DEPARTMENT OF THE INTERIOR ALBERT B. FALL, Secretary UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, Director Bulletin 679 THE MICROSCOPIC DETERMINATION OF THE NONOPAQUE MINERALS BY ESPER S. LARSEN WASHINGTON GOVERNMENT PRINTING OFFICE 1921 CONTENTS. CHAPTER I. Introduction.................................................. 5 The immersion method of identifying minerals........................... 5 New data............................................................. 5 Need of further data.................................................... 6 Advantages of the immersion method.................................... 6 Other suggested uses for the method.................................... 7 Work and acknowledgments............................................. 7 CHAPTER II. Methods of determining the optical constants of minerals ....... 9 The chief optical constants and their interrelations....................... 9 Measurement of indices of refraction.................................... 12 The embedding method............................................ 12 The method of oblique illumination............................. 13 The method of central illumination.............................. 14 Immersion media.................................................. 14 General features............................................... 14 Piperine and iodides............................................ 16 Sulphur-selenium melts....................................... 38 Selenium and arsenic selenide melts........................... 20 Methods of standardizing media for measuring indices of refraction. 20 Measurement of all indices of refraction of crystals............. .^...... 22 Measurements of axial angles.......................................... 24 Dispersion of the optic axes............................................. 25 Optical character...................................................... 25 Optical orientation, dispersion of bisectrices, and crystal system.......... 26 Other tests............................................................ 27 CHAPTER III. Some statistics on the optical properties of minerals. .......... 28 Distribution of minerals with regard to optical character................. 28 Distribution of minerals with regard to index of refraction and birefringence. 28 Relation between index of refraction, density, and chemical composition . 30 CHAPTER IV. Measurements of optical properties of minerals.................. 33 Completeness and accuracy of the data................................. 33 The new data.......................................................... 34 CHAPTER V. Tables for the determination of minerals from their optical prop­ erties. ............................................................... 161 Arrangement of the data in the tables................................... 161 Completeness of the data.............................................. 162 Tables................................................................ 16 3 List of minerals arranged according to their intermed iate indices of refraction, /?, and showing their birefringences. .... .\............. 163 Data for the determination of the nonopaque minerals................ 171 Isotropic group............................................... 171 Uniaxial positive group....................................... 185 Uniaxial negative group...................................... 192 Biaxial positive group.......................................... 205 Biaxial negative group...................................... 241 Minerals of unknown optical character......................... 279 INDEX ................................................................... 285 3 ILLUSTRATIONS. PLATE I. Case carrying dropping bottles with index of refraction media...... 16 FIGURE 1. The error in V'« as computed from the approximate formula Cos2 V'«=^-.............................................« j a 11 2. Indices of refraction of mixtures of piperine and iodides............. 17 3. Indices of refraction of mixtures of sulphur and selenium......... 19 4. Diagram showing density of distribution of the nonopaque minerals with respect to the intermediate index of refraction............ 29 5. Diagram showing density of distribution of the nonopaque minerals with respect to index of refraction and birefringence........... 29 6. (Optical orientation and zonal growths of annabergite on tabular face {010}.................................................... 40 7. Optical orientation of tabular crystals of churchite................ 58 8. Optical orientation of common cleavage fragments {010} of haiding- erite........................................................ 82 9. Optical orientation of tabular crystals {100} of larderellite......... 98 10. Optical orientation of tablets {100} of martinite.................. 105 11. Optical orientation of tablets {010} of artificial sodium bicarbonate. 134 12. Optical orientation of tabular crystals of voglite................... 154 13. Optical orientation of tabular {010} crystals of walpurgite.......... 156 14. Optical orientation of tabular crystals of zinkosite................. 159 4 THE MICROSCOPIC DETERMINATION OF THE NONOPAQUE MINERALS. By ESPER S. LARSEN. CHAPTER I. INTRODUCTION. THE IMMERSION METHOD OF IDENTIFYING MINERALS. Optical methods of determining minerals with the petrographic microscope have long been used and have been carried to a high state of development in studies of the minerals in thin sections of rocks and ores, yet out of about 1,000 mineral species comparatively few can be identified readily in thin sections. A mineral whose optical properties are known can be accurately and quickly identi­ fied, however, by the immersion method that is, by immersing its powder in liquid media whose indices of refraction are known and determining its optical constants. In this bulletin the author gives a set of tables for the systematic determination of minerals from their optical constants, describes briefly some methods for the rapid determination of optical constants, gives the results of measurements of the optical constants of more than 500 species for which data was not previously available, and presents statistics on the optical properties of minerals. The first tables prepared for general use in determinations of minerals by the immersion method were those of Van der Kolk,1 published in 1900. Somewhat similar tables, prepared by A. F. Rogers,2 were published in 1906, and an optical mineralogy con­ taining tables and description of all minerals for which optical data were then available, prepared by N. H. and A. N. Winchell,3 was published in 1909. However, the method has not received the attention that it deserves and has not come into general use, largely because the optical constants of over half the known minerals had not been determined. NEW DATA. In attempting to employ the immersion method some years ago the writer assembled all the data then available for its use with the petrographic microscope in determining minerals but found them so 1 Schroeder van der Kolk, J. C., Tabellen zur mikroskopischen Bestimmung der Mineralien nach ihrem Brechungsindcx, Wiesbaden, 1900; 2d ed., revised and enlarged by E. H. M. Beekman, 1906. 2 Rogers, A. F., School of Mines Quart., vol. 27, pp. 340-359,1906. ' Winchell, N. H. and A. N., Elements of optical mineralogy, D. Van Nostrand Co., 1909. 5 6 MICROSCOPIC DETERMINATION OF NONOPAQUE MINERALS. incomplete that the method was applicable to but few species. Since then he has measured the chief optical constants of over 500 mineral species for which the data were lacking or incomplete, so that such data are now lacking for only about 30 very rare species. No attempt at great accuracy was made in these measurements, and compara­ tively few of the specimens studied represented analyzed minerals. Only a small number of minerals of many isomorphous series were examined, in some series only a single member. NEED OF FURTHER DATA. Much further work on the optical constants and more complete and accurate data on nearly all the minerals are needed, as well as detailed studies of isomorphous series and of the effect of solid solution. Another need is a fuller appreciation of the fact that most minerals are of variable chemical composition and therefore have variable optical and other properties. The science of mineralogy needs also good connected and consistent data on minerals. Chemical analyses, crystallographic studies, and determination of physical properties, optical constants, and paragenesis should be made on identical material. A highly accurate determination of the optical or other constants of a mineral is of comparatively little value unless the data obtained are definitely tied to a chemical analysis. Greater care should be taken in examining minerals for lack of homogeneity, whether it is due to zonal growths or to admixed or included foreign material. As few specimens of minerals are entirely free from foreign bodies and without zonal growths or other elements of heterogeneity no description of a mineral is adequate which does not show clearly that its material has been carefully examined microscopically. Every published analysis of a mineral should include a clear statement of the approximate amount and the character of the foreign material it contains and of the degree or extent of zonal growths or other elements of heterogeneity; and this statement should be the result of a careful microscopic examination of a sample of the same powder that furnished the material for the chemical