MICA, GROUP: BIOTITE, LEPIDOLITE, MUSCOVITE
MICAS group (Glimmers—Micas—Слюды), from Lat. “mico” – to glisten. Russ. name is from Old Slavic “sluda” – a stone. A group of sheet silicates. Monoclinic system. Hardness 2-4. Density 2.8-3. Glass luster to pearl one, along cleavage planes. Cleavage in one direction. From this group biotite, lepidolite, muscovite are described here. They are more often than other micas represented as inclusions in gemstones and sometimes produce aventurescence in them. Some of them are used as ornamental stones when in massive aggregates.
Biotite (Biotit—Biotite—Биотит) (Hausmann, J.F.L. 1847), after J.B. Biot (1774-1862), Fr. physicist and chemist, who first called attention to the optical differences in micas. Composition – K(Mg,Fe2+)3[(OH,F)2|(Al,Fe3+)Si3O10]. It is found as columnar and plate crystals with maximum size of plates up to 7 m2. Non-transparent to translucent. Color: black, more rarely brownish, sometimes with reddish or golden tone. Of special interest are its inclusions in feldspars, chrysolite and other gemstones. At one of the deposits of Norway, to the north from Bergen, Hordaland, inclusions of biotite change to brown the color of chrysolite and produce asterism in it.
Lepidolite (Lepidolit—Lépidolite—Лепидолит) (Klaproth, M.N. 1792), from Gk. “lepidos” – scale, alluding to the scaly appearance of many specimens. Composition – K(Li,Al)3[(F,OH)2|(Si,Al)4O10]. Translucent to transparent. Color: white, gray, yellow, pink, violet of different tones. Pink color depends on the admixture of manganese. Plate individuals of lepidolite reach the size of 6x9 cm. It is not rarely produces flaky-shelly sphaerolites – Barbot’s-eye. A variety close to lepidolite in the content is zinnwaldite, which is found as flaky masses of pale violet, yellowish and brown hue with greenish tone.
Deposits. Lepidolite is found in granite pegmatites, greisens and ore veins. In Russia, its accumulations are known in the Middle Urals, in pegmatites of Lipovka, Mokrusha and Murzinka. In the South Urals, near Annenskoye, not far from the railway station of Kartaly, they found lepidolite Barbot’s-eye. In East Transbaikalia, accumulations of lepidolite are found in pegmatites of the Borshchovoshnyi Ridge, at the deposits Zavitninskoye, Voron’ya Gora, Urchugan River and others. At the Kibirevskie mines, they found lepidolite Barbot’s-eye up to 1 cm. in size. In East Kazakhstan, lepidolite was discovered at the Belogorskoye deposit as solid masses of lilac-purple jade. There, in the region of Ust’ Kamenogorsk, lepidolite forms significant accumulations in pegmatites of the deposits Asu Bulak and Ak-Kezen. In Mongolia, it was discovered in pegmatites of Khukh del Ula. In Tadzhikistan, lepidolite is found in the East Pamirs Mts., in pegmatites of the Mika deposit, in the upper reaches of the Kukurt River. The best samples of zinnwaldite in crystals up to 6 cm. in cross-section were found in Czech Rep., in tin-ore veins of the Rozná deposit, Moravia. In Germany, lepidolite is known in the Penig deposit, Saxony; in Sweden – in pegmatites Utö Is., and at the Varuträsk deposit, Väserbotten. In South-East Asia, lepidolite in the form of large sphaerolites is extracted in India, in the Jammu and Kashmir State (disputed area), in pegmatites of the region of Gilgit. In Mozambique, in miarolitic pegmatites of the Alto Ligonha region, at the Muiana deposit they find spherolitic aggregates of lepidolite up to 5-7 kg. in mass, and the size of some crystals reaches 30 cm. In pegmatites of Monaro, near the Muiane deposit, massive ornamental lepidolite was discovered as lenses, 11x3 m. in size, and as separate crystals up to 18x10 cm.
