Common Forms/Occurrences: Widely distributed in mountian belts and deeply eroded shield areas of continental crust Useful Identification Test: Optically negative character and large optical axial angle and extinction angle Chemical formula: calcium ferromagnesian aluminum silicate Crystal System: Prisms Colour: Dark gray to black Streak: White to pale gray Lustre: Nonmetallic (NM) Cleavage: Good at 60° and 120° Fracture: Subconchoidal to uneven Hardness: 5.5 Hardness Search: five Specific Gravity: 3.0 - 3.3
