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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.
Manganese also substitutes for ) within cummingtonite amphibole, replacing B site atoms. These minerals are found in high-grade metamorphic banded iron formation and form a compositional series between (tirodite) and (dannemorite).
Calcium, sodium and potassium concentrations in cummingtonite are low. Cummingtonite tends toward more calcium substitution than related anthophyllite. Similarly, cummingtonite has lower ferric iron and aluminium than anthophyllite.
Amosite is a rare asbestos variety of grunerite that was mined as asbestos only in the eastern part of the Transvaal Province of South Africa. The origin of the name is Amosa, the acronym for the mining company "Asbestos Mines of South Africa".
Cummingtonite has also been found in some felsic volcanic rocks such as . Manganese rich species can be found in metamorphosed Mn-rich rock units. The grunerite end member is characteristic of the metamorphosed iron formations of the Lake Superior region and the Labrador Trough. With prograde metamorphism cummingtonite and grunerite morph to members of the olivine and pyroxene series.
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