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Formula: Al2O3
Oxide, hematite group
Varieties
Ruby is the red gem variety of corundum
Sapphire is the gem variety of corundum which is any colour but red, although usually blue
Crystal system: Trigonal
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.
Localities
At lots 10 and 11 of concession 1, Bathurst Township, Lanark County, Ontario, Canada (DeWitts corner), the deposit is
located in the Grenville Geological Province, which consists mostly of
marble, gneiss, and
quartzite.
Syenite-migmatite was
also reported in the area where the vein-dikes are located. Characteristic features of the vein-dikes include the
fact that perfectly formed euhedral crystals of different minerals can often be found floating in
calcite with no points of contact with the walls. Sometimes these crystals
have inclusions of calcite, irregular or rounded in shape. It has been argued
that at least some of the vein-dikes were formed as a result of melting of Grenville
marble.
Corundum forms pale grey, grey-green, or grey-yellow granular aggregates replacing
spinel. Most pseudomorphs are
only partially replaced, resulting in aesthetic and unique specimens with corundum replacing mostly vertices and
edges of spinel crystals. Complete
pseudomorphs are rare and are mostly found replacing smaller
spinel crystals to 1.5 cm. However, several larger
pseudomorphs are also known. Corundum, as colourless to pale
green crystals to 10 mm displaying fragments of the common squat-barrel shape, has been observed rarely either free in
the calcite or surrounded by cavernous masses of
spinel. These crystals are of an earlier generation than the corundum
that replaces the spinel
(R&M 97.6.556-564).
At the Gutz corundum occurrence, near Jewelville, Renfrew county, Ontario, Canada, black
corundum occurs in white syenite
(R&M 94.5.410).
At Luc Yen, Myanmar, corundum pseudomorphs after
spinel have been found
(KL p137).
Amity, Town of Warwick, Orange county, New York, USA, is an area of
granite intrusions into
marble and associated
gneiss. The marble is
mostly composed of white crystalline calcite that often has small flakes
or spheres of graphite and
phlogopite. Corundum occurs sporadically as pink crystals less
than 2 cm across in marble. The corundum fluoresces bright red
(R&M 96.5.436).
The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in
marble. The development of veins of large crystals probably occurred as
a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during
this primary event, with additional species resulting from the
subsequent alteration of scapolite. There seems to be little, if any,
secondary, late-stage mineralisation present.
Corundum occurs as pink to red anhedral masses to 1.5 cm in the walls of the
scapolite vein, associated with
nepheline and meionite
(R&M 96.6.548-549).
It is deep red under short wave UV and even brighter red under long wave UV. The fluorescence does not reveal any
distinct boundaries between the corundum and the enclosing mineral, often nonfluorescing
nepheline
(R&M 97.5.442).
Alteration
anorthite and CO2 to meionite
(scapolite series), corundum and quartz
4Ca(Al2Si2O8) + CO2 ⇌
Ca4Al6O24(CO3) + Al2O3 + 2SiO2
(DHZ 4 p334)
augelite to berlinite,
corundum and H2O
2Al2(PO4)(OH)3 to 2Al(PO4) + Al2O3 + 3H2O
(AM 64.1175-1183)
augelite to trolleite, corundum and
H2O
3Al2(PO4)(OH)3 to Al4(PO4)3(OH)3 +
Al2O3 + 3H2O
(AM 64.1175-1183)
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
2Mg
(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)
trolleite to berlinite, corundum and H2O
2Al4(PO4)3(OH)3 to 6Al(PO4) + Al2O3 + 3H2O
(AM 64.1175-1183)
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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