corundum(redirected from Corrundum)
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a mineral, a natural anhydrous alumina, Al2O3. Corundum crystallizes in the trigonal system. Its crystal structure consists of aluminum atoms surrounded by six oxygen atoms, which form a highly dense hexagonal packing. The oxygen layers are located parallel to the (0001) face of corundum. The aluminum occupies two-thirds of the octahedral cavities in the packing of the oxygen ions. Corundum is rarely found as large dipyramidal, barrel-shaped, tabular, or rhombohedral crystals; most often it forms, together with other minerals, coarse and fine granular aggregates. Corundum has a hardness of 9 on the mineralogical scale and a density of 3,950–4,100 kg/m3. It is characterized by high chemical stability and high melting point (2020°-2050°PC). The color of corundum varies depending on the composition of the admixtures (Fe, Cr, Ti). The most common colors are gray-brown, pinkish to red, and blue-gray to dark blue.
Corundum is found in silica-deficient igneous rocks (syenites), in anorthite-corundum dikes, in the contacts between syenites and gneisses, in regionally metamorphic rocks (emeries), and in secondary quartzites and desilicated pegmatites.
There are large deposits of corundum in the USSR in Kazakhstan and the Urals. Abroad, there are major deposits in Africa (Malagasy Republic, Southern Rhodesia, Republic of South Africa) and in India, Burma, Sri Lanka, Greece, and Turkey.
Beautifully colored transparent or translucent natural crystals (red, ruby; blue, sapphire; and colorless, leucosapphire) have been used since antiquity as high-quality gems. Granular aggre-gates of nontransparent corundum are used as abrasives in the production of grinding and polishing powders (for polishing and cutting metal, hard rocks, and glass) and as a refractory. Artificial corundum is obtained on an industrial scale by melting bauxite in an electric furnace with a reducing agent (iron filings). It is also used as an abrasive. Cutting tools that are used for the mechanical treatment of metals at high temperatures are made from corundum by the methods of powder metallurgy.
Monocrystalline transparent synthetic corundum in the form of boules and rods is prepared by melting and recrystallizing alumina in an oxygen-hydrogen flame. The boules thus produced may be colored by Cr to red, by V to grayish green in daylight and violet under artificial illumination, by Mn to yellowish pink, by Ni to yellow, and by Ti to pinkish violet. Upon faceting, synthetic corundum is used in jewelry-making under various names, such as sapphire, ruby, topaz, alexandrite, and amethyst. Red corundum, known as ruby, is used as jewel bearings in watches and other precision instruments while corundum rods are used in lasers.