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The production of metal via the iron disproportionation reaction in the deep Earth has been a long debated topic with important implications for the geochemistry of the lower mantle. To explore the occurrence of the iron disproportionation reaction from 25 to 65 GPa, a natural almandine‐pyrope‐grossular garnet was studied with in situ X‐ray diffraction (XRD) in the laser‐heated diamond anvil cell and ex situ scanning electron microscopy (SEM) techniques. Upon heating the natural almandine‐pyrope‐grossular garnet up to 3000 K up to 65 GPa, the formation of phase assemblage consisting of bridgmanite, stishovite, and davemaoite was confirmed by XRD, but because of the low abundance of Fe metal and small grain size, XRD was determined not to be effective in detecting the disproportionation reaction. Examination of the samples recovered between 39 and 64 GPa by SEM analysis revealed the presence of nm‐scale disproportionated iron metal grains as an additional product of this reaction that was not detectable in the XRD patterns. Volume compression data of bridgmanite synthesized in the experiments were fit to the Birch‐Murnaghan equation of state and compared to similar compositions. Bridgmanite was found to decompress to the LiNbO₃‐type structure, indicating a high FeAlO₃content, in accordance with the occurrence of a disproportionation reaction. The experimental confirmation of disproportionated metallic Fe has significant implications for the distribution of siderophile and volatile elements in the lower mantle.

Super‐Earths ranging up to 10 Earth masses (M_E) with Earth‐like density are common among the observed exoplanets thus far, but their measured masses and radii do not uniquely elucidate their internal structure. Exploring the phase transitions in the Mg‐silicates that define the mantle‐structure of super‐Earths is critical to characterizing their interiors, yet the relevant terapascal conditions are experimentally challenging for direct structural analysis. Here we investigated the crystal chemistry of Fe₃O₄as a low‐pressure analog to Mg₂SiO₄between 45–115 GPa and up to 3000 K using powder and single crystal X‐ray diffraction in the laser‐heated diamond anvil cell. Between 60–115 GPa and above 2000 K, Fe₃O₄adopts an 8‐fold coordinated Th₃P₄‐type structure (I‐43d,Z = 4) with disordered Fe²⁺and Fe³⁺into one metal site. This Fe‐oxide phase is isostructural with that predicted for Mg₂SiO₄above 500 GPa in super‐Earth mantles and suggests that Mg₂SiO₄can incorporate both ferric and ferrous iron at these conditions. The pressure‐volume behavior observed in this 8‐fold coordinated Fe₃O₄indicates a maximum 4% density increase across the 6‐ to 8‐fold coordination transition in the analog Mg‐silicate. Reassessment of the FeO—Fe₃O₄fugacity buffer considering the Fe₃O₄phase relationships identified in this study reveals that increasing pressure and temperature to 120 GPa and 3000 K in Earth and planetary mantles drives iron toward oxidation.