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Crystal habit of corundum

Dehydration sequence of hydrated alumina in air
Fig.1 Dehydration sequence of hydrated alumina in air

Formula Al2O3
Molecular mass 101.96 g/mol
Density  3.98 g/cm3
Hardness 9
Melting point 2053°C
CAS-No. 1344-28-1
EINECS-No. 2156916

Aluminium oxide appears in nature as a-Al2O3 (=Corundum). The hexagonal a-Al2O3 is the only thermodynamically stable modification. Besides there is the cubic face-centered  g-Al2O3, which develop from aluminium hydroxide at 400-800°C and is finaly converted to a-Al2O3 at 1100°C. Chemical data of calcinated aluminas are given on a separate page.

Technically corundum is manufactured today by electric melting of bauxite, which is the starting material for refractory products. Thus, for example crucibles, protective tubes and refractory bricks are produced as sintered corundum by pressing a-Al2O3 with bonding agents such as clays, which are fired at approx 1300°C. Alumina coatings are available as Aluminium-Oxide-Coating AA and Aluminium-Oxide-Coating AW.

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© 2004 Büro für angewandte Mineralogie · Dr. Stephan Rudolph · D-47918 Tönisvorst
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