Zoisite - Encyclopedia

    Class : Silicates
    Subclass : Sorosilicates
    Crystal system : Orthorhombic
    Chemistry : Ca2Al3(SiO4)3(OH)
    Rarity : Fairly common

Zoisite is the orthorhombic dimorph of clinozoisite and, like it, belongs to the epidote group. It is a mineral of clayey limestones and marls having undergone a not very intense regional metamorphism, more rarely eclogites and hydrothermalized magmatic rocks. It owes its name to the Austrian naturalist Siegmond Zois, an amateur of mineralogy and who provides the defining samples. It usually occurs in flattened fibers with perfect cleavage, frequently assembled in radiate groups, rarely in terminated crystals striated parallel to the elongation. The color is variable : white to grayish white and greenish, sometimes brown-green, exceptionally pink when it is manganiferous (thulite variety) or of a magnificent purplish blue (tanzanite variety). The particularly aesthetic zoisites are used for ornamental or jewelry purposes.

Zoisite from Canari, Bastia, Haute-Corse, France
7.00 ct tanzanite from Arusha, Tanzania
22.00 ct Norwegian Thulite
39.00 ct ruby in zoisite from Tanzania

Zoisite in the World

The most spectacular zoisite is that of the Longido deposit, near Arusha (Tanzania), which provides extraordinary apple-green masses rich in centimetric crystals of red rubies, exploited for ornamentation. Less spectacular but nevertheless exceptional, we should note the 6 cm green gemstone crystals from Alshuri, near Shigar (Pakistan), and, to a lesser extent, the pinkish centimetric crystals from the Jeffrey mine in Abestos (Canada). Good specimens are also known in the eastern American states (Massachussets, Maryland, Tennessee, etc...).

Tanzanite is the variety of zoisite from Merelani Hills (Tanzania), the only locality where it is known in superb crystals, sometimes decimetric, particularly revealing the trichroism of the species which is in this case blue (most often improved by heating), purple and gray.

Thulite is mostly known in large clusters at Thule in Norway.

Zoisite in France

In France, very beautiful gemmy and pink crystals come from pegmatites and aplites from Petches in Ariège. Asbestos from Canari (Haute-Corse) also provided very pretty pale green gemmy crystals up to 2 cm in length.


No twinning known for this mineral species.

Fakes and treatments

No fake inventories for this mineral species. Blue tanzanite is almost systematically treated (heated).

Hardness : 6 to 7
Density : 3.15 to 3.36
Fracture : Irregular to conchoidal
Trace : White

TP : Translucent to transparent
RI : 1.696 to 1.718
Birefringence : 0.006 to 0.018
Optical character : Biaxial +
Pleochroism : Strong
Fluorescence : None

Solubility : Hydrofluoric acid

Magnetism : Paramagnetic
Radioactivity : None