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

Titelangaben

Criniti, Giacomo ; Boffa Ballaran, Tiziana ; Kurnosov, Alexander ; Ishii, Takayuki ; Rogmann, Elena-Marie ; Glazyrin, Konstantin ; Fedotenko, Timofey ; Frost, Daniel J.:
Effect of chemistry on the compressibility and high-pressure structural evolution of the CaFe₂O₄-type aluminous silicate phase.
In: Physics of the Earth and Planetary Interiors. Bd. 361 (2025) . - 107331.
ISSN 1872-7395
DOI: https://doi.org/10.1016/j.pepi.2025.107331

Volltext

Link zum Volltext (externe URL):

Angaben zu Projekten

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 Na0.90(1)Al1.03(2)Si1.00(2)O4 (NaCF), Na0.66(4)Mg0.28(4)Al1.22(3)Si0.78(3)O4 (MgCF) and Na0.62(2)Mg0.19(1)Fe2+0.17(1)Fe3+0.080(4)Al1.20(3)Si0.70(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 (KT0) 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.