Phase transformation of ferric-iron-rich silicate in Earth’s mid-mantle Available to Purchase

The incorporation of ferric iron in mantle silicates stabilizes different crystal structures and changes phase transition conditions, thus impacting seismic wave speeds and discontinuities. Recent experiments of MgSiO₃-Fe₂O₃ mixtures indicate the coexistence of fully oxidized iron-rich $(Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3$ with Fe-poor silicate (wadsleyite or bridgmanite) and stishovite at 15 to 27 GPa and 1773 to 2000 K, conditions relevant to subducted lithosphere in the Earth’s transition zone and uppermost lower mantle. X-ray diffraction (XRD) shows that $(Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3$ recovered from these conditions adopts the R3c LiNbO₃-type structure, which transforms to the bridgmanite structure again between 18.3 and 24.7 GPa at 300 K. XRD data are used to obtain the equation of state of the LiNbO₃-type phase up to 18.3 GPa. Combined with multi-anvil experiments, these observations suggest that the stable phase of $(Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3$ is bridgmanite at 15–27 GPa, which transforms on decompression to LiNbO₃-type structure. Our calculation revealed that ordering of the ferric ion reduces the kinetic energy barrier of the transition between $(Mg0.5Fe0.53+)(Fe0.53+Si0.5)O3$ LiNbO₃ structure and bridgmanite relative to the MgSiO₃ akimotoite-bridgmanite system. A dense Fe³⁺-rich bridgmanite structure is thus stable at substantially shallower depths than MgSiO₃ bridgmanite and would promote subduction.