Grunerite

inosilicate

metamorphic

Formula: ☐Fe2+2Fe2+5Si8O22(OH)2 inosilicate (chain silicate) amphibole
Specific gravity: 3.4 to 3.5
Hardness: 5 to 6
Streak: Colourless
Colour: Ashen, brown, brownish green, dark gray
Environments:

Metamorphic environments

Grunerite is common in iron formations due to medium to high grade contact metamorphism, and it also occurs in some blueschist facies quartzite.
Upon further metamorphism cummingtonite and grunerite give way to orthopyroxene or olivine.

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 p138)

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

fayalite, SiO2 and H2O to grunerite
7Fe2+2(SiO4) + 9SiO2 + 2H2O → 2Fe2+2Fe2+5Si8O22(OH)2 Fayalite may undergo partial regression to grunerite according to the above reaction. DHZ 1A p265)

ferro-actinolite to hedenbergite, grunerite, quartz and H2O
7Ca2Fe2+5Si8O22(OH)2 → 14CaFe2+Si2O6 + 3Fe2+2Fe2+5 Si8O22(OH)2 + 4SiO2 + 4H2O
In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, probably due to the instability of ferro-actinolite with rising temperature. The association of hedenbergite and grunerite has been widely described, and its formation may be due to the above reaction DHZ 2A p273).

Mg-rich greenalite to olivine, Mg-rich grunerite and H2O
18(Fe2+, Mg))3Si2O5(OH)4 → 20(Fe,Mg)2SiO4 + 2(Fe2+,Mg)7Si8O22(OH)2 + 34H2O
DHZ 1A p266)

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