Compressibilities along the magnetite–magnesioferrite solid solution

Abstract

To calculate the thermodynamic properties of recently discovered high-pressure mixed valence iron oxides in the system Fe–Mg–O, information on the equation of state of precursor inverse spinel phases along the magnetite–magnesioferrite join is needed. The existing equation of state data, particularly for magnesioferrite, are in poor agreement and no data exist for intermediate compositions. In this study, the compressibility of nearly pure magnesioferrite as well as of an intermediate \({{\mathrm{Mg}}_{0.5}}^{\vphantom{2+}}\mathrm{Fe}_{0.5}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{O}}_{4}^{\vphantom{2+}}\) sample have been investigated for the first time up to approximately 19 and 13 GPa, respectively, using single-crystal X-ray diffraction in a diamond anvil cell. Samples were produced in high-pressure synthesis experiments to promote a high level of cation ordering, with the obtained inversion parameters larger than 0.83. The room pressure unit cell volumes, V₀, and bulk moduli, K_T0, could be adequately constrained using a second-order Birch–Murnaghan equation of state, which yields V₀ = 588.97 (8) ų and K_T0 = 178.4 (5) GPa for magnesioferrite and V₀ = 590.21 (5) ų and K_T0 = 188.0 (6) GPa for the intermediate composition. As magnetite has K_T0 = 180 (1) GPa (Gatta et al. in Phys Chem Min 34:627–635, 2007. https://doi.org/10.1007/s00269-007-0177-3), this means the variation in K_T0 across the magnetite–magnesioferrite solid solution is significantly non-linear, in contrast to several other Fe–Mg spinels. The larger incompressibility of the intermediate composition compared to the two end-members may be a peculiarity of the magnetite–magnesioferrite solid solution caused by an interruption of Fe²⁺–Fe³⁺ electron hopping by Mg cations substituting in the octahedral site.