Riebeckite

Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na₂(Fe₃Fe₂)Si₈O₂₂(OH)₂. It forms a series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals 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. Cleavage is perfect, two directions in the shape of a diamond; fracture is uneven, splintery. It is often translucent to nearly opaque.