Almandine | |
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General | |
Category | Nesosilicate |
Formula (repeating unit) | Fe2+ 3Al 2Si 3O 12 |
IMA symbol | Alm[1] |
Strunz classification | 9.AD.25 |
Crystal system | Cubic |
Crystal class | Hexoctahedral (m3m) H–M symbol: (4/m 3 2/m) |
Space group | Ia3d |
Identification | |
Color | reddish orange to red, slightly purplish red to reddish purple and usually dark in tone |
Cleavage | none |
Fracture | conchoidal[2] |
Mohs scale hardness | 7.0–7.5 |
Luster | greasy to vitreous |
Streak | white |
Specific gravity | 4.05+0.25 −0.12[2] |
Polish luster | vitreous to subadamantine[2] |
Optical properties | Single refractive, and often anomalous double refractive[2] |
Refractive index | 1.790±0.030[2] |
Birefringence | none |
Pleochroism | none |
Dispersion | 0.024[2] |
Ultraviolet fluorescence | inert |
Absorption spectra | usually at 504, 520, and 573 nm, may also have faint lines at 423, 460, 610 and 680–690 nm[2] |
References | [3][4][5] |
Almandine (/ˈælməndɪn/), also known as almandite, is a species of mineral belonging to the garnet group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron alumina garnet, of deep red color, inclining to purple. It is frequently cut with a convex face, or en cabochon, and is then known as carbuncle. Viewed through the spectroscope in a strong light, it generally shows three characteristic absorption bands.[6]
Almandine is one end-member of a mineral solid solution series, with the other end member being the garnet pyrope. The almandine crystal formula is: Fe3Al2(SiO4)3. Magnesium substitutes for the iron with increasingly pyrope-rich composition.
Almandine, Fe2+
3Al
2Si
3O
12, is the ferrous iron end member of the class of garnet minerals representing an important group of rock-forming silicates, which are the main constituents of the Earth's crust, upper mantle and transition zone. Almandine crystallizes in the cubic space group Ia3d, with unit-cell parameter a ≈ 11.512 Å at 100 K.[7]
Almandine is antiferromagnetic with the Néel temperature of 7.5 K. It contains two equivalent magnetic sublattices.[8]