Water speciation in rhyolitic melt determined by in-situ infrared spectroscopy

Near-IR spectra obtained from hydrous silicate melts, close to natural rhyo lite, were acquired at both high temperature using a heating stage and simu ltaneously at high pressure and temperature using a hydrothermal diamond ce ll. The temperature dependence of the extinction coefficients for the peaks due to OH and H2O is negligible. In both sets of experiments, the speciati on reaction H2O + O = 2OH is shifted to the right with increasing temperatu re above the glass transition but changes below this are negligible, within experimental uncertainty. For a sample containing 3.93 wt% total water, th e temperature dependence of the speciation equilibrium can be described by two equations with temperature in K: In k = -36.74/T - 4.02 for the glass p hase (giving enthalpy and entropy of reaction values of Delta H = 0.31 kJ/m ol and Delta S = -33.45 J/mol-K), and In k = -3821.83/T + 1.61 for the melt phase (where Delta H = 31.77 kJ/mol and Delta S = 13.41 J/mol-K) respectiv ely). The glass transition temperature of 670 K is defined by the intersect ion of these curves. Our values are in good agreement with previously publi shed glass transition temperatures for similar compositions. Similar values for the enthalpy and entropy of reaction were obtained from all the other experiments including those at pressures up to 4.5, 5, and 10 kbars, sugges ting that the speciation depends negligibly on pressure for this pressure r ange.