Enstatite

inosilicate

augite

Formula: Mg2Si2O6 inosilicate (chain silicate) pyroxene group
Specific gravity: 3.2 to 3.9
Hardness: 5 to 6
Streak: Grey to white
Colour: Green, brown, white
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Volcanic igneous environments
Metamorphic environments

Enstatite is a widespread mineral of the pyroxene group. It usually occurs in magnesium- and iron-rich igneous rocks and in both iron and stony meteorites.
In igneous rocks, a high silica melt will give augite, hornblende and enstatite or biotite.
Metamorphic enstatite is a mineral of the greenschist, amphibolite, eclogite and granulite facies.

Alteration

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopite
2.5Ca(Al2Si2O8) + 5Mg2Si2O6 + 6MgAl2O4 + K2O + 3H2O → Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs as the metamorphic grade decreases from the granulite to the amphibolite facies (DHZ 2A p631).

anthophyllite to enstatite, quartz and H2O
2☐Mg2Mg5Si8O22(OH)2 ⇌ 7Mg2Si2O6 + 2SiO2 + 2H2O
At 10 kbar pressure the equilibrium temperature is about 810oC (granulite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

anthophyllite and forsterite to enstatite and H2O
2☐Mg2Mg5Si8O22(OH)2 + 2Mg2SiO4 ⇌ 9Mg2Si2O6 + 2H2O
At 2 kbar pressure the equilibrium temperature is about 690oC (pyroxene-hornfels facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

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
2K(Mg,Fe)3(AlSi3O10)(OH)2 + 6SiO2 → 3(Mg,Fe2+)2Si2O6 + 2KAlSi3O8 + 2H2O
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 → (Mg,Fe2+)2Si2O6 + (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 = 2(Mg,Fe2+)2Si2O6 + 2Ca(Al2Si2O8) + SiO2
(DHZ 2A p126)

corundum and forsterite to spinel and enstatite
2Al2O3 + 2Mg2SiO4 ⇌ 2MgAl2O4 + Mg2Si2O6
At 10 kbar pressure the equilibrium temperature is about 570oC (amphibolite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

cummingtonite-grunerite and olivine to enstatite-ferrosilite and H2O
2(Fe,Mg)7Si8O22(OH)2 + 2(Mg,Fe)2SiO4 ⇌ 9(Mg,Fe2+)2Si2O6 + 2H2O
(DHZ 2A p138)

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

dolomite, tremolite and forsterite to diopside, enstatite and H2O
Ca2Mg5Si8O22(OH)2 + Mg2SiO4 ⇌ 2CaMgSi2O6 + H2O
At a pressure of 4 kbar the equilibrium temperature is about 840oC (granulite facies) (JVW p97).

enstatite and H2O to forsterite and cummingtonite
9Mg2Si2O6 + 2H2O = 2Mg2SiO4 + 2Mg2Mg5Si8O22(OH)2
Cummingtonite may be formed by retrograde metamorphism according to the above reaction (DHZ 1A p259).

enstatite and H2O to serpentine and talc
3Mg2Si2O6 + 3H2O → Mg3Si2O5(OH)4 + Mg3Si4O10(OH)2
talc may occur as a pseudomorph after enstatite by the above reaction (DHZ 3 p185).

enstatite and calcite to forsterite, diopside and CO2
3Mg2Si2O6 + 2CaCO3 ⇌ 2Mg2SiO4 + 2CaMgSi2O6 + 2CO2
Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above (DHZ 2A p135).

enstatite, calcite and quartz to diopside and CO2
3Mg2Si2O6 + 2CaCO3 + 2SiO2 ⇌ + 2CaMgSi2O6 + 2CO2
Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above (DHZ 2A p135).

