Diaspore

diaspore

bohmite

kyanite

gibbsite

Images

Formula: AlO(OH)
Oxide containing hydroxyl, paramorph of böhmite. both minerals are orthorhombic, but they have a different arrangement of atoms in their structure.
Specific gravity:
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).

Back to Minerals