From HandWiki
Short description
Nesosilicate mineral species of garnet
Single crystal (4.2cm) – Diakon, Nioro du Sahel Circle, Kayes Region, Mali
CategoryGarnet group
(repeating unit)
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
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
Fractureconchoidal to uneven
Mohs scale hardness6.5 to 7
|re|er}}Adamantine to resinous, dull
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]
Major varieties
Demantoidtransparent light to dark green to yellow-green
Melaniteopaque black
Topazolitetransparent to translucent yellow, may show chatoyancy

Andradite is a mineral 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 Brazil ian statesman, naturalist, professor and poet José Bonifácio de Andrada e Silva (1763–1838).[2][6]


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 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 3}}d, 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


  1. 1.0 1.1 Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN:0-87311-019-6
  2. 2.0 2.1 2.2 2.3 Handbook of Mineralogy
  3. 3.0 3.1 Andradite,
  4. Webmineral data
  5. Melanite,
  6. 6.0 6.1 6.2 6.3 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/, 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