Corundum is an aluminum oxide mineral of the oxides and hydroxides group, with structural formula Al2O3. The structure consists essentially of a dense arrangement of oxygen ions in hexagonal closest-packing with Al3+ ions in two-thirds of the available octahedral sites.
Because Al is trivalent, each oxygen atom is bonded to only two Al, and thus, only two out of three available octahedra are occupied. This arrangement makes the structure neutral; therefore, no charge balancing cations are needed. The sheets are held together by strong covalent bonds and this results in a very hard and dense structure. In fact, corundum is one of the hardest and densest minerals after diamond.
Minor substitutions of Fe3+ for Al3+ are common. Crystals of corundum are commonly well formed, often rather large, showing hexagonal and rectangular sections. Crystals are commonly bounded by hexagonal prisms or steep hexagonal di-pyramids resulting in barrel-shaped forms. Tabular and rhombohedral forms are also found, but much less common. Corundum is normally colorless, but colored crystals are common due to various element impurities. Crystals can be red, blue, pale, pink, green, violet, and yellow.
Corundum is found in a wide variety of rock types. It is characteristic of A-rich and silica poor environments, and appears in association with other aluminous and/or silica-poor minerals. Plutonic igneous rocks not containing quartz and poor in Fe and Mg usually contain corundum. Syenites, nepheline syenites, and monzonite may bear corundum in association with andesine-oligoclase, orthoclase, nepheline, and scapolite. Quartz-free pegmatites commonly contain corundum in association with orthoclase, muscovite, Na-plagioclase, andalusite, tourmaline. Corundum is rarely found in volcanic rocks. Metamorphic rocks, both regional and contact, especially those derived from aluminous or carbonate sediments, may yield corundum.
Corundum is not usually listed as a gemstone; however, two of its varieties are: the deep red variety of corundum is well known as ruby, the rarest and most valuable of gemstones. All other colored corundum are known as sapphires, which may be blue, pink, green, violet, yellow, and others. The most well known are the deep blue sapphires. Much of the gem quality corundum is found as individual isolated crystals in metamorphosed crystalline limestone. Gem quality corundum comes mainly from Thailand and Sri Lanka; some good quality crystals have been found in several places in the US (North Carolina and Montana).
Fine grained corundum is often separated in the heavy fraction of sedimentary rocks and used as an abrasive in many industries.
Importance in Soil Environments
Corundum occurs in soils by inheritance from the parent rocks. It alters readily under surface conditions and yields other secondary minerals depending on the geochemistry of the soil environment. Commonly it yields Al-rich minerals such as kaolinite, gibbsite, diaspore, zoisite and sillimanite. Fine grains of corundum may be abundant in the heavy fraction of soils formed on recent detrital deposits. Corundum and other heavy minerals are often used to study the origin and genesis of soils formed on transported materials.