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Formula: Fe2+2Si2O6
Inosilicate (chain silicate), orthopyroxene subgroup,
pyroxene group
Crystal System: Orthorhombic
Specific gravity: 3.60 to 4.00 measured, 4.00 calculated
Hardness: 5 to 6
Streak: Pale grey brown
Colour: Dark brown to black
Common impurities: Ca,Na,K,Al,Co,Ni,Mn,Ti,Cr
Environments
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
(DHZ 2A p139).
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)
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