DEFINITION OF MICA

A group of platy minerals with the general formula

(K,Na,Ca) (Mg,Fe,Li,Al)2-3 (Al,Si)4 O10(OH,F)2, that have perfect

basal cleavage and exhibit a tendency to split into thin

elastic sheets. The basal cleavage reflects the layered atomic

structure of complex phyllosilicates that form repeating sheets

and typically show one type of mica interlayered with another.

Some common micas include muscovite (K2A14Si6A12), Paragonite

(Na2Al4Si6A12), phlogopite (K2[Mg,Fe+2]6Si6A12, and

biotite (K2[Mg,Fe,A1]6Si6–5A12–3).

Micas are common constituents of a number of different

types of igneous, metamorphic, and sedimentary rocks. Muscovite

is found in many granites, pegmatites, phyllites,

schists, gneisses, and as a detrital sedimentary mineral. Phlogopite

is found in ultramafic peridotites and metamorphosed

carbonates. Biotite is a common component in gabbros, diorites,

granites, pegmatites, in schists, phyllites, and gneisses.

Paragonite is found in schists, gneisses, and as detrital and

authigenic sediments.

See also IGNEOUS ROCKS; METAMORPHISM; MINERALOGY.

mid-latitude cyclone See EXTRATROPICAL CYCLONES.

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