ENGINEERING RESEARCH INSTITUTE UNIVERSITY OF MICHIGAN ANN ARBOR QUARTERLY REPORT NATURAL MICA STUDIES (Covering period December 1, 1952, to February 28, 1953) By E. WM. HEINRICH Project M978 SIGNAL CORPS, U. S. ARMY CONTRACT DA 36-039 sc-15357, SC PROJECT NO. 152B-0, DA PROJECT NO. 3-99-15-022 March, 1953

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN QUARTERLY REPORT NATURAL MICA STUDIES (Covering period December 1, 1952, to February 28, 1953) INTRODUCTION During the quarterly period December 1, 1952, to February 28, 1953, several additional phases of research on the natural micas have been completed. These include a study of the zinnwaldites and the problem of zoning and overgrowths in all mica species. The work on both topics consisted of an extensive survey of the literature supplemented by new data obtained from specimens in the University of Michigan mica collections. With the acquisition and completion of x-ray studies on several critical specimens, the manuscript "Studies in the mica group; The structure of the high-silica muscovites (phengites)" by E. Wm. Heinrich and A. A. Levinson was completed. Work has also been completed on the study of roscoelite and oellacherite, and the manuscript for this study is practically complete. Electron photo-micrographs of several alleged specimens of barium-and mangan-muscovi-tes were taken and are now being interpreted. It is planned to submit both papers for publication in the near future. Since the publication of the conclusions on the relationship between chemistry and composition in the. muscovite-lepidolite series, interest in our work has been aroused at the Corning Glass Works, Corning, New York. Recently a Corning representative visited our laboratory to discuss the subject with particular reference to their problem of obtaining a source of lepidolite with a consistent Li2O content for glass manufacture. As a result of these discussions the Corning Company will analyze several specimens in the muscovite-lepidolite range, and we shall study them structurally. During this quarterly period a study of the world-wide distribution, geologic relationships, and paragenesis of all known lepidolite and lithiumbearing muscovite (> 2.0% Li20) deposits has been completed. Emphasis was

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN placed on paragenesis and on recommendations for potentially economically valuable deposits. It was hoped that the x-ray study of the dark-colored micas would be completed by the end of the quarter, but about 20 biotites from extrusive rocks still remain to be studied. The structures of all analyzed specimens, however, have been determined and are discussed in this report. During the next quarter, in addition to completion of the biotite study, the subject of mica symplectites will be investigated. Thin sections are now being made of about a dozen specimens showing symplectic intergrowths. Work will also begin on several initial phases of the final report. This will include cataloguing the many x-ray films and compiling data. A comprehensive study of manganophyllites is tentatively scheduled for completion, and if the results of the final group of spectrochemical analyses become available, a study of trace elements in micas may be ready. STRUCTURE OF ANALYZED BIOTITES, PHLOGOPITES, AND MANGANOPHYLLITES In the First Annual Report, mention was made of the work being carried on in an attempt to correlate chemistry, polymorphism, and paragenesis in the dark-colored micas. This work has been completed with the determination of the structures of the approximately 60 analyzed micas in this category. Most specimens have been described in the literature and were received from foreign investigators. The results of the study are presented in Table 1. A survey of the data will indicate that there is no evident relationship between chemistry, polymorphism, and paragenesis among the dark-colored micas, In the near future an attempt will be made to correlate the cell dimensions with chemistry. A SUMMARY OF THE MINERALOGY AND PARAGENESIS OF ZINNWALDITE Crystal Chemistry General Composition. The composition of zinnwaldite may be expressed by the following general formula: K2 (Fe21-2, Li2-5, A12)6 (Si6-7, A12-l) 020(F3-2, OH1-2)4 The (Al, PF, Li) group may be considerably deficient. The possible ionic substitutions that can take place are as follows: 2

TABLE 1 STRUCTURE OF ANALYZED BIOTITES, PHLOGOPITES, AND MANGANOFPHYLLITES ANALYZED BIOTITES Number Reference Structure Paragenesis FeO Fe203 MgO TiO2 728 Glass (1955' 5-layer pegmatite 26.72 2.87 n.d. 5.60 729 Stevens (1946) 1-layer pegmatite 8.96 5.51 16.15 111 797 Grout (1924) No. 1 1-layer (+5- basic segreg. in 7.72 7.44 16.55 i.67 layer?) granite 798 Grout (1924) No. 2 5-layer (mod. granite 14.80 4.05 10.21 2.2 scattering) 799 Grout (1924) No. 5 2-layer. granite P5.25 5.05 9.24 5.52,800 Grout (1924) No. 4 1-layer granite 25.75 1.14 6.16 4.4 801 Grout (1924) No. 5 1-layerperidotite 12.96 8.67 n.d. 1.50 n(1924) Nog -lyrpridnite 15.58 4.25 12.16.8 994 vanBiljdn(1940) 2-layer granite. 158 ^ T 960 Pagliani (1949) 2-layer mica schist 9.10 5.10 10.44 0.56 1001 Inoue (1950) 1-layer nepheline syenite 19.9 4.55 6.26.10 1002 Inoue (1950) 1-layer (very nepheline syenite 21.94 8.55 5.2 197 heavy scatt.) 1005 Inoue (1950) 1-layer nepheline syenite 21.02 12.45 4.19.04 1004 Inoue (1950) 1-layer (mod. cancrinite syenite 16.05 20.22 1.57 0.70 scattering) pegmatite 1010 Kavano (193355) 1-layer metamorphosed xenolith 16.58 5.28 8.99 2.45 1011 Kawano (1942) 1-layer lepidomelane-quartzfels 25.27 7.81 4.52 2.42 1084 Jakob (1951) No. 57 2-layer (weak 2 mica pegmatite 10.47 4.09 15.19 2.06 scattering)'1085 Jakob (19N1) No. 58 2-layer 2 mica pegmatite 9.72 2.24 14.22 1.64 i086 Jakob (19l1) No. 59 (5-layer?) 2 mica pegmatite 6.26 4.0 8.46.6 1087 Jakob (1951) No. 60 2-layer (veak 2 mica pegmatite 16.85 4.68 8.06 2.71 scattering) 1088 Jakob (1951) No. 61 i-layer lamprophre 15.84 5.05 11.17 1.95 1089 Jakob (1957) 1-layer feldspar pegmatite 28.61 0.00 6.72 1.99

