Biotit

Biotite is a common member of the mica group, a phyllosilicate mineral that typically forms dark, flexible sheets. It is commonly referred to as black mica due to its dark brown to black color, though some varieties can appear greenish. Biotite occurs in a wide range of igneous and metamorphic rocks and is often abundant in granitic and granodioritic rocks as well as in schists and gneisses.

Chemically, biotite is a potassium aluminum silicate with iron and/or magnesium in octahedral sites, giving the general formula K(Mg,Fe)3(AlSi3O10)(OH)2. The iron-to-magnesium ratio yields two well-known varieties: annite (Fe-rich) and phlogopite (Mg-rich). Biotite belongs to the mica family and has a layered sheet silicate structure. It exhibits perfect basal cleavage, allowing sheets to be peeled from rocks, and typically shows a vitreous luster. Its hardness is about 2.5–3 on the Mohs scale, and its specific gravity ranges roughly from 2.7 to 3.4. It often displays distinct pleochroism, appearing different colors when viewed from different angles.

Biotite forms during magmatic crystallization in granitic systems and during metamorphism at relatively high temperatures, in rocks such as schists and gneisses. It weatheres to clays and iron oxides and can serve as an indicator mineral for metamorphic grade in field geology.

Uses of biotite have declined relative to other micas; historically it was mined for mica in electrical insulators and as a filler, but it is now mainly of geological and academic interest. Biotite is named after the French physicist Jean-Baptiste Biot.