Mica

Any one of a group of hydrous aluminum silicate minerals with platy morphology and perfect basal (micaceous) cleavage.

Structure

 Sheets can be produced from silicate (SiO4) tetrahedra by having each tetrahedron share each of its three basal oxygen atoms with a different adjacent SiO4 tetrahedron. The overall stoichiometry of these sheets will be (Si2O5)2-. Such sheets can also be considered to be produced by the linking together of double chains of SiO4 tetrahedra such as occur in the amphibole structure. The structures of trioctahedral phyllosilicates can be thought of as being derived from that of brucite [Mg3(OH)6], a three-layer structure in which a layer of octahedrally coordinated magnesium ions (Mg2+) is sandwiched between layers of hydroxyl (OH−) groups. If the sheets described above replace those two OH- layers in brucite, the mineral talc [Mg3(Si4O10)(OH)2] is produced; in this structure the Mg2+ ions remain octahedrally coordinated, this time by the unshared apical oxygen ions of the sheet-forming tetrahedra and by the remaining hydroxyl ions between the sheets. In the micas, the net negative charge on the sheets is increased by the substitution of one aluminum ion (Al3+) for one of every four silicon ions (Si4+), and this is compensated for by the addition of one alkali metal ion (for each Al3+ ion) between the two layers of basal oxygen ions that form the top and bottom of two three-layer sandwiches stacked on top of each other. If potassium ion (K+) is the alkali, the trioctahedral mica phlogopite [KMg3(AlSi3O10)(OH)2] results. The structure of dioctahedral micas is related in an analogous way to that of the three-layer structure of gibbsite [Al2(OH)6], giving such phases as muscovite [KAl2(AlSi3O10)(OH)2]. In the brittle micas, there is a divalent ion (for example calcium, Ca2+) between the sandwiches. Each mica can exist in several structural types or polytypes. This occurs because the sheets can be rotated relative to one another, and different stacking sequences can result from repeating layers that have been rotated by specific amounts at regular intervals, that is, at every so many layers. Micas crystallize in the monoclinic system, but lepidolite can be trigonal. See also: Amphibole; Crystal structure; Lepidolite; Muscovite; Phlogopite; Talc

 

Chemistry

 The most common micas are muscovite [KAl2(AlSi3O10)(OH)2], paragonite [NaAl2-(AlSi3O10)(OH)2], phlogopite [K(Mg,Fe)3(AlSi3O10)-(OH)2], biotite [K(Fe,Mg)3(AlSi3O10)(OH)2], and lepidolite [K(Li,Al)2.5-3.0(Al1.0-0.5Si3.0-3.5O10)(OH)2]. Calcium (Ca), barium (Ba), rubidium (Rb) and cesium (Cs) can substitute for sodium (Na) and potassium (K); manganese (Mn), chromium (Cr), and titanium (Ti) for magnesium (Mg), iron (Fe), and lithium (Li); and fluorine (F) for hydroxyl (OH).

 

Physical properties

 Mica is commonly found as small flakes or lamellar plates without a crystal outline. Muscovite and biotite sometimes occur in thick books, tabular prisms with a hexagonal outline that can be up to several feet across. The prominent basal cleavage is a consequence of the layered crystal structure. Thin cleavage sheets of micas, particularly muscovite and phlogopite, are flexible, elastic, tough, and translucent to transparent (isinglass). They have low electrical and thermal conductivity and high dielectric strength. Percussion figures may be developed on cleavage plates by striking the surface sharply with a dull-pointed tool. This yields a six-rayed star, the rays of which are parallel to certain crystallographic directions.

Micas have Mohs hardnesses of 2–3 and specific gravities of 2.8–3.2. Upon heating in a closed tube, they evolve water. They have a vitreous-to-pearly luster. Muscovite is colorless to pale shades of brown, green, or gray. Paragonite is colorless to pale yellow. Phlogopite is pale yellow to brown. Biotite is dark green, brown, or black. Lepidolite is most often pale lilac, but it can also be colorless, pale yellow, or pale gray. See also: Biotite

 

Occurrence

The three major species, muscovite, biotite, and phlogopite, are widely distributed rock-forming minerals, occurring as essential constituents in a variety of igneous, metamorphic, and sedimentary rocks and in many mineral deposits.

Muscovite is found in regionally metamorphosed aluminous rocks that formed under a wide range of physical conditions. In igneous rocks, it occurs in some types of granites, in aplites, and as books in pegmatites. It is a characteristic phase of greisens which are produced when fluorine and other volatiles are introduced from granitic melts into adjacent rocks. Sericite is the name given to fine-grained white mica, usually muscovite. This mineral is a widespread gangue mineral in many hydrothermal ore deposits, either in the deposits themselves or in the adjacent altered wall rocks. Paragonite occurs in schists, gneisses, quartz veins, and fine-grained sediments. Phlogopite is a product of regional metamorphism of impure magnesian limestone. It is also characteristic of mantle-derived kimberlites and inclusions in kimberlites. Of all the micas, biotite occurs in the widest range of geological settings. It is common in the thermally metamorphosed rocks adjacent to granitic intrusions. In regionally metamorphosed rocks, it occurs in schists with chlorite, garnet, staurolite, kyanite, and sillimanite, although not all of these simultaneously. In plutonic igneous rocks, biotite is most common in intermediate and acid rocks, but it even occurs in some norites. Biotite books are found in pegmatites. Lepidolite is the most common lithium-bearing mineral and occurs almost exclusively in pegmatites with beryl, topaz, tourmaline, and other lithium minerals such as spodumene and amblygonite. It is also found occasionally in granites and aplites. See also: Aplite; Granite; Ore and mineral deposits; Pegmatite

 

Uses

Commercial mica is of two main types: sheet, and scrap or flake. Sheet muscovite, mostly from pegmatites, is used as a dielectric in capacitors and vacuum tubes in electronic equipment. Lower-quality muscovite is used as an insulator in home electrical products such as hot plates, toasters, and irons. Scrap and flake mica is ground for use in coatings on roofing materials and waterproof fabrics, and in paint, wallpaper, joint cement, plastics, cosmetics, well drilling products, and a variety of agricultural products. See also: Electric insulator; Silicate minerals

Bibliography

  • W. A. Deer, R. A. Howie, and J. Zussman, An Introduction to the Rock-Forming Minerals, 2d, ed., 1992   Alifazeli = egeology.blogfa.com
  • C. Klein and C. S. Hurlbut, Jr., Manual of Mineralogy, 21st ed., rev. 1999  Alifazeli = egeology.blogfa.com
  • J. J. Papike, Chemistry of the rock-forming silicates: Multiple-chain, sheet, and framework structures, Rev. Geophys., 26:407--444, 1988 Alifazeli = egeology.blogfa.com
  • Alifazeli = egeology.blogfa.com