In Canada, large accumulations of lepidolite were found at the Bernic Lake deposit, Manitoba Prov. In the U.S.A., in California, at the pegmatite deposits Pala and Tourmaline Queen, they found large sphaerolites of lepidolite up to 15 cm. in diameter. In the states of Maine and Colorado, they found massive lepidolite. In Brazil, in Minas Gerais State, in the region of Governador Valadares, at the Cruzeiro mine, they extract crystals of translucent purple-raspberry-red lepidolite, 12x6 cm. in size. Near Mimoso do Sol, Espírito Santo State, lilac lepidolite is represented with translucent crystals in the shape of hexagonal pyramids, up to several kg in mass. Similar findings are known also near Teófilo Otoni, at the Shanda mine.
Synonyms. Barbot’s-eye, after the Russ. mineralogist Barbot de Marni | Lilalith, Germ., after the lilac color | Liliathite, obs. | Lithium mica, after the content | Lilac stone | Zinnwaldite, from Germ. “Zinn” – tin, after the discovery location in tin-ore veins of the Zinnwald (now Cinovec) deposit in Czech Rep., syn. scaly stone.
Treatment. Faceted stones for collection purposes were made only from large crystals of lepidolite from Brazilian deposits. Its massive aggregates, including sphaerolites, are used in cabochons, but more often for stone-carving work of art. balls, ash-trays and other souvenirs.
Muscovite (Muscovit—Muscovite—Мусковит) (Wallerius, 1747). The name derives from its usage in windows as Muskovy glass, Muskovy being an Old Russ. Composition – KAl2[(OH)2|AlSi3O10]. Transparent to translucent. Color: colorless in fine plates, in thicker plates it is gray, rose, often with yellowish, reddish or greenish in tone. Its color depends on admixtures: rose and red-brown one – on iron (Fe3+), manganese (Mn3+) and titanium (Ti3+), light lilac one – on lithium; bright green one of fuchsite – on chromium (Cr3+). It is represented with plate, exfoliating along cleavage crystals, shelly-flaky sphaerolites muscovite Barbot’s-eye and leprous sericite, forming solid masses oncosine (oncosite). The biggest plate crystals of muscovite reach 20 m. wide. Muscovite is a rock-forming mineral for muscovite greisen – pinite, the main component of prasiolite, and mica quartzite – silvery aventurine. Green chromium-containing variety of muscovite is fuchsite, or mariposite forms massive rock – verdite. Fuchsite is also a rock-forming mineral of such ornamental stones as listwanite (See dolomite), green aventurine (See aventurine) and fuchsite schist. Sodium-containing variety of muscovite – paragonite is often represented as admixture in crystals of rock crystals and it forms fine-flaked solid masses of colorless and pale yellow oncosine, which is easy processed. We should also mention one more variety of muscovite – hydro mica celadonite, which is formed in the process of hydrothermal metamorphism of the basic rocks. It is found as solid, fine-flaked masses of green hue and as cryptocrystalline formations, filling amygdales in effusive rocks. Celadonite is also represented as inclusions in agate, quartz – smaragd prase, chrysoprase, calcite, producing blue color in them.
