Ferrosilite

inosilicate

granulite facies

Formula: Fe2+2Si2O6 inosilicate (chain silicate) pyroxene group
Specific gravity:
Hardness: 5 to 6
Streak: Pale grey brown
Colour: Dark brown to black

Plutonic igneous environments
Volcanic igneous environments
Metamorphic environments

Ferrosilite is found in basic and ultrabasic igneous rocks and also in high-grade metamorphic rocks, such as the granulite facies. Ferrosilite is common in metamorphosed iron formation in association with grunerite.

Alteration

augite and andalusite to enstatite-ferrosilite and anorthite
2Ca(Fe,Mg)Si2O6 + 2Al2SiO5 → (Mg,Fe2+)2Si2O6 + 2Ca(Al2Si2O8) (DHZ 2A p126)

biotite and quartz to enstatite-ferrosilite, orthoclase and H2O
K(Mg,Fe)3(AlSi3O10)(OH)2 + 3SiO2 → 3(Mg,Fe2+)SiO3 + KAlSi3O8 + H2O Enstatite-ferrosilite may develop from the breakdown of biotite according to the above reaction. (DHZ 2A p134)

chlorite and quartz to enstatite- ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)4Al4Si2O10(OH)8 + 5SiO2 → 2(Mg,Fe2+)SiO3 + (Mg,Fe2+2)2Al4Si5O18 + 4H2O
Enstatite-ferrosilite also occurs in medium grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content according to the above equation (DHZ 2A p134).

Fe-rich cordierite and diopside- hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2
(DHZ 2A p126)

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)

diopside-hedenbergite and CO2 to enstatite-ferrosilite, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe2+)SiO3 + CaCO3 + SiO2
(DHZ 2A p136)

enstatite-ferrosilite and H2O to serpentine and quartz
6(Mg,Fe2+)SiO3 + 4H2O ⇌ (Fe,Mg)6Si4O10(OH)8 +2SiO2
(DHZ p138)

enstatite-ferrosilite and andalusite to Fe-rich cordierite and spinel- hercynite
5(Mg,Fe2+)SiO3 + 5 Al2SiO5 → 2(Mg,Fe2+)2Al4Si5O18 + (Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction (DHZ 2A p134).

enstatite-ferrosilite, andalusite and SiO2 to Fe-rich cordierite
2(Mg,Fe2+)SiO3 + 2Al2SiO5 + SiO2 → (Mg,Fe2+)2Al4Si5O18
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction (DHZ 2A p134).

enstatite-ferrosilite, diopside- hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2 This reaction represents metamorphic reactions between the granulite and amphibolite facies .

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4
(DHZ 2A p258)

enstatite-ferrosilite, K-feldspar and H2O to biotite and quartz
3(Mg,Fe2+)SiO3 + K(AlSi3O8) + H2O ⇌ K(Mg,Fe)3(AlSi3O10)(OH)2+ 3SiO2
The forward reaction leads to an amphibolite facies assemblage (DHZ 2A 139).

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

ferrosilite to fayalite and quartz
Fe2Si2O6 ⇌ Fe2SiO4 + SiO2
Ferrosilite is unstable at pressure less than about 15 kbar (DHZ 1A p55)

gedrite- and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al4Si6O22(OH)2 + 2Si2 → 3(Mg,Fe2+)SiO3 + (Mg,Fe2+)2Al4Si5O18 + H2O In the pyroxene-hornfels facies enstatite-ferrosilite may develop from gedrite-ferro-gedrite according to the above reaction (DHZ 2A p134).

olivine and CO2 to enstatite-ferrosilite and magnesite-siderite
(Mg,Fe)2SiO4 + CO2 → (Mg,Fe2+)SiO3 + (Mg,Fe)CO3
(DHZ 2A p139)

olivine and quartz to enstatite-ferrosilite
(Mg,Fe)2SiO4 + SiO2 → 2(Mg,Fe2+)SiO3
(DHZ 2A p139)

Common impurities: Ca,Na,K,Al,Co,Ni,Mn,Ti,Cr

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