| Andradite [Adr] | |
|---|---|
 Single crystal (4.2 cm) – Diakon, Nioro du Sahel Circle, Kayes Region, Mali | |
| General | |
| Category | Garnet group |
| Formula | Ca3Fe2(SiO4)3 |
| IMA symbol | Adr[1] |
| Strunz classification | 9.AD.25 |
| Crystal system | Cubic |
| Crystal class | Hexoctahedral (m3m) H-M symbol: (4/m3 2/m) |
| Space group | Ia3d |
| 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 |
| 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 – 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 | |
| Demantoid | transparent light to dark green to yellow-green |
| Melanite | opaque black |
| Topazolite | transparent to translucent yellow, may showchatoyancy |
Andradite is amineral species of thegarnet group. It is anesosilicate, with formula Ca3Fe2Si3O12.
Andradite includes three varieties:
It was first described in 1868 for an occurrence inDrammen,Buskerud,Norway.[3][4][7] Andradite was named after theBrazilian statesman, naturalist, professor and poetJosé Bonifácio de Andrada e Silva (1763–1838).[3][7]
It occurs inskarns developed incontact metamorphosed impurelimestones or calcicigneous rocks; in chloriteschists andserpentinites and in alkalicigneous rocks (typicallytitaniferous). Associated minerals includevesuvianite,chlorite,epidote,spinel,calcite,dolomite andmagnetite.[3] It is found inIran,Italy, theUral Mountains ofRussia,Arizona andCalifornia and inDnipropetrovsk Oblast inUkraine.
Like the other garnets, andradite crystallizes in the cubicspace group [[Ia3d]], with unit-cell parameter of 12.051 Å at 100 K.[9]
Thespin structure of andradite contains two mutually canted equivalentantiferromagnetic sublattices[10] below theNéel temperature (TN=11 K[11]).
Media related toAndradite at Wikimedia Commons
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