TABLE 1 (cont.) Number Reference Structure Paragenesis FeO Fe203 MgO TiO2 1117 Coats and Fahey (1944) 1-layer (mod. pegmatite (siderophyl- 30.16 tr. 0.22 0.02 scattering) lite ll4 C. 0. Hutton (unpubl.) 2-layer from sands derived from l14.49 9.50 5.80 3.47 granitic rocks, Monterey Pen., Calif. 1145 Hallimond (1947) 1-layer marble 6.8 1.1 18.7 1.9 1257 Hutton and Seeyle 2-layer pegmatite-like lense 14.4l 3.92 11.11 3.02 (1947) 1262 Mauguin (1928) 1-layer (heavy from Tschebarkul 12.77 7.09 13.50 1.16 F-^~~~~~~ ~scattering) 1550 Yamada and Sugirua 1-layer (veak pegmatite 3.49 0.67 24.24 0.64 (1950) scattering)

TABLE 1 (cont.) ANALYZED PHLOGOPITES Number Reference Structure Paragenesis FeO Fe203 MgO TiO2 F2.Jakob.,...7. 1054 Jakob (1938)? Very diffuse from Morawitza 0.64 1.05 26.77 0.10 0.00 scattering 1055 Jakob (1931) No. 55 1-layer (mod. mica peridotite, 2.52 0.00 25.45 2.80 0.00 scattering) Italy 1056 Jakob (1932) No. 73 1-layer Revision of 55 2.52 0,00' 25.45 2.80 0.00 1057 Jakob (1932) No. 70 1-layer Rossie, N. Y. 4.79 0.00 22.30 4.07 0.12 1058 Jakob (1932) No. 69 1-layer Hull, Quebec 2.49 0.71 24.60 1.74 2.04 n 1059 Jakob (1932) No. 68 l-layer Burgess, Ontario 0.30 0.00 27.32 0.39 6.74 1060 Jakob (1932) No. 67 i-layer Burgess, Ontario 1.50 0.00 26.14 0.86 2.37 1061 Jakob (1932) No. 66 1-layer Isolo, Madagascar 3.28 0.00 25.29 2.19 1.15 1062 Jakob (1932) No. 65 2-layer (mod. Ampandrandara, 2.96 1;18 23.40 1.69 0.68 scattering Madagascar 1063 Jakob (1932) No. 64 1-layer Mandridano, Madag. 2.09 0.00 24.48 1.64 0.08 1064 Jakob (1932) No. 63 2-layer Saharakara, Madag. 2.79 1.68 23.78 1.11 0.56 1065 Jakob (1932) No. 62 1-layer Ambatoaba, Madag. 1.42 0.66 24.80 0.86 0.86 1066 Jakob (1928) No. 19 1-layer (heavy Simplon-Tunnel 0.00 1.71 24.79 0.39 0.00 scattering 1067 Jakob (1928) No. 20 1-layer dolomite-Tessin 0.00 1.31 25.81 0.83 0.00 1068 Jakob (1928) No. 22 1-layer contact met. car- 2.72 0.97 25.05 0.66 0.58 bonate rock, S.W. Africa 1069 Jakob (1928) No. 23 1-layer 0.58 1.92 28.18 1.27 1071 Jakob-not published 1-layer Skrabbole, Pargas, 0.71 1.01 27.80 0.12 2.12 Finland 1072 Jakob-not published 1-layer Pargas, Finland 5.59 1.54 22.00 1.41 2.80 1073 Jakob-not published 1-layer Patteby, Pargas, 1.47 1.29 26.16 0.33 1.87 Finland

TABLE 1 (cont.) Number Reference Structure Paragenesis FeO Fe203 MgO TiO2 F2 1074 Jakob-not published 1-layer Ontala, Pargas, 1.68 1.87 25.91 0.68 1.30 Finland 1075 Jakob-not published 1-layer Skrabb'ole, Pargas, 0.59 1.75 27.22 0.10 0.48 Finland 949 Dana (1892) Anal. 12 l-layer Rossie, N. Y. 7.62 1.12 21.47 1.16 4.00 p.633 1135 Pagliani (1940) 1-layer in crystalline 1.55 2.65 27.62 2.83 limestone 1139 Hutton and Seeyle 1-layer from marble, New 2.58 0.43 22.95 0.82 0.62 (1947) Zealand 1252 Pieruccini (1950) 1-layer pneumatalytic, Mt. 7.89 Tr. 15.66 0.33 2.57 Somma 1261 (a) Mauguin (1928) 1-layer (b and c) Mauguin (1928) 3-layer hexagonal Ambotoaba, 2.30 24.42 0.74 ach^~~~~~~~ ~~~~Madagascar 730 Prider (1940) 1-layer leucite lamproite, 3.75 2.18 19.66 8.97 0.66 731 West Australia 1325 Cross (1897) 1-layer in Wyomingite 0.90 2.73 22.40 2.09 1.03

TABLE 1 (cont.) ANALYZED MANGANOPHYLLITES Number Reference Structure Paragenesis FeO Fe203 MgO MnO Mn203 TiO2 1076 Jakob (1925, No. 8) 1-layer contact met. 0.00 16.94 26.79 4.52 0.00 0.00 1077 Jakob (1925, No. 7) 1-layer contact met. 2.54 0.00 29.22 Tr. Tr. 0.00 10i8 Jakob (1925, No. 6) l-layer contact met. 0.00 3.95 22.60 0.00 8.50 Tr. 1079 Jakob (1925, No. 5) 1-layer Varmland, Sweden 0.00 4.68 21.18 9.25 2.96 0.00 1080 Jakob (1925, No. 4) 1-layer contact met. 0.00 2.97 24.80 0.00 5.77 0.55 1081 Jakob (1925, No. 3) 1-layer contact met. 0.00 2.68 26.65 0.00- 4.07 0.41 1082 Jakob (1925, No. 2) 1-layer contact met. 0.00 0.91 29.28 0.00 3.92 0.00 1083 Jakob (1925, No. 1) 1-layer Varmland, Sweden 0.00 2.81 27.87 0.00 4.93 0.22 726 Kauffman, et al. (1925) 1-layer pegmatite ---- Spectrographic ----- -.

