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riebeckite A member of the alkali amphiboles Na2(Fe2+ 3Fe3+2)[Si4O11]2(OH,F)2 and end-member of an isomorphous series with glaucophane Na2(Mg3Al2)[Si4O11]2 (OH,F)2; sp. gr. 3.43; hardness 5; forms either tiny prismatic crystals or large, poikilitic, subhedral, prismatic crystals; dark bluish-green or black; occurs in alkaline igneous rocks, especially granites, in association with aegirine. Fibrous riebeckite (called crocidolite or blue asbestos) is formed by metamorphism of massive ironstone deposits. When infiltrated with silica it constitutes the semi-precious cat's eye or tiger's eye.