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Corundum

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Corundum
General
CategoryOxide mineralHematite group
Formula
(repeating unit)
Aluminium oxide, Al
2
O
3
Strunz classification04.CB.05
Dana classification4.3.1.1
Crystal symmetryTrigonal (32/m)
Unit cella = 4.75 Å, c = 12.982 Å; Z=6
Identification
ColorColorless, gray, brown; pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown
Crystal habitSteep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular
Crystal systemTrigonal (Hexagonal Scalenohedral)
Symbol (32/m)
Space group: R3c
TwinningPolysynthetic twinning common
CleavageNone – parting in 3 directions
FractureConchoidal to uneven
TenacityBrittle
Mohs scale hardness9 (defining mineral)[1]
LusterAdamantine to vitreous
StreakWhite
DiaphaneityTransparent, translucent to opaque
Specific gravity3.95–4.10
Optical propertiesUniaxial (–)
Refractive indexnω = 1.767–1.772
nε = 1.759–1.763
PleochroismNone
Melting point2044 °C
FusibilityInfusible
SolubilityInsoluble
Alters toMay alter to mica on surfaces causing a decrease in hardness
Other characteristicsMay fluoresce or phosphoresce under UV light
References[2][3][4][5]
Major varieties
SapphireAny color except red
RubyRed
EmeryBlack 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 a ruby. Other impurities can make a sapphire.

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References

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  1. "Mohs' scale of hardness". Collector's corner. Mineralogical Society of America. Retrieved 10 January 2014.
  2. Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. and Nichols, Monte C., ed. (1997). "Corundum". Handbook of Mineralogy (PDF). Vol. III(Halides, Hydroxides, Oxides). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209724.{{cite book}}: CS1 maint: multiple names: editors list (link)
  3. Corundum. Mindat.org
  4. Corundum. Webmineral
  5. Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, pp. 300–302 ISBN 0-471-80580-7