Zoisite

Zoisite
Yellow zoisite crystal (1.7 × 1 × 0.8 cm)
General
Category	Sorosilicate – epidote group
Formula (repeating unit)	Ca2Al3(SiO4)(Si2O7)O(OH)
IMA symbol	Zo[1]
Strunz classification	9.BG.10
Dana classification	58.2.1b.1
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pnma
Identification
Color	White, gray, greenish brown, greenish gray, pink, blue, purple
Crystal habit	Prismatic crystals with striations; massive to columnar
Cleavage	Perfect {010} imperfect {100}
Fracture	Uneven to conchoidal
Mohs scale hardness	6 to 7
Luster	Vitreous, pearly on cleavage surfaces
Streak	White or colorless
Diaphaneity	Transparent to translucent
Specific gravity	3.10–3.36
Optical properties	biaxial positive
Refractive index	nα = 1.696 – 1.700 nβ = 1.696 – 1.702 nγ = 1.702 – 1.718
Birefringence	0.006–0.018
Pleochroism	X = pale pink to red-violet; Y = nearly colorless to bright pink or deep blue; Z = pale yellow to yellow-green
References	[2][3][4]
Major varieties
Tanzanite	Gem-quality zoisite, blue-purple
Thulite	Pink

Zoisite, first known as saualpite, after its type locality, is a calcium aluminum hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca₂Al₃(SiO₄)(Si₂O₇)O(OH).

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. Blue crystals are known under the name tanzanite. 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. The mineral is between 6 and 7 on the Mohs hardness scale, and its specific gravity ranges from 3.10 to 3.38, depending on the variety. It streaks white and is said to be brittle. Clinozoisite is a more common monoclinic polymorph of Ca₂Al₃(SiO₄)(Si₂O₇)O(OH). Transparent material is fashioned into gemstones while translucent-to-opaque material is usually carved.

The mineral was described by Abraham Gottlob Werner in 1805. He named it after the Carniolan naturalist Sigmund Zois, who sent him its specimens from Saualpe in Carinthia.^[5] Zois realized that this was an unknown mineral when it was brought to him by a mineral dealer, presumed to be Simon Prešern, in 1797.^[6]

Sources of zoisite include Tanzania (tanzanite), Kenya (anyolite), Norway (thulite), Switzerland, Austria, India, Pakistan, and the U.S. state of Washington.