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

Phase X, ascribed by Luth (1995) to a hydrous K-rich silicate formed from t he breakdown of K-amphibole at high pressures, was synthesized at 1250-1300 degreesC and 10-16 GPa in four different compositions: Na-1.78(Mg1.89Al0.1 3)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 K1.54Mg1.9 3Si1.89O7H1.04 (phase X). A general chemical formula for these phases can b e 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 diffrac tion data shows that anhydrous sodic phase X is trigonal with space group P (3) over bar 1m. whereas the other three have an identical structure with space group P6(3)cm. Both P (3) over bar 1m and P6(3)cm structures are char acterized by MgO6 octahedral layers that are stacked along the c axis and i nter-linked together by Si2O7 tetrahedral dimers and K or Na cations. Withi n the MgO6 layers, each MgO6 octahedron shares three edges with neighboring MgO6 octahedra to form brucite-like layers with one out of three octahedra l sites vacant. Large K or Na cations are situated right below and above ea ch occupied octahedron in the MgO6 layers, whereas the Si2O7 groups are loc ated 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 tetrahe dral dimers. By comparison, the Na2Mg2Si2O7 phase synthesized by Gasparik a nd Litvin (1997) appears to have the stoichiometry identical to anhydrous s odic phase X. Hence, these two high-pressure phases are likely to possess t he same structure, or at least are closely related to each other structural ly.