Riebeckite
Chemical Formula	[][Na2][(Fe2+)3(Fe3+)2][(OH)2|Si8O22]
Species	Silicates
Crystal System	Monoclinic
Mohs Scale	5
Specific Gravity	3.28-3.44
Color	Usually black, also commonly light blue to blue-black, gray-blue, gray, brown
Streak	Pale gray to bluish-grey.
Luster	Vitreous, Sub-Vitreous
Refractive Index	n = 1.680 - 1.698 n = 1.683 - 1.700 n = 1.685 - 1.706
Diaphaneity	Translucent, Opaque
Cleavage	PerfectPerfect on
Fracture	Splintery
Crystal Habit：Prismatic, frequently very elongated. May be acicular, especially in late cavities. Sometimes is very acicular in fine-grained, light colored aggregates. Well-terminated crystals are uncommon.
Geological Setting：Syenite and undersaturated rocks