ENGINEERING RESEARCH INSTITUTE UNIVERSITY. OF MICHIGAN ANN ARBOR STUDIES IN THE NATURAL HISTORY OF MICAS SEPTEMBER 1951 - AUGUST 1953 By E. Wm. Heinrich A. A. Levinson D. W. Levandowski C. H, Hewitt Project M978 CHEMICAL-PHYSICS BRANCH, SQUIER SIGNAL LABORATORY UNITED STATES SIGNAL CORPS, FORT MONMOUTI, N. J. DA 36-039 sc-15357, SC PROJECT NO. 152B-0, DA PROJECT NO. 3-99-15-022 Under the administration of' S. Benedict Levin, Deputy Chief August, 1953

TABLE OF CONTENTS Page LIST OF TABLES viii LIST OF FIGURES xii ABSTRACT xiii PART I. GENERAL INFORMATION I. INTRODUCTION 1 A. General Statement 1 B. History and Personnel of the Project 3 C. Acknowledgements 4 II-. CRYSTAL CHEMISTRY OF MICAS 7 III. X-RAY TECHNIQUES FOR IDENTIFICATION OF POLYMORPHS 10 IV NOMENCLATURE 11 Vc CLASSIFICATION 26 A. History of Mica Studies 26 B. Chemical-Structural Systematization of the Natural Micas 34 PART II. MUSCOVITE I. MINERALOGY OF NORMAL MUSCOVITE 44 A. Chemistry 44 B. Muscovite Structure 45 1. General and Polymorphism 45 2. Morphology 50 3. Interferometric Data 55 C. Optical Properties 57 1. Indices, 2V 57 a. Variation of optical properties with composition 57 b. Changes of optical properties with temperature 58 2. Variation of Color with Other Optical Properties and Composition 59 D. Zoning 61 II. MINERALOGY OF VARIETAL MUSCOVITES 66 A. Oellacherite and Other Reported Barium-Containing Micas 66 B. Ferrian and Manganian Muscovite 69 C,. Chromian Muscovite 71 iii

TABLE OF CONTENTS (cont.) Page D. Phengite 74 lo Introduction 74 2. Alurgite 76 3. Mariposite 78 4. Phengite 79 5. Muscovite, Sultan Basin, Washington 80 6. Other Reported Uniaxial or Near-Uniaxial Kuscovites 80 7o Discussion 82 E. Lithian Muscovite 85 III o Occurrence 91 A. Granites and Related Rocks 91 B. Pegmatites 91 1. General 91 2. Deposits 92 a. Disseminated books in unzoned pegmatites 92 b. Wall-zone deposits 92 c. Intermediate-zone deposits 93 d. Central-unit deposits 93 e. Fracture-controlled veins and replacement deposits 94 fo Combined deposits 94 35 Exomorphic Mica 95 Co Hydrothermal Deposits 95 D. Metamorphic Rocks 95 E. Sedimentary Rocks 96 Fo Variation of Composition with Occurrence 96 IV. Pegmatitic Muscovite 98 A. Structural Defects 98 o1 Muscovite-Biotite overgrowths and Intergrowths 99 B. Inclusions 104 1. Magnetite and Hematite 104 2. Other Minerals 104 3. Other Materials 104 C. Variations in Individual Pegmatites 104 1o Concentrations 104 2. Structural Defects and Color 105 3. Composition 106 D. Variations in Pegmatite Districts 118 1. Occurrence and Color 118 2. Composition 118 E. Commercial Quality Factors 129 iv

TABLE OF CONTENTS (cont.) Page PART III. LEPIDOLITE I. MINERALOGY 153 A. Composition 130 B. Structure 131 1. Polymorphism 131 2. Single-Crystal Variations 133 3. Uniaxial Lepidolite 135 4. Powder X-Ray Studies 138 5. Variation of Structure with Composition 139 6. Discussion 142 7. Summary 146 C. A New 3-Layer Monoclinic Lepidolite Polymorph 147 D. Optical Properties 150 1. Indices, 2V, and Variation with Composition 150 2. Variation of Color with Composition — 154 II. OCCURRENCE 160 A. Non-Pegmatite 160 B. Pegmatites 160 1. Localization 160 2. Macrostructural Features 161 a. General 161 b. Overgrowths 161 3. Compositional Variation in Individual Pegmatites and Districts 164 PART IV. PHLOGOPITE I. MINERALOGY 169 A. Composition 169 1. General 169 2. Mahadevite 169 B. Structure 170 1. Polymorphism 170 2. Relation of Structure to Composition 172 C. Optical Properties 172 D. Inclusions 178 E. Zoning 178 II. OCCURRENCE 182 A. Non-Pegmatitic 182 B. Pegmatitic and Vein Occurrences 182 III. MANGANOPHYLLITE 183 A. Introduction 183 B. Structure 183 C. Composition 183 D. Optics 184

TABLE OF CONTENTS (cont.) Page Eo Single Crystal Variation 184 F. Occurrence 185 Go Conclusions 185 PART V. BIOTITE I. MINERALOGY 187 Ao Composition 187 B. Structure 187 1. Introduction 187 2. X-Ray Technique 188 3, New Structural Data 189 4. Variation of Structure in Pegmatite Districts 189 C. Optical Properties 191 1o Indices, 2V, Optic Plane; Variation with Composition 191 2. Color; Variation with Composition 191 3. Zoning 192 II. OCCURRENCE 194 Ao Igneous Rocks 194 B. Metamorphic Rocks 194 C. Variation of Composition with Occurrence 194 III PEGMATITIC BIOTITE 198 A. Localization 198 B. Macrostructural Features 199 PART VI. ZINIWALDITE I. MINERALOGY 200 A, Composition 200 B.o tructure 201 Co Optical Properties 204 Do Zoning and Overgrowths 205 II. OCCURRENCE 208 PART VII. MISCELLANEOUS MICAS I. PARAGQNITE 211 II. ROSCOELITE 212 IIIo TAENIOLITE 216 vi

TABLE OF CONTENTS (cont.) Page PART VIII. PARAGENESIS OF PEGMATITIC MICAS I, SEQUENCE OF CRYSTALLIZATION OF MICAS 217 Ao Zonal Growths 217 B. Overgrowths between Micas 219 C. Lpcalization in Pegmatites 219 II. CHEMICAL EVOLUTION 225 REFERENCES 229 vii

LIST OF TABLES Table Page I. New Spectrochemical Data on 168 Muscovites.....o................. 45 II. Peacock and Ferguson (1943) Measurements on Muscovite o......... 53 III. Variation of Color with Composition in Muscovite (New Data)...... 60 IV. Optical Data on Selected Zoned Muscovites.................o....o 63 Vo Spectrochemical Analysis of Mangan-Muscovite (Eskola, 1914)...... 71 VI. Phengites with the Two-Layer Structure, Analyzed by Jakob (1925B, 1929B, 1929C).............o........o............... * 79 VII. Occurrences of Uniaxial or Nearly Uniaxial Phengites and Muscovites o.o...eooooo.......................................oooe...o... 84 VIII. Approximate Observed Intensities of Some (Ok,) Reflections of Normal Muscovite and Lithian Muscovite,....o.o.............. 88 IX. Spectrochemical Analyses of Muscovite Sheets from Opposite Ends of a Single Thick Book. (Tilley Pegmatite, FranklinSylva District, N. C.).........................o........... 106 Xo Spectrochemical Analyses of Different Muscovites from the Same Zone Within the Big Ridge Pegmatite (Franklin-Sylva District, No C.)OO.oe eo....ooo.......oo...........oooo............ 107 XI. Spectrochemical Analyses of Different Muscovites from the Same Zone within the Poll Miller Pegmatite (Franklin-Sylva District, N. C.)................... o. o o oo a..a..... 108 XIIo Spectrochemical Analyses of Different Muscovites from the Same Zone within the Tilley Pegmatite (Franklin-Sylva District, No C. )O o o * 9 0 0 0 O a O.. a o. o..... o o o a...o a.. o 0 0 0 a *.o * o.*. 109 XIII. Spectrochemical Analyses of Different Muscovites from the Same Zone within the Ruby, tNorton, Sheep Mountain and Doc Nicols Pegmatites, 7( Franklin-Sylva District, N. C.)*.......... 110 XIV. Spectrochemical Analyses of Different Muscovites from the Same Zone within the Mitchell Creek and Johnson Pegmatites (Thomaston-Barnesville District, Georgia).....a................ 111 Vili

LIST OF TABLES (cont.) Table Page XV. Spectrochemical Analyses of Different Muscovites from the Same Zone within the School Section Pegmatite (Eight Mile Park, Colorado)......................................... 112 XVI. Spectrochemical Analyses of Different Muscovites from the Same Zone within the Meyers Quarry Pegmatite (Eight Mile Park, Colorado)..........................o............. 113 XVII. Spectrochemical Analyses of Different Muscovites from the Same Zone within the Globe Pegmatite (Petaca District, New Mexico).................................................. l114 XVIII. Spectrochemical Analyses of Muscovites from the Gregory Pegmatite (Franklin-Sylva District, N. C.)....................... 116 XIX. Spectrochemical Analyses of Muscovites from the KXier Pegmatite (Franklin-Sylva District, N. C.).0......................... 117 XX. Minor and Trace Elements in Muscovites from Pegmatites (Franklin-Sylva District, North Carolina and North Georgia)...........................................o.............. 120 XXI. Minor and Trace Elements in Muscovites from Pegmatites (Spruce Pine District, North Carolina)....................... 121 XXII. Minor and Trace Elements in Muscovites from Pegmatites (Shelby-Hickory District, North Carolina)...,,.................... 122 XXIII. Minor and Trace Elements in Muscovites from Pegmatites (Thomaston-Barnesville District, Georgia)..0..................... 123 XXTV. Minor and Trace Elements in Muscovites from Pegmatites (Alabama District )........................... 124 XXV. Minor and Trace Elements in Muscovites from Pegmatites (Latah County District, Idaho).................0.................. 125 XXVI. Minor and Trace Elements in Muscovites from Pegmatites (Petaca District, New Mexico)...................................... 126 XXVII. Minor and Trace Elements in Muscovites from Pegmatites (Eight Mile Park District, Colorado)............................. 127 XXVIII. Minor and Trace Elements in Muscovites from Pegmatites (Southwestern Montana),**.................. **...*.... 128 ix

LIST OF TABLES (cont.) Table Page XXIXo New Spectrochemical Data on 26 Lepidolites o...................... 132 XXXo Structure of Micas Analyzed by Stevens (1938)..... o...e....... 134 XXXI. Extent of Polymorphic Variation in One Lepidolite Book from Opportunity Pegmatite, Gunnison County, Colorado.....o.......o... 136 XXXIIo Spacings of Polymorphic Forms,.o.,o.....o ooo....oo........ 141 XXXIII. Structure of Micas Described by Berggren (1940, 1941) and Lundblad (1942 ) o..................e......oo.. oeo o o o.. o e o o 14 XXXIV. Spectrochemical and X-Ray Data on Lepidolite from Skuleboda, Sweden (No. 476)...oo o.....,...o o o o o.. o............... 151 XXXV. Indices of Refraction and 2V of Lepidolites and Lithian Muscovite O o..o o o.o o... o o oo o o...o o o..o oo * 153.... 155 XXXVI. Relationship between Color and Composition in Lepidolites, New Analyses.........o. o.e e *. * o *,...o. 156 XXXVIIo Relationship between Color and Composition in Lepidolites, Reanalyses of Lepidolites Reported in the Literature.1........ 158 XXXVIIIo Relationship between Color and Composition in Lepidolites; Specimens in Our Laboratory for which Analyses are Recorded in the Literature o oo.,.ooo.o...,ooo,,oooooooooooo ooooooooooo Oo 159 XXXIXo Analyses of Lepidolites from Brown Derby Pegmatite, Gunnison Coulnty, COlOrad1o~..o o..<. e.... oebeoooeoooo oo oeooooe o.o.~1 65 XL. Spectrochemical Analyses of Micas from Newry, Maine......,....... 166 XLIo Reanalyses of Two Lepidolites from Varutrask, Sweden Analyzed by Berggren o.o e o...o.eo.o o O O O O O O O a O O O O. O O o....... O........ 167 XLII. Spectrochemical Analyses of Lepidolites from Two Other Pegmatites in Brown Derby Districto8ooo..., oeoooo.. e.............. 168 XLIIIo 2- and 3-Layer Phlogopiteso....o.o.......o............ 171 XLIV. Structure of Analyzed Biotites, Phlogopites and ManganophylliteSoooooo. o. o o o o a a a o a a 173 XLV. Spectrochemical Analysis of Light-Green Phlogopite from Snake Creek, Utaho e,.. o e e o.. d o.o. o o o o o o oo o o Q o o o o o o o o o o o o. * O 179 XLVIo Spectrochemical Analysis of ZonedPhlogopite, Perth, Ontario. 179 XLVII. Occurrence of Manganophyllitee.........,.......... 186 XLVIII Structure of Biotite Varieties oo o o o o o o o,.. o o,.. o o o........ 190 x

LIST OF TABLES (concl.) Table Page XLIX. Chemical Relati6n of Zinnwaldite to Lepidolite and Biotite..... 200 L* New Structural Studies of Zinnwaldite............................ 202 LI.- Approximate Observed Intensities of Some (Ok;) Reflections of 2-Layer Forms....................... 204 LII~ Optical Properties of Zinnwaldite (from the Literature).......... 206 LIII. Occurences of Zinnwaldite....0.............................0 *.. 209 LIV. X-Ray Powder Data for Roscoelite and Barium Muscovite........... 214 LV.6 Spectrochemical Analyses of Muscovites from Jasper Pegmatite, (Franklin-Sylva District, N. C.)............................... 9* 218 LVI. Comparison of Average Composition of Muscovites from FranklinSylva, N. C., and Petaca, N. Mex., Districts.................. 222 LVII. Two Muscovites Analyzed by Liashchenko (1940)................... 223 LVIII. Micas from the Schuttenhofen Pegmatite Analyzed by Scharizer (1887B)............................ 223 LIX. Two Micas from Sakihama, Japan, Analyzed by Shibata (1952A)...... 224 LX. Average Contents of Various Oxides in Micas..................... 226 LXI. Chemical Evolution of Muscovite.................................. 227 xi

