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Phase relations in MgFe2O4 at 20-27 GPa and 1,000-1,200 degrees 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)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)Ti2-xO4 phase (0.75 <= x <= 0.9) with (Fe,Ti)O-6 Z-shape framework and tricapped-prism sites for Na. Compression data showed similar to 6% volume reduction at 16-20 GPa. The recovered phase may have transformed during decompression from the Na-x (Fe,Ti)(x)Ti2-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.

Plain Language Summary The high-pressure behavior of spinel-structured minerals is an important issue in Earth science to understand physics and chemistry of the mantle, particularly its redox state. Spinel-type MgFe2O4 and its high-pressure phase are found in diamonds and shocked meteorites. During the past two decades, high-pressure forms of spinel-type MgFe2O4 have been intensively investigated, but uncertainty about their stability fields and crystal structures persists. We examined the phase relations in a MgFe2O4 system up to 27 GPa, which is representative of the uppermost lower mantle pressures, at 1,000-1,200 degrees C, and synthesized a previously unknown MgFe2O4 phase. So far, CaFe2O4-, CaTi2O4-, and CaMn2O4-type structures have served to define as the high-pressure spinel-type structures. Structural analysis of the novel compound revealed that it has features similar to those of conventional high-pressure structures, but its cation arrangement is very different. Based on known phase relations, this phase may occur in impact craters and shocked meteorites and possibly plays an important role as a P-T indicator for shock events. The finding of the novel MgFe2O4 phase highlights the necessity of careful phase identification of high-pressure phases of spinel-type structures by crystal structure refinement in any experimental products and natural samples due to their crystallographic similarity.