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We report in situ structural measurements of shock‐compressed single crystal orthoenstatite up to 337 ± 55 GPa on the Hugoniot, obtained by coupling ultrafast X‐ray diffraction to laser‐driven shock compression. Shock compression induces a disordering of the crystalline structure evidenced by the appearance of a diffuse X‐ray diffraction signal at nanosecond timescales at 80 ± 13 GPa on the Hugoniot, well below the equilibrium melting pressure (>170 GPa). The formation of bridgmanite and post‐perovskite have been indirectly reported in microsecond‐scale plate‐impact experiments. Therefore, we interpret the high‐pressure disordered state we observed at nanosecond scale as an intermediate structure from which bridgmanite and post‐perovskite crystallize at longer timescales. This evidence of a disordered structure of MgSiO3 on the Hugoniot indicates that the degree of polymerization of silicates is a key parameter to constrain the actual thermodynamics of shocks in natural environments. Plain Language Summary: The study of silicate materials at extreme pressures and temperatures provides insight on the evolution of planetary bodies evolution during solar system formation. During their accretion, rocky bodies have undergone several collisions and possibly planetary impacts that have transformed their minerals. The microscopic processes occurring during such events are not fully understood. In this study, we used high‐power lasers to generate shock waves into MgSiO3 enstatite crystals, creating conditions comparable to the deepest part of the early Earth mantle and large planetary impacts. During the shock wave transit, within few nanoseconds, we probed the structure of shocked enstatite between 14 ± 3 and 337 ± 55 GPa using intense X‐ray pulses from Linac Coherent Light Source X‐ray free electron laser facility. We found that, when shocked between 80 ± 13 GPa and the conditions of equilibrium melting (above 170 GPa), enstatite transforms into a disordered structure instead of forming bridgmanite or post‐perovskite—the expected equilibrium phases. This disordered structure is similar to MgSiO3 glass or liquid and is observed up to 337 ± 55 GPa. This study provides the first direct measurement of shocked enstatite structure and suggests that the observed disordered state is an intermediate phase on the transformation pathway of bridgmanite in natural impacts. Key Points: We performed ultrafast X‐ray diffraction of orthoenstatite crystals shock compressed up to 337 ± 55 GPa on the HugoniotA disordered structure was observed from 80 ± 13 GPa along the Hugoniot, well below the melting temperatureThe degree of polymerization strongly influences the kinetics of recrystallization in shocked silicates [ABSTRACT FROM AUTHOR]

Recent experiments have demonstrated the existence of previously unknown iron oxides at high pressure and temperature including newly discovered pyrite‐type FeO2 and FeO2Hx phases stable at deep terrestrial lower mantle pressures and temperatures. In the present study, we probed the iron oxidation state in high‐pressure transformation products of Fe3+OOH goethite by in situ X‐ray absorption spectroscopy in laser‐heated diamond‐anvil cell. At pressures and temperatures of ~91 GPa and 1,500–2,350 K, respectively, that is, in the previously reported stability field of FeO2Hx, a measured shift of −3.3 ± 0.1 eV of the Fe K‐edge demonstrates that iron has turned from Fe3+ to Fe2+. We interpret this reductive valence change of iron by a concomitant oxidation of oxygen atoms from O2− to O−, in agreement with previous suggestions based on the structures of pyrite‐type FeO2 and FeO2Hx phases. Such peculiar chemistry could drastically change our view of crystal chemistry in deep planetary interiors. Plain Language Summary: Iron oxides are important end‐members of the complex materials that constitute the Earth's interior. Among them, FeO and Fe2O3 have long been considered as the main end‐members of the ferrous (Fe2+) and ferric (Fe3+) states of iron, respectively. All geochemical models assume that high oxygen concentrations are systematically associated to the formation of ferric iron in minerals. The recent discovery of O22− peroxide ions in a phase of chemical formula FeO2Hx stable under high‐pressure and high‐temperature conditions challenges this general concept. However, up to now, the valences of iron and oxygen in FeO2Hx have only been indirectly inferred from a structural analogy with pyrite FeS2. Here we compressed goethite (FeOOH), an Fe3+‐bearing mineral, at lower mantle pressure and temperature conditions by using laser‐heated diamond‐anvil cells, and we probed the iron oxidation state upon transformation of FeOOH in the pressure–temperature stability field of FeO2Hx using in situ X‐ray absorption spectroscopy. The data demonstrate that upon this transformation iron has transformed into ferrous Fe2+. Such reduced iron despite high oxygen concentrations suggests that our current views of oxidized and reduced species in the lower mantle of the Earth should be reconsidered. Key Points: Ferrous iron is evidenced in coexistence with high oxygen concentration modifying our understanding of deep Earth geochemistryIn situ X‐ray absorption spectroscopy shows the reduction of iron into Fe2+ when FeOOH transforms into the pyrite‐structured phase FeO2HxSubduction of FeOOH‐type hydrated iron oxides releases zero‐valent hydrogen (H2) instead of mono‐valent hydrogen (H2O) in the lower mantle [ABSTRACT FROM AUTHOR]

There is a direct relation between the composition of a host star and that of the planets orbiting around it. As such, the recent discovery of stars with unusual chemical composition, notably enriched in carbon instead of oxygen, supports the existence of exoplanets with a chemistry dominated by carbides instead of oxides. Accordingly, several studies have been recently conducted on the Si–C binary system at high pressure and temperature. Nonetheless, the properties of carbides at the pressure‐temperature conditions of exoplanets interiors are still inadequately constrained, effectively hampering reliable planetary modeling. Here we present an in situ X‐ray diffraction study of the Si–C binary system up to 200 GPa and 3,500 K, significantly enlarging the pressure range explored by previous experimental studies. The large amount of collected data allows us to properly investigate the phase diagram and to refine the Clapeyron slope of the transition line from the zinc blende to the rock salt structure. Furthermore, the pressure‐volume‐temperature equation of state is provided for the high‐pressure phase, characterized by low compressibility and thermal expansion. Our results are used to model idealized C‐rich exoplanets of end‐members composition. In particular, we derived mass‐radius relations and performed numerical simulations defining rheological parameters and initial conditions which lead to onset of convection in such SiC planets. We demonstrate that if restrained to silicate‐rich mantle compositions, the interpretation of mass‐radius relations may underestimate the interior diversity of exoplanets. Key Points: We experimentally determined the SiC thermal equation of state up to 200 GPa and 3,500 KWe constrain the SiC B3‐B1 phase transition and expand the Si‐C binary phase diagram mappingCarbon‐enriched planets can have the same mass‐radius relationship as Earth's‐like planets but different dynamical behavior [ABSTRACT FROM AUTHOR]