LIST OF FIGURES Page lo Muscovite structure after Grim and Bradley (1951)...o... s*. & 8 2. Muscovite showing some diffuse scattering along 02 reciprocal lattice line. 0-level a-axis. Compare with Figure 5.......,. 48 3. Stereographic projection of established forms for muscovite after Peacock and Ferguson (1943)e.........o...o............. 54 4. Crystal drawing of muscovite forms found by Peacock and Ferguson (1943)...........o. ooo..o.oo..0o0,0ooo,... 54 5. O-level a-axis Weissenberg photograph of normal muscovite... o.,< 86 6. 0-level a-axis Weissenberg photograph of lithian muscovite,.....o 87 7. X-ray powder photographs of polymorphs in muscovite-lepidolite series............o..eoooooo..oooooooooo.....................0. 140 8. Plot of the Li20 content of 35 micas in muscovite-lepidolite series against the various polymorphs Oo o oo oo....o.....o. 8,.A 144 9. Idealized representation of relationship between Li20 content and polymorphism in the muscovite-lepidolite series.o........, 144 100 O-level pseudo a-axis Weissenberg photograph obtained from 3layer monoclinic lepidolite............................. 148 11o O-level b-axis Weissenberg photograph obtained from 3-layer monoclinic lepidolite............. oo o o oo o o............o o o o o.o o o 149 12. 1-level a-axis Weissenberg photograph obtained from 3-layer monoclinic lepidolite.....o o o o o e...... o o o o o.............o o o o o..............o e oo 149 13. O-level a-axis Weissenberg photograph of zinnwaldite with 2-layer monoclinic structure,......e..ee..., oeo o o o..,... oo 203 x 1i

ABSTRACT The micas, a group of sheet-structure alumino-silicates, are composed mainly of K,Mg,Fe2,Fe,Mn and Li with OH and F but also contain varying small amounts of Na,Ba,Rb,V,Cr, and Ti as well as minute amounts of a large variety of trace elements. The main species are muscovite, lepidolite, phlogopite, biotite, zinnwaldite, paragonite, roscoelite, and taeniolite. Most micas are monoclinic; some are hexagonal; a few may be triclinic. In muscovite the 2-layer form is essentially invariant. Lepidolites crystallize in 1-layer, 3-layer and 6-layer forms which can be correlated with the Li content. Phlogopites, biotites, and zinnwaldites also show considerable polymorphic variation but correlations with composition or occurrence are not yet possible. The micas of commercial significance are muscovite and phlogopite. Economically significant deposits of sheet-size, high-quality muscovite are confined to pegmatites, principally in wall zones and core-margin units. Muscovites from different zones in individual pegmatites are characterized by distinct differences in color, structural defects, and composition. Similar variations occur among the micas of different districts. The peratitic micas show systematic differences in composition, mainly in Fe3,Fe,Mg,Li, and F, which reflect the stage of Differentiation of the pegmatite magma at the time each type crystallized. xiii

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN PART I. GENERAL INFORMATION I. INTRODUCTION A. General Statement The micas constitute a closely related group of minerals with both common and rare representatives. This report gives the results of research on this group of minerals undertaken mainly as Project M978, Natural Mica Studies, of the University of Michigan Engineering Research Institute, sporsored by the United States Army Signal Corps. The purpose of the project was "To conduct investigations on physical properties, crystal structure, and composition of natural micas as related to geological occurrences and genesis." Thus we have attempted to learn as much as possible concerning the natural history of micas. A study of the mica group is of significance for two reasons: (1) Certain members of the group are widely distributed rock-forming minerals and therefore a study of these species leads not only to information regarding the conditions of their own formation, but also helps in establishing the environmental conditions under which the enclosing rocks were developed. (2) Several of the micas are of crucial and strategic significance, either as raw materials themselves or as a source for rare metals. We have attempted to concentrate our work mainly on those micas that are of some economic significance; these include muscovite, phlogopite, and lepidolite. But because of the difficulty of defining the limits of the various species, and also because of the structural and chemical relationships between the economically significant micas and others of lesser or no commercial importance, some of our work has necessarily included micas of no direct economic value. Members of the mica group are widespread in igneous rocks and mineral deposits of magmatic derivation; for example, muscovite in certain granites and particularly in granitic pegmatites; phlogopite in certain peridotites; biotite in gabbros, norites, diorites, and granites as well in many granitic pegmatites and other types of pegmatites; and lepidolite and 1

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN zinnwaldite, mainly in certain rare pegmatites and some high-temperature veins. Muscovite and its fine-grained variety, sericite, are widespread essential constituents and accessory minerals of hydrothermal veins and replacement deposits. Among the metamorphic rocks, biotite and/or muscovite are common and widely distributed in many types of slates, phyllites, schists, and gneisses, and phlogopite is a common constituent of some marbles. In sedimentary rocks, fine-grained detrital muscovite may be an important mineral in clastic rocks and authigenic muscovite may also become a significant constituent in some fine-grained elastics. We have not attempted to study the micaceous minerals that lie between the true micas and the clay-minerals group, namely, members of the illite group of minerals of which the most widespread representative is hydromuscovite or hydromica. The micas are also of scientific significance in that they are the most outstanding representatives of a type of silicate structure, namely, the sheet structure or disilicate type, in which the silica tetrahedra all are placed in one plane with each tetrahedron being joined to others by the three oxygen atoms that lie within the common plane. A continuous extension of such a linkage provides a hexagonal network within the plane. Other mineral groups that are characterized by this basic sheet-type structure include the chlorites, brittle micas, and vermiculiteso By studying representatives of the mica group it is possible to obtain more information on the fundamental properties of silicates showing this type of linkage. The micas are of scientific significance also because of their highly diversified composition, being hydrous or fluosilicates of alkali metals, principally potassium, and of divalent as well as trivalent metals. Conse — quently, a great many isomorphous substitutions are possible within the group. From the economic standpoint, muscovite and phlogopite represent substances that require an essentially unique type of mining and recovery, for they are desirable riot because of their composition, but mainly because of their unusual combination of physical properties and because of the size of-.the crystals in which they occur, Consequently, the mining of muscovite and phlogopite demands that the crystals as they occur in the deposit be removed without damage or without diminution of grain size. In this respect the micas are similar to quartz crystals in the requirements of their mining. The unique combination of properties possessed by muscovite and phlogopite which makes them economically desirable is: (1) their perfect basal cleavage, (2) their flexibility and elasticity, (5) their extremely low electrical conductivity and high dielectric property, (4) their very low heat conductivity, and (5) their occurrence in large, relatively perfect, single crystals. Lepidolite, the other mica of commercial value, is not mined because of its properties as a mica, but because it contains the element lithium which is extracted from it. Thus it is unimportant whether it occurs in large single crystals or relatively free of mineralogical impurities. 2

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN The importance of the mica studies is therefore two-fold; scientific, leading to information on mineral and rock genesis, silicate structures, and isomorphism; and economic, leading to data on occurrences, recovery, and effective utilization. B, History and Personnel of Project The investigations of Heinrich on micas began in 1941 at the suggestion of the late Professor Harry Berman of the Department of Mineralogy and Petrography of Harvard University. It was originally intended to attempt a study of the micas for a doctoral dissertation, but due to the death of Professor Berman and various other factors, this was abandoned. However, this preliminary work led directly to the compilation of a relatively complete collection of analyses of micas and a bibliography of articles relating to all aspects of mica research. From this work, one paper (Heinrich, 1946) resulted. During the period 1942 to 1947 Heinrich, as a member of the United States Geological Survey, was assigned during much of this time to the investigation and mapping of pegmatite mineral deposits, particularly mica pegmatites, and participated in such investigations in the southeastern mica districts of North Carolina, Georgia and Alabama as well as those of Idaho~ Colorado and New Mexico. Subsequently, he has had opportunity to study mica deposits in New England, Montana and other western states, in Canada, and in various European countries, particularly those of Scandinavia and Germany. Most of the mica specimens which have been studied as part of this investigation have been personally collected by Heinrich and were carefully labeled and studied as to their detailed geological occurrence within their deposits. One group of Colorado specimens was collected by Levinson and the remaining studied specimens are analyzed micas which have been obtained from numerous investigators both within the United States and abroad. In all cases, however, we have attempted to confine our studies to specimens whose detailed paragenesis either was known to us from our own field work or whose geological occurrence was a matter of record. General mica investigations were resumed in 1950 by Heinrich and Levinson in an attempt to learn more regarding the variation in optical and physical properties of muscovites within individual pegmatites and individual pegmatite districts. Attention was soon directed toward the widespread polymorphism existing in the mica group, and it became clear that in order to learn more regarding the genesis of pegmatitic muscovites, basic information on structural and compositional variations was required. In September, 1951, a large part of the investigation was transferred to Project M978 of the Engineering: Research Institute of the University of Michigan sponsored and supported by the U. S. Army Signal Corps, without whose assistance the wide scope of our work would have been impossible. 5

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN At the Mineralogical Laboratory of the University of Michigan, many techniques were utilized in the quest for new data on natural micas. Foremost of these were X-ray techniques, by means of which the structures of over 1000 specimens of mica were identified. The refractive indices of about 500 specimens, particularly muscovites, were determined by means of the Abbe" refractometer and immersion liquids, and 2V was measured by means of the Fuess axial-angle apparatus and the Mallard method. Extensive surveys of the literature were undertaken in connection with such problems as the study of overgrowths, zoning, paragenesis, and trace elements, in an attempt to correlate and interpret the older data on these subjects with information secured from our measurements and from the large number of new chemical analyses. A number of apparently significant Russian papers were translated in their entirety. The project during the two years of its sponsorship by the U. S. Army Signal Corps has been under the administrative supervision of Dr. S. Benedict ILevin, Deputy Chief, Chemical-Physics Branch, Squier Signal Laboratory. The project as a whole has been guided and supervised by Heinrich. Levinson has personally carried out nearly all of the X-ray work. The other two writers, Levandowski and Hewitt, have been employed as general assistants in varied aspects of the research work, but have also contributed specifically to the writing of certain sections of this report. C. Acknowledgments We are extremely grateful for the assistance rendered by many indi — viduals and organizations. Technical assistance and advice were given by Dr. S. B. Levin, Dr. H. Kedesdy (Squier Signal Laboratory); Mr. C. E. Harvey (Amer. Spectrographic Laboratories); Dr. R. A. Hatch, Dr. J. A. Kohn (U. S. Bur. of Mines); Prof. L. S, Ramsdell, Prof. R. M. Denning, Prof. C. B. Slawson, Prof. G. B. B. M. Sutherland (Univ. of Michigan); Dr. H. S. Yoder (Geophysical Lab.); Dr. W. T. Schaller, Dr. M. Fleischer (U. S. Geological Survey); and Prof. S. Goldich (Univ. of Minnesota). Through the courtesy of the following investigators we have been fortunate in obtaining samples of about 200 different micas, of which approximately 150 have been analyzed and described in the literature: (1) Dr. J. M. Axelrod (U.S.G.S., Washington) (2) Dr. F. A. Bannister (Brit. Mus. Nat. History, London) (3) Prof. L. G. Berry (Queens Univ., Kingston, Ontario) (4) Mr. J. E. Bever (Univ. of Michigan) (5) Dr. Harald Bjorlykke (Norwegian Geol. Survey) (6) Dr. H. V. Ellsworth (Geol. Survey of Canada, Ottawa) (7) Mr. R. C. Erd (U.S.G.S., Washington) 4

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN (8) Prof. H. W. Fairbairn (Mass. Inst. Techn.) (9) Prof. R. R. Franco (Univ. of Sao Paulo, Brazil) (10) Prof. C. Frondel (Harvard Univ.) (II) Prof. E. N. Goddard (Univ. of Michigan) (12) Dr. C. H. Greene (Corning Glass Works, Corning, N.Y.) (13) Mr. W. R. Griffitts (U.S.G.S., Washington) (14) Dr. E. Grip (Boliden, Sweden) (15) Dr. A. F. Hallimond (London, England) (16) Dr. S. B. Hendricks (U. S. Dept. of Agriculture, Washington) (17) Dr. P. Hidnert (Nat. Bur. of Standards, Washington) (18) Prof. W. F. Hunt (Univ. of Michigan) (19) Prof. C. 0. Hutton (Stanford Univ., California) (20) Mr. R. Jacobson (Geol. Surv. of Nigeria, Kaduna Junction) (21) Prof. J. Jakob (Tech. Univ. of Zurich, Switzerland) (22) Dr. Y. Kawano (Geol. Surv. of Japan) (23) Dr. A. Knopf (Yale University, New Haven, Conn.) (24) Prof. L. Lokka (Helsinki, Finland) (25) Mr. A. M. Macgregor (Geol. Survey of Sourthern Rhodesia) (26) Prof. C. Mauguin (Univ. of Paris, France) (27) Dr. V. B. Meen (Royal Ontario Museum, Toronto) (28) Dr. and Mrs. A. Miyashiro (Geol. Surv. of Japan, Tokyo) (29) Prof. J. Murdoch (Univ. of California, Los Angeles) (30) Prof. T. Noda (Nagoya Univ., Japan)(31) Prof. E. W. Nuffield (Univ. of Toronto, Canada) (32) Dr. O. H. Odman (Royal Inst. of Technol., Stockholm) (33) Prof. I. Oftedal (Univ. of Oslo, Norway) (34) Prof. G. P. Pagliani (Univ. of Milan, Italy) (35) Prof. R. L. Parker (Techn. Univ. of Zurich, Switzerland) (36) Prof. G. Pehrman (Abo Academy, Abo, Finland) (37) Prof. R. Pieruccini (Univ. of Florence, Italy) (38) Dr. F. H. Pough (Amer. Mus. Nat. Hist., New York) (39) Prof. R. T. Prider (Univ. of Western Australia, Perth) (40) Prof. Percy Quensel (Univ. of Stockholm) (41) Mr. A. H. Roberson (U. S. Bur. of Mines, Albany, Oregon) (42) Dr. H. P. Rowledge (Govt. Chem. Laboratories, Perth, West. Australia) (43) Dr. A. Schiener (Mus. Nat. Hist., Vienna, Austria). (44) Dr. F. T. Seeyle (Dominion Laboratories, Wellington, New Zealand). (45) Dr. K. Sugiura (Toyko Inst. of Technology, Japan). (46) Dr. G. Switzer (Smithsonian Institution, Washington) (47) Prof. S. Tyler (Univ. of Wisconsin, Madison, Wis.j (48) Dr. S. van Biljon (Heidelberg, Transvaal, South Africa) (49) Dr. F. Wickman (Royal Mus. of Stockholm, Sweden) (50) Prof. H. Winchell (Yale Univ., New Haven, Conn.) (51) Dr. H. Yamada (Tokyo Inst. of Tech., Japan).. ---------. --------------------— 5

