Andradite

Andradite [Adr]
Single crystal (4.2 cm) – Diakon, Nioro du Sahel Circle, Kayes Region, Mali
General
CategoryGarnet group
Formula
(repeating unit)
Ca3Fe2(SiO4)3
IMA symbolAdr[1]
Strunz classification9.AD.25
Crystal systemCubic
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupIa3d
Unit cella = 12.056 Å; Z = 8
Identification
ColorYellow, greenish yellow to emerald-green, dark green; brown, brownish red, brownish yellow; grayish black, black; may be sectored
Crystal habitCommonly well-crystallized dodecahedra, trapezohedra, or combinations, also granular to massive
Cleavagenone
Fractureconchoidal to uneven
TenacityBrittle
Mohs scale hardness6.5 to 7
LusterAdamantine to resinous, dull
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity3.859 calculated; 3.8–3.9 measured
Optical propertiesIsotropic, typically weakly anisotropic
Refractive indexn = 1.887
Absorption spectrademantoid – 440 nm band or complete absorption at 440 nm and below, may also have lines at 618, 634, 685, 690 nm[2]
References[2][3][4][5]
Major varieties
Demantoidtransparent light to dark green to yellow-green
Melaniteopaque black
Topazolitetransparent 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:

  • Colophonite: a historical variety found in the Scandinavian islands, brownish or reddish in color, often opaque or translucent.[6]: 61 
  • Demantoid: Vivid green in color, one of the most valuable and rare stones in the gemological world.[7]
  • 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.[8]
  • 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]

  1. ^ 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.
  2. ^ a b Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  3. ^ a b c Handbook of Mineralogy
  4. ^ a b Andradite, Mindat.org
  5. ^ Webmineral data
  6. ^ Olga Bortnik. All about precious stones. ― Moscow: Harvest, 2011.
  7. ^ 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.
  8. ^ Melanite, Mindat.org