BIOTITE, French Alps

Biotite is a widely recognized group of phyllosilicate minerals that falls under the mica category, with a general chemical formula of K(Mg,Fe)3AlSi3O10(F,OH)2. It mainly exists as a solid-solution series between two end-members: annite, which is rich in iron, and phlogopite, which contains more magnesium. There are also more aluminum-rich varieties like siderophyllite and eastonite. Although the International Mineralogical Association classified biotite as a distinct mineral species until 1998, it is now considered a mineral group. However, the term biotite is still commonly used to refer to untested dark micas found in nature. The name biotite was given by J.F.L. Hausmann in 1847 to honor the French physicist Jean-Baptiste Biot, who conducted early studies on the optical characteristics of mica.