enstatite and corundum to cordierite and spinel
5Mg2Si2O6 + 10Al2O3 ⇌ 2Mg2Al4Si5O18 + 6MgAl2O4
At 6 kbar pressure the equilibrium temperature is about 715oC (amphibolite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

enstatite and corundum to pyrope
3Mg2Si2O6 + 2Al2O3 ⇌ 2Mg3Al2(SiO4)3
At 14 kbar pressure the equilibrium temperature is about 810oC (eclogite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

Al-rich enstatite and Al-rich diopside to forsterite, enstatite, diopside and anorthite
Mg9Al2Si9O30 + Ca5Mg4Al2Si9O30 ⇌ 2Mg2SiO4 + 3Mg2Si2O6 + 3CaMgSi2O6 + 2Ca(Al2Si2O8)
This reaction occurs at fairly low temperature and pressure (DHZ 1A p233).

enstatite and kyanite to spinel and cordierite
5Mg2Si2O6 + 10Al2OSiO4 ⇌ 2MgAl2O4 + 4Mg2Al4Si5O18
Increasing temperature favours the forward reaction (SERC).

enstatite, kyanite and quartz to cordierite
Mg2Si2O6 + 2Al2OSiO4 + SiO2 ⇌ Mg2Al4Si5O18
At 6 kbar pressure the equilibrium temperature is about 475oC (greenschist facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

enstatite and spinel to forsterite and cordierite
5Mg2Si2O6 + 2MgAl2O4 ⇌ 5Mg2SiO4 + Mg2Al4Si5O18
At 4 kbar pressure the equilibrium temperature is about 715oC (amphibolite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

enstatite-ferrosilite and H2O to serpentine and quartz
3(Mg,Fe2+)2Si2O6 + 4H2O ⇌ (Fe,Mg)6Si4O10(OH)8 +2SiO2
(DHZ p138)

enstatite-ferrosilite and andalusite to Fe-rich cordierite and spinel-hercynite
5(Mg,Fe2+)2Si2O6 + 10Al2SiO5 → 4(Mg,Fe2+)2Al4Si5O18 + 2(Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, in an SiO2 deficient environment the association of andalusite with enstatite-ferrosilite is excluded by the above reaction (DHZ 2A p134).

enstatite-ferrosilite, andalusite and quartz to Fe-rich cordierite
(Mg,Fe2+)2Si2O6 + 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, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
(Mg,Fe2+)2Si2O6 + 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, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)2Si2O6 + 2Ca(Mg,Fe2+)Si2O6 + 2NaAlSi3O8 + 2Ca(Al2Si2O8) + 2H2O ⇌ 2NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 8SiO2 This reaction represents metamorphic reactions between the granulite and amphibolite facies (DHZ 2A p139).

enstatite-ferrosilite, K-feldspar and H2O to biotite and quartz
3(Mg,Fe2+)2Si2O6 + 2K(AlSi3O8) + 2H2O ⇌ 2K(Mg,Fe)3(AlSi3O10)(OH)2+ 6SiO2
The forward reaction leads to an amphibolite facies assemblage (DHZ 2A 139).

enstatite-ferrosilite, quartz and H2O to cummingtonite- grunerite
7(Mg,Fe2+)2Si2O6 + 2SiO2 + 2H2O ⇌ 2(Fe,Mg)7Si8O22(OH)2
(DHZ 2A p138)

forsterite and CO2 to enstatite and magnesite
2Mg2SiO4 + 2CO2 ⇌ Mg2Si2O6 + 2MgCO3
(DHZ 2A p105)

forsterite and anorthite to enstatite, diopside and spinel
2Mg2SiO4 + Ca(Al2Si2O8)= Mg2Si2O6 + CaMgSi2O6 + MgAl2O4
(DHZ 1A p242)

forsterite, enstatite and H2O to serpentine
2Mg2SiO4 + Mg2Si2O6 + 4H2O → 2Mg3Si2O5(OH)4
serpentine is not stable in the presence of carbon dioxide, and may further react with it to form talc and magnesite (R&M 90.6.521).

