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Andradite

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Andradite

Andradite
single crystal (4.2cm) - Diakon, Nioro du Sahel Circle, Kayes Region, Mali
General
Category Garnet group
Formula
(repeating unit)
Ca3Fe2(SiO4)3
Strunz classification 09.AD.25
Crystal symmetry Isometric 4/m 3 2/m
Unit cell a = 12.056 Å; Z = 8
Identification
Color Yellow, greenish yellow to emerald-green, dark green; brown, brownish red, brownish yellow; grayish black, black; may be sectored
Crystal habit Commonly well-crystallized dodecahedra, trapezohedra, or combinations, also granular to massive
Crystal system Cubic - Hexoctahedral
Cleavage none
Fracture conchoidal to uneven
Tenacity Brittle
Mohs scale hardness 6.5 to 7
Luster Adamantine to resinous, dull
Streak White
Diaphaneity Transparent to translucent
Specific gravity 3.859 calculated; 3.8 - 3.9 measured
Optical properties Isotropic, typically weakly anisotropic
Refractive index n = 1.887
Absorption spectra demantoid - 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
Demantoid transparent light to dark green to yellow-green
Melanite opaque black
Topazolite transparent 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.[3][2][6] Andradite was named after the Brazilian statesman, naturalist, professor and poet José Bonifácio de Andrade e Silva (1763–1838).[2][6]

Occurrence

Black crystals of andradite : melanite

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.

As the other garnets andradite crystallizes in the cubic space group , 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.  
  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.
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