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Mrs. E. G. Smith provided Russian translations, and Miss M. J. Abels and J. I. Smith, French translations. S. J. Lefond, A. A. Giardini, -and A. Geyer assisted in the laboratory, while Mrs. S. Roof and Miss J. L. Everett furnished secretarial aid. The Faculty Research Fund of the Horace H. Rackham School of Graduate Studies, University of Michigan, furnished the funds for five new chemical analyses. 6

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN II. CRYSTAL CHEMISTRY OF MICAS Mauguin (1927, 1928A, 1928B) was the first to apply X-ray analysis to the study of micaso He determined their unit cells and symmetry and discussed the types of isomorphous replacement. Knowing the cell dimensions, the density and composition of micas, and the weight of each atom, he calculated the average number of atoms of each kind in the unit cell rOne of the main points noted was that the total number of oxygen and fluorine atoms in the structural unit is always 12. It is upon this value that modern formulas of the micas are based, Pauling (1930) advanced the general concept of the structure of mica and other layered silicates by proposing on the basis of Alis coordination theory, an outline of the general structural scheme for these mineralso Paulingls (1930) work gives, in particular, the sequence of atomic planes parallel to the cleavage plane. The first complete analysis of a mica structure on this basis was made independently by Jackson and West (1930, 1933)o Their X-ray work on muscovite showed conclusively that the ideas proposed by Pauling (1930) were correct. The basis of the mica structure, which has been described in detail by Bragg (1937), Hendricks and Jefferson (1939), and others, is the silica tetrahedron (Si04) arranged in sheets to form a hexagonal network (Fig. l). Silicon has a coordination number of four, In the tetrahedron, aluminum may replace one-fourth of the silicon. Inasmuch as the silicon-oxygen sheet, by sharing oxygen atoms of adjacent tetrahedra, has the composition (Si4010); the substitution referred to may result in the silicon-oxygen. sheet having the formula (A1Si3010)o Two of these sheets are placed together with the vertices of the tetrahedra pointing toward each other. In muscovite these sheets are held together by aluminum atoms which have octahedral coordination with respect to the neighboring hydroxyl groups and oxygen atoms. The hydroxyl groups are situated at the centers of the plane hexagons formed by the oxygen atoms of the silica tetrahedra, The octrahedral groups may also be built around magnesium as in phlogopite, lithium as in lepidolite, and so forth. This constitutes one firmly bound double-sheet. The structure of the micas is completed by joining the above-mentioned double-sheets with potassium atoms which have twelve-fold coordination with respect to oxygen atoms. The perfect cleavage of the micas lies between these double-sheetso With the possible exception of titanium, the position occupied by the many elements reported in all good analyses is no longer i.n doubt, Berman (1937) indicates the general formula for the mica group as~ ----------------------- 7 —----------

-— __+215)l' 1 I A1:K......-4(.;(i -3Si4-, ]AI3+ 7 -3Si,6 02— 6 ~ " L _ —4. O0.201 1 Fig. 1.Muscovite -strutr — 3S-, r1 A13+ Fig. 1. Muscovie s —-t r 2K Fig. 1. Muscovite structure after Grim and Bradley (1951).

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN W(X,Y)2_3z40lo( (OHF)2 in which W = K predominantly; Na, Ba, Ca, subordinate to rare; X = Mg, Fe2, Mn2 Li; Y = Al, Fe3, Ti subordinate, Cr, Mn3, V rarely; Z = Si-Al 5:3 to 7:1 Atoms represented by W have twelve-fold coordination, those represented by X and Y have octahedral coordination, and those represented by Z have tetrahedral coordination. Mauguin (1927, 1928A, 1928B) noted in his original X-ray studies that the c-axis of biotite was apparently only half as long as that of muscovite and he was thus the first to encounter polymorphism in the micas. It remained for Hendricks and Jefferson (1939) to give a full account of the scope and magnitude of polymorphism in this complex group. In all, among 100 different specimens, they discovered seven polymorphic variations which embrace three different crystal systems. They described the seven different structures (hexagonal, monoclinic, and triclinic) in detail and showed how they might be derived from the single-layer form by the application of various combinations of symmetry operations. Hendricks and Jefferson (1939) pointed out that muscovite crystallized in only the two-layer muscovite structure, but Axelrod and Grimaldi (1949) have reported a three-layer monoclinic muscovite (see page 47). Levinson (1953), as a result of work carried out on this project, has described a new variation of the muscovite structure for which the term "lithian muscovite" has been proposed. These are the only X-ray studies on micas since the classic contribution of Hendricks and Jefferson (1939). 9

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN III. X-RAY TECHNIQUES FOR IDENTIFICATION OF POLYMORPHS It has been necessary to do a considerable amount of X-ray work, consisting mainly of identifying the structures of as many micas as possible. Whenever possible, Weissenberg photographs were taken about the O-level a-axis, as this type of photograph is unique for the different polymorphs. In many cases, however, upper level photographs were taken as further verification of the structural interpretation. In determining the orientation of the flakes used in the single crystal work, an optical procedure was used whenever practicable. Lines corresponding to the desired crystallographic axes were scratched upon the cleavage flakes with a razor. The exact positions of these axes were determined from interference figures and from extinction positions in those micas which have observable birefringence on cleavage planes. For all practical purposes, the crystallographic axes may be assumed to correspond with the extinction postion. However, many micas, particularly biotites and phlogopites, are uniaxial or have small 2V. In this case the orientation had to be determined from Laue photographs or by means of percussion figures. In several instances crystal development was poor, crystals were minute, or extinctions were poor. This made single-crystal methods difficult if not impossibleand left powder X-ray methods as the only study technique. Unfiltered copper radiation was used in most single-crystal work and a nickel filter (with copper radiation) was used in powder studies. Iron radiation was used whenever powder photographs of biotites were taken. Only a large diameter (114.6 mm) powder camera is suitable for the mica studies. Polymorphs of muscovite and lepidolite can be distinguished by means of X-ray powder photographs but those of biotite and phlogopite cannot be differentiated by this method. 10

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN IV. NOMENCLATURE In every complex mineral group there exists a large number of varietal names based on major or minor chemical variations, optical distinctions, grain size, occurrences, localities, and misidentification. The mica group is no exception for it is burdened with a surplus of trivial names. The list that follows defines the varietal names that have been employedin the series. Most of these are listed in Hey (1950), Chemical Index of Minerals, but the following references also were consulted in constructing the complete synonymy. 1. Dana (1892), A System of Mineralogy. 2. Chester (1896), A Dictionary of the Names of Minerals. 3. Winchell and Winchell (1951), Elements of Optical Mineralogy, Part II, Description of Minerals. The following scheme is employed to differentiate the various categories: Synonyms and varieties: lower case (e.g., adamsite) End-members: first letter capitalized (e.g., Ferrimuscovite) Pseudomorphs: capital delta preceding name (e.g,, Aachlusite Doubtful species: underlined (e.g., anhydromuscovite) Valid species, chem. varieties: capitals (e.g., MUSCOVITE) Terms which are useful or sufficiently well defined, in the opinion of the Writerp, and should therefore be retained: asterisk preceding name (e.g., *MUSCOVITE). Aachlusite: a green alteration product of topaz, resembling steatite, but near soda mica in composition; pseudomorphous after topaz. adamsite: syn. of muscovite. agalmatolite: syn. of pinite, pyrophyllite or talc. alurgite: var. of muscovite near K2(Mg,Al)4_5(Al,Si)8020(OH)4J also contains about 1% Mn oxides. 11

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN ammochrysos: syn. of muscovite. amphilogite: syn. of didymite. anhydrobiotite: artificially dehydrated biotite, nature unknown. anhydromuscovite artificially dehydrated muscovite, nature uncertain. annite: var, of lepidomelane; syn, of hydroxyl-anniteo anomite: varo of biotite with optic plane normal to b-axis (see meroxene). anthrophyllite: "A mica (?)" Hey (1950). Aaspasiolite: aluminosilicate of Mg, an altered cordierite. aspidolite: near (NaK)2(Mg,Fe+2) l2Si 00(OH)4 with Na:K 53 and Mg:Fe2e 5; near phlogopite. avalite: near K(Al,Cr)4(Al,Si)8020(0H);4 probably a mixture (Chester, 1896). baddeckite: a mixture of hematite and clay, originally considered to be iron mica related to muscovite (Schaller and Henderson, 1926)o barium muscovite: syno of oellacherite. barium-phlogopite: var. of phlogopite with 1% BaO, barytbiotite: var. of phlogopite; 2 (KBa)2(Mg,Al)4_6(A, Si)8020(0H)4] containing 6.84% BaO and also some Fe. Analysis in Dana (192) shows it to be a phlogopite rather than biotite (Hey, 1950, p. 160). basonite: near a hydrobiotite or a vermiculite, near K1/2(Mg,Fe3,Fe2Al) (SiAl)802 (OH)41-1/2H20 with Mg and Fe o2 and Sie 5-l/2. bauerite: SiO2 pseudomorphs after mica; the nature of the material is unknown. (Hey, 1950) biaxial mica:~ syno of muscovite. bildstein: syn. of agalmatolite. *BIOTITE: K2(Fe2,Mg) (Fe,Al,Ti)O (Si6 Al )0 (OHF)4 2 6-4 0-2 6-5 2-3 20-22 4-2. Abonsdorffite: aluminosilicate of Fe2 and Mg, an altered cordierite, pinite group. 12

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN brammallite: var. of illite, aluminosilicate of Na and K; syn.is sodium illite. buldymite: aluminosilicate of Mg,Fe2,Fe3, and K; probably a hydrobiotite. caesium-biotite: var. of biotite with as much as 3% Cs20 and 1-1/2% Rb20 (Hess and Fahay, 1932). calciobiotite: var. of biotite containing as much as 14% CaO. Acataspilite: aluminosilicate of Mg,Ca,Na, and K, pseudomorphous after cordierite. cat gold: syn. of muscovite; an early popular name for gold-colored mica. catlinite: a mixture of sericite, pyrophyllite, etc. cat silver: syn.of muscovite. chrome mica: (1) syn. of fuchsite (!Daha, 1892, p. 614); (2) syn. of fuchsite and mariposite (Whitmore et al., 1946). chromglimmer: syn. of chrome-mica and fuchsite. chromglimmer (of Schafhautl): in part var. of biotite, near K2(Mg,Fe,Cr,Al)4(AlSi)8020(OH)4; the rest fuchsite (Hey 1950). *CHROMIAN MUSCOVlTE: name propose'd for fuchsite since fuchsite has the typical muscovite structure (Whitmore, et al., 1946). CHROMIAN PHENGITE: name proposed for mariposite as it has a high silica content and a small (less than 1%) Cr203 content. (Whitmore, et al., 1946), chromochre: appears to be similar to niscovite but has more Cr203 and has a very small optic angle; it may be a chromian phengite (Winchell and Winchell, 1951). colomite: syn. of roscoelite. common mica: syn. of muscovite. Acordierite-pinite: pinitic pseudomorph after cordierite. cryophyllite: var. of zinnwaldite often with some deficiency in (Li,A1,Fe) group; typically Li %2-1/2, (Fe2,Fe3) 1, and Si - 7. Also the name has been used for 2 hypothetical end-members; K2Li2FeA12Si7O18 (OH)4 and K2Li2FeAl2Si6016(OH)4 (Hey, 1950). cymatolite: muscovite and albite mixture, perhaps pseudomorphous after spodumene. 13

