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.
Phlogopite, KMg3Fe3AlSiO10(OH)2, is any of a range of white or brown, magnesium-rich varieties of mica, grading into biotite. It is found in limestones as monoclinic crystals. Hardness 2.5–3; relative density 2.8.