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Zircon


Properties
Chemical Composition: 
Silicate of Zirconium with traces of Uranium and Thorium - ZrSiO4
Classification / Type: 
Classified into 3 categories - high, medium and low type. The presence of radioactive elements within the structure may cause the breakdown of the internal structure (metamictization). The extent of breakdown defines the three types.
Colors / Varieties: 
  • High Type: Blue, white, reddish brown
  • Medium Type:
  • Low Type: Green, Brown, Yellow (Metamict Zircon)
  • Chatoyant Varieties
Crystal System / Forms: 
Tetragonal System / Prismatic habit and also as water-worn pebbles. Tetragonal prisms with pyramidal and / or pinacoidal terminations.
Hardness: 
6.5 - 7.5 (low to high type)
Specific Gravity: 
  • High: 4.70
  • Medium: 4.32
  • Low: 4.00
Cleavage / Fracture: 
None / Conchoidal fracture.
Optic Character: 
Anisotropic, D.R.; Uniaxial Positive. Low type metamict zircon maybe almost amorphous.
Lustre: 
Adamantine to vitreous.
Refractive Index / Birefringence: 
  • High: 1.925 - 1.984 / 0.059
  • Medium: 1.875 - 1.945 / 0.006 - 0.050
  • Low: 1.810 - 1.815 / 0.002 - 0.005
Pleochroism: 
Variable dichroism with blue zircon showing the strongest effect.
Dispersion: 
Weak to Moderate. Maximum: 0.038
Magnification: 
Angular zoning and streaks (are sometimes seen in low type), crystals, doubling of facet edges, abraded facet edges.
U.V. Fluorescence: 
Variable, commonly mustard yellow in shortwave and dull yellow in longwave. It may phosphoresce also.
Spectrum: 
  • The strongest line is at 653.5nm due to Uranium.
  • Many narrow lines and strong bands across the whole spectrum, ranging from more than 40 lines (in Myanmar Zircon) to only a few lines (in Australian Zircon). Other lines are 653.5nm, 691.0nm, 683.0nm, 662.5nm, 660.5nm, 621.0nm, 615.0nm, 589.5nm, 562.5nm, 537.5nm, 516.0nm, 484.0nm, 460.0nm, 432.7nm
  • Heat treated and low type zircon has a weak spectrum.
  • Colorless, blue and golden brown (all heat treated) zircons exhibit one fine line at 653.5nm
  • Some of zircons may display no spectrum at all.
Cause of Color: 
Color centers.
Treatment (Enhancement): 
Heating brown to red, yellow, colorless and blue.
Specific Tests & Remarks: 
Metamict Zircon: Low type zircons in which the internal structure is broken down due to residual radioactive decay. Though doubly refracting, they become almost singly refracting. Their properties are lower (R.I., S.G.) and they may exhibit weak doubling.
Synthesis: 
-
Simulants (with key separation tests): 
Sphene (optic figure, dispersion, spectrum), Synthetic Cubic Zirconia (optic character, dispersion, spectrum), Y.A.G. (optic character, spectrum), G.G.G. (optic character, spectrum, S.G.), Strontium Titanate (optic character, S.G. dispersion), Diamond (optic character, S.G., spectrum), Demantoid (optic character, spectrum, inclusions), Malaya Garnet (optic character, S.G., spectrum), Topaz (R.I., S.G., spectrum, dispersion), Natural / Synthetic Sapphire (R.I., spectrum, doubling), etc.
Geological Occurrence: 
In igneous rocks and alluvial deposits.
Sources: 
Sri Lanka, Myanmar, Thailand, Cambodia, France, India, Australia, Madagascar.
Cuts & Uses: 
Facetted cuts, cabochons, beads, etc.
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