The effects of heat on zircon from Kanchanaburi

Abstract

The naturally occurring reddish brown zircon from many localities such as from Ratana Kiri, Cambodia can be heat-treated to blue, yellow and colorless which are more desirable in gem trade. It is therefore interesting to know and understand the effect of heat treatment on coloration and other properties of zircon from Amphoe Bo Phloi, Changwat Kanchanaburi which has not been investigated before. Thirty five zircon samples from Bo Phloi were used in this study. The samples can be subdivided into 2 groups based on their color; dark reddish brown to orange brown and light orange and reddish brown. Their specific gravities average about 4.569 and 4.486 which fall into a high to intermediate-type zircon. The samples are mostly inert both under short and long wave UV. The dominant internal features are negative crystals, finger prints, color zones and fractures. All samples were heat-treated at 900oC in reduction condition for 3 hours. After treatment, the color mostly was changed from originally brown to colorless and some dark gray and violet in both groups. The UV-VIS-NIR absorption band from 600nm to UV range, which is related to brown color center or structure damaged by radioactive element, was disappeared after heating. Beside, peaks of U5+ appear at 1150 nm and 1157 nm both before and after heating. In addition, the structural damage appear to improve after heating as seen from FTIR absorption spectra; the O-H stretching peaks at 2800-3200 cm-1 disappear and some also sharper after heating. Beside, the Si-O stretching peaks at 1400-2000 cm-1are sharper after heating. Furthermore, the Raman spectra of zircon samples before heating give the average V3 (anti-symmetric SiO4 stretching) values of 1007.23 cm-1 and 1007.29 cm-1 which represent a well crystalline structure or slight degree of metamictization. After heat-treatment, the average V3 values increase to 1007.67 cm-1 and 1007.46 cm-1 and the average FWHM (Full with at Half Maximum) values before heating decrease from 5.60 cm-1 to 5.46 cm-1. Both values suggest that heat treatment can improve their structure. The chemical analyzed by LA-ICP-MS shows the average contents of elements such as Zr, Si, Hf, Th and U etc. The dark brown group has lower U concentrations (average about 112.6 ppm) than that of the light brown group (average about 276.2 ppm). Their formulas vary from ZrSiO4 to (Zr0.99,Hf0.01)SiO4.