Almandine

Almandine
General
Category	Nesosilicate
Formula (repeating unit)	Fe2+3Al2Si3O12
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
Mohs scale hardness	7 - 7.5
Luster	greasy to vitreous
Streak	white
Specific gravity	4.05 (+.25, -.12)
Polish luster	vitreous to subadamantine
Optical properties	Single refractive, and often anomalous double refractive
Refractive index	1.790 (+/- .030)
Birefringence	none
Pleochroism	none
Dispersion	.024
Ultraviolet fluorescence	inert
Absorption spectra	usually at 504, 520, and 573nm, may also have faint lines at 423, 460, 610 and 680-690nm
References

Almandine /ˈælməndɪn/, also known incorrectly 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.

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⁺₃Al₂Si₃O₁₂, 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.

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