THE EQUATION OF STATE OF WATER TO 200-DEGREES-C AND 3.5 GPA - MODEL POTENTIALS AND THE EXPERIMENTAL PRESSURE SCALE

The velocity of sound in liquid water has been measured as a function of temperature and pressure in the heated diamond-anvil high-pressure cell. The equation of state is determined at temperatures up to 200 de grees C and pressures as high as 3.5 GPa. Above 500 MPa, the results o f molecular-dynamics simulations based on the TIP4P potential agree wi th the experimental density within 1%. Such simulations, however, do n ot achieve the accuracy required to usefully predict the thermodynamic s of hydration reactions in the upper mantle. At pressures below 1 GPa a comparison is possible up to 150 degrees C between velocities measu red using the ruby fluorescence pressure scale and velocities based on an absolute pressure determination. Under the assumption that velocit ies deviations are a result of pressure uncertainty, the mean error of the ruby scale is 4 MPa (40 bars) with a standard deviation of 8 MPa (80 bars). (C) 1998 Elsevier Science B.V. All rights reserved.