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

Approximately 22–26 vol% of a basaltic phase assemblage at lower mantle conditions is comprised of a (Na,Mg,Fe 2+ )(Al,Si,Fe 3+ ) 2 O 4 phase with CaFe 2 O 4 -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) O 4 (NaCF), Na 0.66(4) Mg 0.28(4) Al 1.22(3) Si 0.78(3) O 4 (MgCF) and Na 0.62(2) Mg 0.19(1) Fe 2+ 0.17(1) Fe 3+ 0.080(4) Al 1.20(3) Si 0.70(1) O 4 (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 Fe 3+ . We found the isothermal bulk modulus ( K T0 ) to increase with decreasing NaAlSiO 4 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 Mg 2+ and Fe 2+ content, as well of the two B sites with increasing Al 3+ and Fe 3+ 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 Fe 3+ , in agreement with some previous observations. Finally, trends in the elastic properties from experimental studies conducted along the NaAlSiO 4 -MgAl 2 O 4 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 NaAlSiO 4 -rich CF-type phases with basaltic composition. • We determined the isothermal equation of state of three CF-type solid solutions. • Structural refinements reveal how cation substitutions affect bulk properties. • The Fe 3+ spin crossover was detected from anomalous octahedral compressibility. • Recent mineral physics models may overestimate the bulk modulus of CF-type phases.