Andradite
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Andradite
Andradite
Andradite-Mali.jpg
Single crystal (4.2cm) - Diakon, Nioro du Sahel Circle, Kayes Region, Mali
General
CategoryGarnet group
Formula
(repeating unit)
Ca3Fe2(SiO4)3
Strunz classification9.AD.25
Crystal systemCubic
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupIa3d
Unit cella = 12.056 Å; Z = 8
Identification
ColorYellow, greenish yellow to emerald-green, dark green; brown, brownish red, brownish yellow; grayish black, black; may be sectored
Crystal habitCommonly well-crystallized dodecahedra, trapezohedra, or combinations, also granular to massive
Cleavagenone
Fractureconchoidal to uneven
TenacityBrittle
Mohs scale hardness6.5 to 7
LusterAdamantine to resinous, dull
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity3.859 calculated; 3.8-3.9 measured
Optical propertiesIsotropic, typically weakly anisotropic
Refractive indexn = 1.887
Absorption spectrademantoid - 440nm band or complete absorption at 440nm and below, may also have lines at 618, 634, 685, 690nm [1]
References[1][2][3][4]
Major varieties
Demantoidtransparent light to dark green to yellow-green
Melaniteopaque black
Topazolitetransparent to translucent yellow, may show chatoyancy

Andradite is a species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12.

Andradite includes three varieties:

  • Melanite: Black in color, referred to as "titanian andradite".[5]
  • Demantoid: Vivid green in color, one of the most valuable and rare stones in the gemological world.[6]
  • Topazolite: Yellow-green in color and sometimes of high enough quality to be cut into a faceted gemstone, it is rarer than demantoid.[6]

It was first described in 1868 for an occurrence in Drammen, Buskerud, Norway.[2][3][6] Andradite was named after the Brazilian statesman, naturalist, professor and poet José Bonifácio de Andrada e Silva (1763-1838).[2][6]

Occurrence

It occurs in skarns developed in contact metamorphosed impure limestones or calcic igneous rocks; in chlorite schists and serpentinites and in alkalic igneous rocks (typically titaniferous). Associated minerals include vesuvianite, chlorite, epidote, spinel, calcite, dolomite and magnetite.[2] It is found in Italy, the Ural Mountains of Russia, Arizona and California and in Dnipropetrovsk Oblast in Ukraine.

Like the other garnets, andradite crystallizes in the cubic space group [[Ia3d]], with unit-cell parameter of 12.051 Å at 100 K.[7]

The spin structure of andradite contains two mutually canted equivalent antiferromagnetic sublattices[8] below the Néel temperature (TN=11 K[9]).

See also

References

  1. ^ a b Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  2. ^ a b c d Handbook of Mineralogy
  3. ^ a b Andradite, Mindat.org
  4. ^ Webmineral data
  5. ^ Melanite, Mindat.org
  6. ^ a b c d Grande, Lance; Augustyn, Allison (2009). Gems and Gemstones: Timeless Natural Beauty of the Mineral World. University of Chicago Press. pp. 188-91. ISBN 978-0-226-30511-0.
  7. ^ Thomas Armbruster and Charles A. Geiger (1993): "Andradite crystal chemistry, dynamic X-site disorder and structural strain in silicate garnets." European Journal of Mineralogy v. 5, no. 1, p. 59-71.
  8. ^ Danylo Zherebetskyy (2010). Quantum mechanical first principles calculations of the electronic and magnetic structure of Fe-bearing rock-forming silicates, PhD Thesis, Universal Publishers/Dissertation.com, Boca Raton, Florida, USA, p. 136. ISBN 1-59942-316-2.
  9. ^ Enver Murad (1984): "Magnetic ordering in andradite." American Mineralogist 69, no. 7-8; pp. 722-24.

External links

Media related to Andradite at Wikimedia Commons


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Andradite
 



 



 
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