ETYM From Biot, a French naturalist.
Dark brown to black mica found in igneous and metamorphic rock.
Dark mica, K(Mg, Fe)3Al Si3O10(OH, F)2, a common silicate mineral. It is brown to black with shiny surfaces, and like all micas, it splits into very thin flakes along its one perfect cleavage. Biotite is a mineral of igneous rocks such as granites, and metamorphic rocks such as schists and gneisses.