Andradite

Andradite
single crystal (4.2cm) - Diakon, Nioro du Sahel Circle, Kayes Region, Mali
General
Category	Garnet group
Chemical formula	Ca₃Fe₂(SiO₄)₃
Crystal symmetry	Isometric 4/m 3 2/m
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
Crystal system	Cubic - Hexoctahedral
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 - 440nm band or complete absorption at 440nm and below, may also have lines at 618, 634, 685, 690nm ^[1]
References	^[1]^[2]^[3]^[4]
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 species of the garnet group. It is a nesosilicate, with formula Ca₃Fe₂Si₃O₁₂.

Andradite includes three varieties:

Melanite: Black in color, referred to as "titanian andradite".^[5]
Demantoid: Vivid green in color, one of the most valuable and rare stones in the gemological world.
Topazolite: Yellow-green in color and sometimes of high enough quality to be cut into a faceted gemstone.

It was first described in 1868 for an occurrence in Drammen, Buskerud, Norway.^[3] Andradite was named after the Brazilian mineralogist José Bonifácio de Andrade e Silva (1763–1838).^[2]

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.^[2] It is found in Italy, the Ural Mountains of Russia, Arizona and California and in Dnipropetrovsk Oblast in Ukraine.

As the other garnets andradite crystallizes in the cubic space group [[Ia3d]], with unit-cell parameter of 12.051 Å at 100 K.^[6]

The spin structure of andradite contains two mutually canted equivalent antiferromagnetic sublattices ^[7] below the Néel temperature (T_N=11 K ^[8]).

^ ^a ^b Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
^ ^a ^b ^c Handbook of Mineralogy
^ ^a ^b Mindat.org
^ Webmineral data
^ http://www.mindat.org/min-7443.html Mindat
^ Thomas Armbruster and Charles A. Geiger (1993): Andradite crystal chemistry, dynamic X-site disorder and structural strain in silicate garnets. European Journal of Mineralogy v. 5, no. 1, p. 59-71.
^ Danylo Zherebetskyy (2010). Quantum mechanical first principles calculations of the electronic and magnetic structure of Fe-bearing rock-forming silicates, PhD Thesis, Universal Publishers/Dissertation.com, Boca Raton, Florida, USA, p. 136. ISBN 1599423162.
^ Enver Murad (1984): Magnetic ordering in andradite. American Mineralogist 69, no. 7-8; p. 722-724.

Simple	Anatase · Brookite · Geikielite · Rutile

Mixed	Aeschynite-(Y) · Armalcolite · Betafite · Euxenite · Freudenbergite · Haggertyite · Ilmenite · Keilhauite · Latrappite · Perovskite · Polycrase · Zimbabweite · Zirconolite · Zirkelite

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