The shock wave equation of state of brucite Mg(OH)_2

Abstract

New equation of state (EOS) data for brucite Mg(OH)_2 shocked between 12 and 60 GPa are reported. When combined with earlier data of Simakov et al. (1974), it is found that brucite EOS data between 12 and 97 GPa can be fit with a single linear U_s-u_p relationship: U_s = 4.76(0.11) + 1.35(0.05) u_p. The third order Birch-Murnaghan equation parameters are: K_(os) = 51 ± 4 GPa and K′_(os) = 5.0 ± 0.4. The lack of a U_s-u_p discontinuity indicates that no phase transformation with a significant volume change occurs to at least 97 GPa. However, thermodynamic and theoretical Hugoniot calculations suggest brucite may dehydrate with only a small volume change. A lower bound for this dehydration pressure under shock conditions is inferred to be 26 GPa. We report the first partial release states measured for this material. The data are in quantitative agreement with earlier shock recovery experiments (Lange and Ahrens, 1984). Volatilization upon release begins at pressures as low as 12 GPa, much less than predicted by the shock entropy method. Calculated phase boundaries using the present EOS data are consistent with experimental data and indicate that brucite is unlikely to be stable under lower mantle conditions. However, brucite data, in conjunction with data for silicates and oxides, can be used to infer the effect of H_2O on lower mantle properties. At high pressure, bulk sound velocities calculated for MgO and Mg(OH)_2 are very similar, indicating that the presence of hydrous assemblages in the lower mantle may not produce anomalous bulk seismic velocities. Comparison of densities in brucite and other high-pressure phases under mantle conditions indicates that the water content of the lower mantle is between 0 and 3 wt %.

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