other names: Corundum
mineral class: Oxides and hydroxides
chemical formula: Al2O3
Chemical elements: Aluminum, oxygen
Similar minerals: ?
colour: pure corundum is white or colorless
shine: Diamond shine possible
crystal structure: trigonal
mass density: 4,0
magnetism: not magnetic
Mohs hardness: 9
stroke color: White
transparency: opaque to transparent
use: Gemstone, abrasive
General information about corundum:
corundum describes a widely used mineral from the group of oxides, which consists of alumina and may appear in different colors depending on the chemical composition. In its pure form, corundum is colorless and is also called leukosaphor, but it is rare. Significantly more frequent is due to defects in the crystal lattice or chemical admixtures with iron, chromium and titanium colored corundum, which may appear red, pink, purple, blue, green or yellow. The most sought-after varieties of corundum are, above all, the bright blue sapphire and the ruby made from chrome.
The corundum forms long crystals, which usually appear columnar, pyramidal or prismatic. Twinning is very common in corundum, the aggregates are grainy and rough. With a Mohs hardness of 9, the corundum is considered the diamond after the diamond as the hardest mineral that exists on Earth. It has a glassy, silky or diamond-like luster, a reddish blue, yellow or purple fluorescence as well as a mussel, brittle or splintered rupture. Cleavage is not present, the transparency ranges from opaque to completely transparent. Corundum only melts at temperatures of over two thousand degrees Celsius and is completely insensitive to acids.
Origin, occurrence and localities:
Corundums are formed exclusively under high temperatures and pressure conditions of magmatic rock such as granite or pegmatite, which has high sodium and aluminum concentrations, but only small amounts of silica. Most corundum is associated with diamond, calcite, chlorite, graphite, or hornblende. The most important deposits of corundum include India, Australia, the Ural Mountains of Russia and Canada. Other important sites are Greenland, the British Isles, Scandinavia, Germany, Austria and Switzerland, as well as much of Eastern Europe, Africa and South Africa, the United States and South America. In Sri Lanka and Burma, corundum is also often found in river sediments in large quantities.
History and use of corundum:
Corundum are particularly economically important because of their high aluminum content and are processed in powdered form into grinding and polishing agents and as extremely hard material to tools and serve as a medium for sandblasting. Also in the production of non-slip tiles, as an additive in concrete and as a material for the production of ceramics used in the art, corundum plays an important role. Particularly beautiful specimens of the varieties sapphire and ruby are used for the production of precious jewelry, which achieve high prices. Corundum also serve as abrasion-resistant and hard bearing stones and glass components in watches and are processed like diamonds to scanned needles in phonograms. Since the late 19th century, corundum, in particular ruby, has also been produced artificially in the course of the so-called Verneuil process or by hydrothermal cultivation. The synthetic stones have the same characteristics as their natural counterparts and are used in both industry and jewelery production.