Corundum facts for kids

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Corundum
Several corundum crystals.jpg
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)
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
Major varieties
Sapphire Any color except red
Ruby Red
Emery Black granular corundum intimately mixed with magnetite, hematite, or hercynite

Corundum is a mineral. It is very hard, with a Mohs hardness of 9 (diamond is 10). It is clear. Its chemical formula is aluminium oxide, Al2O3. Sometimes some of the aluminium atoms get replaced with chromium atoms. It then turns into ruby.

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