-Sylva district, N. the Southeastern States (abstract): Geol. Soc. America C.: North Carolina Dept. Cons, and Devel., Div. Min. Re­ Bull., vol. 56, p. 1170. sources, Bull. 49. ————, 1946, Mica deposits of the Petaca district, Rio Arriba Pegau, A. A., 1929, The pegmatites of the Amelia, Goochland, County, N. M.: New Mexico School of Mines, State Bur. and Ridgeway areas, Va.: Am. Jour. Sci., 5th ser., vol. 17, Mines and Min. Resources, Bull. 25. pp. 543-547. -, Strategic pegmatite minerals from the Southwestern ————, 1932, Pegmatite deposits of Virginia: Virginia Geol. and Southeastern States during World War II: Am. Inst. Survey Bull. 33. Min. Met. Eng. Tech. Pub. (in press). Phillips, W. B., 1888, Mica mining in North Carolina: Eng. Judd, D. B., 1939, Specification of uniform color tolerances for and Min. Jour., vol. 45, pp. 286, 306-307, 322, 324, 382-383, textiles: Textile Research, vol. 9, p. 258. 398, 418, 436. ————, 1945, Color standard for ruby mica: Nat. Bur. Stand­ Ridgway, Robert, 1912, Color standards and color nomencla­ ards Jour. Research, vol. 35, pp. 245-256. ture, 43 pp., 53 color pis., Washington, D. C. Judd, D. B.} and Kelly, K. L., 1939, Method of designating Rose, G., 1862, Asterism: Preuss. Akad. Wiss. Monatsber., pp. colors: Nat. Bur. Standards Jour. Research, vol. 23, p. 355. 614 et seq. Keith, Arthur, 1903, U. S. Geol. Survey Geol. Atlas, Cranberry Schaller, W. T., 1925, The genesis of lithium pegmatites: Am. folio (no. 90). Jour. Sci., 5th ser., vol. 10, pp. 269-279. ————, 1904, U. S. Geol. Survey Geol. Atlas, Asheville folio Schaller, W. T., and Henderson, E. P., 1926, Purple muscovite (no. 116). from New Mexico: Am. Mineralogist, vol. 11, p. 12. ————, 1905, U. S. Geol. Survey Geol. Atlas, Mount Mitchell Smith, C. D., 1876, Ancient mica mine in North Carolina: folio (no. 124). Smithsonian Inst. Rept., pp. 441-443. ————, U. S. Geol. Survey Geol. Atlas, Cowee folio (unpub­ Smythe, D. D., 1946, Muscovite mica in Brazil: Am. Inst. Min. lished manuscript, 1906). Met. Eng. Tech. Pub. 1972. ————, 1907, U. S. Geol. Survey Geol. Atlas, Nantahala folio Snow, H. A., 1937, Losses in mica and simple test procedure: (no. 143). Inst. Radio Eng. Proc., vol. 17, p. 19. ————, 1907, U. S. Geol. Survey Geol. Atlas, Pisgah folio (no. Spence, H. S., 1929, Mica: Canada Dept. Mines, Mines Branch, 147). Pub. 701. ————, 1907, U. S. Geol. Survey Geol. Atlas, Roan Mountain ————, 1946, Mica as a critical war mineral: Canadian Min. folio (no. 151). Jour., vol. 67, pp. 611-617, 710-717. -, U. S. Geol. Survey Geol. Atlas, Lincolnton folio (un­ Sterrett, D. B., 1907, Mica deposits of western North Carolina: published manuscript, 1911). U. S. Geol. Survey Bull. 315-M, pp. 400-422. Keith, Arthur, and Sterrett, D. B., 1931, U. S. Geol. Survey ————, 1910, Mica deposits of North Carolina: U. S. Geol. Geol. Atlas, Gaffney-Kings Mountain folio (no. 222). Survey Bull. 430-J, pp. 593-638. Kennard, T. G., and Howell, D. H., 1941, Types of coloring in -, 1923, Mica deposits of the United