Cummingtonite

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.