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| Corundum | |
|---|---|
 | |
| General | |
| Category | Oxide mineral – Hematite group |
| Formula | Al2O3 |
| IMA symbol | Crn[1] |
| Strunz classification | 4.CB.05 |
| Dana classification | 4.3.1.1 |
| Crystal system | Trigonal |
| Crystal class | Hexagonal scalenohedral (3m) H-M symbol: (3 2/m) |
| Space group | R3c (No. 167) |
| Unit cell | a = 4.75 Å, c = 12.982 Å; Z = 6 |
| Identification | |
| Color | Colorless, gray, golden-brown, brown; purple, pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown |
| Crystal habit | Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular |
| Twinning | Polysynthetic twinning common |
| Cleavage | None – parting in 3 directions |
| Fracture | Conchoidal to uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 9 (defining mineral)[2] |
| Luster | Adamantine to vitreous |
| Streak | Colorless |
| 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 | 2,044 °C (3,711 °F) |
| 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 light |
| References | [3][4][5][6] |
| Major varieties | |
| Sapphire | Any color except red |
| Ruby | Red |
| Emery | Black granular corundum intimately mixed with magnetite, hematite, or hercynite |
Corundum is a crystalline form of aluminium oxide (Al2O3) typically containing traces of iron, titanium, vanadium, and chromium.[3][4] It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure.[7] Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present.[7] A rare type of sapphire, padparadscha sapphire, is pink-orange.
The name "corundum" is derived from the Tamil-Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda).[8][9]
Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. It is a black granular form of corundum, in which the mineral is intimately mixed with magnetite, hematite, or hercynite.[6]
In addition to its hardness, corundum has a density of 4.02 g/cm3 (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.[10]
- ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- ^ "Mohs' scale of hardness". Collector's corner. Mineralogical Society of America. Retrieved 10 January 2014.
- ^ a b Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1997). "Corundum". Handbook of Mineralogy (PDF). Vol. III Halides, Hydroxides, Oxides. Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209724. Archived (PDF) from the original on 5 September 2006.
- ^ a b "Corundum". Mindat.org.
- ^ "Corundum". Webmineral.com. Archived from the original on 25 November 2006.
- ^ a b Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). Wiley. pp. 300–302. ISBN 0-471-80580-7.
- ^ a b Giuliani, Gaston; Ohnenstetter, Daniel; Fallick, Anthony E.; Groat, Lee; Fagan; Andrew J. (2014). "The Geology and Genesis of Gem Corundum Deposits". Gem Corundum. Research Gate: Mineralogical Association of Canada. pp. 37–38. ISBN 978-0-921294-54-2.
- ^ Harper, Douglas. "corundum". Online Etymology Dictionary.
- ^ Jeršek, Miha; Jovanovski, Gligor; Boev, Blažo; Makreski, Petre (2021). "Intriguing minerals: corundum in the world of rubies and sapphires with special attention to Macedonian rubies". ChemTexts. 7 (3): 19. doi:10.1007/s40828-021-00143-0. ISSN 2199-3793. S2CID 233435945.
- ^ "The Mineral Corundum". galleries.com.
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