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN damourite: var. of muscovite; an altered muscovite, usually contains more water or more easily removed water (Winchell and Winchell, 1951). Folia less elastic; includes most hydro-mica in general; does not necessarily contain more water; they may, however, give it off more easily (Dana, 1922). didrimite: syn. of didymite. didymite: syn. of muscovite. Adysyntribite: syn. are dysintribite, dyssintribite, dyssyntribit; aluminosilicate of K and Na; pseudomorphous substance of pinite group. Eastonite (of Winchell): theoretical end member, K2Mg5A14Si520p(OH)4; some natural micas apparantly approach this composition closely. epileucite:' pseudomorphs of orthoclase and muscovite after leucite. epi-sericite: syn. of sericite; sericite formed in epi (upper) zone of metamorphism. euchlorite (of Shephard): var. of biotite. eukapt alinosilicat of g and K; a mica or chlorite, not well defined. euphyllite: near (Na,K)A13Si010 (OH)2; may be a mica intermediate between muscovite and paragonite, or a brittle mica, or perhaps a mixture. Afahlunite: aluminosilicate of Mg, Fe, and K near (Mg,Fe2) A12Si3012H20;O pinite group. ferribiotite: syn. of lepidomelane. Ferri-muscovite: hypothetical end-member used by Winchell (1951) (K2Fe32A12 (OH)4Si6A1202o); also by Wahl (1925)(K Fe33Si3010(OH)2). ferrititanbiotite: syn. of ferriwotanite. ferriwotanite: var. of biotite high in Fe3 and Ti (see wodanite). ferro-ferri-muscovite: syn. of monrepite. Ferrophengite: hypothetical end-member used by Winchell and Winchell (1951) (K2Fe2A1 (OH )4Si7A102) o Fluor-annite: var. of lepidomelane; the mica end-member KFe2Al1Si5O10F2. -----------------— ^ -------------------

ENGINEERING RESEARCH INSTITUTE ~ UNIVERSITY OF MICHIGAN Fluor-biotite: var. of biotite; the mica end-member K(MgFe2)AlSi3O10F2 Fluor-lepidomelane: var. of lepidomelane; end-member K(Mg,Fe2)3(Al,Fe5) Si010F2~ Fluor-meroxene: var. of biotite; the end-member KMg(Al,Fe3)Si3010F2. Fluor-phlogopite: var. of phlogopite; the fluorine end-member KMg3AlSi3010F2. Also the most abundant synthetic phlogopite. Fluor-siderophyllite: var. of biQtite; the end-member KFe2 (AlFe )Si310F 2. frauenglas: syn. of muscovite. fuchsite: var. of muscovite with as much as 4.81% Cr20O3 X-ray studies by Whitmore, Berry, and Hawley (1946) have shown that fuchsite has the muscovite structure. They propose that chromian muscovite be adopted in place of fuchstte which has no structural significance. This proposal is entirely justified. It follows, therefore, that if mariposite is also shown to have structures characteristic of phengite, the term chromian phengite or hexagonal chromian phengite would be appropriate (owing to a high silica content). gaebhardite: syn. of fuchsite. Agieseckite: aluminosilicate of Mg and K; sometimes appreciable FeO; a pinite type pseudomorph. gigantolite:- a mixture of muscovite and biotite; pinite group. gilbertite: var. of muscovite. glimmer: syn. of mica. goeschwitzite: syn. of illite. Agongylite: aluminosilicate of Mg,Fe2. and K; a pinitic pseudomorph. grundite: syn. of illite; trade name for illite clayo gumbellite: var. of hydromuscovite (Hey, 1950); a hydrous silicate of aluminum, near pyrophyllite (Chester, 1896). hallerite: a lithium bearing mica (0.5 to 1.5% Li), but it is not clear whether it should be classed as a variety of paragonite or of muscovite. 15

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN haughtonite: var. of biotite rich in Fe2 and Fe3. helvetan: aluminosilicate of Fe2,Mg,Ca and alkalis; probably an impure muscovite *heptaphyllite: group name for those micas which contain 7 other atoms to 12 (O,OHF) as distinguished from octophyllite micas which contain 8 atoms to 12 (O^OHF); originally applied to muscovite and lepidolite but now applies to muscovite only (and paragonite). The term heptaphyllite is structurally correct, and is thus considered valuable. heterophyllite: var. of biotite; a mica between siderophyllite and annite; somewhat lower in Fe2 than siderophyllite; the name seems of doubtful value o hexagonal mica: syn. of biotite. hydrobiotite: a hydrated biotite low in K, Mg, etc. and OH in place of the 0. Also classed as a vermiculite. hydromica: "Used as a term to designate a micaceous clay mineral of common occurrence resembling sericite but having weaker double refraction. Galpin (1912) indicates that kaolinite alters through metamorphism into hydromica, thence to muscovite... The group of hydrous micas, illite, hydromuscovite or glimmerton has been studied with particular care by Grim, Bray and Bradley (1937) who proposed the name illite..0. The material has less K20 and more water than muscovite," (Kerr and Hamilton, 1949 p. 33) *hydromuscovite: (1) var. of muscovite higher in OH, lower K, or K and Al than muscovite (Hey, 1950) (2) general name for various hydromicas derived from muscovite (Chester, 1896). (3) This mineral seems to lie between muscovite and illite both as regards potassium content and perfection of crystallinity (Brammall, Leach and Bannister, 1937). hydrophlogopite: a var. of phlogopite high in H20. hydrous mica: syn of hydromica. Hydroxyl-annite: var. of lepidomelane; the end-member K2Fe26(A2Si6)020(0H) 4 with little Mg (Grigoriev, 1935). Also a syn. of annite. (Winehell and Winchell, 1951) -- ------- 15 —6

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Hydroxyl-biotite: syn. of normal biotite so named to distinguish it from f luor-biotite. Hydroxyl-lepidomelane: syn. of lepidomelane, in particular the F-free end-member. Hydroxyl-meroxene: var. of biotite; the hydroxyl end-member to be distinguished from fluor-meroxene. Hydroxyl-phlogopite: syn. of normal phlogopite so named to distinguish it from fluor-phlogopite. Hydroxyl-siderophyllite: syn. of normal siderophyllite so named to distinguish between it and fluorsiderophyllite. Ahygrophyllite: aluminosilicate of Fe2,Mg,Ca and alkalis; derived in part, at least, from feldspars; pinite group. iberite: (of Svanberg, 1844) syn. of gigantolite. *illite: "It is not proposed as a specific mineral name, but as a general term for clay mineral constituents of argillaceous sediment belonging to the mica group." (Grim, Bray and Bradley, 1937, p. 816). iron mica: syn. of lepidomelane and biotite. irvingite: lepidolite near (Na,K) (LiAAl)5(Si,A1) 8(,OH,F)24 with Lil 2-1/2 and (0H,F)w3. isinglass: syn. of mica, particularly muscovite and phlogopite. ivigtite: aluminosilicate of Fe and Na(?); analysis imperfect (Hey, 1950); may be near gilbertite (Chester, 1896). kaliglimmer: syn. of muscovite. Akillinite: aluminosilicate of K; pseudomorphous after spodumene; probably identical with muscovite; pinite group. kryptotile: A13(Si04)3H3, extreme hydrogen end of muscovite series (Clarke, 1914). lardite: syn. of steatite and of agalmatolite; pinite group. *LEPIDOLITE: K2 ( Li,A1 ) 5-6 (S i 6_7,A 12_1)020 - 21(FOH )4 17

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN lepidomelane a term commonly employed for iron-rich biotite. The term has been used for biotites rich in Fe3, those rich in Fe2, and those with relatively large amounts of both Fe2 and Fe3 (Heinrich, 1946). lepidomorphite: (1) var. of phengite (Hey, 1950); (2) a fine, scaly mica; the result of the alteration of oligoclase (Chester, 1896). leucophyllite: a high silica var. of muscovite; similar to phengite; original analysis possibly on impure materialo leverrierite: may be the same as kryptotile but of different genesis (Clarke, 1914)o Aliebenerite: aluminosilicate of alkalis of Fe and Ca; a pinite group pseudomorph o lilalite: syno of lepidolite. lilalitho syn. of lepidoliteo lithia mica: syno of lepidoliteo lithioneisenglimmer: syno of zinnwalditeo lithionglimmer: syno of lepidolite. lithionit: syno of lepidolite. lithionite: syn, of lepidoliteo lithionitesilicat: syn. of lepidolite. Lithium muscovite: a hypothetical mica end-member: (1) Stevens (1938), K4Li6AlSi12040 (OH,F ) (2) Berggren (1941), K4Li6All0Si12041F50H. lithium (Li)-phengite: contains 3.02% Li20, 47.44% SiO2, 1.41% Mg0 and 1087% total Fe. These values do not permit it to be classed as a phengite. Perhaps somewhere between muscovite and lepidolite; described by Shibata (1952A and 1952B). lythrodes: an altered nepheline; pinite group. macrolepidolite: var. of lepidolite; daistinguished from microlepidolite by larger 2V, 18

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN magnesia mica: syn. of phlogopite. mahadevite: a mica supposedly between muscovite and phlogopite in composition; near (K,Na)o 97(Al,Fe,Mg) 66(Si,A A)4(OOHj; should be reanalyzed. manandonite: a basic boro-silicate of lithium and aluminum closest to lepidolite in composition LiAl114B4Si6029(OH)24 (?); should be studied further; possibly not a mica, manganese mica: syn. of manganophyllite. manganese muscovite: syn. of mangan-muscovite. mangan-muscovite: var. of muscovite with about 2% MnO. manganophyll: syn. of manganophyllite. manganophyllite (of Igelstrom, 1872): aa Mn- var. of phlogopite; K MgF eFe2 J o 3 Mn3 ))(Si6Al2)020(OH)4; also considered var. of biotite (Hey, 19g07 Manganophyllite (of Yoshimura, 1959): hypothetical mica end-member K2Mn5A14 i5020 (oH)4' margarodit: syn. of margarodite. margarodite: syn. of damourite. marienglas: syn. of muscovite. mariposite: var. of phengite (Whitmore, et al., 1946) with high silica and as much as 1% Cr203; see discussion of fuchsite. meroxene: var. of biotite which has optic plane parallel with b-axis (perpendicular to 010) as contrasted with anomite. Most biotite was considered of this type (Dana, 1892). metabiotite: syn. of bauerite. metasericite: var. of muscovite; a hydrous mica classed under damourite. micarel: mica pseudomorphous after scapolite; pinitic. micarelle: syn. of micarel. microlepidolite: var. of lepidolite distinguished from macrolepidolite by its small optic angle. 19

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN minguetite: near (KNa)0o8(Fe2 MgFe3 A1)5.2(SAl)8020(OH)4 with (FeMg) 53.2 and Si 6.8; belongs either to chlorite or mica groups; nature uncertain. monrepite: KFet1 Fe''Si 3010(0H)2; a ferro-ferrimuscovite; should be studied further. *MUSCOVTITE: [KAJ1(Si,Al)40 (OH)2] nacrite (of Thomson, 1836): syn. of muscovite. natronbiotite: var. of biotite containing appreciable Na. natronphlogopite: var. of phlogopite containing appreciable Na. oblique mica: syn. of muscovite. *octophyllite: group name for those micas which contain 8 other atoms to 12 (0,OH,F) atoms. Biotite, phlogopite and lepidolite generally classed as octophyllites (see heptaphyllite). odenite: syn. of odinite, odite, and oderite. var. of biotite (Hey, 1950) and var. of muscovite (Chester, 1896). oellacherite: muscovite containing barium. oncophyllite: muscovite pseudomorphous after feldspar. oncosine: (1) aluminosilicate of Mg and K; a pinite group pseudomorph; probably a mixture of muscovite, quartz, etc. (Hey, 1950 ); (2) a cryptocrystalline mica classed under damourite (Ford, 1932). onkophyllit: syn. of oncaphyllite. onkosin: syn. of oncosine. Aoosite: aluminosilicate of Fe2 and alkalis; a pinitic pseudomorph. pagodite: syn. of agalmatolite. ~PARAGONITE: (NaK) 2A14 (Si6A2 ) 20 (OH ) parophite: a pinitic rock. Paucilithionite: a hypothetical mica end-member (K2Li3A15Si6020F 41). 20

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN ~PHENGITE: var. of muscovite higher in silica than typical muscovite, and with Mg.Fe in place of some Al; K2(Mg,Fe,Al)4(SiAll)020(0H)4. *PHLOGOPITE: K2(MgFe2)6(Si6A2)020 (H)4 pholidolite: aluminosilicate of Mg, Fe2 and K; perhaps a fluorine-free, Al-poor phlogopite; nature uncertain. phyllite: (1) (of Thiebaut, 1925) syn. of pholidoide (group name for aluminous glauconites); (2) a general term for micas, chlorites, clays and vermiculites used by French writers; (3) (of Thomson, 1828) usually regarded as identical with ottrelite. Picrophengite: a hypothetical mica end-member K2MgAl3(Si7A 1)020(01H)4. *pinite: a group of pseudomorphs, mostly of mica, after cordierite, nepheline, or scapolite; Heinrich (1950, p. 183), after a study of cordierite in pegmatite concludes, "Cordierite that crystallized in the magmatic pegmatite stage is not in equilibrium during the hydrothermal stage and is altered to pinite, which consists chiefly of muscovite, chlorite, and biotite in varying proportions. This change requires chiefly the introduction of potassium and hydroxyl by pegmatite solutions." Apinitoid: aluminosilicate of Fe2 and alkalis; a pinitic pseudomorph. Apolyargite: aluminosilicate of Ca and K; a pinitic group pseudomorph. poly-irvingiteo a var. of irvingite high in Si. Polylithionite: a hypothetical mica end-member. polylithionite: a lithium bearing mica (lepdiolite) with high (as much as 9% Li20) lithium content. potash micat syn. of muscovite. Protolithionite: a hypothetical mica end-member (Kunitz, 1924); K2Fe AlSi3010 (OH)4. protolithionite (of Sandberger): var. of zinnwaldite. pseudobiotite: an altered biotite high in H20; probably a hydrobiotite. pterolite: a mixture of aegirine and lepidomelane. 21

