Discovery of New‐Structured Post‐Spinel MgFe2O4: Crystal Structure and High‐Pressure Phase Relations. Issue 6 (13th March 2020)

• Record Type: Journal Article
• Authors: Ishii, Takayuki; Miyajima, Nobuyoshi; Sinmyo, Ryosuke; Kojitani, Hiroshi; Mori, Daisuke; Inaguma, Yoshiyuki; Akaogi, Masaki
• Is Part Of: Geophysical research letters. Volume 47:Issue 6(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-03-13
• Subjects: high pressure -- Rietveld analysis -- phase transition -- spinel -- mantle -- magnesioferrite; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL087490
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Phase relations in MgFe2 O4 at 20–27 GPa and 1, 000–1, 200 °C were investigated, and a novel post‐spinel phase of MgFe2 O4 (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)x Ti2‐x O4 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)x Ti2‐x O4 ‐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 MgFe2 O4 and its high‐pressure phase are found in diamonds and shocked meteorites. During the past two decades, high‐pressure forms of spinel‐type MgFe2 O4 have been intensively investigated, but uncertainty about their stability fields and crystal structures persists. We examined the phase relations in a MgFe2 O4 system up to 27 GPa, which is representative of the uppermost lower mantle pressures, at 1, 000–1, 200 °C, and synthesized a previously unknown MgFe2 O4 phase. So far, CaFe2 O4 ‐, CaTi2 O4 ‐, and CaMn2 O4 ‐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 MgFe2 O4 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. Key Points: We investigated phase relations in MgFe2 O4 and found a new MgFe2 O4 high‐pressure phase that is stable at 21–27 GPa and 1, 000–1, 200 °C The new phase has features similar to conventional post‐spinel structures, but it is a novel type that can be formed in shock events We stress the risk of misidentification of post‐spinel phases in the absence of structure analysis due to their crystallographic similarity