Common Forms/Occurrences: Present in most igneous and both regional and contact metamorphic rocks Useful Identification Test: Transparent to translucent Chemical formula: K(Mg,Fe)3(Al,Si3O10)(OH,F)2 Crystal System: Very short prisms that split easily into very thin, flexible sheets Colour: Black, green-black, or brown-black Streak: Gray-brown Lustre: Nonmetallic (NM) Cleavage: Excellent Fracture: Uneven Hardness: 2.5 - 3 Hardness Search: two; three Specific Gravity: 2.7 - 3.1
