DEFINITION OF BIOTITE

A common black, dark brown, or green mineral of

the mica group. It is an aluminosilicate with the formula

K(Mg,Fe+2)3(Al,Fe+3)Si3O10(OH)2 and is a sheet silicate that

forms thin cellophane-like sheets that easily tear off underlying

sheets. Biotite is a common constituent of igneous

rocks and also forms as a metamorphic mineral especially in

aluminous metasedimentary and mafic igneous rocks. It

forms common platy minerals in metamorphic gneisses, typically

being one of the main minerals that define tectonic

foliations, where many biotite grains are aligned in a similar

orientation.

Biotite exhibits monoclinic symmetry with pseudo-rhombohedral

forms and has well developed basal cleavage. It typically

has short tabular or prismatic crystals, exhibits striations

on cleavage planes, and exhibits strong pleochroism under the

polarizing microscope. Ionic substitution is common, and

there are many varieties of biotite. When altered by the addition

of water, biotite may exhibit a bronze-colored luster on

cleavage surfaces.

See also MINERALOGY; STRUCTURAL GEOLOGY.

bioturbation See BIOGENIC SEDIMENT.

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