orthorhombic Sorosilicates
Tanzanite
Formula Ca2Al3(SiO4)(Si2O7)O(OH):V
Crystal Structure
#! Species: Tanzanite (Zoisite)
#! System: Orthorhombic (mmm)
#! Habit: Prismatic with extreme trichroism
orthorhombic[mmm]:{100}@1.0[pleochroism:strong] + {010}@0.9 + {001}@1.2 + {110}@0.7
orthorhombic
mmm
{100} {010} {001} {110}
Quick Facts
Hardness
6.5
Specific Gravity
3.1
Refractive Index
1.691
Optical Character
Biaxial +
Physical Properties
- Crystal System
- orthorhombic
- Hardness (Mohs)
- 6.5
- Specific Gravity
- 3.1
- Cleavage
- Perfect in one direction
- Fracture
- Uneven to conchoidal
- Lustre
- Vitreous
Optical Properties
- Refractive Index
- 1.691
- Birefringence
- 0.008-0.013
- Optical Character
- Biaxial +
- Dispersion
- 0.021
- Pleochroism
- very strong: Blue / Purple / Bronze-brown (Strongest trichroism of any common gemstone; all three colours distinctly different)
Characteristics
Colours
Violet-blueBluePurpleBurgundy (untreated)
Localities
Tanzania (Merelani Hills)
Common Inclusions
Growth tubesNegative crystalsFingerprint inclusionsStress cracks (from cleavage)
Known Treatments
Heat treatment (universal, removes brown)
Crystal Forms
prismpinacoid
Diagnostic Features
Extremely strong trichroism (blue/purple/bronze); heat treatment converts brown to blue-violet
Related Minerals
Other minerals in the orthorhombic system
