Biotite

Among the most widespread rock-forming micas, biotite is a monoclinic phyllosilicate with the approximate formula K(Mg,Fe)3(AlSi3O10)(OH)2. Its dark color, whether black, brown, or green, reflects abundant iron and magnesium and separates it from the paler muscovite. Perfect basal cleavage yields thin, flexible sheets, and the mineral is translucent with a vitreous luster and a white-to-gray streak. Hardness ranges from 2.5 to 3 and specific gravity from 2.70 to 3.40. Biotite occurs throughout igneous and metamorphic rocks, where petrologists read it as an indicator of metamorphic grade, its habit typically tabular or pseudo-hexagonal. Splitting into thin elastic laminae along the basal plane is shared with other micas but paired here with a characteristically dark tone.

Biotite is a common phyllosilicate mineral found in igneous and metamorphic rocks. It is distinguished from muscovite by its darker color due to iron and magnesium content. Biotite is an important indicator of metamorphic grade.