Andradite

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

Andradite [Adr]
Single crystal (4.2cm) – Diakon, Nioro du Sahel Circle, Kayes Region, Mali
General
CategoryGarnet group
Formula
(repeating unit)		Ca3Fe2(SiO4)3
IMA symbolAdr[1]
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 [2]
References[2][3][4][5]
Major varieties
Demantoidtransparent light to dark green to yellow-green
Melaniteopaque black
Topazolitetransparent to translucent yellow, may show chatoyancy

Andradite includes three varieties:

  • Melanite: Black in color due to limited substitution of titanium for iron. Also known as "titanian andradite". Forms a solid solution with Morimotoite and Schorlomite depending on titanium and iron content.[6]
  • Demantoid: Vivid green in color, one of the most valuable and rare stones in the gemological world.[7]
  • Topazolite: Yellow-green in color and sometimes of high enough quality to be cut into a faceted gemstone, it is rarer than demantoid.[7]

It was first described in 1868 for an occurrence in Drammen, Buskerud, Norway.[3][4][7] Andradite was named after the Brazilian statesman, naturalist, professor and poet José Bonifácio de Andrada e Silva (1763–1838).[3][7]

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.[3] It is found in Iran, 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.[8]

The spin structure of andradite contains two mutually canted equivalent antiferromagnetic sublattices[9] below the Néel temperature (TN=11 K[10]).

  • Black crystals of andradite: melanite
    Black crystals of andradite: melanite

See also

References
  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  3. Handbook of Mineralogy
  4. Andradite, Mindat.org
  5. Webmineral data
  6. Melanite, Mindat.org
  7. 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.
  8. 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.
  9. 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.
  10. Enver Murad (1984): "Magnetic ordering in andradite." American Mineralogist 69, no. 7-8; pp. 722–24.

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