Equations of state of magnesium silicates anhydrous B and superhydrous B

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 +/- 0.03 Angstrom(3), K(T,0) = 151.5 +/- 0.9 GPa and K' = 5.5 +/- 0.3 for Anhy-B and V(0) = 624.71 +/- 0.03 Angstrom(3), K(T,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 (K(c) = 137 +/- 3 GPa), the b-axis is least compressible (K(b) = 175 +/- 4 GPa), and the a-axis is intermediate (K(a) = 148 +/- 1 GPa). In Shy-B, the a-axis is most compressible (K(a) = 135 +/- 1 GPa), followed by the b- and c-axes which have similar compressibilities (K(b) = 146 +/- 3 GPa; K(c) = 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 MgO(6) and SiO(6) 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 similar to 22% in Anhy-B to similar to 9% in Shy-B.