Feature Gems & Gemology, Summer 2006, Volume 42, No. 2

The Effects of Heat Treatment on Zircon Inclusions in Madagascar Sapphires

Zircon inclusions in sapphires from Madagascar were studied to investigate the effects of heat treatment on their gemological and spectroscopic features. Progressive decomposition of zircon and chemical reactions between zircon and the host sapphire occurred at temperatures between 1400°C and 1850°C. In unheated sapphires, transparent zircon inclusions displayed euhedral slightly elongated forms and clear interfaces with their corundum host. Most were confined within the host under relatively high pressures (up to 27 kbar), and showed evidence of natural radiation-related damage (metamictization). Subsolidus reactions (i.e., the decomposition of zircon into its component oxides without melting) of some zircon inclusions started at temperatures as low as 1400°C, as evidenced by the formation of baddeleyite (ZrO2) and a SiO2-rich phase. Differences in the degree of preexisting radiation damage are the most likely cause for the decomposition reactions at such relatively low temperatures. Melting of zircon and dissolution of the surrounding sapphire occurred in all samples at 1600°C and above. This resulted in the formation of both baddeleyite and a quenched glass rich in Al2O3 and SiO2. From these data and observations, a systematic sequence of both modification and destruction of zircon inclusions with increasing temperature was compiled. This zircon alteration sequence may be used (1) as a gemological aid in determining whether a zircon-bearing ruby/sapphire has been heated, and (2) to provide an estimate of the heating temperature.