Corundum facts for kids

Quick facts for kids 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)
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 usually clear. Its chemical formula is aluminium oxide, Al₂O₃. Sometimes some of the aluminium atoms get replaced with chromium atoms. It then becomes ruby. Other impurities can make sapphire.

Related pages

• Bauxite
• Aluminium
• Chromium(III) oxide

Images for kids

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  Corundum from Brazil, size about 2 cm × 3 cm (0.8 in × 1 in)

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  Crystal structure of corundum

See also

In Spanish: Corindón para niños