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN For Si: One-fourth of the Si may be replaced by Al. Minor Ti may also be substituted in this position. For Al: Ti, Fe2, Fe3, Mn, Mg, and Li. For K: Na, Ba, Rb, Cs, and Minor Ca. Trace Elements. Glass (1935) in studying zinnwaldite from Amelia, Virginia, reports the largest variety of trace elements ever recorded in a zinnwaldite: B, Be, Sn, Zn, Rb, Li, Cs, Ti, Cs2O = 0.10%, and Rb20 = 1.04o. Other trace elements reported in zinnwaldites are P, He, Mg, Mn3, Ga, Ba, Sc, T1, and Y (Rankama and Sahama, 1950). Polymorphism Three structural varieties of zinnwaldites are now known: (a) 1-layer monoclinic, (b) 2-layer monoclinic muscovite-type, and (c) 3-layer hexagonal, Hendricks and Jefferson (1939) report zinnwaldites with the 1-layer structure from Amelia, Virginia: Zinnwald, Bavaria; and Brambach, Saxony. They also report a 3-layer hexagonal structure zinnwaldite from Amelia, Virginia. The x-ray data of 14 zinnwaldites studied in the Mineralogical Laboratory of the University of Michigan are tabulated in Table 2. Eight samples studied have crystallized as the 1-layer polymorph, five as the 3-layer polymorph, and one as a 2-layer polymorph very similar to that of normal muscovite. An excellent Weisenberg photograph obtained from a flake of zinnwaldite (Spec, 1251) from Ubini, West Australia (Murry and Chapman, 1931) indicates the 2-layer monoclinic structure with the presence of 061 reflections with ~ odd. The significance of these reflections has been discussed in detail by Hendricks and Jefferson (1939) and Levinson (1953). The significant point is that 06i reflections with I odd are present in the 2-layer muscovite (heptphyllite) type of micas, and their presence indicates considerable distortion from the ideal mica structure. These reflections should be absent in the ideal mica structure on the basis of structure-factor calculations. Hendricks and Jefferson (1939) believe that the muscovite-like micas produce these reflections because of an incomplete filling of the octahedral positions which results in a distortion, the extent and nature of which are not known. Muscovite has only two-thirds of the octahedral positions filled, whereas the ideal octaphyllite micas have all these positions occupied. Therefore, the presence of 06~ reflections with I odd recorded on the Weissenberg photograph of Spec. 1251 -8

TABLE 2 NEW STRUCTURAL STUDIES OF THE ZINNWALDITES Identification Polymorphic 2V* ColorLocality Remarks Number Form 778 1-layer Medium Green Bohemia Borrowed from Harvard 779 1-layer ND Pale green- Zinnwald Yale No. 1504 brown 780 1-layer ND Green-black Fredrickstown, Mo. American Museum 786 l-layer Medium Pale brown Amelia, Va. Harvard 787(a) 3-layer ND Pale brown Amelia, Va. Analyzed by Glass (1935) 1128 1-layer ND Rust brown Martha Mine, U. S. Nat'l Pongo. Bolivia Musuem R8089 1242(a) 1-layer Medium Brown Amelia, Va. outer edge zone 1242(b) 3-layer Small Dark brown Amelia, Va. inner zone 1251 2-layer ND Pale brown Ubini, W. Murry and Australia Chapman (1931) 1324 3-layer ND Dark brown Brown Derby, Colorado 1329 1-layer Large Amber brown Bohemia Ontario Museum 1330 3-layer ND Dark brown Altenberg Rabenglimmer, Ontario Museum 1331 1-layer ND Green-black Bohemia Ontario Museum 1337(a) 3-layer Small Brown Brown Derby, Core zone Colorado 1537(b) 3-layer Small Dark brown Brown Derby, Margin zone Colorado *Large 2V means greater than 40~ Medium 2V means 15 -40 Small 2V means 0~-15~ ND means not determined but assumed to be Medium 9

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN implies that there is distortion in the mica structure and that this zinnwaldite approaches the heptaphyllites in structure. This appears to be the only specimen of z.innwaldite which can be definitely shown to be more closely related structurally to heptaphyllite micas than to the octaphyllites. Noteworthy are the facts that the deficiencies in the Al, Fe, Li (octahedral) position have been reported, and that Lemke, et al. (1952) report the occurrence at Amelia, Virginia, of many books intermediate between zinnwaldite and muscovite and some showing gradations from muscovite centers to zinnwaldite borders. This would be expected if zinnwaldite were to approach a heptaphyllite composition. The intensities of the 061 reflections with I odd from the specimen of zinnwaldite from Ubini (No. 1251) are of an order of magnitude somewhere intermediate between normal muscovite and lithian muscovite (apparantly closer to normal muscovite). The intensities of these reflections and other selected Okl reflections are listed in Table 3. TABLE 3 APPROXIMATE OBSERVED INTENSITIES OF SOME (Oki) REFLECTIONS OF 2-LAYER FORMS Plane Normal Muscovite 2-Layer Zinnwaldite Lithium Muscovite 020 W W- a 022 MW MW VW 026 a VW VW 045 a VVVW VW 061 W W- VW 065 VW VW VW 066 W W VVW 067 vw vw vvw 066 w vw vvw o67 VW VW VVW 069 W VW VVW 10

