Non-stoichiometry and local order in minerals with the rockbridgeite structure

Ian Edward Grey, Tim Williams, Anthony R. Kampf, John D. Cashion, Yesim Gozukara, Colin M. Macrae, Erich Keck

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2 Citations (Scopus)

Abstract

A non-stoichiometric frondelite from the Hagendorf Süd pegmatite, Bavaria, has been studied using chemical and thermal analyses, electron diffraction (ED), Mössbauer spectroscopy and synchrotron single-crystal structure analysis. The structural formula, with occupancies of the M1, M2 and M3 sites given in sequence, is (Fe3 +)(Mn2 + 0.67Zn0.20Fe2 + 0.08Fe3 + 0.70)S 1.65(Fe3 + 2.30) (PO4)3(OH)4.9(H2O)0.1. The non-stoichiometric frondelite contains an excess of 0.3 atoms in the M3 site and a deficiency of 0.35 atoms in the M2 site. The ED patterns show rows of reflections that violate the reflection extinctions for Bbmm, and which are due to a local ordering of Fe3 þ and vacancies in the M3 sites. Based on the ED results, a model was developed in space group Pnma, in which planes containing corner-shared octahedral dimers of M3 atoms are ordered on a scale of ∼6 nm. The Pnma model was refined successfully against the synchrotron single-crystal data. Non-stoichiometry can be explained within the Pnma local-order model by distortions at the M3 sites in non-stoichiometric regions, where local, barbosalite-like, trimeric chains of face-sharing octahedra are formed.

Original languageEnglish
Pages (from-to)773-783
Number of pages11
JournalEuropean Journal of Mineralogy
Volume30
Issue number4
DOIs
Publication statusPublished - 2018

Keywords

  • Crystal structure
  • Electron diffraction
  • Frondelite
  • Local order
  • Mössbauer spectra
  • Non-stoichiometry
  • Thermal analyses

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