High-pressure synthetic (Na 0.97Mg 0.03)(Mg 0.43Fe 0.17 3+Si 0.40)Si 2O 6, with six-coordinated silicon, isostructural with P2/n omphacite

Esther S. Posner, Jürgen Konzett, Daniel J. Frost, Robert T. Downs, Hexiong Yang

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The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, Na 0.97Mg 0.03)(Mg 0.43Fe 0.17 3+Si 0.40)Si 2O 6, is isotypic with ordered P2/n omphacite. Its structure is characterized by single chains of corner-sharing SiO 4 tetra-hedra, extending along the c axis, which are crosslinked by bands of edge-sharing octa-hedra (site symmetry 2), statistically occupied by (Mg 2+ + Fe 3+ + Si 4+). Between the bands built up of the octahedra are two non-equivalent highly distorted six-coordinated sites (site symmetry 2), statistically occupied by (Na + Mg). In contrast to omphacites, the great differences in size and charge between Mg 2+ and Si 4+ result in complete, rather than partial, ordering of Mg and Si into two distinct octa-hedral sites, whereas Fe 3+ is disordered between the two sites. The octa-hedron filled by (Mg + Fe) is larger and markedly more distorted than that occupied by (Si + Fe). The average (Mg + Fe) - O and ( VISi + Fe) - O bond lengths are 2.075 and 1.850 Å, respectively.

Original languageEnglish (US)
Pages (from-to)i18
JournalActa Crystallographica Section E: Structure Reports Online
Issue number2
StatePublished - Feb 1 2012



  • R factor = 0.036
  • T = 293 K
  • data-to-parameter ratio = 15.4
  • disorder in main residue
  • mean σ(Si-O) = 0.002 Å
  • single-crystal X-ray study
  • wR factor = 0.085

ASJC Scopus subject areas

  • Chemistry(all)
  • Materials Science(all)
  • Condensed Matter Physics

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