Zoisite is a member of the Epidote mineral group that includes Allanite, Clinozoisite, Epidote, Hancockite, Mukhinite, Piedmontite, Tanzanite, Thulite and Zoisite. The Chemical Formula of Zoisite is Ca2Al3Si3O12(OH) [Calcium Aluminum Silicate Hydroxide] and its molecular weight is 454.36 gm. Moh’s Hardness of Zoisite is 6.0 – 7.0. Actually, Tanzanite and Thulite are types of Zoisite. Zoisite is the only orthorhombic member of the Epidote group. Zoisite is typically found in various shades of green and brown. Tanzanite is the fine gem variety of Zoisite that has been heated and turned to the popular blue and purple colors. Thulite is the pink to dark pink variety of Zoisite. Zoisite has a specific gravity of 3.15 – 3.36 (g/cm3) and the refractive index of 1.685 – 1.725 Biaxial ( + ). Tanzanite is by far, the most popular and well known variety of the Epidote mineral group thanks to its eye-catching blue and purple colors.

Category: Sorosilicate – epidote group
Chemical Formula: Ca2Al3Si3O12(OH)
[Calcium Aluminum Silicate Hydroxide]
Molecular Weight: 454.36 gm
  Calcium 17.64 % Ca 24.68 % CaO
  Aluminum 17.82 % Al 33.66 % Al2O3
  Silicon 18.54 % Si 39.67 % SiO2
  Hydrogen 0.22 % H 1.98 % H2O
  Oxygen 45.78 % O    
    100.00 %   100.00 % = Total Oxide


Crystallography: Orthorhombic – Dipyramidal
Crystal Habit: Crystals prismatic, to 10 cm, typically deeply striated and poorly terminated; columnar to compact, massive.
Twinning: None


Cleavage: [010] Perfect, [100] Imperfect
Fracture: Uneven, Conchoidal
Tenacity: Brittle
Moh’s Hardness: 6.0 – 7.0
Density: 3.15 – 3.36 (g/cm3)
Radioactivity: Not Radioactive


Color: Green, Brown, Blue (Tanzanite), Purple, Pink, Rose red (Thulite), Gray
Transparency: Transparent to Translucent
Luster: Vitreous, Pearly
Refractive Index: 1.685 – 1.725  Biaxial ( + )
Birefringence: 0.006 – 0.018
Dispersion: Relatively Strong
Pleochroism: X = pale pink to red-violet; Y = nearly colorless to bright pink or deep blue; Z = pale yellow to yellow-green