Phase relations of Fe-Mg spinels including new high-pressure post-spinel phases and implications for natural samples

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

“…These results are consistent with those reported by Uenver-Thiele et al (2017b). We found an orthorhombic MgFe 2 O 4 phase with lattice parameters of a = 10.1179(2) Å, b = 2.97638(5) Å, and c = 9.7472(2) Å (Pnma) (see below and Table S2) and/or a temperature decrease, as already discussed in Uenver-Thiele et al (2017b). The molar volume of the Pnma-MgFe 2 O 4 is much larger than those of the other phases (or assemblages) with a MgFe 2 O 4 bulk composition, except for the Sp-type phase (Table S3).…”

“…Solid diamond, orthorhombic MgFe 2 O 4 ; solid square, Mg 3 Fe 4 O 9 + Fe 2 O 3 ; solid circle, MgO + Fe 2 O 3 ; half‐closed diamond, orthorhombic MgFe 2 O 4 phase + Mg 3 Fe 4 O 9 + Fe 2 O 3 . Gray lines are phase boundaries determined by Uenver‐Thiele et al (2017b). Red lines are phase boundaries constrained by this study.…”

“…Hereafter, we refer to Mg 2 Fe 2 O 5 and Mg 3 Fe 4 O 9 as Cmcm ‐Mg 2 Fe 2 O 5 and C 2/ m ‐Mg 3 Fe 4 O 9 , respectively. Uenver‐Thiele et al (2017a, 2017b) also found an orthorhombic MgFe 2 O 4 phase (space group, Pnma ) at 20–23 GPa and 1,300–1,500 °C; however, structural analysis of this phase has not yet been conducted. Andrault and Bolfan‐Casanova (2001) synthesized an orthorhombic MgFe 2 O 4 phase at ~20 GPa and 2,200 °C with a LH‐DAC and analyzed its structure by the Rietveld method under high pressure.…”

“…High-pressure and high-temperature phase relations in MgFe 2 O 4 have been widely investigated in multianvil press (MAP) and laser-heated diamond anvil cell (LH-DAC) experiments (Andrault & Bolfan-Casanova, 2001;Levy et al, 2004;Uenver-Thiele et al, 2017a, 2017bWang et al, 2002). Recent studies of Uenver-Thiele et al (2017a, 2017b Andrault and Bolfan-Casanova (2001) synthesized an orthorhombic MgFe 2 O 4 phase at~20 GPa and 2,200°C with a LH-DAC and analyzed its structure by the Rietveld method under high pressure. They suggested that it was the CM type (Pbcm), but in spite of the Jahn-Teller inactive Fe 3+ , the Fe 3+ O 6 octahedra in the reported structure were largely deformed, which is not realistic.…”

Synthesis and crystal structure of Mg-bearing Fe9O11: New insight in the complexity of Fe-Mg oxides at conditions of the deep upper mantle

“…Although complete Fe 2+ -Mg solid solution remains to be demonstrated, analogy with the structurally related phases (Mg,Fe 2+ ) 2 Fe 3+ 2 O 5 (Lavina et al 2011;Boffa Ballaran et al 2015;Uenver-Thiele et al 2018) and (Mg,Fe 2+ ) 3 Fe 3+ 4 O 9 (Sinmyo et al 2016;Uenver-Thiele et al 2017b) suggests that this may be the case for Fe 9 O 11 as well. Myhill et al (2016) calculated that Fe 4 O 5 (Fe 3+ /ΣFe = 0.5) is stable at oxygen fugacities above those at which diamond is stable, but that the redox stability of Fe 5 O 6 (Fe 3+ /ΣFe = 0.4) overlaps with that of diamond.…”