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The fine-structure of the dehydration boundary of brucite (Mg(OH)₂) up to 3.5 GPa - indication of anomalous PVT behaviour of supercritical H₂O

Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52, A-6020 Innsbruck, Austria

e-mail: peter.mirwald{at}uibk.ac.at

Liquid and supercritical H₂O, the most abundant and important non-crystalline component in rock forming processes, is assumed to exhibit continuous PVT behaviour at high pressures and temperatures. However, a number of experimental observations at high pressure and temperatures challenges this concept. Direct measurements of PVT properties of H₂O at supercritical conditions are very difficult. Information on the PVT behaviour at these P-T conditions may be gained by certain dehydration reactions. An experimental re-examination of the reaction Mg(OH)₂ (brucite) = MgO (periclase) + H₂O has been conducted between 0.4 to 3.5 GPa and 650 to 1200 °C. The study was performed with a piston cylinder apparatus using the in-situ differential pressure analysis (DPA)-technique. The data reveal that the P-T slope of the equilibrium boundary is not continuous but exhibit a fine-structure which is characterised by three inflections at 1.1 GPa/830 °C, 1.9GPa/945 °C and 2.7 GPa/1040 °C. Because the solid phases, Mg(OH)₂ and MgO, exhibit no structural changes at these conditions, these effects are thought to be due to anomalous PVT behaviour of supercritical H₂O.

Key-words: brucite dehydration, not-smooth P-T boundary, anomalous PVT properties of H₂O.

This article has been cited by other articles:

				F. Jiang, S. Speziale, and T. S. Duffy Single-crystal elasticity of brucite, Mg(OH)2, to 15 GPa by Brillouin scattering American Mineralogist, November 1, 2006; 91(11-12): 1893 - 1900. [Abstract] [Full Text] [PDF]