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Formula: ☐Mg2Mg5Si8O22(OH)2
Inosilicate (chain silicate),
magnesium-iron-manganese amphibole subgroup,
amphibole supergroup,
paramorph of anthophyllite,
forms a series with grunerite.
Crystal System: Monoclinic
Specific gravity: 3.1 to 3.6 measured, 3.3 calculated
Hardness: 5 to 6
Streak: Greyish white
Colour: Translucent dark green, brown, grey, colourless
Common impurities: Mn,Ca,Al,Ti,Na,K
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
V-grunerite
7(Mg,Fe2+)SiO3 + SiO2 + H2O ⇌
(Fe,Mg)7Si8O22(OH)2
(DHZ 2A p138)
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