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Biotite (K(Mg,Fe2+)3(Al,Fe3+)Si3O10(OH,F)2) NS4012A      Crystal system monoclinic. Habit commonly in plates or platy aggregates; also in tabular pseudo-hexagonal crystals. Cleavage perfect. Fracture none. Hardness 2.5-3 Specific Gravity 2.8 Color brown. Luster transparent to translucent with a vitreous luster. Streak uncolored. Distinguishing features thin, flexible brown sheets are very typical. Uses of interest to collectors and researchers.

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