- ENGINEERING RESEARCH INSTITUTE ~ UNIVERSITY OF MICHIGAN Optics In Table 4 are listed the optical constants of zinnwaldites as reported in the literature. The range of values is as follows: = 1.555-1.558 Axial plane: parallel with (010) P = 1.570-1.589 Color: gray to amber-brown 7 = 1.572-1.590 Sp. Gravity: 2.916-3.018 2V = 0-70~ Average 25-30~ An attempt was made to correlate the optical data with the chemistry. Unfortunately only three analyzed zinnwaldites have optical determinations sufficiently precise for plotting, which prevents any definite conclusion. There does, however, appear to be a general increase in the indices with an increase in iron content. Single-Crystal Variations Baumhauer (1879) reported a zoned zinnwaldite from Zinnwald, Bavaria. According to his descriptions, crystals of zinnwaldite showed zones of different widths normal to an "A" structure. These zones were yellow, whereas the rest of the crystal was colorless to gray. He postulated the zoning to be due to chemical differences. Upon examination under polarized light he found the various zones to have the same adsorption formulae. Sollas (1889) reports a zinnwaldite found in the granite of Mourne Mountains, Ireland, which shows a zonal structure with a dark-green center having an axial angle of 44004' and a border zone having an angle of 52~06. Sterrett (1923, p. 133) reports zoning in a zinnwaldite from the Palermo Mine, Grafton County, New Hampshire: "The sheets of this mica show a clear brown core and a greenish-blue exterior about the color of indicolite tourmaline. These colors are arranged parallel to planes of crystallization, and the blue contains thin zonal growths of the brown." A zoned crystal from Amelia, Virginia (spec. 1242) upon examination showed optical and structural variations. The dark-brown inner zone had a 2V of 5-4~, P = 1.584, and 7 = 1.586, and crystallized as the 3-layer form. The marginal zone had a 2V of 28~, 5 = 1.583; and 7 = 1.588, and crystallized as the 1-layer form. It has been shown in the lepidolites that the 3-layer and 1-layer forms have almost identical chemical compositions; it is believed that 11

TABLE 4 OPTICAL PROPERTIES OF ZINNWALDITES Locality a y Axil Color References Locality____________ _Angle Zinnwald, Bohemia 1.551 1.578 1.581 2V = 29-30~ Brown Larsen and Berman, 1934 Zinnwald, Bohemia 1.5435 1.5729 1.5750 2V = 30-32~ Winchell, 1942 -1.5450 -1.5737 -1.5757 Zinnwald, Bohemia 1.541 1.571 1.573 2V = 28047' Light Jakob, 1927 Gray Zinnwald, Bohemia 1.5511 1.5777 1.5812 2V = 14048' Kunitz, 1924 2E = 47030t Zinnwald, Bohemia 1.539 1.564 2V = 30~ Hendricks and Jefferson, 1939 Altenberg, Saxony 1.5572 1.5850 1.5876 2V = 10021' 2E = 36~20' Kunitz, 1924 Fichtelgebirge, 2E = 47~10' Durfeld, 1909 Bavaria Brambach, Saxony 1.572 2V = 25-30~ Hendricks and Jefferson, 1939 Volhynia, Russia 1.587 2V = 13-25~ Buryanova, 1940 Erongo Schlucht, 1.573 1.576 2V = 31~+30 Frommurze, Gevers, S.W. Africa +0.002 +0002 and Rossouw, 1942 Umberatana, 1.57 2V = 10~ Mawson and South Australia Dallwitz, 1945 Wakefield, Quebec 1.5357 1.5596 1.5628 2V = 30-39~ Winchell, 1942 -1.5440 -1.5671 -1.5701 New Ross, Nova 2V = 700 (?) Amber Walker and Parsons, Scotia Brown 1924 Amelia, Virginia 1.550 1.580 1.580 2V = 0-33~ Bronze Glass, 1935 -1.558 -1.589 -1.590 to Gray Amelia, Virginia 1.550 1.584 2V = 0~ Hendricks and Jefferson, 1939 12

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN the 3-layer form results from a twinning of the 1-layer form. This twinning also probably accounts for the structural variation in this zinnwaldite, as the chemistry of the 2 zones probably varies very little. A systematic x-ray study of several zoned zinnwaldites from the Brown Derby, Colorado, pegmatite has shown that polymorphic variation does not necessarily exist within strongly zoned crystals. The zoning of these micas is a color zoning with a light outer zone and dark inner zone. Both zones gave the same optical results: P = 1.584, Y = 1.585, and 2V = 0.2~. There are neither optical changes nor structural changes with respect to the zoning in zinnwaldite specimens from this locality. No reference to a chemical study of any of the zoned crystals was found in the literature, although most writers agree that slight chemical differences are probably the main reason for zoning in the zinnwaldites. Lamke, Jahns, and Griffitts (1952) report that many books intermediate between zinnwaldite and muscovite occur in the Amelia District of Virginia and that some grade from muscovite centers to zinnwaldite borders. They concluded that the zinnwaldite formed later than muscovite and in some books formed rims around cores of muscovite. General Paragenesis A survey of the literature describing the occurrences of zinnwaldite leads to the following generalizations concerning the paragenesis of zinnwaldite: 1. Zinnwaldite is associated with granitic magmas, especially with pegmatites, greisens, and granites. 2. Rocks containing zinnwaldite usually contain one or more of the following minerals: topaz, cassiterite, lepidolite, feldspar (cleavelandite), beryl, tourmaline, tantalite, columbite, monazite, spodumene, and fluorite. 3. Zinnwaldites in pegmatites are usually associated with Na-Li replacement units. 4. Zinnwaldite probably crystallizes after muscovite and/or biotite but before lepidolite (Shibate, 1952). 13

