GLAUCOPHANE

common amphibole mineral, a sodium, magnesium, and aluminum silicate that occurs only in crystalline schists formed from sodium-rich rocks by low-grade metamorphism. It also forms from sedimentary rocks by the introduction of sodium oxide (Na2O). Glaucophane typically occurs in folded rocks associated with epidote amphibolites and greenschists. Both ferrous and ferric iron are replaced by magnesium and aluminum in the crystal structure to form magnesioriebeckite and riebeckite. For chemical formula and detailed physical properties, see amphibole (table).