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Topaz


Properties
Chemical Composition: 
Fluorosilicate of aluminium - Al2SiO4(F,OH)2 + Cr, Mn, Vn.
Classification / Type: 
Topaz contains fluorine and / or hydroxyl in the structure. The presence of fluorine increases the S.G. and lowers the R.I., while the presence of hydroxyl lowers the S.G. and increases the R.I.
Colors / Varieties: 
Colorless, white, gray, blue, greenish-yellow, yellow, yellow-brown, orange, pink, red. Chatoyant variety (rare).
Crystal System / Forms: 
Orthorhombic System / Prismatic, Combination of forms, massive, granular, as rolled pebbles.
Hardness: 
8
Specific Gravity: 
  • Pink / Yellow: 3.53 - 3.56
  • Blue / Colorless: 3.56 / 3.57
Cleavage / Fracture: 
Perfect 1 directional basal / Conchoidal fracture.
Optic Character: 
Anisotropic, D.R.; Biaxial Positive.
Lustre: 
Vitreous.
Refractive Index / Birefringence: 
  • Pink / Yellow / Brown: 1.629 - 1.637 / 0.008
  • Blue / Colorless / Green: 1.609 - 1.617 / 0.008
Pleochroism: 
Trichroic; varies with color.
Dispersion: 
0.014
Magnification: 
2 phase inclusions, 2 non miscible liquids, may show 3 phase inclusions or rhomb like etch markings.
U.V. Fluorescence: 
Variable
Spectrum: 
Not characteristic. Heated pink gives Cr spectrum at 682nm
Cause of Color: 
  • Pink, Red - Chromium
  • Orange - Chromium and color centers
  • Blue, yellow, brown - color centers
Treatment (Enhancement): 
  • Coating: a thin film of gold is coasted over the surface of a colorless topaz to give a blue color. (Aqua Aura)
  • Heat treatment: some yellow, orange, brown to pink known as pinking process.
  • Irradiation to change light color topaz to deep brown to deep blue. By gamma rays in linear accelerators gives sky blue color, in nuclear reactors gives London blue color and with combination of linear accelerators and nuclear reactors gives Swiss blue and American blue color.
Specific Tests & Remarks: 
Easy cleavage may cause crackling when exposed to heat or excessive friction.
Synthesis: 
-
Simulants (with key separation tests): 
  • Blue Topaz: Aquamarine (optic figure, R.I., S.G., inclusions), Glass (optic character), Synthetic Spinel (optic character), Barite (S.G., lustre, hardness), Synthetic Quartz (optic figure, R.I., S.G.), Apatite (optic figure, R.I., birefringence, inclusions).
  • Yellow Topaz: Citrine (optic figure, R.I., S.G., inclusions), Yellow Beryl (optic figure, R.I., S.G., inclusions), Synthetic Sapphire (optic figure, R.I., S.G), Glass (optic character), Apatite (optic figure, R.I., birefringence, inclusions), Danburite (S.G.)
Geological Occurrence: 
In pegmatite and high temperature quartz veins, in alluvial deposits as pebbles.
Sources: 
Brazil, Mexico, Sri Lanka, South Africa, Afghanistan, Pakistan, India, etc.
Cuts & Uses: 
Facetted, cabochon, beads, carvings, etc.
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