Equations of state of magnesium silicates anhydrous B and superhydrous B

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 V0 = 838.86 ± 0.04 A3, KT,0 = 151.5 ± 0.9 GPa and K′ = 5.5 ± 0.3 for Anhy-B and V0 = 624.71 ± 0.03 A3, 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.