A monoclinic sodic amphibole with composition close to Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂. This mineral exhibits a characteristic blue color with distinct pleochroism from colorless to lavender blue when viewed in thin section by plane-polarized light. Outcrops of glaucophane-rich metamorphic rocks are commonly blue and tend to have good foliation; these rocks are termed blueschists.