Heat Treating Blue Sapphire of Pailin, Cambodia

Abstract

A total of 54 rough blue sapphire samples from Pailin gem field of Cambodia were studied. The samples can be classified into three groups on the basis of their colors and distinct appearance. They consist of trapiche-like blue sapphire, light blue and blue to dark blue groups. The main aim of this study is to study heat treatment process of blue sapphire samples from Pailin gem field of Cambodia. Gemological properties and chemical compositions of these samples are also investigated. For heat treating experiments, the samples were divided into two batches according to the purpose of the heat treatments. Forty-five samples were heated in oxidizing condition to lighten dark blue color of the samples while nine samples were heated in reducing condition to intensify or develop the blue color. The results show that heating at 1200°C can lighten dark blue color and heating over 1400°C can develop or intensify blue color. The UV-Vis-NIR absorption spectra of all samples exhibit Fe3+ absorption peaks, Fe2+/Ti4+ IVCT and Fe2+/Fe3+ IVCT absorption bands. The more intense blue color intensified after heating, the stronger the intensity of Fe2+/Ti4+ IVCT absorption band. The Fe2+/Fe3+ IVCT absorption band probably contributes to the darker tone in the samples. Chemical compositions reveal iron (Fe) and titanium (Ti) are the most significant minor and trace elements because both of them are necessary for the color-causing mechanism in blue sapphire.