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Download hereGENERAL DESCRIPTION
Zoisite is a hydrated calcium aluminosilicate. It is formed under the effect of a regional metamorphism of medium- to low temperature but also in pegmatites (having also been metamorphized as in the Himalayas).
It generally presents in prismatic crystals aggregates, very elongated to fibrous, more rarely as striated terminated crystals. Their colours vary between white, gray, brown, pink, blue and purple.
Several varieties are distinguished: thulite (pink) and tanzanite (blue). The anyolite is red and green because it is made up of rubies set in a matrix made of green and black zoisite. Zoisite presents a strong pleochroism: tanzanite shows a variation of colour when observed at different angles, from blue to red. This last variety is often heated to accentuate ("to improve") its blue colour and its purple reflections.
Nevertheless, although a growing number of tanzanites that are heat-treated, the gemmologists always regard it as a genuine gem. This last variety is present only at Merelani Hills in Tanzania and was discovered only in 1967. More generally, zoisite is remarkable in Broken Hill in Australia; in Austria; in Canada; in Czech Republic, in Pakistan, China (etc).
In France, it was observed in the deposits of Saint-Yreix la Perche (Limousin), the Brioude-Massiac area (Cerzat, Traignac) in Auvergne or Alsace (Framont-Grandfontaine mining district) or in the Pyrénées (Costabonne).
The conservation location of the type of this species (from Rauris and Saualpe in Austria) is unknown.
HISTORY : Species dedicated to Earl Siegmund Zoïs (1747-1819), Austrian naturalist and mineralogist
Species first described in 1805 by Abraham Gottlob Werner (1749-1817), German geologist, Bergwerk Akademie, Freiberg, Germany
Type-locality: Saualpe, Carinthia, Austria
CHEMICAL FORMULA : Ca2 Al3 (SiO4)3 (OH)
CRYSTAL SYSTEM : Orthorhombic
COLOR : Colorless, gray, white, blue, pink, greenish
DIAPHANIETY : Transparent to translucent
LUSTER : Vitreous
STREAK : White
MORPHOLOGIE : Prismatic crystals
HARDNESS : 6,7
CHEMICAL CLASS: 3,35
DENSITY : VIII - Silicates
GROUP : Zoisite
STRUNZ CLASS BEFORE 2001 : 8/C.23-100
STRUNZ CLASS AFTER 2001 : 9.BG.10
