GEMOLOGICAL AND SPECTROSCOPIC PROPERTIES OF BLUE SAPPHIRES FROM DEN CHAI DISTRICT, PHRAE PROVINCE
Keywords:
Corundum, sapphire, gem depositAbstract
The total of 30 rough samples of very dark, low transparency, or uneven-coloured bluesapphire from basaltic affiliated gem field of Den Chai district, Phrae province, Thailand,were studied to characterize the gemological and spectroscopic properties. The specificgravity ranges from 3.86 to 4.20 (average 3.97) and refractive index are ne = 1.762–1.768and no = 1.770–1.775 respectively. Inclusions are negative crystals, gas bubbles, fingerprint, rhombohedral parting and mineral inclusions. Some of them show a characteristic fixedsix-rayed star-shaped growth zoning or Trapiche. Spectroscopic methods using UV-VISNIRanalysis shows prominent peaks of Fe3+ in the ultraviolet region at 376 nm, as well asin the visible region at 450 nm of Fe3+/Fe3+ pair. The absorption band of Fe2+/Ti4+Intervalence Charge Transfer (IVCT) between 550 and 750 nm and increasing absorption of Fe2+/Fe3+ IVCT towards the near infrared (maximum in the region between approximately 870 to 890 nm) which is typically found in sapphire from basaltic sources.The FTIR results show C/O, C/H and Ti/OH absorption peaks at 2350, 2846, 2915, and3309 cm-1 respectively. The chemical composition analysis using PIXE shows relativelyhigh FeO content and traces of Ti, V, Cr, and Ga that indicates the basaltic origin of thesesapphires.
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