forsterite and quartz to enstatite
Mg2SiO4 + SiO2 → Mg2Si2O6
forsterite is not stable in the presence of free SiO2 and will react with it to form enstatite according to the above reaction.

forsterite and talc to enstatite and H2O
2Mg2SiO4 + 2Mg3Si4O10(OH)2 ⇌ 5Mg2Si2O6 + 2H2O
(JVW p103)
At 10 kbar pressure the equilibrium temperature is about 680oC (amphibolite facies), with equilibrium to the right at higher temperatures and to the left at lower temperatures (for the same pressure) (SERC).

gedrite-ferro-gedrite and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
2(Mg,Fe2+)5Al4Si6O22(OH)2 + 4Si2 → 3(Mg,Fe2+)2Si2O6 + 2(Mg,Fe2+)2Al4Si5O18 + 2H2O
In the pyroxene-hornfels facies enstatite-ferrosilite may develop from gedrite-ferro-gedrite according to the above reaction (DHZ 2A p134).

kyanite and enstatite to cordierite and corundum
3Al2O(SiO4) + Mg2Si2O6 ⇌ Mg2Al4Si5O18 + Al2O3
The equilibrium temperature for this reaction at 6 kbar pressure is about 520oC (amphibolite facies), with equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

kyanite and enstatite to cordierite and pyrope
6Al2OSiO4 + 5Mg2Si2O6 ⇌ 2Mg2Al4Si5O18 + 2Mg3Al2(SiO4)3
Increasing temperature favours the forward reaction (SERC).

kyanite and enstatite to quartz and pyrope
2Al2O(SiO4) + 3Mg2Si2O6 ⇌ 2SiO2 + 2Mg3Al2(SiO4)3
The equilibrium temperature for this reaction at 14 kbar pressure is about 950oC (granulite facies), with equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

kyanite and forsterite to enstatite and spinel
2Al2OSiO4 + 4Mg2SiO4 ⇌ 3Mg2Si2O6 + 2MgAl2O4
Increasing temperature favours the forward reaction (SERC).

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 → (Mg,Fe2+)2Si2O6
(DHZ 2A p139)

Fe and Cr-rich spinel, diopside and enstatite to forsterite, anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 + Mg2Si2O6 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4
This reaction occurs at fairly low temperature and pressure (DHZ 1A p233).

spinel, enstatite and cordierite to pyrope
MgAl2O4 + Mg2Si2O6 + Mg2Al4Si5O18 ⇌ Mg3Al2(SiO4)3
Increasing pressure favours the forward reaction (SERC).

spinel, kyanite and enstatite to pyrope
2MgAl2O4 + 2Al2OSiO4 + 5Mg2Si2O6 ⇌ 4Mg3Al2(SiO4)3
Increasing temperature favours the forward reaction (SERC).

talc to enstatite, quartz and H2O
2Mg3Si4O10(OH)2 ⇌ 3Mg2Si2O6 + 2SiO2 + 2H2O
At 10 kbar pressure the equilibrium temperature is about 790oC (granulite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

talc and enstatite to anthophyllite
Mg3Si4O10(OH)2 + 2Mg2Si2O6 → ☐Mg2Mg5Si8O22(OH)2
At 10 kbar pressure the equilibrium temperature is 750oC (granulite facies).

tremolite to diopside, enstatite, quartz and H2O
2Ca2Mg5Si8O22(OH)2 ⇌ 4CaMgSi2O6 + 3Mg2Si2O6 + 2SiO2 + 2H2O
The equilibrium temperature for this reaction at 8 kbar pressure is 930oC (granulite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (for the same pressure) (KB p421, diagram p420, SERC).

tremolite and forsterite to diopside, enstatite and H2O
Ca2Mg5Si8O22(OH)2 + Mg2SiO4 ⇌ 2CaMgSi2O6 + H2O
The equilibrium temperature for this reaction at 4 kbar pressure is 840oC (granulite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (JVW p97).

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

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