In situ formation of coestite under hydrothermal conditions

We present thein situneutron diffraction data of a water and silica mixture at high pressure and temperature. We show initially the formation of ice VI at 1.5 GPa at 290 K in the presence of crystalline SiO, upon heating we observe its melting at similar to 400 K. Upon further warming to 1200 K, we observe melting of the crystalline silica. Upon cooling to 290 K and recovery to ambient pressure, we obtain a mixture of silica in the coesite structure and liquid water. These results have implications for the phase diagram of the coesite-water solidus and hence the behaviour of fluids at mantle conditions.