Andradite

Andradite [Adr]
Single crystal (4.2 cm) – Diakon, Nioro du Sahel Circle, Kayes Region, Mali
General
CategoryGarnet group
FormulaCa3Fe2(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 – 440 nm band or complete absorption at 440 nm and below, may also have lines at 618, 634, 685, 690 nm[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 garnet - Jiangxi, Nantan, China.

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

Andradite includes three varieties:

  • Colophonite: a historical variety found in the Scandinavian islands, brownish or reddish in color, often opaque or translucent.[6]: 61 
  • Demantoid: Vivid green in color, one of the most valuable and rare stones in the gemological world.[7]
  • 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.[8]
  • 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.[9]

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

  • 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. ^ a b Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  3. ^ a b c d Handbook of Mineralogy
  4. ^ a b Andradite, Mindat.org
  5. ^ Webmineral data
  6. ^ Olga Bortnik. All about precious stones. ― Moscow: Harvest, 2011.
  7. ^ 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.
  8. ^ Melanite, Mindat.org
  9. ^ 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.
  10. ^ 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.
  11. ^ Enver Murad (1984): "Magnetic ordering in andradite." American Mineralogist 69, no. 7-8; pp. 722–24.

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