Riebeckite is a soium-rich member of the amphibole group of minerals, chemical formula Na2(Fe,Mg)5Si8O22(OH)2. 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. It was named after German explorer Emil Riebeck (1853-1885).