INFRARED-SPECTROSCOPY OF HYDROUS SILICATE MELTS TO 1000-DEGREES-C AND10 KBAR - DIRECT OBSERVATION OF H2O SPECIATION IN A DIAMOND-ANVIL CELL

Near-infrared spectra of hydrous silicate melts are reported up to 10 kbar and 1000 degrees C. Measurements were performed using an external ly heated diamond-anvil cell that allows precise control of pressure a nd temperature. In hydrous glasses, changes in H2O speciation occur be low the glass-transformation temperature; however, the standard enthal py of the speciation reaction H2O + O = 2OH is very different for the melt and glass phases. For a peraluminous sodium aluminosilicate glass containing 8.1 wt% total H2O, the temperature dependence of the H,O s peciation equilibrium can be described by two equations: In K = -2.65 - 1.91 x 10(2) K T-1 (glass phase, Delta H = 1.6 kJ/mol) and ln K = 3. 04 - 3.64 x 10(3) K T-1 (melt phase, Delta H = 30.3 kJ/mol). The inter section of these two equilibrium curves defines a glass-transformation temperature of 335 degrees C. The near-infrared spectrum of a peralum inous sodium silicate melt with 30-40 wt% H2O at 10 kbar and 1000 degr ees C shows the presence of both OH and H2O; however, exceptionally st rong speciation changes occur upon quenching. From visual observation, it appears that at 10 kbar and 1000 degrees C there is complete misci bility between this melt and water. Therefore, these data constitute t he first infrared spectra of a phase beyond the critical point in a sy stem ofsilicate melt and H2O.