Corundum |
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Category | Oxide mineral – Hematite group |
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Formula (repeating unit) | Aluminium oxide, Al2O3 |
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Strunz classification | 04.CB.05 |
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Dana classification | 4.3.1.1 |
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Crystal symmetry | Trigonal (32/m) |
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Unit cell | a = 4.75 Å, c = 12.982 Å; Z=6 |
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Color | Colorless, gray, brown; pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown |
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Crystal habit | Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular |
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Crystal system | Trigonal (Hexagonal Scalenohedral) Symbol (32/m) Space group: R3c |
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Twinning | Polysynthetic twinning common |
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Cleavage | None – parting in 3 directions |
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Fracture | Conchoidal to uneven |
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Tenacity | Brittle |
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Mohs scale hardness | 9 (defining mineral) |
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Luster | Adamantine to vitreous |
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Streak | White |
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Diaphaneity | Transparent, translucent to opaque |
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Specific gravity | 3.95–4.10 |
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Optical properties | Uniaxial (–) |
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Refractive index | nω = 1.767–1.772 nε = 1.759–1.763 |
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Pleochroism | None |
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Melting point | 2044 °C |
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Fusibility | Infusible |
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Solubility | Insoluble |
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Alters to | May alter to mica on surfaces causing a decrease in hardness |
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Other characteristics | May fluoresce or phosphoresce under UV light |
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References | |
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Sapphire | Any color except red |
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Ruby | Red |
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Emery | Black granular corundum intimately mixed with magnetite, hematite, or hercynite |
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