Technical Bulletin No. 344 January, 1933 PETROGRAPHIC METHODS FOR SOIL LABORATORIES BY W. H. FRY Soil Petrographer, Division of Soil Chemistry and Physics Soil investigations. Bureau of Chemistry and Soils United States Department of Agriculture, Washington, D, C, For sale by the Superintendent of Documents, Washington, 1). C. -------------._ Price 10 ( piF'T'"' IN MEMORIAM Work on the galley proof of this bulletin was com- pleted by the author on the morning of his death, December 27, 1932. It seems fitting to preface this pub- lication, which the author considered his best effort, with a brief account of his activities. Mr. Frj^ became associated with the Bureau of Soils on December 15, 1911, and, except for a period of three months in 1922, continued this association until his death. He was a native of North Carolina and gradu- ated from the university of that State in 1910. During his undergraduate years and for a year thereafter he served as assistant and instructor in the department of geolog}^ in that university. He published several short articles previous to coming to Washington and since that time has published, either alone or as associate author, some 35 contributions to soil literature. These contributions center around the detection and estima- tion of soil minerals by pétrographie methods, and his name is indissolubly associated with the great advance in the knowledge of soil colloids which has occurred in recent years. In the progress of his work he de- veloped a technic which is unique. The work he has been able to do has had a marked effect on the trend of investigations within the bureau with respect to both soil chemistry and soil physics. Besides his research, his services were frequently required by others in con- nection with their own work, and it is probable that but few names have appeared so frequently as a con- tributor to papers not related directly to his work. He was also called on to identify a wide range of mis- cellaneous materials which came to the bureau and in this line he developed great skill. A very large number of persons, to whom his name is entirely unknown, have been aided by his expert assistance. Personally he was afltable, extremely modest, of the strictest personal and scientific integrity, and his associates will long lament his untimely death. TECHNICAL BULLETIN NO. 344 JANUARY, 1933 UNITED STATES DEPARTMENT OF AGRICULTURE WASHINGTON. D. C. PETROGRAPHIC METHODS FOR SOIL LABORATORIES By W. H. FRY Soil Petrographer, Division of Soil Chemistry and Physios, Soil Investigations, Bureau of Chemistry and Soils CONTENTS Page Optical methods—Continued. -^^S® Introduction 1 Sign of elongation _ 61 Crystal form 4 Color of crystals __ 61 Fundamental properties of light 6 Pleochroism 62 Polarized light 9 Orientation 63 Double refraction 10 AppUcation of methods 63 Becke lines 13 Pure substances 63 Interference of light 14 Identification of minerals in soils 64 The pétrographie microscope and accessories. 16 Percentage estimation of constituents in Mechanical parts 16 a mixture (mechanical analysis) 71 Optical system 18 Differentiation of soil colloids from soil Calibration and measurements 25 crystalline material _ 76 Optical methods .„ __.. 33 Soil organic matter and residue of organ- Immersion media 33 isms -- - 78 Determination of refractive index by Shape of particles 79 Becke line 34 Residues from the evaporation of soil solu- Determination of two indices of refrac- tions 81 tion 36 Vertical illumination 82 Determination of three indices of refrac- Particle surfaces 82 tion 37 Soil structure 83 Interference figures __ 39 Ultramicroscopes 84 Optical character . 45 Fertilizer materials 85 Optic-axial angle 48 Miscellaneous materials 86 Dispersion of optic axes 60 Inspection of samples for purity 86 Measurement of double refraction (bire- Phase-rule problems 87 fringence) 53 Appendix _ _ 89 Extinction angles _ __. 58 Literature cited _ 95 INTRODUCTION For many years, almost since the beginning of microscopy, the ordinary microscope has been used for the observation of crystals too small to be otherwise observed. Certain obvious similarities and dissimilarities between different crystalline substances were early noted and were used as confirmatory methods of identification. For example, the cubes of sodium chloride were seen to be identical in appearance with the cubes of potassium chloride, but both were found to be utterly dissimilar to the monoclinic, frequently twinned, crystals of gypsum. Here, then, were two substances which could not be distinguished from each other by simple observation, no matter how perfect the microscope might be as a magnifying instrument, whereas either could be instantly distinguished from the third sub- stance even with a very crude microscope. In