The crystal structure of the two dense hydrous magnesium silicates Phase D, MgSi2H2O6, and Phase H, MgSiH2O4, synthesized at 45 GPa and 1000 °C and coexisting in the same micrometer-sized grain, was investigated by single-crystal X‑ray diffraction to study the preferential partition of Al between the two structures. In agreement with the literature, Phase D was found to be trigonal, space group P3̅1m, with lattice parameters a = 4.752(2), c = 4.314(2) Å, V = 84.37(6) Å3 (R1 = 0.020), and Phase H was found to be orthorhombic, space group Pnnm, with lattice parameters a = 4.730(2), b = 4.324(2), c = 2.843(2) Å, V = 58.15(5) Å3 (R1 = 0.024). The estimated proportion (vol%) of the two phases from the refinement is 27(2)PhD - 73PhH. The analysis of the geometric details of the two structures shows that Phase D hosts almost all the Al available, whereas Phase H is nearly identical to pure MgSiH2O4. Overexposed electron-microprobe X‑ray maps of the same grain used for the X‑ray diffraction study together with WDS spots on the two phases confirmed the structural results. Thus, our results suggest that when Phase D and Phase H coexist, Al is strongly partitioned into Phase D at the expense of coexisting Phase H. At pressure above ~50 GPa, where Phase D is no longer stable, Phase H is able to incorporate the high aluminum contents present in hydrous peridotitic compositions in the deep lower mantle and be stabilized at the expense of Phase D and magnesium silicate perovskite.
© 2015 by Walter de Gruyter Berlin/Boston