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  1. Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite (FA50) with the highest Fe3+‐Al3+ coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions the FA50 adopted the LiNbO3 structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yields V 0 = 172.1(4) Å3, K 0 = 229(4) GPa with K 0′ = 4(fixed). The calculated bulk sound velocitymore »of the FA50 bridgmanite is ~7.7% lower than MgSiO3 bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe3+‐Al3+ substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing ~6% of Fe3+ cations exchanged with Al3+ and underwent the high‐spin to low‐spin transition at 59 GPa. The low‐spin proportion of Fe3+ increased gradually with pressure and reached 17‐31% at 80 GPa. Since the cation exchange and spin transition in this Fe3+‐Al3+‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe3+ fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.« less
  2. It is believed that the core formation processes sequestered a large majority of Earth’s carbon into its metallic core. Incorporation of carbon to liquid iron may significantly influence its properties under physicochemical conditions pertinent to the deep magma ocean and thus the chemical evolution of terrestrial planets and moons. Compared to available experimental data on the physical properties of crystalline iron alloys under pressure, there is a remarkable lack of data on the properties of liquid iron‐rich alloys, due to experimental challenges. Here we review experimental and computational results on the structure and properties of iron or iron‐nickel liquids alloyedmore »with carbon upon compression. These laboratory data provide an important foundation on which the interpretation of ultrahigh pressure laboratory data and the verification of theoretical data will have to be based. The low‐pressure data can be used to validate results from theoretical calculations at the same conditions, and high‐pressure calculations can be used to estimate and predict liquid properties under core conditions. Availability of the liquid properties of Fe‐C liquids will provide essential data for stringent tests of carbon‐rich core composition models for the outer core.« less