Topic summary
Riebeckite
Riebeckite is a sodium-rich member of the amphibole group of silicate minerals. It has chemical formula Na2(Fe3Fe2)Si8O22(OH)2. It forms a solid solution series with magnesioriebeckite, where Mg substitutes for Fe. It crystallizes in the monoclinic system, usually as long prismaticcrystals showing a diamond-shaped cross section, but also in fibrous, bladed, acicular, columnar, and radiating forms. Its Mohs hardness is 5.0–6.0, and its specific gravity is 3.0–3.4. It has perfect cleavage in two directions in the shape of a rhombus, while its fracture is uneven and splintery. It is often translucent to nearly opaque.