RIEBECKITE

a sodium-iron silicate mineral [Na2Fe2+3Fe3+2Si8O22(OH)2] in the amphibole family. It forms part of a solid-solution series that includes both magnesioriebeckite (formed when iron is replaced by magnesium) and glaucophane (formed when iron is replaced by magnesium and aluminum). Riebeckite is a moderately hard mineral with a glassy lustre. It forms prismatic crystals that are dark blue or black in colour. A fibrous variety, crocidolite, is of metamorphic origin and is commonly called blue asbestos. The mineral is associated with acidic igneous rocks such as granites and syenites. Common deposits are found in Arizona, Colorado, and Massachusetts, U.S.; Greenland; Portugal; Nigeria; South Africa; and portions of western Australia. For detailed physical properties, see amphibole (table).