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN ZONING IN MICAS Many species of mica, particularly those of pegmatitic origin, display zoning of various types in physical, optical, or structural properties within a single crystal. Several of these properties are interrelated. Primary types include those formed during the period of crystallization, whereas secondary types of zoning result from changes occurring after the period of crystallization. The information on different types of zoning in micas given below has been drawn both from the literature and from the examination of specimens in our mica collections. Color Zoning The type of zoning that is the most easily recognized is color zoning, which may or may not be accompanied by significant differences in indices, 2V, etc., in the various zones. Cleavage flakes of mica often show concentric bands displaying slightly different colors. These bands are parallel with the prism faces (110) in the diamond-shaped crystals, parallel with the prism and clinopinacoid in the pseudohexagonal crystals, or parallel with the margin of an irregular crystal. Five different varieties have been noted. They are: 1. Core and Margin. Two distinct zones parallel to the crystal outline or to an irregular outline. The zonal boundary may be sharp or gradational. Visible on the cleavage faces. 2. Oscillatory Zoning. Alternating bands of two different colors. Generally the boundaries are sharp. Zones are parallel with a diamond or pseudohexagonal shape. Visible on cleavage faces. 3. Varicolored Zoning. Several shades of color are involved. From core to margin, each zone tends to become darker or in some specimens lighter than the preceding zone. Visible on the cleavage faces. 4. Three-Dimensional Color Zoning Two shades of color are involved. The cleavage plates at one end of a crystal are one color, whereas those at the other end are of another color. The two colors grade into one another along the length of the c-axis. 5. Crosshatch Zoning. This type appears as linear streaks associated with two sets of reeves or as a plaid pattern resulting from intersecting spots and patches. Visible on the cleavage faces. 14

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Optical Zoning Optical zoning without accompanying color zoning is inconspicuous and hence not easily recognized. Variation in indices of refraction, birefringence, size of optic angle, or optic orientation indicate the presence of optical zoning. Since each differently colored zone generally has slightly different optical properties, optical zones often correspond to color zones. Megastructural Zoning Megastructural zoning results from the systematic distribution of reeves, ruling, "A" structure, or wedging. Reeves are lines or shallow corregations that lie in the plane of cleavage and are oriented perpendicular to the prismatic or clinopinacoid faces. "A" structure has two sets of reeves intersecting at 60~ along the edges of a V-shaped fragment. The interior portion of the "A" may or may not show other types of zoning. Some "A" mica will have color zones developed normal to the reeves. Rulings are parting planes perpendicular to the prismatic or clinopinacoid faces. Wedging is caused by interlayering of unequal-sized sheets and is commonly associated with "A" mica. In many books these imperfections do not extend over the entire width of the crystal but are concentrated either in central or in marginal parts. Polymorphic Zoning The most difficult zoning to detect is that showing variation in polymorphic types. Generally, it is marked by an irregular variation in the crystal structure across the basal cleavage face. Inclusion Zoning Zoning of inclusions is seen either as inclusion-filled cores surrounded by inclusion-free margins, as an alternation of inclusion-rich zones with inclusion-free zones, or as irregularly distributed zones of inclusions. All three types are evident on cleavages. The inclusions occur as needles, flattened crystals, or irregular spots and are commonly crystallographically oriented. As many as 30 different minerals have been reported as inclusions in mica. Frondel (1940) states that the inclusions may have been trapped on a crystal face and buried by continual growth of the mica crystal. Some inclusions are thought to be the result of exsolution. Still others may be secondary. 15

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Three-Dimensional Zoning Variation in color along the c-axis has been mentioned above. In addition, variations in optics and size and shape of the zonal pattern are found. Zoning in Muscovite Color zoning is a rather common phenomenon in muscovites ofpegmatitic origin. Zoning of different types has been reported in muscovites fromthe following localities: Minas Gerais, Brazil. "Symmetrically zoned color patterns, hexagonal'in outline are sharply defined and most commonly are composed of recurrent zones of two different tints of ruby. Rarely, a rhombshaped ruby core is surrounded by hexagonally shaped zones of lightercolored ruby. In some specimens, cores of ruby mica are surrounded by zones of green and yellow-green mica, Three well-defined linear patterns are known. In one, linear bichromatic streaks are oriented along two sets of reeves; in another, called checkerboard or plaid, the bichromatic lines are at right angles to each other; the third, called'hen track'... has two sets of streaks parallel to the adjacent reeve sets, and the third is parallel to the limiting crystal face (010 or 110)." Two irregular color patterns have also been found in this district (Pecora, Klepper, and Larrabee, 1950). Ejiba, Nigeria. Jacobson and Webb (1946) report three-dimensional color zoning in muscovite from an albitized pegmatite. At one end of a crystal the muscovite is apple-green and at the other it is lilac. The colors grade into each other along the length of the caxis. The only chemical difference in the two types is a slightly higher iron-manganese ratio in the former. San Diego County, California. Some zoned crystals have been reported from the Pala District, (Jahns and Wright, 1951) with green centers and intermediate zones that are yellow to white. Madison County, Montana. Stoll (1950) reports sheet mica that is colorless and shows a criss-cross pattern or banding of run-colored areas. Taos County, New Mexico. Optical zoning was reported by Heinrizh and Levinson (1953) in pink muscovite flakes from the Harding pegmatite. Many of the muscovite flakes have a very thin rim that display-s lower birefringence than the main central part of the zrystal. 16

