Cummingtonite
General
Category	Inosilicate
Formula (repeating unit)	(Mg,Fe2+ ) 2(Mg,Fe2+ ) 5Si 8O 22(OH) 2
IMA symbol	Cum[1]
Strunz classification	9.DE.05
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) H-M symbol: (2/m)
Space group	Monoclinic Space group: C2/m
Unit cell	a = 9.53 Å, b = 18.23 Å, c = 5.32 Å; β = 101.97°; Z = 2
Identification
Color	Dark green, brown, gray, beige; colorless to pale green in thin section
Crystal habit	Rarely as distinct crystals. Columnar to fibrous and granular
Twinning	Simple and lamellar – common
Cleavage	Good on {110} intersecting at 54 and 126°
Fracture	Splintery
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous to silky
Diaphaneity	Translucent, will transmit light on thin edges.
Specific gravity	3.1–3.6
Optical properties	Biaxial (+)
Refractive index	nα = 1.639–1.671 nβ = 1.647–1.689 nγ = 1.664–1.708
Birefringence	δ = 0.025–0.037
Pleochroism	With increasing iron content, weak; X = Y = colorless; Z = pale green
2V angle	Measured: 65° to 90°, Calculated: 70° to 90°
Diagnostic features	Characterized by light brown color and needlelike, often radiating habit. Difficult to distinguish from anthophyllite or gedrite without optical and/or X-ray tests.
References	[2][3][4][5]

Cummingtonite (/ˈkʌmɪŋtənaɪt/ KUM-ing-tə-nyte) is a metamorphic amphibole with the chemical composition (Mg,Fe²⁺
)
₂(Mg,Fe²⁺
)
₅Si
₈O
₂₂(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 named after the town of Cummington, Massachusetts, where it was discovered in 1824.^[6][2][3] It is also found in Sweden, South Africa, Scotland, and New Zealand.^[3]

Chemistry

Cummingtonite is a member of the cummingtonite-grunerite solid solution series which ranges from Mg
₇Si
₈O
₂₂(OH)
₂ for magnesiocummingtonite to the iron rich grunerite endmember Fe
₇Si
₈O
₂₂(OH)
₂. Cummingtonite is used to describe minerals of this formula with between 30 and 70 per cent Fe
₇Si
₈O
₂₂(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 Mn
₂Mg
₅Si
₈O
₂₂(OH)
₂ (tirodite) and Mn
₂Fe
₅Si
₈O
₂₂(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".

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.