corundum, color.4, FTIR, gems

Infrared spectra of 159 corundum crystals from various global localities reveal hydroxide (OH) bands arising from intrinsic hydrous defects, with the strongest signals observed in blue sapphires. This suggests a link between OH defects and redox reactions involving iron. Applying a kyanite-based calibration, absolute OH concentrations in corundum are generally ≤ 0.5 wt. ppm. The strongest OH band at 3310 cm⁻¹ never exceeds a linear absorption coefficient of 0.5 cm⁻¹. Additionally, some of the OH absorptions originate from previously identified hydrous inclusions in the crystals.