Riebeckite
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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. It was named after German explorer Emil Riebeck (1853-1885).
It typically forms dark-blue elongated to fibrous crystals in highly sodic granites, syenites, iron formations and schists.
Some forms of riebeckite are asbestiform, notably crocidolite, also known as blue asbestos.
The riebeckite found on the island of Ailsa Craig, known as Ailsite, is prized for its use in making curling stones.
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