Measurement of H₂O Molecule and Hydroxyl Concentrations in Hydrous Rhyolitic Glass by UV–Vis–NIR Dispersive Microspectroscopy

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The speciation of water in volcanic glass, as indicated by the relative proportions of H₂O and OH^-, provides information on the processes of volcanic eruptions. Earlier studies of water species used ultraviolet–visible–near-infrared (UV–Vis–NIR) dispersive spectroscopy to examine the NIR spectra of volcanic glass but were unable to confirm whether areas as small as 100 μm across could be studied. Here, UV–Vis–NIR dispersive microspectroscopy was applied in a study of hydrous rhyolitic glass synthesized by decompression in a cold-seal pressure vessel at 880°C. The concentrations of water species were determined by transmittance spectroscopy, with results consistent with those of Fourier-transform infrared microspectroscopy. The measured total water contents were consistent with the known solubility of water in rhyolitic magma, and, therefore, it is concluded that UV–Vis–NIR microspectroscopy can be applied in determining the concentrations of H₂O and OH^- in hydrous rhyolitic glass.