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Rio Arriba County, New Mexico. Heinrich and Levinson (1953) also report small flakes of muscovite from the Petaca District that show pink and green colors grading into each other. Clear Creek County, Colorado. Muscovite from the Ajax pegmatite was reported by Hanley et al. (1950) to show alternating color strips of clear and ruby color. Southeastern Piedmont Area. Jahns (1945) summarizes the color characteristics of the muscovites of this area as follows: a) Many books are color zoned with gridiron or chessboard patterns, concentric color bands, strips, or faint irregular mottling. b) Nearly all books contain pale rims that are more greenish than the interior. This appears to be secondary zoning. c) Color variations in individual books are more pronounced where the muscovite is light to olive-green. New England. Sterrett (1923) summarizes the types of zoning in New England muscovite as follows: "Some muscovite shows variations in color that accord with crystal structure. The variations generally appear in bands that follow the outline of the crystal. Thus,... one may see a dark rum-colored center surrounded by a fringe of light rum or yellow;... or the center may be light and the border zone dark. In some sheets there are alternating bands of varying color." Woodward (1951) reports an unusual type of megastructural zoning in the green muscovite of the Lord Hill pegmatite near Stoneham, Maine. One book of green muscovite grades from perfectly normal flat folia on one side of the specimen into a gray botryoidal folia on the other. The botryoidal structure may be due to late volume increase because of the addition of materials to the pre-existing muscovite. A group of selected specimens showing especially distinct color zoning was selected from our mica collection for optical study. Indices of refraction were measured on an Abbe refractometer and 2E was determined by the Mallard equation. The optical data are listed below. 17

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN No. 5. Gregory Mine, Jackson County, North Carolina green core clear margin a;= 1.562 = 1. 565 = 1.598 = 1.602 7 = 1.603 = 1.612 2E = 72~ 2E = 67~20' No. 7. Jasper Mica Mine, Jackson County, North Carolina dark-brown medium-brown light-brown core intermediate zone margin = 1.565 a = 1.564 = 1.566 P = 1.602 = 1.601 = 1,601 7 = 1.606 7 = 1.605 = 1.605 2E = 670251 2E = 65015' 2E = 62040' No. 9. Franklin-Sylva District, North Carolina clear core light-brown margin a = 1.569 a = 1.572 = 1.607 = 1.612 7 = 1.613 7 = 1.615 2E = 54~10' 2E = 59015' No. 15. Franklin-Sylva District, North Carolina stained core clear margin a = 1.566a 1.567 = 1.602 = 1.604 7 = 1.608 y = 1.608 2E = 61040' 2E = 60010 No. 46. Franklin-Sylva District, North Carolina stained core clear margin a = 1.562 a = 1565 P = 1.598 = 1.601 7 = 1.6077 = 1.607 2E = 62~50' 2E = 635030 18

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN No. 57. Franklin-Sylva District, North Carolina medium-brown core clear margin a= 1.560 a = 1.560 P = 1.595 P = 1.593 7 = 1.604 7 = 1.607 2E = 70000' 2E = 69045' No. 64. Franklin-Sylva District, North Carolina clear core spotted margin a = 1.564 = 1.559 P = 1.601 o = 1.596 7 = 1.607 = 1.606 2E = 65~30' 2E = 63~ No. 75. Franklin-Sylva District, North Carolina light-brown core medium-brown margin = 1.562 = 1.566 P = 1.599 1 = 1.601 7 = 1.606 7 = 1.606 2E = 65030' 2E = 65~40' No. 82. Franklin-Sylva District, North Carolina light-brown core medium-brown margin = 1.565 = 1.566 = 1.599 B = 1.603 7 = 1.605 7 = 1.606 2E = 64~501 2E = 64 50' No. 124. Spruce Pine District, North Carolina medium-green core light-green margin a = 1.570 Q = 1.569 = 1.610 P = 1.603 7 = 1.615 7 = 1.609 2E = 64~15' 2E = 67010' 19

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN No. 128. Spruce Pine District, North Carolina dark-green core light-green margin a= 1.570 ca 1.569 = 1.610 p = 1.606 y = 1.616 = 1.616 2E = 70000' 2E = 67 30' No. 190. Muscovite Claim, Latah County, Idaho light-brown portion clear portion = 1.562 o = 1.562 = 1.598 1 = 1.599 y = 1.602 = 1.604 2E = 63015' 2E = 63~ No. 374. Hebron, Maine medium-tan light-tan clear core intermediate zone margin a c= 1- 6 = 1.563 = 1.560 D = 1.594 P = 1.598 P = 1.591 7 = 1.603 = 1.601 7 = 1.598 2E = 67010' 2E = 65015' 2E = 61050' The above data indicate that slight differences in index of refraction and size of the optic angle accompany variation in color. The darker-colored zones and those that are stained or contain inclusions tend to have higher indices and a larger optic angle. However, no general tendency is apparent in the order of zoning from this limited number of specimens. A zoned muscovite may have either a lighter or a darker core than the surrounding border. No quantitative correlation has yet been attempted between chemical composition and color in the muscovites. Olson (1942) states that green muscovite probably contains more iron than ruby. Heinrich and Levinson (1953) correlated the pink color in muscovite with an absence of Fe2 and an equality or predominance of Mn3 to Fe3. The chromophores in muscovite are Fe2, Fe3, Mn3, Cr, and, rarely, Ti. Zoning in Lepidolites Polymorphic zoning with associated optical properties is the only type recorded in lepidolites. The following specimens, described in Quarterly Report No. 2, show optical and polymorphic variation: 2Q

