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.