What Is Zoisite?

Chemical Composition

• Zoisite's chemical formula is Ca2Al3(SiO4)(Si4O7)O(OH). This classifies it as a neosilicate, calcium aluminum hydroxy sorosilicate.

  A neosilicate is a kind of simple silicate mineral composition that has ionic bonding of isolated SiO4 tetrahedra. A sorosilicate indicates that it has two silicate tetrahedrons connected by an oxygen ion.

History

• Zoisite was officially discovered by Baron Sigmund Zois von Edelstien (1747-1819). He was shown a mineral specimen by the mineral dealer, Simon Prešern, who had found it in the Saualpe mountains which are located in the Central Alps in 1894.

  This was an unidentified mineral which the Baron told the European mineralogists about. Initially it was named sauaplite after where it was found. It was analyzed and named zoisite in 1905 after the Baron.

Considerations

• In 1967, a new variety of zoisite was found in Tanzania named tanzanite. This is considered gem-quality zoisite.

  Gem-quality zoisite from Tanzania is a blue, violet color and is used for jewelry. Before the 1967 discovery of tanzanite, the only zoisite in use for ornamental stones was the type from Norway that is opaque pink called thulite. Zoisite of various grades is used for decorative stone construction.

Features

• Zoisite rates 6 to 7 on the Mohs scale, which rates gem hardness (a diamond rates 10).

  When it is broken, it doesn't break evenly. It can break in conchoidal fashion, meaning it breaks in a "shell-like" way, circular and with ridges.

  Colors of zoisite range widely, from blue and violet (tanzanite), red and pink (thulite), to green, the most commonly thought of color for zoisite. Brown, grey and white zoisite also have been found.

  One feature of color in zoisite is pleochroism. Pleochroism is a feature of some minerals wherein the color changes depending on the angle you look at it.

Sources

• Zoisite can be found places across the globe. In Africa, zoisite can be found in Tanzania and Kenya. European sources of zoisite include Switzerland, Germany, Scotland, Italy, Austria and Norway. North American sources of zoisite are in Washington, Tennessee and Mexico. In the Middle-East, zoisite has been found in Pakistan.