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Formula: Mg2Al4Si5O18
Cyclosilicate (ring silicate)
Crystal System: Orthorhombic
Specific gravity: 2.6 to 2.66 measured, 2.505 calculated
Hardness: 7 to 7½
Streak: White
Colour: Grey, blue to violet, yellow to brown, greenish, also colourless
Solubility: Slightly soluble in hydrochloric acid
Common impurities: Mn,Fe,Ti,Ca,Na,K
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 and kyanite to
cordierite and spinel
5Mg2SiO4 + 10Al2OSO4 ⇌ 3Mg2Al4Si5O18 +
4MgAl2O4
At 400oC the equilibrium pressure for this reaction is about 4 kbars; increasing temperature 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
2Mg
(DHZ 2A p635)
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