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN biaxial portion uniaxial portion No. 539 1-layer 3-layer rhombohedral No. 679 2-layer lithian muscovite 3-layer rhombohedral No. 476 1-layer 3-layer monoclinic The zoning is irregularly distributed as spots or patches on the basal cleavage. Only specimen No. 559 examined showed any systematic shape or distribution of zoning. It has been shown that the variation in polymorphic forms is related to a slight variation in the Li20 content. Zoning in Biotite Brogger (1921) reports optically zoned biotite from the Fen Region of southern Norway in damkjernite, a porphyritic rock containing phenocrysts of biotite and pyroxene in a fine-grained groundmass of pyroxene, biotite, and magnetite with minor nepheline, microcline, and calcite. In hand specimen the biotite is bronze-brown, but under the microscope it is reddish in color. The biotite flakes are not six-sided, but usually irregular. Pleochroism of the core is from ruby-red or orange-red to bright yellow; pleochroism of the rim is from bright green to bright yellow or colorless. The biotite seems to be uniaxial and usually has weak pleochroism. Johannsen (1948) notes the occurrence of color-zoned biotite in minette, a lamprophyre of the syenite series composed of biotite phenocrysts in an orthoclase groundmass. The biotite is dark brown under the microscope and in both the phenocrysts and the groundmass it shows "a light center surrounded by an iron-rich opaque border." Grout (1924) mentions that biotite in the Minnesota granites commonly shows a zoned structure. Over 100 specimens of biotite in the mica collections were examined for zoning. None of the biotite of pegmatitic origin showed zoning of any type. One thin section of minette from Freiberg, Saxony, contined phenocrysts of zoned biotite as described by Johannsen. Apparently zoning in biotite is very uncommon and is restricted to the phaneritic and porphyritic igneous rocks. Zoning in Siderophyllite None of the specimens of siderophyllite in the mica collection displays zoning. However, two occurrences have been described in the literature. 21

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Clark e (1887) describes a bronzy black mica, from Pikes Peak, Colorado,... resembling phlogopite externally,... but made up of a core composed of a soft rotten material, evidentally derived from the original mica, and surrounded by a broad black margin of the latter. Streaks of rusty alteration products reached into the margin in every direction." Analyses for both zones are given as follows. Margin Center H20 4.54 7.28 SiO2 34.21 54.63 A1203 16.53 17.95 Fe203 20.12 31.25 FeO 14.17 3.01 MnO 0.91 0.34 CaO 0.48 0.81 MgO 1.34 1.08 Na20 1.43 0.89 K20 6.50 1.96 F 0.08 0.54 Coats and Fahey (1944) describe an optically zoned siderophyllite from an alaskite pegmatite near Brooks Mountain, Alaska. In thin section some of the plates show a core having.a double refraction of 0.050 and 2V 6~-8~ O surrounded by a discontinuous rim of pale-blue mica having a double refraction of 0.028 and 2V = 37~ Zoni in Phlogopite Orcel (1924) described a peculiar green phlogopite from Snake Creek, Utah, that displays oscillatory color zoning. A specimen in our mica collection shows alternating light- and dark-green zones. The dominant portion of the irregular crystal is a light yellowish green with fine darker-green bands parallel with the prism and clinopinacoid. There are minor reeves normal to the color bands. Optical properties are as follows. Orcel Project M978 np = 1.5529 o = 1.552 P = 1.592 ng = 1.5910 7 = 1.593 uniaxial 2V = 2-3~ Both color zones have essentially the same optical properties. 22

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Larsen (1941) reports color-zoned phlogopite of hydrothermal origin associated with aegirine and sodic amphibole in contact-metamorphosed limestone. He states that the micas vary moderately in indices of refraction. They are strongly zoned with a pale-brown, or commonly a green core, and one or more outer zones that may be lighter or darker than the core. Some of the outer zones have pleochroism which is the reverse of that normal in phlogopite, Larsen gives the following optical data: Specimen IH- 2 core: a = 1.576 p = 1.613 y = 1.613 pleochroism a, nearly colorless D and 7 faint brown intermediate zone: a = 1.576 P = 1l6135 = 1.614 pleochroism a, deep chestnut brown P and y, nearly colorless outer zone: pleochroism a, deep chestnut brown ( and 7, nearly colorless Generally the indices or refraction are the same for the different zones, but the axial angle may be a little larger in the outer zones. Specimen 1369 from Thorne Township, Quebec, Canada, (No. 462A-11) on loan from Cranbrook Institute) shows four concentric color zones. The specimen varies in color from a deep red-brown at the core to a light green-brown at the margin. The boundaries between zones become sharper toward the core. Optical data for each zone are as follows. Zone _a _ 2V Color 1 (core) 1.575 1.622 1.622 1~-2~ deep red-brown with many rutile inclusions 2 1.575 1.622 1.622 1~-2~ deep red-brown with no inclusions 3 1.568 1.611 1.611 0~ red-brown, inclusion free 4 (margin) 1.559 1.599 1.599 0~ green-brown From margin to core, each zone shows a progressive increase in indices and size of the optic angle. Specimen 1370, an irregularly shaped specimen from Canada, has parallel arrangement of light-brown inclusion-free zones alternating with darkerbrown zones containing numerous inclusions of rutile. Both types are essentially uniaxial. The inclusion-free zones show no asterism and yield a sharp 23

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN optic axis figure whereas the zones bearing inclusions show strong asterism and yield an extremely diffuse figure. Indices ot refraction for each zone are: a a and y Clear zones 1.541 1.570 Zones bearing inclusions 1.544 1.578 Numerous specimens of Canadian phlogopite in the mica collection commonly display concentric color zones. Generally, the optical properties for the different zones in a single crystal are essentially the same. Wherever a variation was noted, the darker-colored zones or zones of inclusion have higher indices of refraction and a larger optic angle. More specimens of phlogopite tend to be darker toward the center and lighter toward the rim than the converse. Correlation between color and chemical composition has been attempted by Hall (1941) and Kennard and Howell (1941). The relative amounts of Fe2, Fe3 and Ti are responsible for the different colors in biotite. Ferrous iron gives the green colors and titania produces red and brown colors, but magnesia tends to mask the colors produced by titania according to Hall (1941), however, it seems unlikely that a nonchromophone such as Mg could seriously influence the pigmenting power of such elements as Fe2 and Ti. Ferric iron imparts shades of yellow and red; whereas a combination of Fe2 and Fe3 causes shades of blue. Small amounts of Fe3 and Ti4 have a greater effect on indices of refraction than a comparable amount of Fe2. As Hall points out, color does not give a good indication of the chemical composition. However, a darker-colored zone showing higher indices of refraction indicates an increase in Fe203 or TiO2. 24

