Topaz
Topaz | |
---|---|
Chemical composition | Al2(F,OH)2SiO4 |
Crystal system | Orthorhombic |
Habit | Prismatic with pyramidal/domed terminations |
Cleavage | Perfect, basal plane |
Fracture | Sub-conchoidal to uneven |
Hardness | 8 |
Optic nature | Biaxial + |
Refractive index | 1.606-1.644 |
Birefringence | 0.008 - 0.010 |
Dispersion | Low, 0.014 |
Specific gravity | 3.49 - 3.57 |
Lustre | Vitreous |
Pleochroism | Distinct to strong (depending on body color) |
Topaz is an aluminium silicate with varying amounts of fluorine (F) and hydroxyl (OH) which replace eachother through isomorphous replacement. Extreme outer limits with only fluorine or only hydroxyl have not been reported.
Contents
Diagnostics
Color
Topaz is allochromatic and occurs in many colors.
- colorless
- yellow
- green
- blue
- red
- pink
Diaphaneity
Transparant
Refractometer
Topaz with high concentrations of fluorine have a lower refractive index thatn those with high concentrations of hydroxyl.
Specific gravity
As with the refractive index, the specific gravity changes with high concentrations of hydroxyl and fluorine.
Hydroxyl causes a lower specific gravity while fluorine raises the specific gravity of topaz.
Imitations
Recently,2007, yellow-orange-pink flame-fusion corundum is offered in Minas Gerais, Brazil as imperial topaz.
Synthetics
Although topaz is synthesized, the material is not commercially available.
Sources
- Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)