Andradite


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.
  • 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.

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. 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 Iad, 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.