MICA

any of a group of hydrous potassium, aluminum silicate minerals. It is a type of phyllosilicate, exhibiting a two-dimensional sheet or layer structure. Among the principal rock-forming minerals, micas are found in all three major rock varietiesigneous, sedimentary, and metamorphic. any of a group of hydrous potassium, aluminum silicate minerals. It is a type of phyllosilicate, exhibiting a two-dimensional sheet or layer structure. Among the principal rock-forming minerals, micas are common in all three major rock varietiesigneous, sedimentary, and metamorphic. Micas may contain various metals besides potassium and aluminum in significant amounts. Such metals include magnesium, lithium, manganese, titanium, and ferrous and ferric iron. Structurally, each mica layer consists of one octahedral and two tetrahedral units. The tetrahedral unit is a network of silicon-bearing tetrahedrons, each sharing three oxygen atoms with other tetrahedrons. The shared oxygen atoms are near-planar, as are the silicon and unshared apex oxygen atoms. The net silicon-to-oxygen ratio of this tetrahedral unit is (Si2O5)-2; aluminum may substitute for silicon. The other structural unit of the mica layer is a two-dimensional array of aluminum- or magnesium-containing octahedrons with the composition (Al2O4(OH)2)-4 or (Mg3O4(OH)2)-4. This octahedral unit lies between and shares apical oxygen atoms with two tetrahedral units. Fluorine may substitute for hydroxide (OH). With trivalent ions (e.g., aluminum) only two-thirds of the available octahedral sites are filled, whereas for divalent ions (e.g., magnesium) all octahedral sites are filled; these two mica types are called di-octahedral and tri-octahedral micas, respectively. Individual mica layers are weakly bonded to adjacent layers by potassium, sodium, or calcium atoms which are 12-coordinated to oxygen; electrical charges are balanced by the substitution of aluminum for silicon in tetrahedral sites. Mica layers can be stacked in several relative orientations with or without a regular repetitive sequence, the former resulting in a larger unit cell than for the ideal structure. The weak bonding between mica layers accounts for the perfect cleavage. The composition of mica minerals can first be based on di- or tri-octahedral layer and then on octahedral species as well as interlayer species. Thus the general formula can be written as X2Y4-6Z8O20(OH,F)4. Here X is the interlayer cation (calcium, sodium, potassium); Y the octahedral cation (aluminum, ferric iron, lithium, magnesium, ferrous iron); and Z the tetrahedral cation (silicon, aluminum). The micas are widespread minerals easily recognized by their cleavage, which gives rise to a relatively soft mineral. Muscovite, the most abundant variety of mica, is common in acid igneous rocks such as granites; it also frequently forms very large books in pegmatites. In metamorphic rocks muscovite commonly occurs at lower grades but reacts to form alkali feldspar and sillimanite at higher grades. Two other principal types of mica are biotite and phlogopite. Unlike the latter, biotite contains more iron than magnesium, resulting in its characteristic brown to black colour. It is very common in granites and intermediate igneous rocks such as gabbros and norites and often forms mica-rich biotite schists during the metamorphism of sediments containing large amounts of iron. Phlogopite is found in ultrabasic rocks, which reflects its high magnesium content. Samples of phlogopite have been discovered in kimberlites, suggesting that it may be a store for volatiles in deep-seated rocks. One other major variety of mica, lepidolite, occors in granitic pegmatites in which lithium is enriched. Fine-grained micas intermixed with clays occur in sediments both as detrital minerals and as the result of diagenesis of clays. Typically these micas are muscovite and glauconite. Micas have various industrial uses. Those varieties containing little iron are employed as thermal or electrical insulators in appliances and certain electrical devices such as capacitors. In ground form, micas are used in the manufacture of wallpaper, roofing paper, and paint. Ground micas also serve as fillers, lubricants, absorbents, and packing material. Additional reading S.W. Bailey (ed.), Micas (1984). R.V. Dietrich