States: U. S. Geol. minerals: Am. Mineralogist, vol. 26, pp. 405-421. Survey Bull. 740. Kerr, W. C., 1880, The mica veins of North Carolina: Am. Stose, G. W., 1926, Geologic map of Alabama, Alabama Geol. Inst. Min. Eng. Trans., vol. 8, pp. 457-462. Survey and U. S. Geol. Survey. ————, 1881, The mica veins of North Carolina: Eng. and Min. Stose, G. W., Jonas, A. I., and others, 1928, Geologic map of Jour., vol. 31, pp. 211-212. Virginia, Virginia Geol. Survey and U. S. Geol. Survey. Kesler, T. L., and Olson, J. C., 1942, Muscovite in the Spruce Stose, G. W., and Smith, R. W., 1939, Geologic map of Georgia, Pine district, N. C.: U. S. Geol. Survey Bull. 936-A. Georgia Div. Mines, Mining and Geol. and U. S. Geol. La Forge, Laurence, and Phalen, W. C., 1913, U. S. Geol. Sur­ vey Geol. Atlas, Ellijay folio (no. 187). Survey. Lancaster, F. W., 1940, Light and color in industry, 291 pp., Townsend, J. R., 1944, Final report on commercialization of Raleigh, N. C., The Technical Press, Inc. mica testing equipment: War Metallurgy Comm., War Landes, K. K., 1925, The paragenesis of the granite pegmatites Production Board, Rept. W-151. of central Maine: Am. Mineralogist, vol. 10, pp. 355-411. ————, 1944, Addendum to War Metallurgy Comm. Rept. W- Lester, J. G., 1946,Inclusions in muscovite from Mitchell Creek 151: War Production Board Rept. W-170. mine, Upson County, Ga.: Am. Mineralogist, vol. 31, pp. Ulmer, Joseph, 1930, International trade in mica, Bur. Foreign 77-81. and Domestic Commerce. Lewis, A. A., Hall, E. L., and Caldwell, F. R., 1931, Some elec­ U. S. Geological Survey, staff members, Mica deposits in the trical properties of foreign and domestic micas and the Blue Ridge province of North Carolina and Georgia (in effect of elevated temperatures on micas: Nat. Bur. Stand­ preparation). ards Jour. Research, vol. 7, research paper 347. ————, Mica deposits of the southeastern Piedmont (in prepa­ Lintner, E. J., 1944, Mica, a war essential mineral: Rock Prod­ ration). ucts, vol. 47, no. 5, pp. 48-50, 92-93; no. 6, pp. 74-76, 114- Walker, T. L., 1896, Observations on percussion figures on 116. cleavage plates of mica: Am. Jour. Sci., 4th ser., vol. 2, Maurice, C. S., 1940, The pegmatites of the Spruce Pine dis­ pp. 5-7. trict, N. C.: Econ. Geology, vol. 35, pp. 49-78, 158-187. War Production Board, 1942, Mica releases 1 and 2. Myers, W. M., 1929, Mica: part 1, general information: U. S. Watts, F. F., 1944, Notes on NRC-537, in Townsend, J. R., Bur. Mines Inf. Circ. 6205. Addendum to War Metallurgy Comm. Rep t. W-151: War Olson, J. C., 1942, Mica-bearing pegmatites of New Hamp­ Production Board Rept. W-170, 3 pp. shire : U. S. Geol. Survey Bull. 931-P. Wayland, R. G., 1944, Mica in war: Am. Inst. Min. Met. Eng. •————, 1944, Economic geology of the Spruce Pine pegmatite Tech. Pub. 1749, 7 pp. district, N. C.: North Carolina Dept. Cons, and Devel., Wierum, H. F., and others, 1938, The mica industry: U. S. Div. Min. Resources, Bull. 43. Tariff Comm. Rept. 130, 2d ser., pp. 11-26.

