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mica(mī`kə), general term for a large group of minerals, hydrous silicates of aluminum and potassium, often containing magnesium, ferrous iron, ferric iron, sodium, and lithium and more rarely containing barium, chromium, and fluorine. All crystallize in the monoclinic system, but mica is most commonly found in the form of scales and sheets. All the micas have an excellent basal cleavage, splitting into very thin, elastic laminae. Some varieties are transparent; resistance to heat is high. Commercially, the most important micas are muscovite (potassium mica) and phlogopite (magnesium mica). Muscovite, the commoner variety, is usually colorless, but it may be red, yellow, green, brown, or gray, with a vitreous to pearly luster. It occurs in granites, syenites, mica schists, and gneisses, but is most common in pegmatite dikes. It is widely distributed. Phlogopite varies in color from yellow to brown, some specimens having a coppery tint and others being greenish. It occurs in crystalline limestones, dolomites, and serpentines in Canada, New York, New Jersey, and Finland. Mica mining, because of the necessity of keeping the crystals intact, is a delicate operation; drills and blasting powder must be used carefully, if at all. The mined crystals are first "cobbed," i.e., roughly trimmed of rock and cut, then split with a hammer into plates, and further split into sheets with a knife. Sheet mica is used as an insulating material and as a resonant diaphragm in certain acoustical devices. Scrap and ground mica is used in wallpaper, fancy paint, ornamental tile, roofing, lubricating oil, and Christmas-tree snow. Ground mica is sometimes pressed into sheets (micanite) that can be used as sheet mica. Most of the sheet mica used in the United States is imported, chiefly from India and also from Brazil. Synthetic mica was produced in the United States after intensive government-sponsored research began in 1946.
any of a group of aluminosilicate minerals with a laminar structure and the general formula R1R2-3[AlSi3O10](OH,F)2, where R1 = K or Na and R2 = Al, Mg, Fe or Li. The basic structural element of micas is a three-layer unit consisting of two tetrahedral layers of [AlSi3O10] separated by an octahedral layer of R2 cations. Two of the six oxygen atoms of the octahedrons are replaced by hydroxyl (OH) groups or fluorine. The units are linked in a continuous structure by K+ (or Na+) ions with a coordination number of 12. A distinction is made between dioctahedral and trioctahedral micas according to the number of octahedral cations in the chemical formula: Al3+ cations occupy two-thirds of the octahedral sites, leaving one-third empty, while cations Mg2+, Fe2+, and Li+ with Al3+ occupy all the octahedral sites. Micas crystallize in the monoclinic (pseudotrigonal) system. The arrangement of the hexagonal cells on the surfaces of the three-layer units results from rotations of the units about the c-axis at various angles, which are multiples of 60°, in conjunction with displacement along the a- and b-axes of the unit cell. This variation in arrangement permits the existence of polymorphic modifications (polytypes) of micas, which can be distinguished by X-ray crystallographic analysis. Polytypes of monoclinic symmetry are common.
The micas are grouped according to their chemical composition into several types. The aluminum micas include muscovite, KAl2[AlSi3O10](OH)2, and paragonite, NaAl2[AlSi3O10](OH)2. The magnesium-iron micas include phlogopite, KMg3-[AlSi3O10](OH,F)2, biotite, K(Mg,Fe)3[AlSi3O10](OH,F)2, and lepidomelane, KFe3[AlSi3O10](OH,F)2. The lithia micas include lepidolite, KLi2_xAl1+x[Al2xSi4-2xO10](OH,F)2, zinnwaldite, KLiFeAl[AlSi3O10](OH,F)2, and taeniolite, KLiMg2[Si4O10] (OH,F)2. Also encountered are a vanadium mica, known as ros-coelite, KV2[AlSi3O10](OH)2, and a chrome mica, known as chrome muscovite or fuchsite.
