Heat treatment of some corundum from Madagascar

Abstract

Heat treatment is an important technique for improving the color and clarity of corundum. It is the most popular method accepted and adopted by the international trade. At present, Thailand imports a large volume of corundum from Madagascar, most of which is heat-treated before it is supplied to the gems and jewelry industry. More than 270 sapphires from secondary gem deposits in Ilakaka-Sakaraha area, southwestern Madagascar, were categorized into 7 color groups including pink, blue, violet, purple, colorless, yellow and green sapphires. Most pink and purple and medium violet sapphires were selected for heat-treatment study. These sapphires were examined in great details on gem characteristics using basic equipment and advanced instruments. Step-heating experiments were performed under oxidizing atmosphere at different maximum temperatures (i.e., 800 ํC, 1000 ํC, 1200 ํC, 1400 ํC and 1600 ํC) for one hour soaking time at each temperature. Characteristics of all samples were examined after each heating step for comparison. Results of this study show that most sapphire samples can be treated ultimately to reduce their blue color in oxidizing atmosphere at 1000 ํC-1200 ํC. The criterion for detecting the heat treatment is the turbidity of zircon inclusion. Causes of color and its changing are obviously explained using UV-VIS-NIR absorption spectra and trace elements. Based on the interaction model of Mg, Ti and Fe by Haeger (2001), the causes of color can be explained in all studied sample groups. Most samples have Mg/Ti ratios close to 1. This data suggest that when heat-treated those sapphires in oxidizing atmosphere, most of Mg and Ti would be tied up with the formation of colorless MgTiO3 clusters and at the same time the Fe2+ might have been oxidized into Fe3+. Hence the blue overcast originated from Fe2+/Ti4+ IVCT process would have been eliminated and the stones should turn in pure pink color.