INDEX

Page Page "A" mica — ———————— ___--_— ______—______8-9; pi. 2 Garnet inclusions——————————————————————————14, 44, 103; pi. 8 Abstract ______.______1—2 General properties ———————————————————————————————4-6; pi. 1 Acknowledgments ———————_—_____—______:______4 Geology ______——————————————————————————————————30-33 Actinolite inclusions — — — __.—______14 Goethite inclusions —————————————————————————————————————14 Air creep ————————————————————————————————___15; pi. 5 Grading and classification————————————————————————17-25; pi. 9 Air stain ————_.———._———______12, 103, 104; pi. 5 Gritty mica —————————————————————————————————————————13 Albite inclusions ——————————____—.—______,._._14 Gummy mica _____——————-—————————————————————————7 Allanite inclusions ————————___—______--_14 American Society for Testing Materials, size-grading chart of _____18, 19 test methods for electrical qualities adopted by—____—______22-24 visual quality classification of ———_——__-_—_—______18, 21-22 Hair cracks (hair lines) ————_———————————————7, 9, 23, 44-45, 104 visual-electrical method of qualification developed by—______24-26 Hair mica ————————————————————————————————————10; pi. 4 Apatite inclusions ———————______14, 44, 103 Hardness —————————————————————————————————————————7-8 Hematite inclusions and stains—————————13-14, 43-44, 103, 104; pis. 5-8 B Herringbone mica ———————————————————————————————9; pi. 2 History of investigations————————————————————————————————3-4 Bell Telephone Laboratories, Q-meter developed by————___28-24 ; pi. 9 of mining operations—————_————————————————————————29-30 series of investigations at-___—__—__—_—______24-25 Horsetail mica. See Herringbone mica. spark-coil test set developed by—_—_—_—__—_—______22-23 Housetop mica. See "A" mica. Beryl inclusions ——————————————_——————_————_—______14 Biotite inclusions ———_____-______14-15, 43-44, 103-104 ; pi. 8 Block mica, defined ———————______—______17, 18 Book mica ————————————————_———————_———______17 See also Block mica. Impurities, primary _———————_——————12-15, 43-44, 103-104 ; pis. 5-8 Brittle mica ______5, 7 secondary ______15-16, 44, 103-104 ; pis. 5, 8 Broad-A mica. See Flat-A mica. Inclusions ______13-15, 43-44, 103-104; pis. 5-8 Buckling _____.______9-10 ; pi. 3 Indian standard sizes, chart of————————————————————————————20 Bursts —————_———————_—____—_—_—______12, 44 Intergrowths, mineral ——————.——————13-15, 43-44, 103-104 ; pis. 5-8 Investigations, history of ——————————————————————————————3-4 Iron stain ______-__——__————15, 44, 103, 104

Chub-A mica ———————————_——___— —— —————— ______9; pi. 3 Cigarette burns ——————______12, 44 Cigarette mica ————————_—_———————_—_.._—_—______16 Kyanite inclusions ——-——_--_————_—————————-———-14 Circle mica, defined ———————_———————_—_——————______18 Clay stain __—______15, 44, 103, 104 ; pis. 2, 8 Cleavage —————————————_—______-_—._—————_____6-7; pi. 1 Cleavage stepping ———————__——______—______10; pi. 3 Location of deposits——————————————————————————————————28-29 Colonial Mica Corporation————————__———_————_—_4, 18, 21, 105 Locky mica ——————————————————————————————————————————7 tests on material loaned by—————————__————_—_____34, 97-103 Color—————_——————————_—______11-12, 42-43, 103, 104; pi. 7 Comparison, Southeastern with India mica—————————______105 M Composition —————————————————————————————————————____.__5 Conclusions ————————————_————————————————______.__103-106 Magnetite inclusions _———_.„-__.—————————43-44, 103, 104; pis. 5, 8 Creping ——————————————————————————————————_—_———_9 Manganese stain ————————————————————————————————15, 44, 103 Cross grains. See Reeves. Marketing ____————————————_———————————————————————27-28 Crystallography ———————————_——————————————______5-6 ; pi. 1 Mine-run (run-of-mine) mica. See Book mica. Cupping ——————————————————____———____————_————___9; pi. 3 Mineral stain ______——_____————————13-15, 43-44, 103 ; pis. 5-8 Mineralogy ——————————————————————————————————————4-5 Mines and prospects from which test specimens were collected——————34-42 Mining operations, history of———————————————————————————29-30 Deposits, distribution of ————______—______.-.28-29 Mottling______12-13. 44, 103-104; pi. 5 location of —————————_--_—-_„-_-—_-____———______29 occurrence of ————————————————————————————______._30-33 N summary data on ————————————————————————_—_——————30 Dielectric constant (K) ______16, 22, 25 Nailed (nail-locked) mica————_——————————————————— ———— —7 Dielectric strength _———_——______—______16, 22, 25 Districts, enumerated ————__—______——______28-29 O location of _————————______29 summary data on —————_—_———_—_———_—_————___———_30 Occurrence of deposits—————————— —————————————————— ———30-33 Double-A mica ———_—————______.__-__—______8; pi. 2

