The effect of dissolved silica on the PVT properties of H2O and structure of silica-saturated aqueous fluids in equilibrium with quartz in the SiO2-H2O system has been determined in situ with the materials at temperature (up to 800 °C) and pressure (up to 1350 MPa) in a constant-volume hydrothermal diamond anvil cell. Pressure was measured with the Raman shift of 13C synthetic diamond and was also calculated from the PVT properties of pure H2O. The two sets of pressures thus obtained differ by ≤50 MPa at T < 500 °C. At higher temperatures (and pressures), the pressure difference increases and reaches about 350 MPa at 800 °C.

The structure of the silica-saturated aqueous fluid was probed with microRaman and microFTIR methods. Coexisting molecular H2O and OH-groups, bonded to Si4+, exist above ~400 °C and ~0.4 GPa with their abundance-ratio, OH/H2O°, positively correlated with temperature. Hydrogen bonding diminishes with temperature and cannot be detected in the silica-saturated aqueous fluid above ~400 °C and ~0.4 GPa. This behavior resembles that of pure H2O under similar temperature and pressure conditions.

Speciation of dissolved silica in the fluid at 400 °C/760 MPa, and above, comprises Qo and Q1 species, whereas at lower temperature and pressure only Qo species were detected. The Q1/Qo abundance ratio is positively correlated with temperature (and silica content). An excess volume of mixing, VH2Oexcess, derived from a comparison of EOS of pure H2O and those of silicate-saturated H2O, is related to the Q1/Qo abundance ratio in silicate-saturated fluid, XQ1/XQo. The VH2Oexcess = MH2O(1/dm−1H2O/0.88), VH2Oexcess ~ 0 when XQ1/XQo approaches 0.

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