High-temperature FTIR spectra of H2O in rhyolite melt to 1300 "C

A rhyolite glass containing about 300 ppm by weight of H'O was heated in a microscope heating stage to 1300 oC, and true infrared absorbance spectra were obtained in situ at various temperatures using an FTIR spectrometer, together with the focusing optics of an IR microscope. Virtually all HrO dissolved as OH at all temperatures. However, large changes both in band shape and integral extinction coefficient suggest that the type of OH species present changes with temperature. At 20 "C, the OH stretching band with a maximum at 3570 cm I was asymmetric with a broad shoulder extending to lower frequencies, which is due to strongly H-bonded OH groups. By 500 "C, still well below the bulk glass transformation temperature (Tu about 800 'C) a large part of the strongly H-bonded species had disappeared, causing the OH stretching band to become more symmetrical and decreasing the integral extinction coefrcient. This trend continues to higher temperatures. At 1300 oC, the spectrum in the OH stretching region consisted of a rather sharp, symmetrical band at 3570 cm-r, superimposed on a broad absorption feature, which is probably due to a highly mobile type of proton. The high mobility of protons means that a distinction between OH groups and molecular HrO in a HrO-rich melt is probably impossible at these temperatures. All changes observed in the spectra are completely reversible upon cooling to room temperature, and no significant amount of HrO was lost from the sample during the entire experiment. Our results show that the speciation and integral extinction coefficients of OH in silicate melts and glasses are strongly temperature dependent and that significant structural changes occur well below the bulk glass transformation temperature.