Corundum Corundum General Category Oxide mineral - Hematite group Chemical formula Aluminium oxide, Al2O3 Strunz classification IV/C.04-10 Dana classification 4.3.1.1 Identification Color Colorless, gray, brown; pink to pigeon-blood-red, orange, yellow, green, blue to cornflower blue, violet; may be color zoned, asteriated Crystal habit Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular Crystal system Trigonal (Hexagonal Scalenohedral) Symbol (-3 2/m) Space Group: R-3c Twinning Polysynthetic twinning common Cleavage None - parting in 3 directions Fracture Conchoidal to uneven Mohs Scalehardness 9 Luster Adamantine to vitreous Streak White Diaphaneity Transparent, translucent to opaque Specific gravity 3.95-4.10 Optical properties Uniaxial (-) Refractive index nω = 1.767 - 1.772 nε = 1.759 - 1.763 Pleochroism None Melting point 2044 °C Fusibility Infusible Solubility Insoluble Alters to May alter to mica on surfaces causing a decrease in hardness Other characteristics May fluoresce or phosphoresce under UV References [1][2][3][4] Major varieties Sapphire Any color except red Ruby Red Emery Black granular corundum intimately mixed withmagnetite, hematite, orhercynite Corundum is a crystalline form of aluminium oxide Al2O3) with traces of iron, titanium andchromium.[1] It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used asgems, called ruby if red, while all other colors are called sapphire. The name corundum probably derived from the Sanskrit "kuruvinda" meaning "ruby".[3] Due to corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral. It is commonly used as an abrasive, on everything from sandpaperto large machines used in machining metals, plastics and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with a lower average hardness near 8.0. In addition to its hardness, corundum is unusual for its high density of 4.02 g/cm³, which is very high for a transparent mineral composed of the low atomic mass elements aluminium andoxygen.[5] Geology and occurrence Corundum from Brazil, about 2 x 3 cm of size Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyredikes and as large crystals in pegmatites.[4] Because of its hardness and resistance to weathering, it commonly occurs as a detrital mineral in stream and beach sands.[4] The largest documented single crystal of corundum measured ~65x40x40 cm3.[6] Corundum for abrasives is mined in Zimbabwe, Russia, and India. Historically it was mined from deposits associated with dunites in North Carolina and from a nepheline syenite inCraigmont, Ontario.[4] Emery grade corundum is found on the Greek island of Naxos and nearPeekskill, New York. Abrasive corundum is synthetically manufactured from bauxite.[4] Synthetic corundum In 1837 Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a small amount of chromium as a pigment. In 1847 Ebelmen made white sapphire by fusingalumina in boric acid. In 1877 Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF2and Al2O3 with a little chromium at temperatures above 2000°C. In 1903 Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process.[7] Crystal structure of corundum The Verneuil process allows the production of flawless single-crystal sapphires, rubies and other corundum gems of much larger size than normally found in nature. It is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market causing a significant reduction of price in recent years. Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant watch crystals and windows for optical equipment, spacecraft as well as for lasers.