Isomorphic substitutions are common in micas: K+ is replaced by Na+, Ca2+, Ba2+, Rb+, or Cs+; Mg2+ and Fe2+ of the octahedral layer are replaced by Li+, Sc2+, or Jn2+; and Al3+ is replaced by V3+, Cr3+, Ti4+, or Ga3+. Complete isomorphism is observed between Mg2+ and Fe2+ (continuous solid solutions of phlogopite and biotite), and restricted isomorphism is seen between Mg2+ and Li+ and between Al3+ and Li+. A variable ratio is also observed between ferric oxide and ferrous oxide. In the tetrahedral layers, Si4+ can be replaced by Al3+, and Fe3+ ions can replace tetrahedral Al3+; the hydroxyl (OH) group can be replaced by fluorine. Micas often contain various rare elements (Be, B, Sn, Nb, Ta, Ti, Mo, W, U, Th, Y, TR, Bi), which are frequently present in the form of such submi-croscopic mineral admixtures as columbite, wolframite, cassi-terite, and tourmaline. The replacement of K+ by Ca+ leads to the formation of the mineral group known as brittle micas. These micas, which include margarite, CaAl2[Si2Al2O10](OH)2, are harder and less elastic than the other micas. The replacement of the interlayer K+ cations by H2O leads to a transition to hydromicas, which are important components of clay minerals.
The laminar structure of mica and the weak bonds between the units are responsible for mica’s platy appearance, perfect (basal) cleavage, and capacity to separate into extremely thin sheets that retain flexibility, elasticity, and strength. Mica crystals can be twinned according to the mica law with a (001) twin plane; the crystals frequently have pseudohexagonal outlines. Micas have a hardness on Mohs’ scale of 2.5–3; the densities of muscovite, phlogopite, and biotite are 2,770, 2,200, and 3,300 kg/m3, respectively. Muscovite and phlogopite are colorless and in thin sheets are transparent; grayish brown, pink, and green shades result from admixtures of Fe2+, Mn2+, and Cr2+. Iron micas are grayish brown, brown, dark green, and black, depending on the content and ratio of Fe2+ and Fe3+. Mica is one of the most common rock-forming minerals of intrusive, metamorphic, and sedimentary rocks and is itself an important mineral.
The three types of commercial micas are sheet mica, flake and scrap mica (wastes from the production of sheet mica), and expanded mica (for example, vermiculite). High-quality commercial deposits of sheet mica (muscovite and phlogopite) are rare. Sheet mica is used in industry because of the perfection and size of the crystals, while flake mica is favored for the purity of the mica material. Large crystals of muscovite are found in granitic pegmatites in Mama Raion of Irkutsk Oblast, the Chupa-Loukhi region of the Karelian ASSR, and the Ena-Kola region of Murmansk Oblast in the USSR and in deposits in India, Brazil, and the USA. Deposits of phlogopite are confined to massifs of ultrabasic and alkalic rocks (Kovdor deposit on Kola Peninsula), with deep, metamorphosed Precambrian rocks primarily of carbonate (dolomite) composition (Aldan mica-bearing region of the Yakut ASSR and the Sliudianka area of the Baikal Region in the USSR), as well as with gneisses (Canada and Madagascar). Muscovite and phlogopite are high-quality electrical insulating materials for which no substitute exists in electrical and radio engineering and aviation technology. Deposits of lepidolite, one of the major industrial minerals of lithium ores, are related to the sodium-lithium type of granitic pegmatites. Special optical glass is made from lepidolite in the glassmaking industry.
Micas are excavated by blasting in underground or open-pit mines. The crystals are separated from the rock mass by hand.
Methods have been developed for the industrial synthesis of micas. Large sheets obtained by the adhesive bonding of mica plates (micanites) are used as high-quality electrical and thermal insulating materials. Ground mica is obtained from scrap and flake mica and is used in the construction, cement, and rubber industries and in the production of paints and plastics. Flake mica is widely used in the USA.
REFERENCESDeer, W. A., R. A. Howie, and J. Zussman. Porodoobrazuiushchie mineraly, vol. 3. Moscow, 1966. (Translated from English.)
Bykhover, N. A. Ekonomikamineral’nogosyr’ia. Moscow, 1969.
Volkov, K. I., P. N. Zagibalov, and M. S. Metsik. Svoistva, dobycha i pererabotka sliudy. [Irkutsk] 1971.
A. S. MARFUNIN and V. P. PETROV