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The structure of the synthetic high-pressure sheet-disilicate Phase-X (PhX), a possible host of H 2 O and K in the mantle, has been determined for a crystal synthesized at 16 GPa/1300 °C/23 h. The composition of the sample is close to K 1.5 Mg 2 Si 2 O 7 H 0.5 , which is 50% PhX/50% Anhydrous-PhX and has 25% of interlayer K sites vacant. The structures of four crystals were determined by singlecrystal X-ray diffraction and had very similar diffraction characteristics and structural results; the structure of one of the larger crystals is reported here. Reflection intensity statistics strongly indicate that PhX is centrosymmetric, space group P6 3 /mcm, in contrast to other studies that have reported non-centrosymmetric space group P63cm. While it was possible to obtain good agreement indices for refinements in P63cm, there were strong correlations between atoms that are equivalent in P6 3 /mcm, suggesting that the correct structure is centrosymmetric. Full anisotropic refinement in space group P6 3 /mcm gave R 1  = 0.036, wR 2  = 0.079, GoF = 1.467. As with all previous studies of PhX, the H atom was not located. Difference-Fourier maps of the residual electron density indicated that the K atom is displaced from the 4c site lying on the sixfold axis on to three split 12j sites 0.2 Å away, each having ¼ occupancy, giving a total of 3 K atoms per unit cell and corresponding to 1.5 K apfu, in good agreement with the content derived from electron microprobe analysis. Diffraction patterns of all four crystals examined, reconstructed from the full-intensity data collection, consistently show the presence of a large hexagonal superstructure with dimensions 8a sub  × 8a sub  × c sub , having Z = 128, compared with Z = 2 for the two-layer subcell. Complex arrays of superlattice reflections occur in layers with l = 2n, but are absent from l = 2n + 1 layers.

New structural features of the high-pressure synthetic sheet-disilicate Phase-X, K (2–x) Mg 2 Si 2 O 7 H x