Biotite

Biotite is a member of the mica group of silicates (sheet silicates), like chlorite and muscovite. It occurs in more geological environments than any of the other micas. It is a common rock forming mineral, being present in at least some percentage in many igneous rocks (e.g. granite and rhyolite), and metamorphic rocks (e.g. schist, gneiss).


Chemical compositionK(Fe, Mg) 3AlSi 310(F, OH) 2 
Hardness - 2.5-3
Specific gravity - 2.9-3.4
Transparency - Transparent to translucent
Colour - Brown to black
Streak - Very pale brown
Lustre - Vitreous to pearly
Cleavage/fracture - Perfect in one direction producing thin sheets or flakes / uneven
Crystal habit/mode of occurrence - Tabular (sheets or flakes) / granular

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