Zoisite

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This article is about the mineral named zoisite. For the Sailor Moon character, see Shitennou.

Zoisite
Anyolite (left) & tanzanite
General
Category	Silicate mineral
Chemical formula	Ca₂Al₃(SiO₄)(Si₂O₇)O(OH)
Identification
Color	Gray, yellow, blue, green.
Crystal habit	Crystals flattened in an acicular manner, may be fibrously curved
Crystal system	Orthorhombic
Cleavage	Perfect {010} imperfect {100}
Fracture	Uneven to conchoidal
Mohs Scale hardness	6.5
Luster	Vitreous, pearly on cleavage surfaces
Refractive index	1.69-1.70
Optical Properties	biaxial positive
Birefringence	0.006-0.018
Pleochroism	Present, dichroism or trichroism depending on color.
Streak	White or colorless
Specific gravity	3.10-3.38
Major varieties
tanzanite	Gem-quality zoisite, blue-purple
thulite	Pink
anyolite	Often found intergrown with ruby
chrome zoisite	Green, uncommon

Zoisite is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca₂Al₃(Si O₄)(Si₂O₇)O(OH). Zoisite is named after the Slovene scientist Baron Sigmund Zois von Edelstein (Žiga Zois), who realized that this was an unknown mineral when it was brought to him by the mineral dealer Simon Prešern, who had discovered it in the Saualpe mountains (Svinška planina) of Carinthia in 1805. Zoisite was first known as saualpite, after its type locality. Transparent material is fashioned into gemstones while translucent-to-opaque material is usually carved into sculptural works. The latter is sometimes shot through with ruby crystals, which are completely opaque and unsuited to use as gems, yet are well colored and contrast strikingly against the green matrix of the zoisite.

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 colorless. It has a vitreous luster and a conchoidal to uneven 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. Clinozoisite is a more common monoclinic polymorph of zoisite.