Corundum

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Corundum
Several corundum crystals.jpg
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 ruby. Other impurities can make sapphire.

Related pages[change | change source]

References[change | change source]

  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). III(Halides, Hydroxides, Oxides). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209724.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