glaucophane

noun

1. a sodium-rich monoclinic mineral of the amphibole family, usually metamorphic.

1. A blue to grayish-blue or bluish-black monoclinic mineral of the amphibole group. Glaucophane occurs as fibrous prisms in schists (especially blueschists) formed by the regional metamorphism of sodium-rich rocks. Chemical formula: Na2(Mg,Fe)3Al2Si8O22(OH)2.