Zoisite is a calcium aluminium hydroxy silicate (Ca2(Al.OH)Al2(SiO4)3) belonging to the epidote group of minerals. Zoisite is named after Austrian scientist Baron Sigmund Zois von Edelstein (Baron von Zois) whom discovered the mineral in the Sau-Alp mountains of Austria in 1805. Zoisite was first known as saualpite after its type locality. Transparent material is fashioned into gemstones whilst translucent to opaque material is usually carved into sculptural works.
Zoisite occurs as prismatic, orthorhombic (2/m 2/m 2/m) crystals or in massive form, being found in metamorphic and pegmatitic rock. Zoisite may be blue to violet, green, brown, pink, yellow, gray, or colourless. It has a vitreous lustre and a conchoidal to ueven fracture. When euhedral, zoisite crystals are striated parallel to the principal axis (c-axis). Also parallel to the principal axis is one direction of perfect cleavage. Zoisite is somewhat higher than 6 in hardness and its specific gravity is between 3.10-3.38, depending on the variety. Zoisite streaks white and is said to be brittle.
Zoisite has a refractive index of c. 1.69-1.70 with a birefringence ranging between 0.007-0.010 (biaxial positive). Zoisite is strongly pleochroic; two or three differing colours are presented depending on crystal orientation and body colour.
There are several varieties of zoisite; the blue-purple tanzanite being the most familiar. Other varieties include mottled pink thulite, bright green anyolite which is often found intergrown with ruby, and the uncommon chrome zoisite (also green).
Sources of zoisite include: Tanzania (tanzanite), Kenya (anyolite), Norway (thulite), Switzerland, Austria, India, Pakistan, and the USA.
See also: mineral list, gemstone
