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Phase X, ascribed by Luth (1995) to a hydrous K-rich silicate formed from the breakdown of Kamphibole at high pressures, was synthesized at 1250-1300 °C and 10-16 GPa in four different compositions: Na 1.78 (Mg 1.89 Al 0.13 )Si 2.02 O 7  (anhydrous sodic phase X), Na 1.16 K 0.01 (Mg 1.89 Al 0.14 ) Si 2.02 O 7 H 0.65  (sodic phase X), K 1.85 Mg 2.06 Si 2.01 O 7  (anhydrous phase X), and K 1.54 Mg 1.93 Si 1.89 O 7 H 1.04  (phase X). A general chemical formula for these phases can be expressed as A 2-x M 2 Si 2 O 7 H x , 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 P3̄̄-1m, whereas the other three have an identical structure with space group P6 3 cm. Both P3̄̄1m and P6 3 cm structures are characterized by MgO 6  octahedral layers that are stacked along the c axis and inter-linked together by Si 2 O 7  tetrahedral dimers and K or Na cations. Within the MgO 6  layers, each MgO 6  octahedron shares three edges with neighboring MgO 6  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 MgO 6  layers, whereas the Si 2 O 7  groups are located below and above each vacant octahedron in the layers. The two types of structures, however, differ in the relative orientation of MgO 6  octahedral layers, the coordination of K or Na, and the configuration of SiO 4  tetrahedral dimers. By comparison, the Na 2 Mg 2 Si 2 O 7  phase synthesized by Gasparik and Litvin (1997) appears to have the stoichiometry identical to anhydrous sodic phase X. Hence, these two highpressure phases are likely to possess the same structure, or at least are closely related to each other structurally

Crystal structure of phase X, a high pressure alkali-rich hydrous silicate and its anhydrous equivalent