Science Fair Project Encyclopedia
Riebeckite is a sodium-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 hardness 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.
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