Discovery of New-Structured Post-Spinel MgFe2O4: Crystal Structure and High-Pressure Phase Relations

Takayuki Ishii, Nobuyoshi Miyajima, Ryosuke Sinmyo, Hiroshi Kojitani, Daisuke Mori, Yoshiyuki Inaguma, Masaki Akaogi

Research output: Contribution to journalArticle

Abstract

Phase relations in MgFe2O4 at 20–27 GPa and 1,000–1,200 °C were investigated, and a novel post-spinel phase of MgFe2O4 (space group, Pnma) was identified. This phase has a Z-shape framework of edge-sharing (Mg,Fe)O6 octahedra, with other Mg and Fe cations randomly occupying one tetrahedral and two octahedral sites in tunnel spaces within the framework. This structure is similar to Nax (Fe,Ti)xTi2-xO4 phase (0.75 ≤ x ≤ 0.9) with (Fe,Ti)O6 Z-shape framework and tricapped-prism sites for Na. Compression data showed ~6% volume reduction at 16–20 GPa. The recovered phase may have transformed during decompression from the Nax (Fe,Ti)xTi2-xO4-type structure due to cations in the tunnel spaces shifting, probably because of cation-size misfit. The novel phase may be found in impact craters and shocked meteorites and may serve as an indicator of shock P-T conditions. Without crystal structure refinement, post-spinel phases are likely to be misidentified due to similar crystallographic features.

Original languageEnglish
Article numbere2020GL087490
JournalGeophysical Research Letters
Volume47
Issue number6
DOIs
Publication statusPublished - 28 Mar 2020

Keywords

  • Rietveld analysis
  • high pressure
  • magnesioferrite
  • mantle
  • phase transition
  • spinel

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