Many geological processes are governed by the migration of hot, mineral-rich water. These "hydrothermal" fluids are stable at high pressures and temperatures and are important in a range of processes that include ore-deposition and the dynamics and evolution of subduction zones at tectonic plate margins. Although silicate minerals such as quartz are completely insoluble at room pressure there is complete miscibility between water and SiO2 at high pressures and temperatures. yet despite this and despite the significance of these fluids no in situ diffraction measurements have been obtained. in this proposal we aim to provide the first measurement of the total structure factor in the simplified water-rock system (SiO2-D2O) at pressures of up to 2GPa and temperatures of 1400K made possible by recent developments in high pressure and high temperature experimental techniques at ISIS (PEARL)