Phase-X, a potassium di-magnesium acid disilicate, is a high-pressure synthetic compound—a potential K-bearing silicate in the mantle—with space group P63cm (no. 185), a = b = 5.028(2) Å, c = 13.216(3) Å, V = 289.34 Å3, Z = 2. The structure has been determined with 1521 CCD measured intensities and refined by the least-square method to R = 0.0187. The structure is built up of octahedral MgO sheets and layers containing disilicate groups, Si2O7, (with distinct Si1 and Si2 tetrahedra linked by the apical O2 atom) alternating along the c axis. The octahedral sheet is based on a hexagonal closest-packed array of two layers of non-equivalent O atoms, O1 and O3; two-thirds of all edge-sharing M octahedra are filled. Within the framework of the Si2O7 groups are channel structures parallel to , , and  that contain K atoms disordered in the middle of a large trigonal cavity (the A site). The FTIR spectrum in the OH stretching region shows a sharp peak at 3602 cm−1 due to OH− ordered in one anion site; the position of hydrogen, which operates in a charge-balancing substitution for the partial occupancy of the A site (K1−x□x)A ↔ Hx□1−x)H, is undetermined. Densification in phase-X is affected by the greater compression of the empty octahedra in the octahedral layer and by constraining the trigonal A cavity containing the K atom to the size of the Si2O7 disilicate group. This dense packing contributes to the relatively high zero-pressure calculated density of 3.38 g/cm3.