process of time such 150470°—33- 1 2 TECHNICAL BULLETIN 3 44, U. S. DEPT. OF AGRICULTURE cases multiplied, and the distinguishing characteristics of the various crystals were studied with greater attention to detail. These studies brought to light a wealth of crystallographic information, some of which extended the lists of definitely identifiable substances, but much of which simply added new members to already large groups of observationally indistinguishable crystals. Such a result was inevitable. There are only six major crystalline systems, to some one of which each crystal must belong. The sub- divisions of these six systems give a somewhat larger number of groups, but at best there is a very large number of crystals falling in each group and therefore having the same general appearance, although they may be completely unlike chemically. Matters are further complicated by the fact that a given substance may, under certain conditions, crystallize in one subdivision of a system, and under other conditions in some other subdivision. For instance, both sodium chloride and potassium alum crystallize in the isometric system, but both may appear as cubes or as octahedrons. It is thus evident that a given crystal not only does not necessarily show defi- nitely diagnostic characteristics on simple observation, but also that it could not be expected to do so. Common sense supplied a solution to the problem by the simple elimination of all substances except those which could be reasonably expected to crystallize from a given solution. In cases where it was known what could crystallize out of a given solution and where these possible crystals were sufficiently different, identification by appear- ance could readily be made, if the crystals came down in sufficiently perfect and characteristic form. But they did not always do this. This method is only infrequently applicable, but when it is applicable it is capable of giving very good results if used with judgment. The methods of qualitative chemical analysis under the micro- scope have abundantly proved their value but they have severe limi- tations. (1) The substance to be analyzed under the microscope must be fairly readily soluble without too much preliminary treatment. This practically eliminates most of the silicate minerals. (2) Some of the methods depend on the visual recognition of crystals formed by the substance under analysis and the reagents applied to it. All that has been said as to the uncertaintv of crystal determination by ordinary observation applies here witli perhaps added force. (3) Microchemical tests on mixed substances leave a degree of uncer- tainty as regards the composition of any particular crystalline com- ponent of the mixture. (4) Microchemical tests do not distinguish between different allotropie forms of the same substance, between different hydrates, or between salts which have qualitatively the same but quantitatively different compositions. Geologists were early confronted by the problem of identifica- tion of crystalline substances in rocks, a problem which combined almost all the difficulties outlined above. The mineral constituents of a granite, for example, usually show no external crystal form whatever and can not therefore be identified even tentatively by their forms. Most microchemical tests are altogether inapplicable. Re- sort was had, of necessity, to methods based on physical properties which were capable of showing internal crystalline structure regard- less of whether or not this internal structure was manifested by external form. PETROGRAPHIC METHODS FOR SOIL LABORATORIES ó As it happened, the optical properties were particularly well adapted for this purpose. Methods and apparatus were devised and improved, and a vast literature on the microscopic constitution and structure of rocks was rapidly built up. This new field was suffi- cient to occupy the attention of geologists and mineralogists for many years, and consequently very little attention was paid by them to artificial crystals obtained in the laboratory. Gradually, how- ever, there appeared a small amount of data on the optics of artificial crystals, and finally the exigencies of certain largely chemical prob- lems, notably those of cement, demanded the accumulation and application of definite data in the identification of specific artificial compounds. It was found that artificial crystals were just as sus- ceptible, and perhaps more so, on account of their purity, to optical identification as were the natural crystalline minerals. Methods predominantly suited to rock sections were modified to apply more accurately and conveniently to loose crystals. Thus optical crystal- lography ceased to be merely a means for rock study and broadened out into its rightful