Discovery of New‐Structured Post‐Spinel MgFe<sub>2</sub>O<sub>4</sub>: Crystal Structure and High‐Pressure Phase Relations
Takayuki Ishii, Nοbuyοshi Miyajima, Ryosuke Sinmyo, Hiroshi Kojitani, Daisuke Mori, Yoshiyuki Inaguma, Masaki Akaogi
Abstract
Abstract Phase relations in MgFe 2 O 4 at 20–27 GPa and 1,000–1,200 °C were investigated, and a novel post‐spinel phase of MgFe 2 O 4 (space group, Pnma ) was identified. This phase has a Z‐shape framework of edge‐sharing (Mg,Fe)O 6 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 Na x (Fe,Ti) x Ti 2‐x O 4 phase (0.75 ≤ x ≤ 0.9) with (Fe,Ti)O 6 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 Na x (Fe,Ti) x Ti 2‐x O 4 ‐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.