Incorporation of Fe3+ in phase-X, A2-xM2Si2O7Hx, a potential high-pressure K-rich hydrous silicate in the mantle

Phase-X, a potential sink for K in the mantle, is a synthetic hydrous K-rich silicate formed by the breakdown of K-amphibole at high pressure. It has the general formula A(2-x)M(2)Si(2)O(7)H(x) where A = K, Na, Ca, rectangle (vacancy), and M = Mg, Al, or Cr. No other isomorphic substitutions, either for the A or the M site, were reported for such a synthetic compound. Here we report the crystal structure and chemical composition of a crystal of phase-X containing large amounts of trivalent Fe. This crystal was synthesized in the model system garnet lherzolite-K2CO3 at lower pressure (P = 7 GPa) and higher temperature (T = 1450-1650 degrees C) with respect to the stability range reported in the literature (i.e. P 9-17 GPa and T = 1150-1400 degrees C). Quantitative analysis led to the following formula: (K1.307Na0.015Ca0.007)(Sigma = 1.329)(Mu(1.504)FeO(0.373)(3+)Al(0.053)(4+)Ti(0.004)(4+)MnO(0.001)(2+))(Sigma = 1.935)Si2O7.00H0.360. The lattice para- meters(hexagonal setting) are a = 5.005(1), c = 13.148(2) angstrom, V = 285.23(9) angstrom(3), Z = 2. The structure was refined in space group P6(3)cm to R = 5.06% using 199 independent reflections and consists Of MO6 octahedra layers stacked along the c axis and linked together by Si2O7 groups. The Si2O7 groups form pillars in the layer that contains A atoms in the cavities between the pillars. The Raman spectrum in the OH-stretching region indicates the existence of two different OH environments. However, the position of H could not be determined. 31 The substitution of Fe3+ for Mg shortens octahedral bond distances. In addition, the entry of Fe in M induces geometrical changes to the adjacent A site. The crystal-chemical characteristics are compared with published data on synthetic Fe-free phase-X.