What is "cummingtonite"

n. (context mineral English) A mineral, magnesium-iron silicate hydroxide, (Mg,Fe)₇Si₈O₂₂(OH)₂.

Cummingtonite is a metamorphic amphibole with the chemical composition (Mg,Fe)(Mg,Fe)SiO(OH), 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- riebeckite. 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.