The dehydration of brucite to produce periclase and H2O [Mg(OH)2 <-> MgO + H2O] was experimentally investigated from 1 to 15 GPa in order to study the compressibility of H2O at very high pressures. Low-pressure experiments were performed in a piston-cylinder apparatus and high-pressure experiments in a multianvil. The dehydration boundary was identified using both differential thermal analysis and quenching experiments. Thermal gradients present in the multianvil assemblies promoted thermal diffusion and allowed thermally compacted periclase to quench as periclase in quenching experiments performed at conditions outside the brucite stability field. Many experimental difficulties were encountered, including obtaining pure brucite starting material, preventing contamination by the pressure medium, and assuring that H2O leakage did not occur during the experiment. This dehydration equilibrium is especially vulnerable to these types of problems, and special care was required to reduce these sources of error. The resulting experimental data indicate that the reaction boundary has a positive dP/dT slope to 15 GPa and is nearly vertical above 6 GPa (approximately 1200-degrees-C). These new data are combined with thermodynamic and PVT properties of brucite and periclase in order to calculate the molar volume of H2O along the dehydration curve. These calculations suggest that the molar volume of H2O decreases from 21 cm3 at 1 GPa and 845-degrees-C to 10 cm3 at 15 GPa and 1280-degrees-C. These volumes are very similar to the molar volumes predicted using empirical modified Redlich-Kwong PVT models.
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