Corundum
Corundum is a mineral. It is very hard, with a Mohs hardness of 9 (diamond is 10). It is usually clear. Its chemical formula is mostly aluminium oxide, Al
2O
3.
| Corundum | |
|---|---|
 | |
| General | |
| Category | Oxide mineral – Hematite group |
| Formula (repeating unit) | Aluminium oxide, Al2O3 |
| Strunz classification | 04.CB.05 |
| Dana classification | 4.3.1.1 |
| Crystal symmetry | Trigonal (32/m) |
| Unit cell | a = 4.75 Å, c = 12.982 Å; Z=6 |
| Identification | |
| Color | Colorless, gray, brown; 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 |
| Crystal system | Trigonal (Hexagonal Scalenohedral) Symbol (32/m) Space group: R3c |
| Twinning | Polysynthetic twinning common |
| Cleavage | None – parting in 3 directions |
| Fracture | Conchoidal to uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 9 (defining mineral)[1] |
| 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 light |
| References | [2][3][4][5] |
| Major varieties | |
| Sapphire | Any color except red |
| Ruby | Red |
| Emery | Black granular corundum intimately mixed with magnetite, hematite, or hercynite |
Different metal atoms can replace some of the aluminium atoms in corundum. These impurities give corundum different colours. Different colours of corundum have different names as gemstones. Rubies are corundum coloured red by chromium impurities. Sapphires are coloured by iron and titanium impurities, and take on many different colours depending on the amount of each element.
Corundum Media
Corundum from Brazil, size about 2 cm × 3 cm (0.8 in × 1 in)
Crystal structure of corundum
Molar volume vs. pressure at room temperature
Related pages
References
- ↑ Mohs' scale of hardness. Collector's cornerMineralogical Society of America. Retrieved 10 January 2014.
- ↑ Handbook of Mineralogy III(Halides, Hydroxides, Oxides) (1997). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209724.
- ↑ Corundum. Mindat.org
- ↑ Corundum. Webmineral
- ↑ Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, pp. 300–302 ISBN 0-471-80580-7
