Phase X, ascribed by Luth (1995) to a hydrous K-rich silicate formed from the breakdown of K-amphibole at high pressures, was synthesized at 1250–1300 °C and 10–16 GPa in four different compositions: Na1.78(Mg1.89Al0.13)Si2.02O7 (anhydrous sodic phase X), Na1.16K0.01(Mg1.89Al0.14) Si2.02O7H0.65 (sodic phase X), K1.85Mg2.06Si2.01O7 (anhydrous phase X), and K1.54Mg1.93Si1.89O7H1.04 (phase X). A general chemical formula for these phases can be expressed as A2−xM2Si2O7Hx, with A = K and/or Na, M = Mg and/or Al, and x = 0–1. Structure determination from single-crystal X-ray diffraction data shows that anhydrous sodic phase X is trigonal with space group P3̅1m, whereas the other three have an identical structure with space group P63cm. Both P3̅1m and P63cm structures are characterized by MgO6 octahedral layers that are stacked along the c axis and inter-linked together by Si2O7 tetrahedral dimers and K or Na cations. Within the MgO6 layers, each MgO6 octahedron shares three edges with neighboring MgO6 octahedra to form brucite-like layers with one out of three octahedral sites vacant. Large K or Na cations are situated right below and above each occupied octahedron in the MgO6 layers, whereas the Si2O7 groups are located below and above each vacant octahedron in the layers. The two types of structures, however, differ in the relative orientation of MgO6 octahedral layers, the coordination of K or Na, and the configuration of SiO4 tetrahedral dimers. By comparison, the Na2Mg2Si2O7 phase synthesized by Gasparik and Litvin (1997) appears to have the stoichiometry identical to anhydrous sodic phase X. Hence, these two high-pressure phases are likely to possess the same structure, or at least are closely related to each other structurally.