A monoclinic mineral, 2(Na₂(Mg,Fe)₃Al₂Si₈O₂₂(OH)₂) ; amphibole group with Mg/(Mg+Fe) = 0.5 to 1.0; prismatic cleavage; bluish-gray to lavender blue; common in low-temperature, high-pressure schists associated with lawsonite, pumpellyite, or epidote. Compare: crossite; riebeckite.