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glaucophane An important alkali amphibole of composition Na2(Mg3Al2)[Si8O22] (OH)2, and end-member of the glaucophane—riebeckite (Na2(Fe2+3Fe3+2)[Si8O22] (OH)2 series; sp. gr. 3.0; hardness 6.0; blue, bluish-black; prismatic crystals, fibrous or granular habit; it is an essential mineral of blueschists under conditions of low temperature and high pressure in metamorphosed sediments at destructive plate margins.

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