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

Hexiong Yang^*, Jürgen Konzett and Charles T. Prewitt

Geophysical Laboratory and Center for High Pressure Research, Carnegie Institution of Washington, 5251 Broad Branch Road N.W., Washington, D.C. 20015-1305, U.S.A.

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: Na_1.78(Mg_1.89Al_0.13)Si_2.02O₇ (anhydrous sodic phase X), Na_1.16K_0.01(Mg_1.89Al_0.14) Si_2.02O₇H_0.65 (sodic phase X), K_1.85Mg_2.06Si_2.01O₇ (anhydrous phase X), and K_1.54Mg_1.93Si_1.89O₇H_1.04 (phase X). A general chemical formula for these phases can be expressed as A_2–xM₂Si₂O₇H_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 P1m, whereas the other three have an identical structure with space group P6₃cm. Both P1m 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.

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