Previous studies of hydrous glasses and melts with infrared spectroscopy have led to the conclusion that the IR combination peaks near 4500 and 5200 cm−1 reflect the existence of OH (hydroxyl) groups and H2Omol water molecules in those materials. Here, we show that the glass chemical composition can impact profoundly the intensities and frequencies of the fundamental O-H stretching signal and, therefore, potentially those of the 4500 and 5200 cm−1 combination peaks. In alkali silicate glasses, compositional effects can give rise to peaks assigned to fundamental O-H stretching at frequencies as low as 2300 cm−1. This expanded range of Raman intensity assigned to O-H stretch is increasingly important as the ionic radius of the alkali metal increases. As a result, the combination of the fundamental O-H stretch in OH groups with the Si-O-H stretch located near 910 cm−1 gives rise to a complex combination signal that can extend to frequencies much lower than 4200 cm−1. This combination signal then becomes unresolvable from the high-frequency limb of the band assigned to fundamental O-H stretch vibration in the infrared spectra. It follows that, when O-H stretch signals from OH groups extend to below 3000 cm−1, the 4500 cm−1 peak does not represent the total OH signal. Under such circumstances, this infrared peak may not be a good proxy for determining the concentration of OH hydroxyl groups for glassy silicate materials.

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