Cordierite

cyclosilicate

hornfels

Formula: Mg2Al4Si5O18
Cyclosilicate (ring silicate)
Specific gravity: 2.5 to 2.8
Hardness: 7 to 7½
Streak: White
Colour: Grey, blue to violet, yellow to brown, greenish, also colourless
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Metamorphic environments (typical)

Cordierite is a common constituent of contact and regionally metamorphosed argillaceous (clay-rich) rocks. It is especially common in hornfels, and it is also found in regionally metamorphosed cordierite-garnet- sillimanite gneiss and schist.
Cordierite may be found in granite, gabbro, phyllite, schist and gneiss. It is also found in river gravel.
Common associations are sillimanite and spinel, or spinel, plagioclase feldspar and orthopyroxene. Cordierite associated with anthophyllite has been described from several localities.
Cordierite is a mineral of the hornblende-hornfels, pyroxene-hornfels, greenschist, amphibolite and granulite facies.

Alteration

chlorite, muscovite and quartz to biotite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al(AlSi3O10)(OH)8 + KAl2(AlSi3O10)(OH)2 + 2SiO2 → K(Mg,Fe2+)3(AlSi3O10)(OH)2 + (Mg,Fe2+)2Al4Si5O18 + 4H2O
This reaction ocurs when the metamorphic grade increases (http://www.tulane.edu/~sanelson/eens212/metaminerals.htm).

chlorite and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)4Al4Si2O10(OH)8 + 5SiO2 → 2(Mg,Fe2+)SiO3 + (Mg,Fe2+)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)

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 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 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, 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
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).

forsterite and cordierite to pyrope and quartz
2Mg2SiO4 + Mg2Al4Si5O18 ⇌ 2Mg3Al2(SiO4)3 + SiO2
Increasing pressure favours the forward reaction (SERC).

forsterite, kyanite and quartz to cordierite
Mg2SiO4 + 2Al2OSiO4 + 2SiO2 ⇌ Mg2Al4Si5O18
At 6 kbar pressure the equilibrium temperature is about 400oC (greenschist facies) (SERC).

gedrite-ferro-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).

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 pyrope to cordierite and spinel
2Al2OSiO4 + Mg3Al2(SiO4)3 ⇌ Mg2Al4Si5O18 + MgAl2O4
Increasing temperature favours the forward reaction (SERC).

muscovite, biotite and SiO2 to K-feldspar, cordierite and H2O
6KAl2(AlSi3O10)(OH)2 + 2K(Fe2+,Mg)3(AlSi3O10)(OH)2 + 15SiO2 → 8KAlSi3O8 + 3(Fe2+,Mg)3Al4Si5O18 + 8H2O
At the high-grade end of the amphibolite facies biotite is no longer stable and reacts with muscovite according to the above reaction. (DHZ 3 p73)

quartz, pyrope and kyanite to cordierite
5SiO2 + 2Mg3Al2(SiO4)3 + 4Al2OSiO4 ⇌ 3Mg2Al4Si5O18
Increasing temperature favours the forward reaction (SERC).

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

spinel, forsterite and cordierite to pyrope
MgAl2O4 + 5Mg2SiO4 + 2Mg2Al4Si5O18 ⇌ 5Mg3Al2(SiO4)3
Increasing pressure favours the forward reaction (SERC).

talc and kyanite to cordierite, corundum and H2O
2Mg3Si4O10(OH)2 + 7Al2OSiO4 ⇌ 3Mg2Al4Si5O18 + Al2O3 + 2H2O
(DHZ 2A p635)

Common impurities: Mn,Fe,Ti,Ca,Na,K

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