Corundum

oxide

Formula: Al2O3 oxide
Specific gravity: 3.98 to 4.1
Hardness: 9
Streak: White
Colour: Colourless, blue, red, pink, yellow, brown
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites
Sedimentary environments
Placer deposits
Metamorphic environments

Corundum is common as an accessory mineral in contact and regionally metamorphosed rocks, such as limestone, mica-schist and gneiss. It is also found as an original constituent of silica deficient igneous rocks such as syenite and nepheline syenite, and in pegmatites. It may be found in large masses in the zone separating peridotite from the adjacent country rock. It is found frequently as a detrital material in sediments, preserved through its hardness and chemical inertness. It also may be found in eclogite and gneiss.
Commonly associated minerals are chlorite, mica, olivine, serpentine, magnetite, spinel, kyanite and diaspore. quartz never occurs with corundum.
It is a mineral of the albite-epidote-hornfels, hornblende-hornfels greenschist, amphibolite, eclogite and granulite facies.

Alteration

anorthite and CO2 to meionite (scapolite series), corundum and quartz
4Ca(Al2Si2O8) + CO2 ⇌ Ca4Al6O24(CO3) + Al2O3 + 2SiO2
(DHZ 4 p334)

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).

diaspore to corundum and H2O
2AlO(OH) ⇌ Al2O3 + H2O
High temperature favours the forward reaction. At 1 kbar pressure the reaction occurs at about 320oC (albite-epidote-hornfels facies), and at 10 kbar it occurs at about 490oC (greenschist facies) (SERC, AM61.699-709).

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) .

grossular and corundum to anorthite and gehlenite
2Ca3Al2(SiO4)3 + Al2O3 ⇌ CaAl2Si2O8 + Ca2Al2SiO7
The equilibrium temperature for this reaction at 5 kbar pressure is about 950oC At 4.3 kbar pressure the equilibrium temperature is about 890oC (granulite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

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 zoisite to anorthite, corundum and H2O
2Al2O(SiO4) + 2Ca2Al3[Si2O7][SiO4]O(OH) ⇌ 4CaAl2Si2O8 + Al2O3 + H2O
The equilibrium temperature for this reaction at 5 kbar pressure is 480oC (greenschist facies), and at 10 kbar it is about 720oC (amphibolite facies). The equilibrium is to the right at higher temperatures, and to the left at lower temperatures (SERC, AM61.699-709).

lawsonite and corundum to margarite and H2O
CaAl2(Si2O7(OH)2.H2O + Al2O3 ⇌ CaAl2Si2Al2O10(OH)2 + H2O
Increasing temperature favours the forward reaction (AM61.699-709).

lawsonite and corundum to zoisite, kyanite and H2O
4CaAl2(Si2O7)(OH)2.H2 + Al2O3 ⇌ 2Ca2Al3[Si2O7][SiO4}O(OH) + 2Al2OSiO4 + 7H2O
The equilibrium temperature for this reaction at 15 kbar pressure is about 570oC (eclogite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (AM61.699-709).

margarite to corundum, anorthite and H2O
CaAl2(Al2Si2O10)(OH)2 ⇌ Al2O3 + Ca(Al2Si2O8)
The equilibrium temperature for this reaction at 6 kbar pressure is about 610oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

margarite to corundum, zoisite, kyanite and H2O
4CaAl2(Al2Si2O10)(OH)2 ⇌ 3Al2O3 + 2Ca2Al3[Si2O7][SiO4]O(OH) + 2Al2OSiO4 + 3H2O
The equilibrium temperature for this reaction at 10 kbar pressure is about 650oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC, AM61.699-709).

muscovite to corundum, K-feldspar and H2O
KAl2(AlSi3O10)(OH)2 ⇌ Al2O3 + K(AlSi3O8) + H2O
(JVW p102)
This reaction takes place above temperatures ranging from 600oC at atmospheric pressure (hornblende-hornfels facies) to about 720oC at pressure above 4 kbar (amphibolite facies) (MOM p517).

staurolite, annite and O2 to hercynite, magnetite, muscovite, corundum, SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3 (AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4 + Fe2+Fe3+2O4 + KAl2 (AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O
(DHZ 1A p860)

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

magnesium-rich tremolite, zoisite, corundum and H2O to tschermakite
3☐Ca2MgSi8O22(OH)2 + 2Ca2Al3[Si2O7][SiO4]O(OH) + 7Al2O3 + H2O ⇌ 5☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2
(AM 76.990)

zoisite to anorthite, grossular, corundum and H2O
6Ca2Al3[Si2O7][SiO4]O(OH) ⇌ 6CaAl2Si2O8 + 2Ca3Al2Si3O12 + Al2O3 + 3H2O
The equilibrium temperature for this reaction at 6 kbar pressure is about 760oC, and at 10 kbar it is about 950oC (granulite facies). For any given pressure, the reaction goes to the right at higher temperatures, and to the left at lower temperatures (SERC).

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