Andradite
| Andradite | |
|---|---|
 single crystal (4.2cm) - Diakon, Nioro du Sahel Circle, Kayes Region, Mali |
|
| General | |
| Category | Garnet group |
| Chemical formula | Ca3Fe2(SiO4)3 |
| 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.
- Topazolite: Yellow-green in color and sometimes of high enough quality to be cut into a faceted gemstone.
It was first described in 1868 for an occurrence in Drammen, Buskerud, Norway.[3] Andradite was named after the Brazilian mineralogist José Bonifácio de Andrade e Silva (1763–1838).[2]
[edit] 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.
As the other garnets andradite crystallizes in the cubic space group [[Ia3d]], with unit-cell parameter of 12.051 Å at 100 K.[6]
The spin structure of andradite contains two mutually canted equivalent antiferromagnetic sublattices [7] below the Néel temperature (TN=11 K [8]).
[edit] References
- ^ a b Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
- ^ a b c Handbook of Mineralogy
- ^ a b Mindat.org
- ^ Webmineral data
- ^ http://www.mindat.org/min-7443.html Mindat
- ^ 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.
- ^ 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 1599423162.
- ^ Enver Murad (1984): Magnetic ordering in andradite. American Mineralogist 69, no. 7-8; p. 722-724.
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