The periclase -wüstite stability field and the exsolution of coherent nano-crystalline spinel ferrite at one atmosphere

Abstract Natural and synthetic samples belonging to the periclase – wüstite system have been shown to contain nano-crystalline exsolutions of spinel ferrite (Fe3O4-MgFe2O4) solid solution. Understanding this exsolution process requires knowledge of the conditions under which it forms in ferric-iron-bearing periclase – wüstite samples. However, for MgO-rich compositions no direct measurements exist of the Fe3+/Fetot ratio of Fe-bearing periclase in equilibrium with spinel ferrite, even at 1 atmosphere. Such measurements would provide a valuable constraint for thermodynamic models describing the MgO–FeO–Fe2O3 system. In this study, Fe3+/Fetot ratios have been determined for periclase – wüstite samples synthesized from two starting materials, (Mg0.87Fe0.13)O and (Mg0.23Fe0.77)O, at 1300 °C in a gas-mixing furnace, at oxygen fugacities up to those where they coexists with macroscopically visible grains of spinel ferrite. A platinum-alloy redox sensor is used to determine the fO2 of each sample. All recovered samples with Fe3+/Fetot ratios > 0.07 contain topotaxial exsolutions of nano-crystalline spinel ferrite, with grain sizes that reach ∼100 nm in more Fe-rich samples but decrease to ∼ 5 nm in samples quenched more rapidly into water. The coherent exsolution of spinel ferrite from Fe-bearing periclase induces lattice strain in both phases, resulting in a smaller unit-cell parameter for Fe-bearing periclase and a larger one for spinel ferrite, than otherwise expected. In Mössbauer spectra, the exsolution appears as sextets showing relaxation, with magnetic hyperfine fields that shift towards lower values and then collapse as the grain size decreases. The initial high-temperature ferric iron content in Fe-bearing periclase obtained by combining the contribution from the spinel ferrite exsolution with the amount of Fe3⁺ remaining in the (Mg,Fe)O, was used to fit a thermodynamic model describing the periclase - wüstite stability field as a function of fO2 in the MgO–FeO–Fe2O3 system. The model reproduces the measurements well, although some deviations remain towards high-MgO compositions at lower fO2. This highlights the need for more sophisticated models that account for differences in Fe3⁺ coordination in (Mg,Fe)O. Microstructures of spinel ferrite exsolutions in Fe-bearing periclase inclusions in natural diamonds are very similar to those observed in this study. The thermodynamic model supports the hypothesis that the exsolution occurs in these (Mg,Fe)O samples as they cool after emplacement near the surface, to temperatures < 1000 °C.