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN pycnophyllite: syn. of pyknophyllite. pyknophyllite: syn. of sericite. Apyrargillite: aluminosilicate of Fe2,Mg, and alkalis; pinite group. Apyrrholite: aluminosilicate of Ca and K; a pinite group pseudomorph after anorthite. rabenglimmer: var. of zinnwaldite high in Fe3. rastolyte: a hydrated biotite similar to voigtite. rhombengliimmer: -syn. of biotite. rhombic mica: syn. of phlogopite. *ROSCOELITE: has been regarded as a vanadium muscovite with as much as approximately 20 = V205; K2(VAl)4(Si6Al2)0 2(0H)4; new data is presented in this report (p. 212 ) rosellan: syn. of rositeo rosellite: syn. of rosellan. Arosite: aluminosilicate of Ca,Mg, and K; a pinitic pseudomorph. rubellan: aluminosilicate of Mg,Fe, and K; probably an altered biotite. sandbergerite-syn. of oellacherite. sarospatakite: syn. of illite, scale stone: syn. of lepidolite. schernikite: pink, fibrous var. of muscovite occurring intergrown with lepidolite. schuppenstein: syn. of lepidolite. *sericite: (1) var. of muscovite; also includes much pinite (Hey, 1950); (2) fine scaly muscovite united in fibrous aggregates and characterized by its silky luster (Ford, 1932); (3) the name usually confined to white mica which is secondary, often the result of alteration of feldspar (it has been suggested that it contains less potash and more water) (Winchell and Winchell, 1951). Sericite as used by Winchell 22

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN and Winchell (1951) is so well established in the field of petrography that any attempt to dislodge or replace it will be futile. Gruner (1948, p. 684) notes, "The exceedingly finegrained micas, which go under such names as sericite, illite, and hydromuscovite, are in a class by themselves. They are defect structures principally with regard to missing alkali cations. What takes the place of these ions? Analyses show definitely that as K decreases H20 increases." shilkinite: described originally by Merkulova (1939) as a new mineral but shown by Tchirvinskii (1948) to be identical with sericite., I siderischer-fels-glimmer. syn. of lepidolite. 35 siderophyllite- var. of biotite high in Fe2 with little Fe. sodium-illite: syn. for brammallite. sterlingite: var. of damourite. taeniolite: syn. of tainiolite. *TAINIOLITE: K Mg4Li2Si8020(F)4; the only member of the mica group without essential aluminum. Classed under phlogopite by Dana (1926) and under lepidolite by Winchell and Winchell (1951). talcite: syn. of damourite. Aterenite: aluminosilicate of Mg and K; an altered scapolite; a pinitic pseudomorph. titanbiotite: syn. of wodanite. titanglimmer: original form of titanmica. titanmica: group name for titaniferous micas. titanobiotite: syn. of wodanite. triclasite: syn.of fahlunite (pinitic pseudomorph), uniaxial mica: syn. of biotite. vanadinglimmer: syn. of vanadium mica. vanadium mica: syn. of roscoelite. 23

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN verdite: impure fuchsite used as an ornamental stone. voigtite: near K(Fe2MgFe3 6SiA (H4-1/2 H2O with Fe2Mg:Fe3:2:1 and Si-i 5-1/2; probably a hydrobiotite. waddoite: a specimen labelled waddoite in the University of Michigan collections appears as typical biotite. Hoy (1950) notes that it is incompletely described. Awilsonite: aluminosilicate of Mg and K; an altered scapolite; a pinite. wodanite: a var. of biotite containing appreciable quantities of TiO2 (as much as 12.5%). wotanite: syn. of wodanite., *ZIiNWALDITE: K2(Fe212Li2 Ak) (Si6_7Al2_1)020(F52 2 There seems to be some confusion as to whether zinnwaldite is more closely related to biotite or lepidolite. kweiaxiger glimmer: syn. of muscovite. The preceding classification is in general agreement with Hey (1950); a few points of difference, however, should be discussed. Hey considers brammallite, for example, a valid species when actually it is no more than a sodium-bearing illite. His opinion on varietal names is expressed as follows: (p. XI): "In the matter of varietal names, the chemical prefixes of W. To Schaller (for example, Plumbian aragonite for Tarnowitzite) have not been adopted; certainly a great multiplication of varietal names based on small differences in chemical composition is to be deprecated, but it seems quite unnecessary to discard such well-known names as Freihergite, Pisanite, or Ceylonite; moreover, it is not really possible to draw a definite line between species and varieties.V Many objections to Hey's position on this matter have been raised and are clearly stated by Heinrich (1951., p. 634): "It is unfortunate that the compiler has seen fit to reject the Schaller System of adjectal modifiers which is rapidly obtaining general adoption by mineralogists in all parts of the world as a satisfactory scheme for freeing mineralogical nomenclature of its present unwieldy and largely meaningless agglomeration of varietal names. It is only by applying the -----------------— ^ ------------------

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN system of adjectal modifiers that mineralogists can remain consonant with a modern conception of minerals as constituting chemical series."' 25

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN V. CLASSIFICATION A. History of Mica Studies Hypotheses advancedprior to 1924 to explain the chemistry of the micas and the other silicates are interesting today only from a historical point of view but a brief resume of tm i u i g a i of them isuseful in giving an idea f the development of some modern theories. Silicates as a group had been recognized about 1780 by Bergmann. He included in this division minerals of the mica, amphibole, garnet, feldspar, and clay groups, as well as individual species. As the discovery of new silicate species continued, several theories were evolved to explain their complex chemistry. Dana's fifth edition of A System of Mineralogy (1868) presented one of the most complete and widely accepted presentations in English up to that time. Dana classified all silicates into two large groups —the anhydrous silicates and the hydrous silicates, each of which had its own subdivisions. The subdivision of the anhydrous silicates was as follows: 1. Bisilicates: oxygen ratio for the bases and silica 1:2 2. Unisilicates: oxygen ratio for the bases and silica 1:1 3. Subsilicates: oxygen ratio for the bases in silica 1: less than 1; mostly 1:2/3- but also 1:1/2 and 1:3/4 The following was the general subdivision of the hydrous silicates. 1. The general section of hydrous silicates. Includes all hydrous silicates except the zeolites and margarophyllites. a. Bisilicates. b. Unisilicates. c. Subsilicates. 2. Zeolite section. 3. Margarophyllite section. 1 ------ --------------— 26 -------------

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN The bases, with reference to the oyxgen ratio (defined by Danaj 1875, p. Xi as "the ratio between -the-number of atoms of oxygen in the different oxygen compounds present") for bases and silica, comprise such elements as potassium, sodium, lithium, magnesium, manganese, aluminum, etc. In relation to the element silicon, which was considered so strongly negative, the oxides of the above named elements were relatively basic. For all practical purposes, the bases were considered mutually replaceable, eight to ten often occurring in the same compound combine-d in either simple or intermediate ratios. Most of the micas were assumed to be unisilicates of the anhydrous silicate group, for the tersilicate subdivision, in which Dana placed them in his fourth edition (1854), was proved to have no existence. A few rare varieties such as margarodite, pinite, paragonite, oellacherite and euphyllite were placed in the margarophyllite section along with talc, serpentine and other hydrous micaceous minerals. Ratios other than for bases and silica formed the foundation for further subdivision within the anhydrous unisilicate group. Chief of these were the RO and R203 ratio, as follows: 1. Oxygen ratio: for bases and silica, 1l; Phlogopite - oxygen ratio for RO, R203, between 2:1 and 5:3; Biotite - oxygen ratio for RO, R203O about 1:1; Lepidomelane -oxygen ratio for RO, R203, about 1:3; Annite - oxygen ratio for RO, R203, 1:2. 2. Oxygen ratio for bases and silica, 1:1-1/4 to 1:2; Muscovite - oxygen ratio for RO, R203, 1:6 to 1:12 and for RO + R203, SiO2, mostly 1 1-1/4; Lepidolite - oxygen ratio for RO + R203, SiO2 mostly 1 1-1/2; Cryophyllite - oxygen ratio for RO + R203, SiO2 12. It is interesting to note that several relatively recent investigators have used similar ratios as the basis for their discussion of mica chemistry. A systematic attempt to explain the chemistry of the micas was made by Tschermak (1878). In brief, Tschermak regarded the micas as consisting of four fundamental molecules to which the following formulas were assigned: 1. R'6A16Si624 R' = KNa,Li or H 2. Mg6Si 624 27

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN 3. H8Si10O24 4. F Si.0 24 10 8 Aluminum may be replaced by ferric iron and chromium; magnesium is equivalent to ferrous iron and manganese. The first of these formulas represents fairly well the composition of muscovite except for Tschermak's (1878) use of the double form of the molecule. Followers of Tschermak used the following formulas to represent the individual molecules: 1. R' 3A3Si3012 2. Mg6Si312 3. H4Si5012 4. F Si04 12 5~4 H2KA13Si3012 was given to represent the formula of common muscovite, which is similar to KA12(SiAl)4010(OH)2, the one used today. Other ratios between K and H, e.g., HK2A13Si3012 were also hypothesized. The chief disadvantage in the former notation is that the true relationship between the various atoms, e.g., substitution of aluminum for silicon or indivuality of the hydroxyl radical, is not recognized. The other Tschermak molecules represent hypothetical compounds. The second is a hypothetical polymer of chrysolite and the third is a hypothetical silicon hydroxide which may also take the form of the fourth, F12Si504. The last two were not analogous to any known substance, and as pointed out by Clarke (1889, p. 584), were chemically improbableo Tschermak believed that the micas were isomorphous mixtures of these molecules. According to Dana (1892, p. 612), Rammelsberg (1878, 1879) "Regards the micas as containing the three silicates R2SiO3, R4SiO4, R6SiO5 in various molecular relation, e.g., Muscovite is R4SiO4 + A14Si3012; the more acidic kinds are R14Si40O1 = R2SiOP + 3R4SiO4, which is further written mR14Si 415 + nRS + pR14Si12045, in which m:n:p = 5:1:5, 7:1:7, 9:1:9 in different cases. Similarly the other micas are resolved into the same three silicates, and the ratios in which they enter are calculated. That this method of calculation is applicable to any silicate, however complex, is obvious, but it is difficult to believe that the results reached really give the true constitution of the compounds." 28

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Tschermak's theory of the mica group remained generally in vogue and unchallenged until Clarke (1889) proposed an alternative. His hypothesis was considered by many to be superior. Dana (1892, p. 613) remarks, "The view of Clarke has the advantage that it assumes only one hypothetical molecule, which, moreover, is analogous to known compounds which play an important part in the Feldspar Group." All orthosilicates (SiO, radical) were considered substitution derivatives of the normal salt Al4(SiO4)3 in which the aluminum is replaced in varying amounts by univalent elements such as hydrogen, potassium, sodium, lithium, or univalent groups as MgF,A10, and AlF2. They are represented in the following formulas by R: Si0 - R 4 3 Al -SiO4 Al 1. R3A13(SiO4)3 Si = Al S iO4 R3 Si04 R Al-Sio 4 --- R 2. R6A12 (SiO04)3 SiO4 A. A/ - 4 3- Al-SiO4 R3 3. R9Al(SiO4)3 Si04 R3 The second of these formulas is not essential as it can be regarded merely as a mixture of equal molecules of the first and third types. Derivatives of the first type include R'R" A1 (SiO4) or R" Al (SiO4)6. R" represents magnesium, iron, manganese, etc. Likewise, erivatives of the second and third types may be obtained. All the orthosilicates may be explained by means of the first and third derivatives. Those micas with fluorine are represented by the univalent groups MgF or AlF2. Excess oxygen is represented by the univalent A10 group. In the case of some muscovites, phlogopites, and many lepidolites, the oxygen-silicon ratio fi low, which indicates a more acid condition. These micas are therefore considered to contain, in part, polysilieie acid, H4Si308, which, like orthosilicic acid, H4Si04, is tetrabasic.c This line 29

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN of reasoning, which is similar to the one used at that time to explain the plagioclase series, necessitates isomorphous miscilibity between orthosilicates and polysilicates (Si308 radical). Thus the general formulas for all micas may be obtained from the theoretical initial units A14(SiO4)3 and A4(Si308)3. Examples of the initial points of the orthosilicates and corresponding polysilicates are given below.: 1. mR3A13(Si04)3 nR Al3 (Si3O8 ) 2. mR6Al2(Si14) nR6A2 (Si38)3 3. mR9Al(SiO4)3 nRgAl(Si308)3 Clarke (1889) notes that if SiO4 and Si108 were represented by X, then all micas fall within the limits R3Al3X3 and RAlX3. Ordinary muscovite was represented by the first type, R3A13(Si04)3,| in which R = H2K, giving H2KA1(SiO4)1. Phengite, the high silica muscovite, was regarded as an isomorphous mixture of normal muscovite and the polysilicate H2KA13(Si308)3; other micas, such as lepidolite, are correspondingly much more complicated. Kunitz (1924) made the first noteworthy modern attempt to correlate the chemical composition of the micas with physical and optical constants. In the course of his work he made 32 chemical analyses of various micas, Three main isomorphous groups of the micas were advocated by Kunitz: 1. Alumina micas, or the muscovite series, in which aluminum is replaceable by as much as 10% of Fe3. 2. Magnesia-iron micas, or the biotite series, in which magnesium and ferrous iron are completely replaceable. 3. Lithia micas, or lithionitesi Here Kunitz assumes a special trivalent complex group of elements consisting of lithium and silicon in variable proportions (Generally 2Li, Si), to which he assigns the symbol Le. This group is completely replaceable by ferrous iron. Lithia micas, therefore, may be written as KH2Al2Le (Si04)3. 130

