OXIDATION POTENTIAL, NOT CRYSTAL STRUCTURE, CONTROLS THE OXIDATION STATE OF IRON IN PEROVSKITE

The mantle’s oxidation state has broad implications on the state and evolution of the earth’s interior. The relatively high oxidation potential of the upper mantle is such that iron is predominantly with small amounts of Fe³⁺. is more stable than Fe²⁺ in the dominant lower mantle mineral, perovskite, despite the fact that the effect of pressure is to reduce the oxidizing potential of a system. It is therefore suggested that iron undergoes a disproportionation reaction of 3Fe²⁺ =2Fe³⁺ +Fe⁰, controlled by the crystallography instead of oxidation potential. We crystallized synthetic enstatite glass with 5% Al₂O₃, 14% FeO, and 3% Fe₂O₃ in the laser-heated diamond anvil cell at 25-63 GPa and 1700-2800 K. We find that for temperatures <2200 K, the sample crystallizes to only perovskite, while at higher temperatures, the sample crystallizes to perovskite and stishovite as evident in x-ray diffraction, with 5-50 nm iron precipitates on grain boundaries. The precipitates have small amounts of dissolved oxygen, but are Mg- and Al- free. We interpret that the stishovite is forming due to the oxidation of the ferric iron to ferrous iron according to (Mg²⁺,Al³⁺)(Fe³⁺,Si⁴⁺) +SiO₂ + Fe⁰ while the lower-temperature samples crystallizing as approximately (Mg²⁺Fe²⁺Al³⁺)(Fe³⁺Al³⁺Si⁴⁺)O₃. We observe 2.8(2) ų volume expansion of the perovskite and a 28(2) GPa decrease in compressibility of the perovskite relative to the perovskite forming at lower temperature, consistent with the proposed compositions of the perovskites. As the increased temperature increases the oxidation potential of the system, we suggest that the oxidation state of iron in perovskite is dependent on oxidation potential as opposed to perovskite’s crystal structure. Transmission Electron Microscopy (TEM) coupled with Electron Energy Loss Spectroscopy (EELS) show iron precipitation on grain boundaries supporting the conclusion.