A range of iron-rich varieties
of mica, grading into phlogopite. Biotite is
an important rock-forming silicate mineral
and a constituent of most igneous rocks
(such as granite) and many metamorphic
rocks (such as schist and gneiss).
It has, in common with other micas, a flaky, sheetlike structure, crystallizing
in the monoclinic system. Biotite is lustrous, opaque, and black, dark brown,
or green in color. It has the approximate chemical formula K(Mg,Fe)3AlSi3O10(F,OH)2.
|Specimen of biotite from the Adirondack Mountains,
New York. Source: Wikipedia / Dave Dyet
AND PLANETARY SCIENCE