Speciation and mixing behavior of silica-saturated aqueous fluid at high temperature and pressure

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 degrees C) and pressure (up to 1350 MPa) in a constant-volume hydrothennal diamond anvil cell. Pressure was measured with the Raman shift of C-13 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 degrees C. At higher temperatures (and pressures), the pressure difference increases and reaches about 350 MPa at 800 degrees C.