BIOTITE (named after the French scientist J. B. Biot * EN: biotite, black mica, iron mica, magnesia mica; DE: Biotit; FR: biotite; ES: biotita; RU: биотит) is the mineral of the class of silicates, which is enriched with iron, the member of the isomorphous series from KFe3(OH,F)2[AlSi3O10] (siderophyllite) to KMg3(OH,F2)[AlSi3O12] (phlogopite), and which is belonging to the group of the trioctahedral micas. The chemical composition is variable. The impurities (%) are: MnO up to 18 (manganophyllite), TiO2 up to 12, Cs2O up to 4, Pb2O and Li2O up to 2, Na2O up to 3, BaO up to 1, Na2O and CaO up to 1.5-2. The content of Fe2O3 and FeO reaches 21 and 27% respectively.

The high-iron biotite is named as lepidomelane. Part of the Fe3+ atoms can replace the Si atoms isomorphically within the tetrahedral positions of the structure of biotite (tetraferribiotite). It crystallizes into the monoclinic crystal system. There are known 3 polymorphic modifications. There are characteristic the tabular, foliated, and scaly crystals with pseudohexagonal habit, with very perfect cleavage. The colour of biotite is black, brown, and reddish-brown. It is transparent within the thin plates. The hardness is 2.5-3. The density is 2700-3300 kilograms per cubic metre.

Biotite is the rock-forming mineral of the magmatic and metamorphic rocks (granites, granodiorites, diorites, gneisses, crystalline schists), and also of pegmatites (the largest crystals). It may be beneficiated with the gravity, magnetic, and flotation methods. It is used for the determination of the absolute age of the rocks with the argon and strontium method. It is used for the manufacturing of lubricants, bronze paint, for the optical instrument engineering, for the production of the electrical insulating materials, and decorative types of cement.