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Mica - group minerals-silicates having a layered structure and having the General formula R1(R2)3 [AlSi3O10](OH, F)2, where R1 = K, Na; R2 = Al, Mg, Fe, Li.

Mica is one of the most common rock-forming minerals Intrusive, metamorphic and sedimentary rocks, as well as important minerals.The main element structure of mica is a three-layer package of two tetrahedral layers [AlSi3O10]4? or [Si4O10]4?, between which is octahedral layer from cations R2. Two of the six oxygen atoms octahedra substituted hydroxyl groups (OH) or fluorine. The packs are connected in continuous structure using ions of K+ (or Na+) with a coordination number of 12. On the number of octahedral cations in the chemical formula distinguish between dioctahedral and trioctahedral mica. In the first cations Al3+ occupy two of the three octahedra leaving one empty; secondly cations Mg2+, Fe2+ and Li+ with Al3+ is all the octahedra. Mica crystallized in monoclinic (pseudotracheal) system and form a columnar or lamellar crystals. The relative position of hexagonal cells of the surfaces of three-layer packets due to their turns around the axis c at various multiples of 60°, in combination with a shift along the axes a and b unit cell. It determines the existence of several polymorphs (polytypes) mica, which has, as a rule, monoclinic symmetry.

The layered structure of mica and the weak link between packages impact on its properties: plastinated, very perfect (basal) cleavage, the ability to split into extremely thin leaves, preserve flexibility, elasticity and strength. The mica crystals can dvoinikovaniya by "mica law" with the plane intergrowths (001) and often have pseudohexagonal outlines.