E

Elasticity _____—______7-8 Pattern mica. See Sheet mica. Electric mica _•———--————______21-22 Pegmatites, mica-bearing ————————————————————————————31-33 Electrical conductivity ————_—_—______—____——_—_—_16, 22, 25 Percussion figures ———————————————————————————6, 7, 10; pi. 1 Petrographic examination of test specimens—————————————————43-45 results correlated with those of electrical tests——————————————47-95 Pinholes______———-———-23, 44-45, 103 Feather mica. See Herringbone mica. Plate mica. See Sheet mica. Field and laboratory work———_—___—_———.—_———_—__—_—_—3-4 Power factor (PF) _————————————_16, 23-24, 25, 26, 45, 104-105 Fishback (fishbone) mica. See Herringbone mica. Preparation _——————————————————————————————————26-27 Fishtail mica. See "A" mica. Prices ______———————————————27-28, 30 Flat-A mica———.--______—______8-9; pis. 2, 4 Production ..._—_—————__—————-—————————————————29-30 Flexibility ______7-8, 16 Punch mica, defined————————————————————————————————————18 Fluorite inclusions _————______-__—_-__———_—_—______14 Pyrite inclusions____———————————————————————14, 44, 103; pi. 8 109 110 INDEX

Page Page Testing program ——————————————————————————————————33-103 Q-meter______.-_--__.______23-24, 45, 97; pi. 9 color tests, discussed———————————————————————————42-43, 103 Q value..—______„___.___..______16, 104-105 results correlated with those of electrical tests————————————47-95 classification based on—————————— ———————————————____—_26 electrical tests, apparatus————————————————————————————45 method of testing for____-______-———————____23-24, 25-26, 45; pi. 9 methods ——————————————————————————————————————45 Quality classification by electrical methods— ——— _ — .—————22-24; pi. 9 results, discussed ______————_————————45-46, 97, 103, 104-106 by visual methods—.—_____———————_———______18, 20-22 tabulated ______-____—_—__————47-103 by visual-electrical method—_ ——————— — ———— ——— ______24-26 mines and prospects represented——————————————————————34-42 Quartz inclusions — ——————————— ————— -——————————_14, 44, 103 petrographie examination, discussed————————————————————43-45 results correlated with those of electrical tests———————————47-95 specimens, preparation of———_—————————————————————————34 source of —————————————————————————————————————34 Thickness ————.———————.————————————————————————————18 Reeves.-.—______8-9 ; pi. 2 Tied mica ———.————————————————————————.——————.——————7 References cited______—————_————__.______106-107 Tourmaline inclusions—————————————————————————14, 44, 103; pi. 8 Ribbing—_—____—_—______———_——————_—______9 ; pi. 3 Ridging ————————————_— ————————— — —— _ ____———.______9 U Rippling ______————————.——___—_—______9, 10 Ruling—_—_—_—__—_„____———————__.______10 ; pi. 4 Upgrading of visually inferior grades—————————————————————105-106 Rutile inclusions————_————___——————————— —————— ______14 Uses __——_—_—————————————————————.——————.——2,16-17

Sandy (sand-pitted) mica—_—____———————————————______13 V-ridge mica. See "A" mica. Scrap mica, defined—_—_—_—_____—_————_——______17 Value __—-_——————_—————————————————-.—————-——————30 Sheet mica, defined______17-18 properties affecting______——————————6-16; pis. 2-8 Silver spots.—_—_—_—_____—______——————______—______12 Vegetable stain, inorganic__—_———————12-13, 44, 103-104; pis. 5-7 Size.—_—.___..______————______18, 19, 20 organic______——_____————-15-16, 44, 103-104 Spark-coil test set______-_____—______22-23, 45 Visually graded mica, tests on————————————————————————34, 97-103 Spearhead mica. See "A" mica. upgrading of ——————————————————————————————————105-106 Spot-locked (spot-welded) mica————— ———————————————_____13 Staining, primary..______—————12-15; 43-44, 103-104; pis. 5-8 W secondary..______.____—————.—15-16, 44, 103-104; pis. 5, 8 Stony mica —————_—_—_—_—_—————————————————______13 Warping______—__—_————————————————————9-10 ; pi. 3 Strategic mica ______-___—______17, 21-22 Washer (washer-punch) mica, defined———————————————————————18 Structural imperfections.______————_———___-_-____8-10 ; pis. 2-4 Wavy mica —————— ——————————————————————————————— ———9 Substitution of other materials______-______2-3, 17, 105-106 Wedging__———_—_—_—_———————————————————————————9 ; pi. 3

Zircon inclusions——————————————————————————————————-44, 103 Tacky mica —...—————_—_—_——————————————_———_.____—7 Zoisite inclusions______-______—————————14, 44, 103 ; pi. 8 Tanglefoot (tangled, tanglesheet) mica—————————————_—_.___._„7 Zoning of pegmatites————————————————————————————————32-33

U. S. GOVERNMENT PRINTING OFFICE: 1950—852911