Hydroxide bands arising from intrinsic hydrous defects have been observed in the infrared spectra of 159 corundum crystals from worldwide localities. The OH defects are most prominent in blue sapphire crystals, suggesting an association of OH with redox reactions involving iron. Using a calibration for OH in kyanite, the absolute concentrations for OH in corundum are found to be generally 0.5 wt.ppm or lower. The linear OH absorption coefficient for the most intense band centered at 3310 cm−1 never exceeds 0.5 cm−1. In addition to the intrinsic OH defects, a number of previously identified hydrous inclusions often contribute to absorptions in the OH stretching region of the corundum infrared spectrum.

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