Cummingtonite

Cummingtonite (/ˈkʌmɪŋtəˌnaɪt/ KUM-ing-tə-nyte with the chemical composition (Mg,Fe2+ ) 2(Mg,Fe2+ ) 5Si 8O 22(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.

Cummingtonite shares few compositional similarities with alkali amphiboles such as arfvedsonite , glaucophane. There is little solubility between these minerals due to different crystal habit and inability of substitution between alkali elements and ferro-magnesian elements within the amphibole structure.