Garnet in Biotite hand-cut disc from Greenland.

Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula K(Mg,Fe)3AlSi3O10(F,OH)2. It is primarily a series of solid solutions between the iron end-member annite and the magnesium end-member phlogopite; more aluminous end members include siderophyllite and eastonite. Biotite was considered a mineral species by the International Mineralogical Association until 1998, when its status was changed to a mineral group. The term biotite is still used to describe unanalyzed dark micas in the field. Biotite was mentioned by J.F.L. Hausmann in 1847 in honor of the French physicist Jean-Baptiste Biot, who conducted early research into the many optical properties of mica. Members of the biotite group are sheet silicates. Iron, magnesium, aluminum, silicon, oxygen and hydrogen form plates that are weakly bonded together by potassium ions. The term "iron mica" is sometimes used for iron-rich biotite, but the term also refers to a flaky, micaceous form of hematite, and the field term Lepidomelane for unanalyzed iron-rich biotite avoids this ambiguity. Biotite is also called "black mica", as opposed to "white mica" (muscovite) - both form in the same rocks, and in some cases next to each other.