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| Corundum | |
|---|---|
| General | |
| Category | Mineral |
| Chemical formula | aluminium oxide, Al2O3 |
| Identification | |
| Colour | Brown to grey, less often red, blue, white, yellow. |
| Crystal habit | Steep bipyramidal, tabular, prismaticl, rhombohedral crystals, massive or granular. |
| Crystal system | trigonal |
| Cleavage | None |
| Fracture | Conchoidal to uneven |
| Mohs Scale hardness | 9 |
| Luster | Adamantine to vitreous |
| Refractive index | 1.76-1.78, Biref 0.009 |
| Pleochroism | None |
| Streak | White |
| Specific gravity | 3.95-4.1 |
| Fusibility | ? |
| Solubility | ? |
| Major varieties | |
| SapphireSapphire is the single crystal form of aluminium oxide (AlO). It can be found naturally as gemstones or manufactured in large crystal boules for a variety of applications. Sapphire Gems Sapphire is any gemstone-quality corundum that is not red in color. | Bluish to blue, pink |
| RubyRuby is a red gemstone, a variety of the mineral corundum in which the color is caused mainly by chromium. Natural rubies are exceptionally rare, though artificial ones can be manufactured which are comparatively inexpensive. Rubies are mined in Africa, A | Red |
| Oriental topazThe mineral topaz is a silicate of aluminium and fluorine with the chemical formula (AlF)SiO. It is orthorhombic and its crystals are mostly prismatic terminated by pyramidal and other faces, the basal pinacoid often being present. It has an easy and perf | Yellow |
| EmeryEmery can refer to the following topics: Emery (mineral) Emery Air Freight Emery (band) Emery, Wisconsin Emery County, Utah. | Granular |
Corundum is the crystalThis article is about the form of solid matter. For other uses of this word, see Crystal (disambiguation . Insulin Crystals A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extendline form of aluminium oxide and one of the rockRock is a substance composed of minerals and classified according to mineral composition. Rocks are generally clasified by the processes that formed them, and are thus separated into igneous, sedimentary, and metamorphic rocks. Igneous rocks are formed fr-forming minerals. Corundum is naturally clear, but can have different colors when impurities are added. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. The word corundum comes from the Hindi kurand.
The oxygen atoms in corundum are arranged in a hexagonal close-packing, with the smaller aluminium atoms occupying 2/3 of the octahedral gaps. The coordination of the atoms are thus 6:4, compared to 4:2 for quartz, which accounts for its greater hardness despite the Al-O bonds being less covalent.
Due to corundum's hardness, it is commonly used in as an abrasive in machining, from huge machines to sandpaper. Emery is an impure and less abrasive variety, with a Mohs hardness of 8.0. Diamond is harder, but much more expensive.
Minerals