Equation of State and Hyperfine Parameters of High-Spin Bridgmanite in the Earth's Lower Mantle

Bridgmanite [(Mg,Fe)(Fe,Al,Si)O3] is one of the most abundant minerals in the Earth's lower mantle. Studies on the combined effect of Fe and Al on the structure and equation of state parameters are important to understand the composition of the lower mantle. In particular, Fe in lower-mantle bridgmanite has been reported to undergo spin pairing transitions which could affect a number of physical properties of bridgmanite. Although high-pressure theoretical and experimental studies have been in agreement for the spin state of Fe3+ in the lower-mantle bridgmanite, the spin state of A-site Fe2+ remains controversial. In this study, we have performed high-pressure synchrotron X-ray diffraction (XRD) and Mӧssbauer spectroscopy (SMS) measurements on two single-crystal bridgmanite samples (Mg0.95Fe0.06Si0.99O3 and Mg0.89Fe0.12Al0.11Si0.89O3) up to 130 GPa and 300 K to investigate the combined effect of Fe and Al on the structure and equation of state parameters of bridgmanite. The SMS results have shown that Fe2+ and Fe3+ in both our Al-free and Al-bearing bridgmanite are predominantly located in the A-site at lower-mantle pressures. The A-site Fe3+ in both compositions remains in the HS state at lower-mantle pressures. In particular, we observed a drastic increase in QS of the A-site Fe2+ in Al-free bridgmanite between 13 and 32 GPa. Together with recent synchrotron X-ray emission spectroscopy results, we support the notion that the occurrence of the extremely high QS component of approximately 4 mm/s in Al-free bridgmanite is due to the enhanced lattice distortion of the high-spin A-site Fe2+. Combining the SMS results with high quality XRD data allows to correlate the the extremely high-quadrupole parameter of the A-site Fe2+ with the anomalous variations in the bond length, tilting angle, and shear strain in the local octahedra of Al-free bridgmanite. The high-quality XRD results also provide crucial constraints on the effect Fe and Al on the equation of state parameters of bridgmanite at lower-mantle pressures.