Effect of chemistry on the compressibility and high-pressure structural evolution of the CaFe2O4-type aluminous silicate phase

Abstract

Approximately 22–26 vol% of a basaltic phase assemblage at lower mantle conditions is comprised of a (Na,Mg,Fe²⁺)(Al,Si,Fe³⁺)₂O₄ phase with CaFe₂O₄-type (CF-type) structure. Previous experimental studies attempted to determine the equation of state of the CF-type phase but reported contrasting compressibility values, even for samples with the same composition. Therefore, the elastic properties of the CF-type phase remain, to date, largely unconstrained. Here, we conducted single-crystal X-ray diffraction (SCXRD) measurements in the diamond anvil cell (DAC) at high pressure and room temperature on three samples of CF-type phase with compositions Na_0.90(1)Al_1.03(2)Si_1.00(2)O₄ (NaCF), Na_0.66(4)Mg_0.28(4)Al_1.22(3)Si_0.78(3)O₄ (MgCF) and Na_0.62(2)Mg_0.19(1)Fe²⁺_0.17(1)Fe³⁺_0.080(4)Al_1.20(3)Si_0.70(1)O₄ (FeCF). A multi-sample loading approach was employed for most DAC runs, where two samples were loaded in the same sample chamber to reduce possible systematic deviations between datasets, thus enhancing internal consistency and corroborating data reproducibility. Experiments on the NaCF and MgCF samples were conducted up to ∼50 GPa, while the FeCF sample was compressed to ∼72 GPa to better characterize the effect of the spin crossover of octahedrally coordinated Fe³⁺. We found the isothermal bulk modulus (K_T0) to increase with decreasing NaAlSiO₄ content, accompanied by only a slight decrease in its pressure derivative (K'_T0). Analysis of the crystal structures of the three samples at high pressure allowed compositional trends to be determined also for the interatomic bonds and polyhedral compressibility, as well as the distortion indices. These suggest an overall stiffening of the A site with increasing Mg²⁺ and Fe²⁺ content, as well of the two B sites with increasing Al³⁺ and Fe³⁺ content. Enhanced compressibility of the unit cell and octahedral B sites was observed between ∼26–42 GPa in the FeCF sample, suggesting a pressure-induced spin crossover of Fe³⁺, in agreement with some previous observations. Finally, trends in the elastic properties from experimental studies conducted along the NaAlSiO₄-MgAl₂O₄ join are discussed and used as a proxy to evaluate the reliability of end-member properties for the CF-type phase employed in most recent mineral physical and thermodynamic databases. Our analysis suggests current mineral physical models might underestimate densities and overestimate bulk sound velocities of NaAlSiO₄-rich CF-type phases with basaltic composition.