Chemical composition: K (Mg, Fe) 3(AlSi3O10) (OH) 2. There is considerable substitution of Fe+2, Fe”3 and A1 for Mg, A1 for Si and Na, Ca, Ba, Rb and Cs for K. F may replace some OH.
(i) Form: Foliated books of pseudo-hexagonal crystals and also disseminated as irregular grains or flakes or foliated masses.
(ii) Colour: Black, greenish black and brownish black.
(iii) Lustre: Vitreous to splendent; pearly on cleavage planes.
(iv) Streak: White.
(v) Hardness: 2-3.
(vi) Cleavage: Perfect 1 set basal.
(vii) Fracture: Uneven.
(viii) Specific gravity: 2.7 – 3.2.
(ix) Diaphaneity: Transparent to translucent, (ix) Tenacity: Flexible and elastic.
Use: Biotite is used as insulator in electrical industry, manufacture of lubricants, wall-finishes, artificial stone, rubber tyres, pharmaceutical industry and to give gloss to wallpaper.