Andradite

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
Strunz classification	9.AD.25
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 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
References
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 mineral species of the garnet group. It is a nesosilicate, with chemical formula Ca₃Fe₂Si₃O₁₂.

Andradite includes four varieties:

• Colophonite: a historical variety found in the Scandinavian islands, brownish or reddish in color, often opaque or translucent.^: 61
• Demantoid: Vivid green in color, one of the most valuable and rare stones in the gemological world.
• Melanite: Black in color due to limited substitution of titanium for iron. Also known as "titanian andradite". Forms a solid solution with morimotoite and schorlomite depending on titanium and iron content.
• Topazolite: Yellow-green in color and sometimes of high enough quality to be cut into a faceted gemstone, it is rarer than demantoid.

It was first described in 1868 for an occurrence in Drammen, Buskerud, Norway. Andradite was named after the Brazilian statesman, naturalist, professor and poet José Bonifácio de Andrada e Silva (1763–1838).

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. 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 Ia3d, with unit-cell parameter of 12.051 Å at 100 K.

The spin structure of andradite contains two mutually canted equivalent antiferromagnetic sublattices below the Néel temperature (T_N = 11 K).

• 
  Black crystals of andradite: melanite

See also