Deposits. Muscovite is found in magmatic rocks and in pegmatites, in greisens and hydrothermal veins, in metamorphic rocks and in weathering crusts. In Russia, in the south of the Kola Penin., near the Turiy Mys (cape), they discovered an ornamental variety of violet gabbro – kataranskite. It is composed with large isolations of anorthite, completely replaced with fine-dispersed manganese-containing mica – fengite. In Karelia, there is the Khizovara deposit of kyanite, where bedding rocks are represented with fuchsite schist with kyanite. These ornamental rocks of light bluish-green color can be processed but with some difficulties, but they are used only for decorative purposes. Besides, there are two deposits of polychromatic mica schists, which display ornamental coloring and necessary solidity for using them as ornamental stones. The first one is the Elgamskoye deposit in the region of Kondopoga, on the river Elgamka, near Lizhmozero. In the color of shists there are alternating white, between dark brown layers. The rock is so massive, that sometimes possesses a shelly fracture. In the same region, by the village of Paloselga, there is the Paloselga deposit of patterned mica schists. There are reddish, dark brown, brown and rose schists with red-brown and gray veins. In the Subpolar Urals Mts., in alpine veins, as well as in the other rock crystal provinces, aggregates of fine-dispersed sericite or paragonite form ornamental inclusions in crystals of rock crystals – so called landscape quartz. Such crystal of rock crystal from the Pyramid deposit with a landscape of snowy taiga is exhibited in the Museum Earth State Moscow Univ. At the Granitnoye deposit, paragonite from rock crystal-containing cavities is cemented in solid masses, and its powder in crystals of quartz produces phantoms. In the Middle Urals, at the Izumrudnue Kopi, they find muscovite-beryl-plagioclase veins with typical rhythmic stripes. This muscovite-albite rock can be of some interest as an ornamental stone. In the South Urals, in pegmatites of the Ilmeny Mts., they find sphaerolites of muscovite of the type of Barbot’s-eye, which are rare collection material. In the Amur Region, in the upper reaches of the Unakha River, to the west from the Zeya water-reservoire, there are ornamental green andesite tuffs with zonal aggregates of opal, chalcedony and celadonite. In the north of the Chita Region, in the Kodar Ridge, they discovered the Levtorskoye deposit of fuchsite quartzite of the ornamental quality. In Georgia, at the Akhaltsikhe deposit, the role of celadonite in the color of agate, enriched with its fine-dispersed admixture, is especially clear. In Finland, near Turku, on the Kimito (Kemiö) Is., they found sphaerolites of muscovite of the type of Barbot’s-eye. In Italy, at deposits of the Piedmont, manganese-containing muscovite rock – brownish-red alurgit is a variety of solid jade; and at the Monte Baldo deposit, Verona Prov., they discovered celadonite in
1847. In South Africa, dark green massive oncosite from a deposit by Johannesburg, Gauteng Prov., is used as an ornamental stone. There, in the Barbeton Dist., since 1907, they have extracted a massive variety of fuchsite verdite. This emerald-green ornamental stone, often with yellow and red spots, is called transvaalite, Transvaal jade or South African jade. Nowadays, it is extracted at mines of Swaziland. In Western Australia, in pegmatites there are findings of ornamental rose muscovite. In India, at the Inukurti deposit, in the region of Nellure, Andhra Pradesh, they have found the biggest crystal of muscovite, 85 tons. In the U.S.A., solid masses of rose muscovite are found in pegmatites of the Harding deposit, New Mexico. In Vermont, near Shrusberi, they extract verdite, which is called here Shrusberi green marble. In Guatemala, they manufacture carved produce from solid fuchsite. As an ornamental stone, it is also known in Venezuela.
Synonyms. Alurgit, from Gk. “alurgos” – violet, after its color | Fengite, from Gk. “fengos” – luster | Frauenglas, Germ. | Fuchsite, after the Germ. mineralogist J.N. Fuchs (1774-1856) | Mariposite, after the discovery location near Mariposa Co. in California, the U.S.A. | Moscovite | Chrome-muscovite | Prismatic mica | Silvery mica | Oncosine, from Gk. “oncosis” – swelling | Paragonite, from Gk. “parago” – to mislead, because it was taken for talc | Picnite, from Gk. “picnos” – solid | Pinite, after the Pinie Stollen deposit near Schneeberg in Saxony, Germany | Celadonite, from Fr. “seladon” – sea verde, after the color of the mineral | Cat’s silver | Verdite, from Ital. “verde” – green, after its color.
Treatment. Massive aggregates of muscovite and its varieties are processed in cabochons, as well as minerals with ornamental mica inclusions. Besides, massive material is used in stone-carving a unique vase, made in 2700 B.C. and discovered in Pakistan by archaeologies, was cut from muscovite-containing rock, close to verdite.