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Kunitz (1924) points out that members of the lithionite series that are rich in lithia contain little alumina, and vice versa, and recognizes that lepidolite can be derived from muscovite by the substitution of Li20 for A1203. He considers that there is a certain kind of isomorphous relationship between magnesium (and ferrous iron) and lithium. Partial isomorphous replacements also occur in the case of Si and Ti; Si and Al; Al, Cr, and Mn; K and Na; but not between K and H, or Al and Mgo His results show also that with increasing iron there is a corresponding, often linear, increase in the specific gravity and in the refractive indices. The refractive indices are to some extent modified by isomorphbus replacement of fluorine and hydroxyl. In 1925, Hallimond (1925, 1926, 1927) and Winchell (1925, 1927) presented almost simultaneously two new theories regarding the chemistry of the micas. In his discussion Hallimond (1925, 1926, 1927) emphasizes two main points, namely, that RO can replace R203, and that the invariant ratio of K20:SiO2 is in the proportion of 1:6. On the basis of these assumptions, calculations are made and ideal formulas for various species and end-members are given. The micas are divided into two main sections; the acidic, which is further divided into the potash and lithia micas, and the basic, which comprises the biotites and phlogopites. That RO can replace R20O, Kunitz (1924 po 389) denies on valency and theoretical grounds. Winchell (1925, 1927), however, believes that the RO:R203 ratio is variable and that K20 is constant, but cannot agree with Hallimond's (1925, 1926) interpretation of either of these facts. Winchell (1925, 1927) observes that the RO:R203 ratio varies because A120 replaces MgSiO3 (MgOSiO2)and vice versa. Winchell (1925, 1927) further regards potash as being constant and contends that not only does the K20:Si02 ratio vary, but that it varies in a regular manner from 1:6 in muscovite and phlogopite to 1:5 in protolithionite and siderophyllite and to 1:7 in phengite. One of the other points stressed by Winchell (1925, 1927) in his early studies of the micas is the belief that isomorphism depends on atomic volumes rather than on the valency of the elements which may proxy for one another (contrary to the opinion of Kunitz, 1924). In the micas, fluorine and hydroxyl may proxy for oxygen; potassium does not proxy for hydrogen, nor titanium for silicon. Winchell (1925, 1927) used many analyses to calculate end-members (many of which were revised in later works) of the biotite-phlogopite and muscovite-lepidolite series. In these contributions Winchell first used the terms heptaphyllite, (muscovite and lepidolite systems) and oatophyllite (biotite system) for the two main groups of micas between which there is no isomorphism. This is the result of the observation that micas fall into the two distinct classes, the fundamental units of which contain seven and eight positive atoms respectively. These terms, somewhat modified, have now been generally accepted. 31

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN In a series of ten papers with the general title, "Contributions on the Chemical Constitution of Micas," Jakob (1925A,B; 1926; 1927; 1928; 1929A BC,D; 1931; 1932) took an entirely different approach to the chemical and structural problems of the micas. The method is based on his theoretical and highly speculative theory of the constitution of the silicates (1920, 1921). A basic part of the theory is that certain atoms or groups of atoms are more closely united than others in a central atom, and may lie in an inner sphere of attraction while others are separately ionized and are possibly more remote. Jakob (1920, 1921) concludes that groups in the outer sphere have little or no influence on the capacity of two minerals to form mixed crystals. The constitution of the silicates he refers to certain prototypes [SiOR'41R [SiO]R'6 and LSiOj R'8 with or without added SiO2 groups. It now appears that this theory has no basis either structurally or chemically. With the application of X-ray diffraction technique to the study of natural micas between 1927 and 1932 (see section II of this part), new data was brought to light and with these new facts in mind Winchell (1932, 1935) revised some of his previous studies on the lepidolites and biotites. Lepidolites which had been classed as heptaphyllites are now, on the basis of Mauguin's (1927, 1928A, 1928B) X-ray results, regarded as octophyllite micas along with the biotites and phlogopites. New end-members in the biotite system were given as: annite 5(H4K2Fe6Al2Si6p24), siderophyllite 6(H4K2Fe5A14 Si 0.), eastonite 6(H K2Mg Al4Si 04 ), and phlogopite 5(H4K^Mg6Al Si6024). The end-members suggested by Winchell (1932, 1955) in these later investiations are in essential agreement with the conclusions of Berman (1937). In his final paper on the micas, Winchell (1942) made his fourth attempt (others1925, 1927, 1932) to correlate the chemical composition and optical properties of the lithia micas in which he took cognizance of the polymorphism established by Hendricks and Jefferson (1939). Newly calculated end-members include polylithionite (K2Li4Al2Si8o20oF), protolithionite (K2LiFe4Al3Si6020F4), and paucilithionite (K2Li3Al5Si602oF4). Winchell (1942) has also noted that in most analyses of lepidolite the Li2O content is deficient by almost one percent. This, he considers is due to the presence of some interlayered, but not isomorphously combined, muscovite. The work of Stevens (1938) is the only other important contribution to the chemistry of the micas before it was realized that the micas crystallize in many polymorphic forms. This work on the lepidolites resulted in the interpretation of seventeen new analyses on the basis of isomorphous mixtures between four arbitrarily chosen end-members. They are polylithionite (KLi2AlSiO4010F2), biotite K(Mn,FeMg) AlSi3010 (F,OH)2, lithium muscovite (K4Li6Al6A1 Si12 O0 (FOH )8), and muscovite (KA12AlSi01 (OHF)2). Volk (1939) studied the optical properties and chemistry of muscovite and suggested that the variations in composition and correlations with optical data may be interpreted with reference to the three end-members; 52 32

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN potassium muscovite, H4K2A16Si6024; phengite H6K2(Fe,Mg)2Al4Si6024; and ferric iron muscovite H4K2Fe32A14Si6024o The formula proposed for phengite is new and was used in order to get satisfactory correlation with the chemical analyses. The ratio of R20:SiO2 remains almost constant in all three members. The chemical constitution and classification of micas has also been discussed by Serdiuchenko (1948A). He proposed the idea that an almost continuous isomorphous series between muscovite and phlogopite is characterized by the replacement R32 —R23(Mg — A12) in the octahedral layers. The composition of biotite is considered intermediate between these two micas, ioe., isomorphism exists between heptaphyllites and octophyllites. This idea stems from the theory that in the biotite-phlogopite series, isomorphism takes place not only between magnesium and ferrous iron, or aluminum, ferric iron and chromium, but also between R32 and R23. This latter type "characterizes a limited isomorphism in the alternation... 6Mg(OH)2... and.. 4Al(OH)3... layers in the lattices of the minerals." (In translation.) The concept of alternating layers with octahedral coordination having different chemistry is similar, in a way, to that of Holzner (1936), who believed that biotite is intermediate in composition between phlogopite and muscovite. Furthermore, he suggested that the crystal structure of biotite is that of alternating layers of these two micas. Chemically he considered biotite as having the approximate composition of two phlogopites to one muscovite, which is KMg2Al0.667 (AlSi3010OH) 2 with the usual isomorphous replacements. Serdiuchenko (1948A), moreover, suggests broad isomorphous replacements in the tetrahedral layers after noting that the Si:Al ratio fluctuates widely from 2.4:1.6 to 3.6:0.4. Silica rich micas such as phengite and alurgite are explained on the basis of the relatively large silicon content in these sheets. Selected biotites are cited as examples of micas intermediate in composition between biotite and muscovite. Serdiuchenko (1948A) also states (in translation), "In connection with the presence of polymorphic modification in the group of micas, the points of the heterostructural micas may be ranked on the line of our diagram." This is interpreted as implying that the various polymorphic forms of biotite, for example, are direct consequences of their chemical position between muscovite and phlogopite. From this discussion of the chemical history of the micas it is clear that the end-member concept is firmly engraved on the minds of many mineralogists. The requirements necessary for end-members are indicated by Winchell (1925, 1942) and Stevens (1938, 1946), and a method for calculating the ratios of end-members in the micas is well explained and illustrated by Stevens (1946). Heinrich (1946, p. 846), however, has pointed out that 33

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN "The theoretical'end-molecules' used by Kunitz (1924) and Winchell (1935) have little significance in the structure of the biotite molecule, nor can variation in chemical composition of the series be expressed adequately in terms of them," Rabbitt (1948, p. 284), in his work on the anthrophyllite seriesi concurs and notes that the molecular end-member concept is founded on false premises "No such end-members in the amphiboles are known in nature. No such mixing on a molecular basis occurs. However convenient it may be to plot the different properties in that way it must be admitted that it engenders and perpetuates a false notion of the variations in a mineral series." B. Chemical-Structural Systematization of the Natural Micas A workable systematization of the micas has long been one of the aims of the project. A classification based on structural and chemical data, along with an account of the very voluminous synonomy of the micas, has been completed. In the following pages this classification is presented. The numerous attempts at mica classification in the past have been based almost entirely on their chemistry and fall generally into two categories: (1) those employing complex quasistructural formulas (e.g., Clarke, 1889; Jacob, 1925); and (2) those involving calculation of theoretical endmember molecules (e.g., Tschermak, 1878; Kunitz, 1924; Stevens, 1946). Berman (1937) approached the problem in more realistic fashion by correlating mica formulas with unit cell contents and expressed the chemical variation by means of atomic ratios of different elements occupying equivalent structural positions. However, the work of Hendricks and Jefferson (1939), Levinson (1953), and others has demonstrated the complex polymorphism in the mica group and the necessity for both chemical and X-ray studies of micas as a basis for a sound classification. Because of the complex chemistry of the group, the numerous textural and other minor varieties, and the repeated misapplication of names, the nomenclature and synonomy of the micas have been ponderous and involved. The purpose of this work is threefold: (1) to present a reasonable and usable subdivision of the micas into naturally occurring species and major varieties on the basis of both chemistry and structure; (2) to review the synonomy of the group and thus assign minor varietal names or duplications to the species or major variety to which they correspond; and (3) to list those micas that are of indeterminate status owing to incomplete studies. ----------- ^ -----------

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN In the annotated classification that follows we have used for chemical varieties the prefixes of Schaller (1930). Wherever possible for each entry we listo (1) a simplified formula based on 24 negative ions in the unit cell; (2) the major isomorphous substitutions; (3) the structure; (4) notes; and (5) synonymy. For the structural varieties, the method of polymorphic notation suggested by Ramsdell (1947) is adopted. The symbols and their meanings used in this work are as follows: 1M 1-layer monoclinic structure 2M muscovite 2-layer monoclinic muscovite structure 2M 2-layer monoclinic octophyllite mica structure 3M 3-layer monoclinic structure* 3H 3-layer hexagonal structure 6M 6-layer monoclinic structure 6T 6-layer triclinic structure 24T 24-layer triclinic structure 18T 18-layer triclinic structure (Amelinckx, 1952B) The chemical-structural classification of the micas is as follows. Species 1. Muscovite, K2A14(Si6Al2)020(OH)4. Minor NaRb,Cs,Ba, and Ca for K; minor MgFe2,Mn2 and Li; Minor Fe3,Ti, and Cr for Al; minor F for OH; maximum Li20 = 3.30, occupying vacant octahedral positions. Structure: 2M muscovite. Synonyms: adamsite, ammochrysos, amphilogite, biaxial mica, cat gold. cat silver, common mica, damourite (in part), didrimite, did-ymite, ferroc-ferri-muscovite, frauenglas, helvetan, heptaphyllite, isinglass, kaliglimmer, marienglass, morrepit nacrite, oblique mica, potash mica, schernikite, zwefxi'ger glimmer. Hypothetical end-members: ferri-muscovite, kryptotile, leverrierite, lithium muscovite. Varieties. a. Barian muscovite. Ba with reported maximum of 5991% BaO (Doelter, 1914) for Ko Structure: probably 2M muscovite, material labeled oellacherite from Tyrol has the 2M muscovite structure. *There is reason for believing that the 3-layer monoclinic structure reported by Axelrod and Grimaldi (1949) is not truly monoclinic but just a distorted 3-layer hexagonal structure., This point is explained in further detail on page 47. 35

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN The barium-muscovite from Franklin Furnace, New Jersey, described by Bauer and Berman (1933), was examined by us by means of the X-ray powder method. The resultant pattern is not that of a muscovite, nor even that of a mica. The material is too finegrained for detailed optical studies (See p. 68). Synonyms: barium muscovite, oellacherite, sandbergerite. b. Manganian-muscovite. Reported maximum MnO = 2532% (Ellsworth, 1932); usually some Li20 present. Structure: 2M muscovite. The very fine-grained,- deep purple mangan-muscovite of Eskola (1914) with 2.35 MnO has been checked by means of X-ray powder photographs, and the pattern does not correspond to any known mica structure. The photograph shows some quartz lines; see page 69 Synonyms: mangan-muscovite, manganese muscovite. c. Ferrian muscovite. Fe+3 with maximum of 5.70% Fe203 (Tschermak, 1878) reported, for Al. Structure: probably 2M muscovite, for the "alurgite" from Cajon Pass (Webb, 1939) with 5.32% F-e20 has been shown to have this structure. d. Ferroan muscovite. Fe+2, with maximum of 6.55% FeO (Wulfing, 1886) reported. Structure: probably 2M muscovite. e. Chromian muscovite. Reported maximum of 4.81% Cr203 (Whitmore, et al., 1946), for Al. Structure: 2M muscovite. This follows the usage of Whitmore et al. (1946). Synonyms: chromglimmer, chrome glimmer, fuchsite, gaebhardite, verdite. f. Lithian muscovite. K2(Al,Li) ca. 5.0 (Si6 7,A1 1) 020(OH + F)4. Li2O, at least 3530% occupying vacant octahedral positions. Usually small amounts of F for OH. Structure: modified 2M muscovite (Levinson, 1953). g. Phengite. K2(Mg,FeAl)4(Si7,Al1)020(OH)4. High-silica muscovite with considerable MgO (7.96% Pagliani, 1937) and in some cases FeO; some Fe3 for Al. Structure: 2M muscovite. ------------- ^~~3

