Specimen of biotite from the Adirondack Mountains, New York. Source: Wikipedia / Dave Dyet.
Biotite is 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.