Cummingtonite


Cummingtonite is a metamorphic amphibole with the chemical composition (Mg,Fe)2(Mg,Fe)5Si8O22(OH)2, magnesium iron silicate hydroxide.
Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being metastable.