Andradite

Andradite [Adr]
Andradite [Adr]
	Single crystal (4.2 cm) – Diakon, Nioro du Sahel Circle, Kayes Region, Mali
General
Category	Garnet group
Formula; (repeating unit)	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 formula Ca₃Fe₂Si₃O₁₂.

Andradite includes three varieties:

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.^[6]
Demantoid: Vivid green in color, one of the most valuable and rare stones in the gemological world.^[7]
Topazolite: Yellow-green in color and sometimes of high enough quality to be cut into a faceted gemstone, it is rarer than demantoid.^[7]

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]

Occurrence

Like the other garnets, andradite crystallizes in the cubic space group [[Ia3d]], with unit-cell parameter of 12.051 Å at 100 K.^[8]

The spin structure of andradite contains two mutually canted equivalent antiferromagnetic sublattices^[9] below the Néel temperature (T_N=11 K^[10]).

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ ^a ^b Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
^ ^a ^b ^c ^d Handbook of Mineralogy
^ ^a ^b Andradite, Mindat.org
^ Webmineral data
^ Melanite, Mindat.org
^ ^a ^b ^c ^d Grande, Lance; Augustyn, Allison (2009). Gems and Gemstones: Timeless Natural Beauty of the Mineral World. University of Chicago Press. pp. 188–91. ISBN 978-0-226-30511-0.
^ 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 1-59942-316-2.
^ Enver Murad (1984): "Magnetic ordering in andradite." American Mineralogist 69, no. 7-8; pp. 722–24.

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