| Andradite [Adr] | |
| Category: | Garnet group |
| Formula: | Ca3Fe2(SiO4)3 |
| Imasymbol: | Adr[1] |
| Strunz: | 9.AD.25 |
| System: | Cubic |
| Class: | Hexoctahedral (mm) H-M symbol: (4/m 2/m) |
| Symmetry: | Iad |
| Unit Cell: | a = 12.056 Å; Z = 8 |
| Color: | Yellow, greenish yellow to emerald-green, dark green; brown, brownish red, brownish yellow; grayish black, black; may be sectored |
| Habit: | Commonly well-crystallized dodecahedra, trapezohedra, or combinations, also granular to massive |
| Cleavage: | none |
| Fracture: | conchoidal to uneven |
| Tenacity: | Brittle |
| Mohs: | 6.5 to 7 |
| Luster: | Adamantine to resinous, dull |
| Diaphaneity: | Transparent to translucent |
| Refractive: | n = 1.887 |
| Opticalprop: | Isotropic, typically weakly anisotropic |
| Absorption: | demantoid – 440nm band or complete absorption at 440nm and below, may also have lines at 618, 634, 685, 690nm |
| Streak: | White |
| Gravity: | 3.859 calculated; 3.8–3.9 measured |
| References: | [2] [3] [4] [5] |
| Var1text: | transparent light to dark green to yellow-green |
| Var2: | Melanite |
| Var2text: | opaque black |
| Var3: | Topazolite |
| Var3text: | transparent 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:
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]
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 3}}d, 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]).