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN In addition to the 2M muscovite and 3H types of phengites, Dr. H. S. Yoder has informed us that some of the sericites from Amelia, Va. described by Glass (1935) have the 1M structure. The nomenclature of the silica-rich end of the muscovite series as proposed by Schaller (1950) is regarded by us as unsatisfactory because: (1) there is some evidence that the original leucophyllite (Starkl, 1883) is a mixture, and in any event a new analysis and an X-ray study of the type material are needed to check its validity; and (2) the term, alurgite, which Schaller (1950) suggests as a substitute for leucophyllite in the event that the latter should prove untenable, also has been erroneously employed to indicate a normal, i.e., low-silica, ferrian muscovite (Webb, 1939; Odman, 1950). X-ray studies, however, indicate that not all true alurgite is two-layer monoclinic in structure; some is three-layer hexagonal. Thus the term alurgite has at various times been used for: (i) a red high-silica muscovite with minor Fe3 and Mn (Penfield, 1893); (ii) a red ferrian low-silica muscovite (Webb, 1939 and Odman, 1950); (iii) a manganian high-silica muscovite (Winchell and Winchell, 1951); and (iv) a three-layer hexagonal polymorph of (i), discovered by Hendricks and Jefferson (1939) and verified by us on material from St. Marcel, Italyo Because of this confusion, the use of the term alurgite for the high-silica end-member of the muscovite series is also undesirable. Less confusion accompanies the term phengite, which has been generally employed to mean high-silica muscovite. Hypothetical end-members: ferrophengite, picrophengite. h. Hexagonal phengite (3H phengite), Differs from 2M phengite (g) in having the three-layer hexagonal structure (3H) and 2V = 0~ - low. No well authenticated analysis of all-uniaxial material is available. i. Chromian phengite. Cr, with maximum of 0.78% Cr203 (Whitmore, etoal., 1946), for Al. Structures 2M muscovite. This follows the usage of Whitmore et al. (1946). 37

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Synonyms: chromochre, mariposite. j. Hexagonal chromian phengite (3H chromian phengite). Presumably chemically similar to ii, but with the three-layer hexagonal structure and 2V = 0~ - low. No analysis of alluniaxial material available. k. Sericite. Fine-grained muscovite. Structure: 2M muscovite. The term-sericite has been used for: (i) -fine-grained muscovite, either primary or secondary; (ii) fine-grained phengite; and (iii) hydromuscovite. Thus it cannot be defined exactly on a chemical basis. Iti;' remains, however, highly useful as a general, nonspecific term for fine-grained muscovite whose exact chemical nature is unknown. Synonyms: (including pinitic pseudomorphs). achlusite, agalmatolite, aspasiolite, avalite, bildstein, bonsdorffite, catalinite, cataspilite, cordierite-pinite, cymatolite, damourite (in part), dysyntribite, epileucite, epi-sericite, fahlunite, giesickite, gigantolite, gilbertite, glimmer, gongylite, helvetan hygrophyllite, iberite,:ivgitite, killinite, lardite, lepidomorphite, liebenerite, lythrodes, margarodite, micarel, micarelle, oncophyllite, oncosine, onkophyllit, onkosin, oosite, pagodite, parpphite, pinite, pinitoid, polyargite, pyknophyllite, pyrargilliter pyrrholite, rosellan, rosellite, rosite, shilkinite, sterlingite, talcite, terenite, triclasite, wilsonite. Species 2. Paragonite (Na,K)2A14(Si6,A12)020(OH)4. Structure: 2M; probably 2M muscovite. Schaller and Stevens (1941) have pointed out that the series muscovite-paragonite is not completely represented in nature. If intermediate types are discovered, it might be better to regard paragonite as sodian muscovite and reduce it to varietal status. Synonyms: hallerite, natronglimmer, pregrattit, soda mica. Species 3. Roscoelite. K2(V,Al)4(Si6A12)20p(OH)4. Maximum V205 = (ca.) 20%. Structure: 1M. 58

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Although roscoelite has previously been regarded as a vanidiferous muscovite, it deserves full species rank because it is structurally distinct (see p 212 ), - Synonyms: colomite, vanadinglimmer, vanadium muscovite. Species 4, Lepidolite. K2(LiAl) 6(Si,onAl )O l(FOH)t Rb and Cs replace K, in some types in considerable amountsRb Cs20 = 1.90%; Lunblad, 1942). Small amounts of Mn,Mg,Fe2,Fe are normally present. The OH:F ratio varies considerably, and OH may become negligible. Synonyms: irvingite, lilalith, lilalite, Li-phengite, lithia mica, lithionglimmer, lithionit, lithionite, lithionitesilicat, macrolepidolite, microlepidolite, poly-irvingite, scale stone, s ideris cher-fels -glimmer. Hypothetical end-members: paucilithionite,' polylithionite (in part), protolithionite. Varieties a. a.' Six-layer: m6onoclinic' l6pidoiite (6M lepidolite). Li 0 = (ca.) 4.0 - 5.1%. b. One-layer monoclinic lepidolite (1M lepidolite). Li20 = 5.1- 7.26%. c. Three-layer hexagonal lepidolite (3H lepidolite). Composition approaches 4b. Due to twinning (?) (Levinson, 19553). 2V = O -small'. d. Three-layer monoclinic (3M lepidolite). Li20 = 4.1%. One example from Skuleboda, Sweden (see p. 147 ) * n e. Manganian-lepidolite, Maximum reported MnO = 7.55% (Shibata, 1952B). Structure: probably variable, depending on Li content. f. Magnesian-lepidolite (cited in Berman, 1937). We are unable to determine if any natural material of this composition has been discovered. g. Polylithionite (in part). K2Li4A12Si8020(F,OH)4. A silicon- and lithium-rich, thus aluminum-poor, lepidolite. Structure: 1M. Species 5. Taeniolite. K2Mg4Li2Si80O20F4 Structure: 1Mo 39

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN - Species 6. Zinnwaldite. K2(Fe21_2Li2_A12) (Si6-7,A12_-1 020 (F53-20 Hl-2). The (AlFe,Li) group usually contains 6 atoms but may be considerably deficient. Zinnwaldites are chemically much more closely related to the biotites than to the lepidolites. In fact, there is no sharp natural compositional boundary between zinnwaldite and lithian biotite. If a demarcation is to made, we believe the line can best be drawn on the amount of Li and suggest that lithium-iron micas with Li> 1 atoms per unit cell formula be regarded as ziannaldites,those with Li < 1 be placed with the lithian biotites. Structurally, however, the problem is more complex. No zinnwaldites have been found with the 6M lepidolite structure. so common in the lepidolites. On the other hand, the discovery of a zinnwaldite with a structure very similar to that of lithian muscovite (p. 201 ) does not permit one to overlook the possibility of at least some zinnwaldites resembling lepidolite structurally. Synonyms: cryophyllite (in part), lithioneisenglimmer, polylithionite (in part), protolithionite, rabenglimmer. Varieties. a. One.layer monoclinic zinnwaldite (1M zinnwaldite). b. Two-layer monoclinic zinnwaldite (2M zinnwaldite). c. Three-layer hexagonal zinnwaldite (3H zinnwaldite). d. Ferrian zinnwaldite maximum report Fe203 = 10.06% (Shibata, 19523). Species 7. Phlogopite. K2(MgFe2)6(Si6A12)0Oo(OH)4. Na can substitute for K up to nearly K:Na =1:1 Harada, 1936); minor RbCs, Ba, and very minor Ca also may proxy for K. Fe is almost always present, but Mg predominates greatly over Fe2. Small amounts of Mn,Fe3 and Ti may be present. The Si:tetrahedral Al ratio may be larger than 6:2. There is no well-defined, natural, compositional boundary between ferroan phlogopite and magnesian biotite. Because Fe2 is a strong chromophae, micas of this type even with only a small per cent of Fe are dark colored and are thus commonly classed as biotites. If a division is required, we suggest that where the ratio of Mg:Fe2> 4:2, the mineral should be classed as phlogopite. _40

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Synonyms: aspidolite, barium-phlogopite, barytbiotite, hydrophlogopite, magnesia mica, natronophlogopite, octophyllite, pholidolite, rhombic mica. Hypothetical end-members: fluor-phlogopite, hydroxylphlogopite. Varieties. a. One-layer monoclinic phlogopite (1M phlogopite). The most common type. b. Two-layer monoclinic phlogopite (2M phlogopite). c. Three-layer hexagonal phlogopite (3H phlogopite). d. Manganophyllite. K2(Mg5_4,Mn21 Fe2 0oFe3_ 0-1) (Si6Al2)0O0( 0H)4. Structure- Generally 1M; some approach a three — layer hexagonal structure. 2M reported by Hendricks and Jefferson (1939). Although some investigators (e.go, Hey, 1950) class manganophyllites as varieties of biotites, most manganophyllites have little or no Fe2 and only small amounts of Fe3. An exception is a Langban, Sweden, mica analyzed by Jakob (No. 8, po 157, 1925A)which contains 16.941 Fe203o Apparently Mn is present commonly as Mn3, rarely as Mn2o Synonyms: Manganese mica, manganophyllo e. Titanian phlogopiteo K2Mg5Ti(SiAl12)020(OHF)2 (Prider, 1940) and Fe2 and Fe3 are minor; (OHF) is deficient; 0.15 atoms of Ti are allottedto tetrahedral positions and 0.84 atoms to octahedral positions on basis of original analysiso Structure: 1M1 Species 8. Biotite. K2(Fe2,Mg)64(Fe3,AlTi)O2(Si65,Al2-)0222(OHF)4-2 Some Na,CaBa,Rb, and Cs for K; Mn for Fe2, F for OH; Mg may be almost absent. Total F + OH may be very low (Walker and Parsons, 1926). Synonyms: annite, anomite, caesium-biotite, chromglimmer (in part), euchlorite, eukamptite, ferromuscovite, haughtonite, heterophyllite, hexagonal mica, iron mica, lepidomelane, meroxene, natronbiotite, octophyllite, odenite, odinite, odite, oderite, pterolite,.hombenglimmer, rubellan, siderophyllite, titanglimmer, titanmica, uniaxial mica, waddolte. 41

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Hypothetical end-members: cryophyllite (in part), eastonite, fluor-annite, fluor-biotite, fluor-lepidomelane, fluor-meroxene, fluor-siderophyllite, hydroxyl-annite, hydroxyl-biotite, hydroxyllepidomelane, hydroxyl-meroxene, hydroxyl-siderophyllite, manganophyllite (in part), Varieties. a. One-layer monoclinic biotite (1M biotite). The most common type. b. Two-layer monoclinic biotite (2M biotite). c. Three-layer hexagonal biotite (3H biotite). d. Six-layer triclinic biotite (6T biotite). Our studies of the supposed type 6T biotite described by Hendricks and Jefferson (1939) from Sterling, N. Y. (U.S.N.M.C3675) indicates the 1M structure. Dr. H. S. Yoder has studied other material from the same specimen and has found the 2M structure. e. Twenty-four-layer triclinic biotite (24T biotite). f. Eighteen-layer triclinic biotite (18T biotite), Amelinckx (1952B). g. Calcian biotite. A biotite from Kaiserstuhl, Germany, has 14.335 CaO; Zambonini (1919). Structure unknown. The validity of calcian biotite as a major variety is doubtful. The existence of Ca in the biotite structure has been challenged by Jacob (1929A). Several specimens labelled calciobiotite, from Italian localities, have been found to have the 1M structure. h. Ferroan biotite. Mg is very minor or absent, Fe is present mainly as Fe2. Synonyms: siderophyllite, lepidomelane (in part). i. Manganian-biotite. Mn as much as 1 atom per unit cell formula. Fe present as Fe2 or Fe5. j. Ferrian biotite. K2(Fe2 Mg)3 4Fe32_ (Si,AlTi)8020_21 (OR,F)43. Synonyms: ferribiotite, lepidomelane (in part). ( 4 Lepidomelane is commonly employed for iron-rich biotite, but the term has been used to embrace biotites rich in Fe3, those rich in Fe2 and those with relatively large amounts of both Fe2 and Fe3 (Heinrich 1946), -----------------------— 4 2 --------------

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN k. Lithian biotite. K2 (Fe2,Mg)5 (Li,Al,Fe35)(SiAl)802 (OHjF)4. 1. Titanian biotite. K2(Fe2,Mg3)5Ti(Si6,A12)8022 (OHF)2. Synonyms: ferrititanbiotite, ferriwotanite, titanbiotite, titanobiotite, wodanite, wotanite. Freudenberg (1919) reports a biotite with 12.5% TiO2. Both titanian biotite and titanium pholgopites are relatively poor in Fe3 and also are very low in OH and F. Hydrous Micas. Because most investigators class these minerals with the clay group, their crystal chemistry is not considered here. However, it is interesting to note that a new interpretation (Brown and Norrish, 1952) of the chemistry of one of the species in this group, hydromuscovite, postulates the replacement of K by oxonium (hydronium) ions H30+. Species and varietal names included in this group are: bastonite, brammallite, bravaisite, buldymite, damourite (in part), goeschwitzite, grundite, gumbelite, hydrobiotite, hydro-mica, illite, metasericite, Mg-illidromica, rastolyte, sarospatakite, sarospatite, sericite (in part), sodium-illite, voigtite. Micas of Indeterminate Status. 1. Euphyllite - near (Na,K)A3SiOl o(OH)2; may be a mica intermediate between muscovite and paragonite; or brittle mica or perhaps a mixture. 2. Mahadevite -near (K,Na)o.97(Al,Fe,Mg)2.66(Si,A1)4(O,OH)2; supposedly between muscovite and phlogopite in composition, A specimen of mahadevite has been received from Dr. H. S. Yoder. X-ray studies of this material indicate the 1M phlogopite structure. 3. Manandonite - a borosilicate of Li and Al closest to lepidolite in composition; Li4A114B4Si60O29(OH)24(?); possibly- not a- mica. 4. Leucophyllite - a high silica phengite; perhaps a mixture. 5. Anthrophyllite - a "mica (?)" (Hey 1950, p. 283). L —-------------------— 45 --------------

