New structural features of the high-pressure synthetic sheet-disilicate Phase-X, K(2-x)Mg2Si2O7Hx

Mark D. Welch*, Juergen Konzett, Luca Bindi, Simon C. Kohn, Daniel J. Frost

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

4 Citations (Scopus)

Abstract

The structure of the synthetic high-pressure sheet-disilicate Phase-X (PhX), a possible host of H2O and K in the Mantle, has been determined for a crystal synthesized at 16 GPa/1300 degrees C/23 h. The composition of the sample is close to K1.5Mg2Si2O7H0.5, which is 50% PhX/50% Anhydrous-PhX and has 25% of interlayer K sites vacant. The structures of four crystals were determined by single-crystal X-ray diffraction and had very similar diffraction characteristics and structural results; the structure of one of the larger crystals is reported here. Reflection intensity statistics strongly indicate that PhX is centrosymmetric, space group P6(3)/mcm, in contrast to other studies that have reported non-centrosymmetric space group P6(3)cm. While it was possible to obtain good agreement indices for refinements in P6(3)cm, there were strong correlations between atoms that are equivalent in P6(3)/mcm, suggesting that the correct structure is centrosymmetric. Full anisotropic refinement in space group P6(3)/mcm gave R-1 = 0.036, wR(2) = 0.079, GoF = 1.467. As with all previous studies of PhX, the H atom was not located. Difference-Fourier maps of the residual electron density indicated that the K atom is displaced from the 4c site lying on the sixfold axis on to three split 12j sites 0.2 angstrom away, each having 1/4 occupancy, giving a total of 3 K atoms per unit cell and corresponding to 1.5 K apfu, in good agreement with the content derived from electron microprobe analysis. Diffraction patterns of all four crystals examined, reconstructed from the full-intensity data collection, consistently show the presence of a large hexagonal superstructure with dimensions 8a(sub), x 8a(sub) x c(sub), having Z = 128, compared with Z = 2 for the two-layer subcell. Complex arrays of superlattice reflections occur in layers with = 2n, but are absent from l = 2n + 1 layers.

Unpolarized infrared spectra of single crystals of PhX were obtained that are similar to those reported previously in the literature. Spectra in the OH-stretching region consist of a major absorption band at 3595 cm(-1) and three much weaker bands at 3690, 3560, and 3405 cm(-1). Bond-valence analysis of PhX indicates that O1 is very over-bonded, whereas O2 is slightly under-bonded and a possible site for protonation. We present geometrical and crystal-chemical arguments that exclude O1 as a candidate for protonation, whereas a much better case can be made for O2. In PhX structures, H must be located at a partially occupied site with a multiplicity 4

Original languageEnglish
Pages (from-to)1849-1857
Number of pages9
JournalAmerican Mineralogist
Volume97
Issue number11-12
DOIs
Publication statusPublished - 2012

Keywords

  • structure
  • Phase-X
  • BOND
  • GPA
  • X-ray diffraction
  • CRYSTAL-STRUCTURE
  • ZONE
  • MANTLE
  • infrared spectroscopy
  • superlattice
  • SOLIDS
  • RICH HYDROUS SILICATE
  • MOLECULES

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