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The crystalline structure of aluminium oxide is an important property for this compound. This is because it determines its refractoriness and strength, making it ideal for producing alumina ceramics. In addition, it is an excellent electrical insulator and is highly resistant to corrosion. In fact, alumina is used for manufacturing protective equipment such as helmets, masks, and clothing. It is also used to make abrasive tools such as grinding wheels and cutting compounds.
The most common crystalline form of aluminum oxide is corundum, which has a trigonal Bravais lattice. Its octahedral oxygen ions have a hexagonally closest packed arrangement, with two-thirds of the octahedral interstices occupied by aluminium ions.
This structure is found in igneous and metamorphic rocks. Rubies and sapphires are gem-quality forms of corundum, whose characteristic colors come from the presence of trace impurities. A less common, but very hard crystalline form of aluminium oxide is the mineral deltalumite, which has a rhombic crystal structure and is found in the bauxite ore.
Unlike many other metal oxides, aluminium oxide does not dissolve in water. This is due to the strong passivating effect of the oxide layer, which prevents aluminium from reacting with air. However, it can be reacted by exposing it to high temperatures or by adding caustic soda.
Aluminium hydroxide minerals are the main component of bauxite, which is the principal ore of aluminium. After being mined, these minerals are processed to form alumina by the Bayer process. This involves filtering the basic solution and heating it to remove the silicates. The resulting solution is then cooled to precipitate the alumina. The alumina crystals are sized and purified to produce different grades of aluminium oxide.
