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Equations of state of magnesium silicates anhydrous B and superhydrous B
Authors:W. A. Crichton  N. L. Ross  T. Gasparik
Affiliation:(1) Department of Geological Sciences, Univ. College London., Gower St., London, WC1E 6BT, UK, GB;(2) Center for High Pressure Research (CHiPR) and Department of Earth Sciences, State University of New York at Stony Brook, Stony Brook, New York 11794, USA, US
Abstract:High-pressure single crystal X-ray diffraction experiments of phase anhydrous B and superhydrous B have been carried out to 7.3 and 7.7?GPa, respectively, at room temperature. Fitting a third-order Birch-Murnaghan equation of state to the P-V data yields values of V 0?=?838.86?±?0.04?Å3, KT,0?=?151.5?±?0.9?GPa and K′?=?5.5?±?0.3 for Anhy-B and V 0?=?624.71?± 0.03?Å3, KT,0?=?142.6?±?0.8?GPa and K′?=?5.8?±?0.2 for Shy-B. A similar analysis of the axial compressibilities in Anhy-B reveals that the c-axis is most compressible (Kc?=?137?±?3?GPa), the b-axis is least compressible (Kb?=?175?±?4?GPa), and the a-axis is intermediate (Ka?=?148?±?1?GPa). In Shy-B, the a-axis is most compressible (Ka?=?135?±?1?GPa), followed by the b- and c-axes which have similar compressibilities (Kb?=?146?±?3?GPa; Kc?=?148?±?3?GPa). The fact that the b-axis of Shy-B is approximately 16% more compressible than Anhy-B is primarily due to differences in the O-T layer in which the H atoms are located and the linkages with the adjacent O layers. The rigid edge-sharing chains of MgO6 and SiO6 octahedra in the O layer control compressibility along the a- and c-axes in both structures. The net result is a reduction in the overall anisotropic compression from ~22% in Anhy-B to ~9% in Shy-B.
Keywords:X-ray diffraction  Equation of state  High pressure  Anhydrous B  Superhydrous B  Magnesium silicates
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