Fe2+ and Fe3+ Oxidation States on Natural Sapphires Probed by X-ray Absorption Spectroscopy

Sapphire is the inorganic gemstone in varieties of gem corundum with the pure alpha-alumina (a-Al2O3) structure showing colorless. The various colors of sapphire are caused by some trace elements such as Cr, Fe, and Ti. Natural sapphires occur in different geological origins such as basaltic and metamorphic host rocks. High-quality sapphires contain low inclusion. The sapphire qualities are based on their sources that are hardly identified by the gemological instrument. In this study, X-ray absorption spectroscopy (XAS) focused on X-ray absorption near edge structure (XANES) technique using the Synchrotron radiation is employed for identifying the origins of sapphire samples. In particular, Fe K-edge XANES spectra are applied to analyze the Fe oxidation states on sapphires. As the results, we conclude that there were the Fe2+ from some mineral inclusions and Fe3+ which substitute Al3+ on the sapphire structure from both basaltic and metamorphic sources.