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

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.

Pressure Destabilizes Oxygen Vacancies in Bridgmanite

“…The MgFe 2 O 4 ‐phase was previously assigned to be a CaMn 2 O 4 ‐type structure (Andrault & Bolfan‐Casanova, 2001; Fei et al., 2020) or CaTi 2 O 4 ‐type structure (Greenberg et al., 2017), and recently suggested to be a new structure (modified Na‐Fe‐Ti oxide‐type) of post‐spinel (Ishii et al., 2020). Our study primarily focused on the bridgmanite phase without considering the structural complexities of the MgFe 2 O 4 ‐phase.…”

“…The reaction of Fe 3+ between bridgmanite and MgFe 2 O 4 ‐phase can be written as: $$${\text{MgFe}}_{2}{\mathrm{O}}_{4}\enspace ({\text{MgFe}}_{2}{\mathrm{O}}_{4}\mbox{-}\text{phase})+{\text{FeFeO}}_{3}\enspace (\text{bridgmanite})=\text{MgO}\enspace (\text{periclase})$$$ The FeFeO 3 component has a molar volume of 29.55 cm 3 /mol (Huang, Boffa‐Ballaran, McCammon, Miyajima, & Frost, 2021). Although the volume change of the above reaction is negative at ambient conditions ( ΔV = −0.6 cm 3 /mol), it becomes positive (+0.5 to +0.8 cm 3 /mol) after adjusting to 27–40 GPa using the equation of state for each phase (Dorogokupets, 2010; Ishii et al., 2020; Tange et al., 2012). As a result, the FeFeO 3 content in bridgmanite decreases with pressure.…”

“…The suppression of Fe 3+ -linked oxygen vacancies with increasing pressure can be understood by the volume increase associated with MgFeO 2.5 formation. Partial molar volume of the MgFeO 2.5 component in bridgmanite is estimated to be 27.65 cm 3 /mol , whereas the molar volume of the MgFe 2 O 4 -phase is 41.4 cm 3 /mol (Ishii et al, 2020), and that of MgO is 11.25 cm 3 /mol at ambient conditions (Dorogokupets, 2010;Tange et al, 2012). Thus, the volume change in the reaction,…”

“…(2) is about +2.7 cm 3 /mol at ambient conditions. This volume change will be even larger (4.3-4.7 cm 3 /mol) by adjusting the pressure to 27-40 GPa using reported equation of states (Dorogokupets, 2010;Ishii et al, 2020;Tange et al, 2012), because the MgFe 2 O 4 -phase has a much smaller bulk modulus (164 GPa) than bridgmanite (257 GPa) (Ishii et al, 2020;Tange et al, 2012).…”

“…The FeFeO 3 component has a molar volume of 29.55 cm 3 /mol . Although the volume change of the above reaction is negative at ambient conditions (ΔV = −0.6 cm 3 /mol), it becomes positive (+0.5 to +0.8 cm 3 /mol) after adjusting to 27-40 GPa using the equation of state for each phase (Dorogokupets, 2010;Ishii et al, 2020;Tange et al, 2012). As a result, the FeFeO 3 content in bridgmanite decreases with pressure.…”