Corundum is an oxide mineral mainly composed of alumina (Al2O3), with colors such as colorless or gray, yellow gray, blue, etc. It has a Mohs hardness of nine, is transparent or semi transparent, and has a glassy luster.
Corundum is produced at the contact surface of metamorphic rocks (such as siliceous rocks), granulites, and their alluvial layers. It has characteristics such as high hardness, high melting point, and corrosion resistance. It is widely used in various industrial fields as a composite material and also has certain ornamental value.
Mineral components
The main chemical composition of corundum is Al2O3 (alumina). The mass fraction of aluminum element is 53.2%, and the mass fraction of oxygen element is 46.8%. Containing Cr3+, Ti4+, Fe3+, Fe2+, Mn2+, V3+, etc., they replace Al3+with equivalent or isovalent isomorphic isomers. Meanwhile, Al2O3 has multiple variants, with α - Al2O3 being the most stable in nature.
Physical properties
Corundum is in the form of barrels, columns, cones, and plates, with a hard texture and often diagonal stripes on the surface. Pure corundum is colorless or gray, yellow gray, and presents different colors due to the presence of different trace elements, such as black for iron, red for chromium, blue for titanium, purple for vanadium and chromium, and green for iron and chromium. Glass luster. No cleavage; Often caused by the cracking of {0001} or {1011} due to the formation of clustered twin crystals or fine inclusions. Under long and short wavelength ultraviolet light, it emits red fluorescence, while those with high Fe content have weaker fluorescence. Containing Cr, it exhibits pink or orange yellow fluorescence. It is relatively stable at both room temperature and high temperature in air.
The interference color between orthogonal polarizations of corundum is similar to that of quartz, but due to its high hardness, its thickness is often slightly larger than that of other minerals in the same thin film. Therefore, the interference color is often more than first-order white, and sometimes can reach second-order. The columnar cutting surface has parallel extinction and negative elongation symbols. However, it must be noted that for corundum with a plate-like habit, when the thin film is cut perpendicular to the plane surface, the elongation symbol of this section is exactly opposite, but the elongation direction is parallel to the slow light. Often, parallel {1011} twin crystals can be seen. Uniaxial crystals have negative photosensitivity, but sometimes appear as biaxial crystals, with a generally small optical axis angle (up to a maximum of 58 °). Even in artificial ruby crystals, this situation exists, and the optical axis angles at the center and edges of the crystal are not equal.
Chemical property
Corundum is acid and alkali resistant, but under high pressure conditions and concentrated alkali solutions, it may undergo reactions such as Al2O3+2NaOH+3H2O=2NaAl (OH) 4, and the reaction is affected by temperature. Taking a 60% sodium hydroxide solution and a reaction time of 6 hours as an example, when the temperature increases from 180 ℃ to 200 ℃, the dissolution rate of corundum increases by 12.5%. However, as the reaction temperature increases from 200 ℃ to 220 ℃, the dissolution rate increases by 46%.
