Aluminum Oxide
Al2O3
Corundum is the second hardest natural mineral known to science.
The History Says
One century ago, the richest corundum mine in the world was in Canada, at Craigmont, located several kilometres south of Combermere, Ontario, in the woodsy hill country of Raglan and Carlow Townships.
The Present Scenario
Notable Occurrences now include Burma; Sri Lanka; North Carolina and Montana, USA; many African localities; several localities in India, and Middle Eastern and Southeast Asian countries.
CORUNDUM is a natural occurring oxide of alumina (AL2O3) with 52.9 per cent aluminium and 47.1 per cent oxygen. In hardness (moh's scale 9) it is next only to diamond. It is valued mostly for its abrasive property. It is also refractory, the melting point being 2010ºC and hence it is used in a sintered form in the manufacture of special refractory crucibles, rods and other materials. Corundum is found in rocks containing a high percentage of alkalies, deficient in silica and excess of alumina. It is an original constituent of various igneous rocks. It is generally found associated with rocks like syenite. It may result from the metamorphism of high aluminous clay and is often found associated with andalusite, kyanite and sillimanite.
| Hardness | Associated Minerals | Chemical/Typical composition | Colour | characteristics | Luster | Field Indicators |
| 9 | calcite zoisite feldspars micas garnets | Al52.93 % O47.07 % | Highly variable. The color can be white or colorless, blue, red, yellow, green, brown, purple, and pink; there are also instance of color zonation. | color intensity is variable from different viewing directions | vitreous to adamantine | extreme hardness, density and crystal habit |
Industry and Specifications
Corundum having a bright and glassy lustre, splintery and devoid of cleavage plane and inclusions is preferred by industry for the manufacture of superior grade abrasives. The minimum Al2O3 content in the mineral should be 84%, though generally, a content of above 90% is preferred. The corundum is crushed, ground and screened to grain sizes varying from 100 to 300 mesh. The grains are utilized in the manufacture of grinding wheels for flour and rice mills, and workshops. Grinding wheels are manufactured either by calcining a mixture of corundum, clay and felspar or by using sodium silicate as a bonding agent. Finer grains are utilized for the preparation of grinding-pastes for the automobile industry. MgO and MgCl2 are used as bonding agents. Corundum paper and cloth finished to belts, discs, rolls, sheets and other shapes are prepared by giving a suitable coating generally of sodium silicate.
Due to its hardness corundum also finds use in mortars, wire drawing dies, thread guides and gauge blocks. Gem varieties are sometimes used for pivot supporteres in delicate scientific instruments, as jewel bearing in watches.
Corundum in small quantities is used in the manufacture of special sparking plugs, muffles, pyrometer tubes, rod and insulator for vacuum tubes of all kinds and crucibles. Finely ground corundum, passing through 200 mesh, the alumina content not below 90%, is used for teh manufacture of the above products by giving a bond of clay. The presence of iron and felspar in corundum for the manufacture of above products is regarded objectionable.
Substitute
Natural abrasives are being fast replaced by manufactured abrasives. The common manufactured abrasives are silicon carbide, fused alumina, emery and boron carbide. Silicon carbide is sold under the trade names:
- Carborundum
- used in plating to produce a hard, beautiful surface and to prevent corrosion.
- Crystolon
- Carbolon, etc.
Emery is an intimate mixture of alumina and iron oxide. It is manufactured by fusing bauxite. Emery also occurs in the natural state. It is mined in Greece and Turkey. Boron carbide known as Norbide, is the hardest known manufactured abrasive. In hardness, if not equal, it is very close to diamond. Although great advancement has been made in some countries in the production of synthetic or manufactured abrasives, yet natural abrasives have retained their eminent place and are still being used in bulk, because of their easy availability and characcteristics. The reason for natural corundum being particularly suitable for certain kinds of grinding is its prominent basal cleavage that causes the crystal to break readily with a smooth flat surface at right-angles to the axis of elongation through successive reductions in size unlike synthetic corundum (alumina) which has a conchoidal fracture.