Andradite

Andradite is a species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12. It is a member of the Garnet group, and although not as well-known as other Garnets such as Almandine or Pyrope, it is still fairly abundant and can produce fine Garnet gemstones.

Andradite includes three varieties:

  • Melanite: Black in color, referred to as “titanian andradite”.
  • 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 is rarer than demantoid.

It was first described in 1868 as an occurrence in Drammen, Buskerud, Norway.

General Information

  • Category: Garnet group
  • Formula: Ca3Fe2Si3O12
  • Crystal system: Cubic
  • Crystal class: Hexoctahedral (m3m)

Andradite is named in honor of José Bonifácio de Andrada e Silva (1763-1838), a Brazilian mineralogist, statesman, professor, and poet, famous for his discovery of Andradite as well as several other important minerals such as Spodumene.

Properties

  • 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

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 Italy, the Ural Mountains of Russia, Arizona, and California and in Dnipropetrovsk Oblast in Ukraine.

 

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