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 P3̄1m, 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.