Formula: AlO(OH)
Hydroxide, diaspore group,
paramorph of
böhmite.
Both minerals are orthorhombic, but they have a different arrangement of atoms in their structure.
Crystal System: Orthorhombic
Specific gravity: 3.2 to 3.5 measured, 3.38 calculated
Hardness: 6½ to 7
Streak: White
Colour: White, brown, colourless, pale yellow, greyish, greenish grey, lilac, pinkish
Solubility: Insoluble in hydrochloric, sulphuric and nitric acids
Common impurities: Fe,Mn,Cr,Si
Environments
Pegmatites
Metamorphic environments
Hydrothermal environments
Diaspore occurs most commonly in metamorphic bauxite deposits associated with
gibbsite and
böhmite.
It is a mineral of the blueschist,
prehnite-pumpellyite,
greenschist and
albite-epidote-hornfels facies.
At the Saga 1 pegmatite, near Porsgrunn, Telemark, Norway, minute diaspore crystals formed through the hydrothermal alteration
of nepheline in the nepheline syenite
pegmatite host rock
(R&M 95.2.145-151).
In the Russian Urals, diaspore is found in emery schist
(R&M 86.3.242).
At Campolonga, Switzerland, diaspore occurs in marble
(R&M 86.3.242).
At the bauxite deposit in the İlbir Mountains, Muğla Province, Turkey, medium to high
grade metamorphism changes
syenite and
nepheline syenite to gneiss,
limestone to marble, and
mudstone to
schist. The diaspore occurs in hydrothermally mineralised fractures formed below
515oC in a deposit of metamorphosed bauxite in
marble,
associated with
calcite, muscovite and
chloritoid on a goethite-rich
matrix. The diaspore crystals here are the finest in the world, and may reach 8 cm in length
(R&M 86.3.242, 95.2.145-151).
At the Chester emery mines, Hampden county, Massachusetts, USA, diaspore occurs in fissures crossing veins of
emery, which at Chester is a mixture of
magnetite, corundum and
ilmenite
(R&M 95.2.145-151).
Alteration
Hydrothermal diaspore is a characteristic mineral of advanced alteration resulting from the reaction of low pH
(acid) fluids with rocks. It forms at intermediate temperatures associated with
pyrophyllite. Diaspore does not exist in equilibrium with
quartz, but
both minerals are usually present in specimens or outcrops of diaspore
(AofA).
diaspore to corundum and H2O
2AlO(OH) ⇌ Al2O3 + H2O
The equilibrium temperature for this reaction at 1 kbar pressure is about 320oC
(albite-epidote-hornfels facies), and at 10 kbar
pressure it is about 490oC
(greenschist facies). The equilibrium is to
the right at higher temperatures, and to the left at lower temperatures
(SERC, AM61.699-709).
diaspore and quartz to kyanite
and H2O
2AlO(OH) + SiO2 ⇌ Al2OSiO4 + H2O
The equilibrium temperature for this reaction at 10 kbar pressure is about 420oC
(greenschist facies), with the equilibrium to
the right at higher temperatures, and to the left at lower temperatures
(SERC).
diaspore and quartz to pyrophyllite
2AlO(OH) + 4SiO2 ⇌ Al2Si4O10(OH)2
The equilibrium temperature for this reaction at 5 kbar pressure is about 160oC
(zeolite facies), and at 10 kbar it is
about 300oC (blueschist facies).
The equilibrium is to
the right at higher temperatures, and to the left at lower temperatures
(SERC).
kaolinite to diaspore, SiO2 and H2O
Al2Si2O5(OH)4 ⇌ 2AlO(OH) + 2SiO2 (aqueous) +
H2O
At 10 kbar pressure the equilibrium temperature is about 300oC
(blueschist facies).
At 1 kbar pressure kaolinite is stable at temperatures less than
300oC; it can be in equilibrium with
quartz and water in solutions both saturated and undersaturated with
quartz. Diaspore is stable at temperatures
less than 400oC but only in solutions undersaturated with quartz.
High temperature and low quartz
saturation favours the forward reaction
(KB p 93).
kaolinite to
pyrophyllite, diaspore and H2O
2Al2Si2O5(OH)4 →
Al2Si4O10(OH)2 + 2AlO(OH) + 2H2O
In the absence of quartz, kaolinite
breaks down on heating according to the above reaction
(KB p431).
At 5 kbar pressure the equilibrium temperature for the reaction is about 320oC
(prehnite-pumpellyite facies), and at 9 kbar it
is about 380oC
(greenschist facies)
(SERC, AM61.699-709).
kaolinite and diaspore to andalusite and
H2O
Al2Si2O5(OH)4 + 2AlO(OH) ⇌ 2Al2OSiO4 +
3H2O
At 1 kbar pressure the equilibrium temperature for the reaction is about 320oC (albite-epidote-hornfels facies), with the equilibrium to the right
at higher temperatures and to the left at lower temperatures
(SERC, AM61.699-709).
kaolinite and diaspore to kyanite and
H2O
Al2Si2O5(OH)4 + 2AlO(OH) ⇌ 2Al2OSiO4 +
3H2O
At 5 kbar pressure the equilibrium temperature for the reaction is about 370oC
(greenschist facies), with the equilibrium to the right
at higher temperatures and to the left at lower temperatures
(SERC, AM61.699-709).
kyanite, zoisite and
diaspore to margarite
Al2OSiO4 + Ca2Al3[Si2O7][SiO4]O(OH) + 3AlO(OH) ⇌
2CaAl2(Al2Si2O10)(OH)2
Increasing temperature favours the forward reaction
(SERC)
lawsonite and diaspore to margarite
and H2O
CaAl2(Si2O7)(OH)2.H2O + 2AlO(OH) ⇌
CaAl2(Al2Si2O10)(OH)2 + 2H2O
The equilibrium temperature for this reaction at 10 kbar pressure is about 460oC
(greenschist facies), with the equilibrium to
the right at higher temperatures, and to the left at lower temperatures
(AM61.699-709).
pyrophyllite and diaspore to
andalusite and H2O
Al2Si4O10(OH)2 + 6AlO(OH) ⇌ 4Al2SiO5 +
4H2O
(JVW p 553)
This reacton is a low pressure reaction, occurring below about 1.9 kbar. Increasing temperature favours the
forward reaction
(SERC).
pyrophyllite and diaspore to
kyanite and H2O
Al2Si4O10(OH)2 + 6AlO(OH) ⇌ 4Al2OSiO4 +
4H2O
(JVW p 553)
This reacton is a higher pressure reaction, occurring above about 1.9 kbar. Increasing temperature favours the
forward reaction. At 9 kbar pressure the equilibrium temperature is about 380oC
(greenschist facies)
(SERC, AM61.699-709).
zoisite, kyanite and diaspore to
margarite
Ca2Al3[Si2O7][SiO4]O(OH) + Al2OSiO4 +
3AlO(OH) ⇌ 2CaAl2(Al2Si2O10)(OH)2
The equilibrium temperature for this reaction at 12 kbar pressure is about 480oC
(blueschist facies), with the equilibrium to
the right at higher temperatures, and to the left at lower temperatures
(SERC).
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