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Formula: ☐Mg2Mg5Si8O22(OH)2
Inosilicate (chain silicate), double chains,
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:
Plutonic igneous environments
Volcanic igneous environments rare
Metamorphic environments common
Cummingtonite is commonly found in medium-grade
regionally metamorphosed rocks; it is characteristic of
metamorphosed iron formations, a late-stage mineral in
some gabbros and norites,
and found rarely in silicic volcanic rocks. Associated minerals include
hornblende, anthophyllite,
actinolite,
ferro-actinolite,
tremolite, arfvedsonite,
magnesio-arfvedsonite,
gedrite, glaucophane,
quartz and garnet
(HOM).
Localities
At the Waiho River, Westland Tai Poutini National Park, Westland District, West Coast Region, New Zealand,
cummingtonite has been found with garnet
(Mindat photo).
Cummingtonite from the Waiho River -
Image
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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