Cummingtonite

inosilicate

anthophyllite

Formula: ☐Mg2Mg5Si8O22(OH)2 inosilicate (chain silicate) amphibole
Cummingtonite is a dimorph of anthophyllite.
Hardness: 5 to 6
Streak: not determined
Colour: Translucent dark green, brown, grey, colourless
Environments:

Metamorphic environments

Cummingtonite is found in both contact and regional metamorphic rocks

Alteration

cummingtonite-grunerite and H2O to serpentine and quartz
6(Fe,Mg)7Si8O22(OH)2 + 22H2O ⇌ 7(Fe,Mg)6Si4O10(OH)8 + 20SiO2 (DHZ 2A p138)

cummingtonite-grunerite and olivine to enstatite-ferrosilite and H2O
(Fe,Mg)7Si8O22(OH)2 + (Mg,Fe)2SiO4 ⇌ 9(Mg,Fe2+)SiO3 + H2O (DHZ 2A p135)

enstatite and H2O to forsterite and cummingtonite
9MgSiO3 + H2O = Mg2SiO4 + Mg2Mg5Si8O22(OH)2
Cummingtonite may be formed by retrograde metamorphism according to the above reaction (DHZ 1A p259).

enstatite-ferrosilite, SiO2 and H2O to cummingtonite-grunerite
7(Mg,Fe2+)SiO3 + SiO2 + H2O ⇌ (Fe,Mg)7Si8O22(OH)2 (DHZ 2A p138)

Common impurities: Mn,Ca,Al,Ti,Na,K

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