Cummingtonite is a metamorphic amphibole with the chemical composition25Si8O222, 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.
Cummingtonite is a member of the cummingtonite-grunerite solid solution series which ranges from Mg7Si8O222 for magnesiocummingtonite to the iron richgruneriteendmember Fe7Si8O222. Cummingtonite is used to describe minerals of this formula with between 30 and 70 per cent Fe7Si8O222. Thus, cummingtonite is the series intermediate. 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 Mn2Mg5Si8O222 and Mn2Fe5Si8O222. 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 asbestiformvariety 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".
Occurrence
Cummingtonite is commonly found in metamorphosed magnesium-rich rocks and occurs in amphibolites. Usually it coexists with hornblende or actinolite, magnesium clinochlore chlorite, talc, serpentine-antigorite minerals or metamorphic pyroxene. Magnesium-rich cummingtonite can also coexist with anthophyllite. Cummingtonite has also been found in some felsicvolcanic rocks such as dacites. 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 Superiorregion and the Labrador Trough. With prograde metamorphism cummingtonite and grunerite morph to members of the olivine and pyroxene series.
