Basin and Range Province, United States

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.