TABLE 5 TYPES AND RELATIVE ABUNDANCE OF ZONING IN MICAS Mineral Type of Zoning Frequency Occurrence Muscovite color-indices common pegmatite inclusion-indices common pegmatite megastructural very common pegmatite Lepidolite polymorphic not uncommon pegmatite Zinnwaldite color rare pegmatite optical rare pegmatite megastructural rare pegmatite polymorphic rare pegmatite Phlogopite color common pegmatite inclusions common pegmatite optical common pegmatite Biotite color uncommon igneous rocks optical uncommon igneous rocks polymorphic uncommon igneous rocks 25

BIBLIOGRAPHY Baumhauer, H., (1879), Zeit. Krist., 3, pp. 113-121. Brogger, W. C., (1921), Die Eruptivgesteine des Cristianiagebietes, IV, p. 294. Buryanova, E. S., (1940), Mem. Soc. Russe Min., 69, pp. 519-540. Coats, R. R. and Fahey, J. J., (1944), Am. Mineral, 29, pp. 373-377. Clarke, F. W., (1887), Amer. Jour. of Sci., 34, pp. 131-137. Cross, W., (1897), Amer Journ. Sci., ser. 4, vol. 4, pp. 115-141. Dana, E. S. (1892), A System of Mineralogy, 5th ed., p. 633. Durrfeld, V., (1909), Zeit. Krist., 46, pp. 563-598. Frommurze, H., Gevers, T. W., and Rossouw, P. J., (1942), Geol. Survey South Africa, Explanation of Sheet No. 79, pp. 107-114. Frondel, C., (1940), Am. Mineral, 21, 777-799. Glass, J., (1935), Am. Mineral, 20. pp. 741-768. Grout, F. F., (1924), Am. Mineral, 9, pp. 159-165. Hall, A. J., (1941), Am. Mineral, 26, p. 29. Hallimond, A. F., (1947), Min. Mag., 28, pp. 230-242. Hanley, J. B., Heinrich, E. Wm., and Page, L. R., (1950), U. S. Geol. Survey Prof. Paper 227, p. 29. Heinrich, E. Wm., (1946), Am. Jour. of Sci., 244, pp. 836-848. Heinrich, E. Wm., and Levinson, A. A., (1953), Am. Mineral, 38, 25-49. Hendricks, S. B. and Jefferson, M. E., (1939). Am. Mineral, 24, pp. 729-771. Hutton, C. 0. and Seeyle, F. T., (1947), Trans. Roy. Soc. New Zealand, 76, pp. 48I-491. Inoue, T., (1950), Jour. Geol. Soc. Japan, 56, pp. 71-77. Jacobson, G., and Webb, J. S., (1946), Geol. Surv. of Nigeria, Bull. 17. Jahns, R. H., (1945), Geol. Soc. of Amer. Bull.,Vol. 56, No. 12.pt. 2, p. 1170. Jahns, R. H., and Wright, L. A., (1951), Calif. Div. of Mines, Special Report 7 A. Jakob, J., (1925), Zeit. Krist., 61, pp. 155-163. Jakob, J., (1927), Schweiz Min. Pet. Mitt., 7, pp. 139-141. 26

Jakob, J., (1928), Zeit. Krist., 69, p. 219, Jakob, J,, (1931), Zeit. Krst., 79, pp. 367-368. Jakob, J., (1937), Schweiz, Min. Pet, Mitt., 17, pp, 149-153. Jakob, J., (1938), Schweiz, Min. Mitt., 18, p. 4735 Jakob, J., (and Parga-Pondal, I.), (1932), Zeit. Krist., 82, pp. 273 and 282. Johannsen, A., (1948), A Descriptive Petrography of the Igneous Rocks, I, University of Chicago Press. Kauffman, A. J., Mortimore, D. M. and Hess, H. D., (1950), U. S. Bur. Mines, Dept. of Investig. 4721. Kawano, Y., (1933) Proc. Imp. Acad. Tokyo, 9, PP. 613-616. iKwano, Y., (1942), Jouro Jap. Assoc. Mineralogists, Petrologists and Econo Geologists, 27, pp. 283-90. Kennard, T. G., and Howell, D. H., (1941). Am. Min. 26. p. 405. Kunitz, W., (1924),NeuesJahrb., Beilage Band, 50, pp. 365-413. Larsen, E. S., and Berman, H., (1934), U. S. Geol. Surv., Bull., 848, pp. 1653237. Larsen, E. S,, (1941), U. S. Geol. Survey Prof., Paper 197 A, p. 55. Lemke, R. W,, Jahns, R. H. and Griffitts, W. R., (1952), U. S. Geol. Surv., Prof. Paper 248-B. Levinson, A. A., (1953), Am. Mineral, 38, pp. 88-107. Mauguin, C., (1928), Bull. Soc. Fr. Min., 51, pp. 285-332. Mawson, D. and Dallwitz, W. B., (1945), Trans. Roy, Soc., South Australia, 69, pp. 22-49. Murry, D. G., and Chapman, F. E., (1931), Jour, Roy. Soc. West Australia, 17, pp. 151-155. Olson, J. C., (1942), U. S. Geol. Survey Bull.,, 91 P, p. 379. Orcel, M. J., (1926), Bull. Soc. Fr. Min., 48, pp. 362-366. Pagliani, G., (1940), Atti. Soc. Ital. Sci. Nat. MEs. Civico Milano, 79, pp. 20-22. Pagliani, G., (1949), Atti. Soc. Ital. Sci. Nat. Mus. Civico Milano, 88, pp. 191198, Pecora, W. T. Klepper, M. R,, and Larrabee, D. M., (1950), U. S. Geol. Survey B11u. 964 C. Pieruccini, R., (1950), Atti. Soc. Toscana Sci. Nat. Mem., 57, Ser. A. pp. 145-175, 27