Cummingtonite ( ) is a metamorphic amphibole with the chemical composition , 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.
Name and discovery
thumb|left|Fibrous, brownish crystals of cummingtonite – Locality: Dannemora Mine, Uppsala Län, Uppland, Sweden
Cummingtonite was named after the town of Cummington, Massachusetts, where it was discovered in 1824.