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Cummingtonite
Chemical Formula	(Mg,Fe)7Si8O22(OH)2
Species	Silicates
Crystal System	Monoclinic
Mohs Scale	5-6
Specific Gravity	3.1-3.6
Color	Dark green, brown, gray, beige; colorless to pale green in thin section
Streak	nd
Luster	Vitreous
Refractive Index	n = 1.639 - 1.671 n = 1.647 - 1.689 n = 1.664 - 1.708
Diaphaneity	Translucent, will transmit light on thin edges.
Cleavage	Perfecton
Fracture	Splintery
Crystal Habit：Rarely as distinct crystals. Columnar to fibrous and granular

Cummingtonite is a metamorphic amphibole with the chemical composition (Mg,Fe)₇Si₈O₂₂(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-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

Cummingtonite was first discovered in 1824 near the town of Cummington, Massachusetts, and it is from this town that the mineral takes its name. It is also found in Sweden, South Africa, Scotland, and New Zealand.

Chemistry

Cummingtonite is a member of the cummingtonite-grunerite solid solution series which ranges from Fe₂Mg₅Si₈O₂₂(OH)₂ for cummingtonite to the iron rich grunerite endmember Fe₇Si₈O₂₂(OH)₂. Cummingtonite is used to describe minerals of this formula with between 30 and 70 per cent Fe₇Si₈O₂₂(OH)₂.

Manganese also substitutes for (Fe,Mg) within cummingtonite amphibole, replacing B site atoms. These minerals are found in high-grade metamorphic banded iron formation and form a compositional series between Mn₂Mg₅Si₈O₂₂(OH)₂ (tirodite) and Mn₂Fe₅Si₈O₂₂(OH)₂ (dannemorite).

Calcium, sodium and potassium concentrations in cummingtonite are low. Cummingtonite tends toward more Ca substitution than related anthophyllite. Similarly, cummingtonite has lower ferric iron and aluminium than anthophyllite.

Amosite is a rare asbestiform 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."

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 felsic volcanic 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 Superior region and the Labrador Trough. With prograde metamorphism cummingtonite and grunerite morph to members of the olivine and pyroxene series.