How do I identify Cummingtonite?

Cummingtonite can be identified by its hardness of 5 - 6 on the Mohs scale, Translucent dark green color. Look for these key characteristics when examining specimens.

What color is Cummingtonite?

Cummingtonite typically appears in Translucent dark green, brown, grey, colourless. Color can vary depending on impurities and formation conditions.

How hard is Cummingtonite?

Cummingtonite has a hardness of 5 - 6 on the Mohs scale. This gives it moderate hardness.

Cummingtonite

A variety of Minerals

What is Cummingtonite?

Cummingtonite (/ˈkʌmɪŋtəˌnaɪt/ KUM-ing-tə-nyte) is a metamorphic amphibole with the chemical composition (Mg,Fe)2(Mg,Fe)5Si8O22(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.

Cummingtonite Market Value Calculator

Estimate the market value of Cummingtonite using size, quality, and finish. This preview calculator is for quick context and is not a formal appraisal.

Cummingtonite Localities Map

See where Cummingtonite is found with a localities map, collecting zones, and geology context. Generate a sample map preview below.

Map preview

North ZoneCentral RidgeSouth Basin

Geochemistry

Cummingtonite is a member of the cummingtonite-grunerite solid solution series which ranges from Mg7Si8O22(OH)2 for magnesiocummingtonite 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. Thus, cummingtonite is the series intermediate. 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 calcium 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".

Key Characteristics

Formation of Cummingtonite

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.

More Images

Quick Facts

Physical Properties

Color: Translucent dark green, brown, grey, colourless
Hardness (Mohs): 5 - 6
Streak: nd

Chemical Properties

Chemical Formula: ☐{Mg2}{Mg5}(Si8O22)(OH)2 Cummingtonite is defined as a monoclinic amphibole belong to the magnesium-iron-manganese amphibole subgroup. It is defined with Mg as the dominant element both in the B and C positions.
Elements: H, Mg, O, Si