Yellow zoisite crystal (1.7 × 1 × 0.8 cm)
CategorySorosilicateepidote group
(repeating unit)
IMA symbolZo[1]
Strunz classification9.BG.10
Dana classification58.2.1b.1
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPnma
ColorWhite, gray, greenish brown, greenish gray, pink, blue, purple
Crystal habitPrismatic crystals with striations; massive to columnar
CleavagePerfect {010} imperfect {100}
FractureUneven to conchoidal
Mohs scale hardness6 to 7
LusterVitreous, pearly on cleavage surfaces
StreakWhite or colorless
DiaphaneityTransparent to translucent
Specific gravity3.10–3.36
Optical propertiesbiaxial positive
Refractive indexnα = 1.696 – 1.700 nβ = 1.696 – 1.702 nγ = 1.702 – 1.718
PleochroismX = pale pink to red-violet; Y = nearly colorless to bright pink or deep blue; Z = pale yellow to yellow-green
Major varieties
TanzaniteGem-quality zoisite, blue-purple

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.

See also


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ http://rruff.geo.arizona.edu/doclib/hom/zoisite.pdf Handbook of Mineralogy
  3. ^ http://www.mindat.org/min-4430.html Mindat
  4. ^ http://webmineral.com/data/Zoisite.shtml Webmineral data
  5. ^ Flint-Rogers, Austin (1937). Introduction to the Study of Minerals. McGraw-Hill Book Company. p. 478.
  6. ^ Faninger, Ernest (1988–1989). "Neue Daten über die Entdeckung des Zoisits" [New Data About the Discovery of Zoisite]. Geologija: Razprave in poročila (in German and Slovenian). 31, 32. Državna založba Slovenije [State Publishing House of Slovenia]: 609–615. ISSN 0016-7789.