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN PART II. MUSCOVITE I. MINERALOGY OF NORMAL MUSCOVITE A. Chemistry The ideal formula for muscovite is K2A14(Si6Al2)020(OH)4. However, all micas exhibit a large number of isomorphous replacements. The following isomorphous substitutions are of major proportions: for K —Na,Rb,Cs,Ba, and (Ca) for octahedral Al —Mg,Fe2,Fe3,Mn,Li,Cr,V and Ti for OH —F for (Si6A12) —the proportion may approach (Si7All) as in phengite For normal pegmatitic muscovite the percentages of isomorphously substituted elements may reach relatively large proportions. Many such micas with over 4 percent total iron, for example, have been reported or are now known on the basis on new spectrochemical data. It appears that rose muscovites, which are the last micas to crystallize in the hydrothermal sequence of complex pegmatites, approach most closely to the ideal muscovite composition (Heinrich and Levinson, 1953)o They contain very little Fe2,Fe3,Mg, or Mn, etc. Schaller (1950) has pointed out that the high-silica sericites (phengites) contain appreciable amounts of a divalent element, usually Mg* Pagliani (1937) reports a phengite with 7.96 percent MgO. It has been reported by several authors (Gruner, 1948 and Sudo, 1949) that the fine-grained micaceous minerals which go under such names as illite, -hydronica, sericite (in part) and hydromuscovite commonly show a deficiency of K and an excess of H20. Sudo (1949) showed that the (OH) content is roughly inversely proportional to the combined content of K,Na, and Ca. Gruner (1948) postulated that the hydronium ion (H30+) may occupy the vacant K portions, and Brown and Norrish (1952) supported this idea further by means of calculations. Several occurrences in which large books of normal pegmatitic muscovite are replaced by fine-grained sericite aggregates may be explained by a hydronium sericitic mica replacing a potassium muscovite (Liashchenko, 1940). 44

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN For this study 168 muscovites were analyzed spectrochemically. The results are shown in Table I. TABLE I NEW SPECTROCHEMICAL DATA ON 168 MUSCOVITES Present in % Range Average of samples Fe20 100 0.63 - 5.1 2.904 Mg 100 0.008 - 1.9 0.931 MnO 100 0.009 - 0.83 0.106 Ti02 99 0.003 - 0.52 0.128 CaO 100 0.0005 - 0.43 0.0069 SrO 21 0.001- 0.005 0.0005 BaO 96 0.0002 - 1.1 0.114 Na20 100 0.33 - 1.7 1.02 Li20 26 0.01 - 1.6 0.089 Rb2O0 O Cs2Q 0 Ga203 100 0.003 - 0.062 0.0219 Sc203 31 0.001 - 0.011 0.0011 Co203 37 0.0002 - 0.0008 0.0002 Cr203 32 0.0001 -.4 0.0001 SnO2 76 0.0007 - 0.10 0.0067 V205 65 o.0001 - o.o36 0.0004 F 44 0.10 - 1.48 0.2361 B. Muscovite Structure 1. General and Polymorphism. Jackson and West (1930, 1933) made the first detailed study of the structure of mica (muscovite) and, in addition, confirmed in greater detail the structure of mica and other layered silicates proposed by. Pauling (1930) on the basis of his coordination theory. The results obtained by Jackson and West (1930, 1933) on the unit cell dimensions of muscovite are as follows: ao = 5.18 A, bo = 9.02 A, co = 20.04 A.

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN - They selected 95~30' as the P angle in contrast with about 100~ chosen by Mauguin (1927, 1928A, 1928B). The monoclinic space group determined by Mauguin as C2/c was verified. The most detailed study of the mica group as a whole was conducted by Hendricks and Jefferson (1939) who found numerous polymorphs. In all, seven different polymorphic modifications embracing the hexagonal, monoclinic and triclinic systems were found among 100 investigated specimens of named micas. Each species, with the exception of muscovite, was found to occur in more than one polymorph. Biotite, for example, appears in five modifications, lepidolite in four, etc. Muscovite, however, Hendricks and Jefferson (1939) found only as the 2-layer monoclinic muscovite type (2M muscovite). They also found one "lepidolite" and one alurgite with this structure. The distinguishing feature of the muscovite structure is its distortion from that of the ideal mica. This is revealed by the presence of the reflections, (06G)with e odd, which should normally be absent in the ideal arrangement on the basis of the structure-factor calculations of Jackson and West (1930, 1933); see Fig. 5. This distortion results from an incomplete filling of the octahedral positions and is considered by Hendricks and Jefferson (1939) to be the factor permitting only the 2-layered structure for muscovite to form. Herein also lies the reason why Winchell's (1925, etc.) grouping of the micas into heptaphyllite and octophyllite divisions appears correct, for these terms simply mean that the unit cell contains seven and eight atoms respectively, for twelve 0,OH, and F. Muscovite, KA12(AlSi3)010(OH,F)2, is the type heptaphyllite, and phlogopite, KMg3(AlSi3)010(0HF)2, is the type octophyllite. This relation may also be expressed by saying that the heptaphyllites have only 2/3 of the octahedral positions filled, whereas the octophyllites have all such positions filled. The presence of(063ereflect ions with e odd must imply departure from the ideal muscovite structure given by Jackson and West (1930, 1933). Hendricks and Jefferson (1939) reported these reflections absent in the two-layered biotite-like micas (2M). It follows therefore that muscovite (two-layered) has a structure different from that of the two-layered biotites. Diffuse scattering is reported in all the micas except muscovite. The scattering is observed in Weissenberg photographs along those reciprocal lattice lines in which h and k are constant, but in which k is not divisable by three (hakbl, kbi3n). Laue photographs taken normal to the cleavage demonstrate scattering in the form of asterism or radial streaks. Mauguin (1927, 1928A,1928B) first noticed this effect and its significant absence in muscovit and theorized that it was the result of some type of randomness in the positions of the heavy ions. Hendricks and Jefferson (1939) explained it in terms of constant h and k indices with an apparently continuous variation of the ~ index, _____________________46 ___________

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN This results from a variable periodicity in the stacking of mica layers in such a manner that those planes in which the k index is a multiple of three are undisturbed. They further state that if one half of a mica layer is translated by nb/3 with respect to the other half, it leaves the layer unchanged with respect to itself but results in a change of the successive layers This is the: factor which permits polymorphism in the micas (but in the muscovite structure distortion prohibits translation)O Therefore, continuous scattering arises from destruction of the lattice periodicity perpendicular to the cleavage, resulting from a translation of some layers along the b-axis, parallel with the cleavage, by nb/3. Scattering has also been reported in other layered silicate minerals such as stipnomelane, vermiculite, cronstedite, and the chlorites. Essentially the same explanation is given for the phenomenon in these minerals. In our work we have found at least one example of an apparently normal pegmatitic muscovite with small amounts of diffuse scattering (Fig. 2; compare with Fig. 5). The scattering, however, is observed only along the 02e reciprocal lattice line (only O-level a-axis photographs have been taken of most muscovites). Scattering of the type described in the preceding paragraphs would require scattering along both the 02 and 042 reciprocal lattice lines. The exact significance of this type of scattering has not yet been determined. Noteworthy perhaps is the fact the specimen (Noo 6) from which the photographs of Fig. 2 were obtained has a SiO2 content of 48 percent which is about 3 percent above normal. Phengites and other high-silica muscovites, as will be shown later in this report are known to crystallize as more than polymorph: the two-layer normal muscovite and the 3-layer hexagonal. It is suggested that specimen No. 6 may be transitional between normal muscovite and the high-silica muscovites that crystallize in several polymorphs, and the diffuse scattering may be an embryonic attempt at a stacking arrangement other than the 2-layer type. In a recent paper, Axelrod and Grimaldi (1949) have reported a new polymorph of muscovite that contains three layers in the monoclinic unit cell and has a very small, variable 2V of from 3~ to 12~. A portion of this analyzed material has been obtained, and a re-study of it substantiates the existence of this new polymorph. However, there is a striking similarity between the "3-layered monoclinic muscovite" (space group C2 determined by Axelrod and Grimaldi, 1949) Weissenberg photographs and those of the 3-layered hexagonal mica polymorph (space group C3112 or C3212 determined by Hendricks and Jefferson 1939) recorded for biotite, alurgite, phlogopite, zinnwaldite and lepidolite. There is apparently room for a difference of opinion in the interpretation of the X-ray photographs of this muscovite. Axelrod and Grimaldi (1949) chose the former space group because of the presence of diffuse scattering in the (hakal) zone lines of one of the pseudo a- and pseudo b-axes, for they state (p. 569): -------------------— 47 -----------

oH'..........~ ~~~~~~~~~~~~~~~~~~~~IJ I~)~ ~~~~~~~~~~~~~~~~oc~ ~~~~~~~~~~~~~~~o ~~ kj1 ~ ii~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~........ ~.0 0' 0z 0 0 0~~~~~~~~~~~~ 00 c~~~~~~~~~~~~~~~~~~+C i~. Ii. o~~~~~~ H *'x,... —............................. -.............................. t.t t t tXHO co............................O ~ 0-............. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ D~

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN "The X-ray symmetry, based on the symmetry of the individual Weissenberg patterns, was C2h or D3d. If the differencesin diffuseness between Weissenberg patterns are neglected, a threefold axis is demonstrated and the Laue symmetry is D3do If, as we prefer, the optic axial angle of 12~ is not ascribed to strain and the differences in diffuseness are not neglected, the Laue symmetry must be taken as C2h with the structure very close to trigonal." Many Weissenberg photographs of this polymorph have been taken and.studied by the writers in an attempt to assign definitely a space group to this muscovite. No distinct differences in diffuse scattering along the reciprocal lattice lines described by Axelrod and Grimaldi (1949) were found. Physical distortion or corrugation of the crystals X-rayed may produce effects that could outweigh the diffuse scattering inherent in the crystal structure. Although Axelrod and Grimaldi (1949) consider strain unlikely, we are inclined to favor this as the cause of the 2V of 12~. However, it is our opinion that the significant fact concerning this mica is that it contains 5 layers in the unit cell and not the fact that it is monoclinic because of its 2V of 12~, or because of diffuse scattering. We are convinced that if measurements are made with sufficient accuracy, many micas will be shown to be triclinic by a few minutes or seconds. On the basis of personal communication with several investigators we have every reason to believe that the P angle of the 3-layer lepidolite described by Hendricks and Jefferson (1939) varies slightly from 90~ and thus must be considered as having a symmetry no higher than monoclinic. (We have been informed of v angles of 90005' by Dr. J. Smith of the Geophysical Laboratory and 90~30' by Mr. S. A. Forman as measured on photographs from Hendricks and Jeffersonts original material). Hendricks and Jefferson (1939) note that some of the micas they describe as hexagonal show a small 2V and that all give some diffuse scattering. They state, however, (p. 762), "There is no doubt but that the hexagonal description is accurate as a limiting case." We agree with this statement and believe that the monoclinic nature of these 3-layer micas is probably due to physical defects and is not inherent in the mica structure. It may be practical, in the future, if precision of measurement continues to increase, to class and discuss micas merely as 1-layer, 2-layer, 3-layer, etc., without regard to crystal system, inasmuch as all multi-layer forms might be derived from the single-layer form by the application of various combinations of symmetry operations. Postel and Adelhelm (1944) have described a white muscovite of late hydrothermal origin from the Wissahickon complex in which 2V varies from 22~ to 50~. The explanation advanced is that random shift in the structural planes of mica may have some bearing on the low and variable 2V. In their paper, they 49

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN include a Laue photograph showing asterism which, if not due to some physical distortion, may indicate the existence of another muscovite whose structure is aberrant'. A study of the crystal structure in muscovite, as well as in many other micas, was one of the principal objects of this study. The Weissenberg method was used primarily and usually one photograph about the a-axis was sufficient to identify the polymorph. The structures of about 150 muscovites, chiefly of pegmatitic origin, have been identified and include "normal" muscovites, rose muscovites, sericites and several uncommon varieties such as pinite, margarodite, adamsite and alurgite. Almost all have the 2-layer monoclinic muscovite structure described by Jackson and West (1930, 1933) and Hendricks and Jefferson (1939), except: 1. The 3-layer polymorph of Axelrod and Grimaldi (1949) 2. Some alurgites and other high-silica muscovites and phengites 3. Lithian muscovite 2. Morphology. After X-ray studies revealed the prevalence of polymorphism in the micas it was evident that the long standing differences of opinion as to the correct axial ratios and P angle were a natural result of studies by investigators unaware of the fact that there are approximately as many correct interpretations as there are polymorphs. For almost a century, mineralogists had tried to obtain one set of constants applicable to all the micas, or in other words, to force unimorphism onto a group now known to be characterized by polymorphism. Muscovite is the only mica for which a set of constants may be obtained, as it alone, for all practical purposes, crystallizes in but one polymorphic form. Hendricks and Jefferson (1939) showed how the 3 angle, for instance, varies with the number of layers in the unit cell. If structural identity is contained in one layer, than the P angle will be 1000 (approximately); if two layers are required, the P angle will be 95 and will be 90~ if there are three or more layers in the unit cell. With these problems in mind, Peacock and Ferguson (1943) reviewed the whole field of morphology of the micas and muscovite in particular. In the following discussion constant reference is made to their work. Goniometric measurements of lepidolite and other micas are few compared with those for muscovite, and for that reason they are practically omitted in the following discussion. However, a complete understanding of the morphology of mica will be forthcoming, as the problems and methods are essentially the same as those for muscovite. Before a