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biotite An important rock-forming silicate mineral; a member of the mica group K2(Mg,Fe)6[Si3AlO10]2(OH,F)4, in which the Mg/Fe ratio is less than 2:1; biotite forms a series with phlogopite (of similar composition, but with Mg/Fe greater than 2:1) and may have Fe3+, Ti4+, and Al3+ also present in the formula; sp. gr. 2.7–3.3; hardness 2–3; monoclinic; black, dark brown, or greenish-black; vitreous to sub-metallic lustre; crystals tabular and pseudo-hexagonal, with lamellar aggregates and flakes; cleavage perfect basal mica; occurs in granites, syenites, and diorites, in schists and gneisses, in contact metamorphic rocks, and often in sedimentary rocks.