Determination of water speciation in hydrous haplogranitic glasses with partial Raman spectra

We use a mathematical approach to decompose the Raman water band at 3000 cm⁻¹ to 3750 cm⁻¹ into two partial Raman spectra corresponding to the individual Raman activity of the two water species, i.e., molecular water (H₂O_m) and OH-groups, present in hydrous rhyolitic glasses. The approach is based on a least-squares optimization algorithm and the assumption that the water band can be expressed as a linear combination of two partial Raman spectra. Our model makes no assumptions regarding the shape of the partial Raman spectra. The model input consists of about 700 Raman spectra from hydrous haplogranitic (HPG8) compositions with total water contents from 0.6 to 3.1 wt% and with known water species distributions. Water speciation and concentrations were derived from the equilibrium constant of the speciation reaction (determined by infrared spectroscopy). This speciation was used to calibrate the Raman spectra. The resulting data show that partial Raman spectra have strong spectral overlap, particularly in the high-frequency region of the water band, but both shape and position of the maxima vary. The stretching vibrations of OH yield a very broad and highly asymmetric band, whereas the main vibrational response of H₂O_m yields a narrower, near-symmetric band in the high-frequency region and a small subordinate band in the low-frequency part. We demonstrate that partial Raman spectra can be used to determine water species concentrations in hydrous glasses.