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Abstract

In this study we report the structure of supercritical H2O–SiO2 fluid composed of 50 mol% H2O and 50 mol% SiO2 at 3000 K and 2400 K, investigated by means of ab initio molecular dynamics of models comprising 192 and 96 atoms. The density is set constant to 1.88 g/cm 3, which yields a pressure of 4.3 GPa at 3000 K and 3.6 GPa at 2400 K. Throughout the trajectories, water molecules are formed and dissociated via the network modifying reaction 2 SiOH = SiOSi + H2O. The calculation of the reaction constant K = [OH-]2/[H2O][O2-] is carried out on the basis of the experimentally relevant Qn species notation and agrees well with an extrapolation of experimental data to 3000 K. After quench from 3000 K to 2400 K, the degree of polymerization of the silicate network in the 192-atom models increases noticeably within several tens of picoseconds, accompanied by release of molecular H2O. An unexpected opposite trend is observed in smaller 96-atom models, due to a finite size effect, as several uncorrelated models of 192 and 96 atoms indicate. The temperature-dependent slowing down of the H2O–silica interaction dynamics is described on the basis of the bond autocorrelation function.