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Iron spin transition directly affects properties of lower mantle minerals andcan thus alter geophysical and geochemical characteristics of the deep Earth.While the spin transition in ferropericlase has been vigorously established atP ~ 60 GPa and 300 K, experimental evidence for spin transitions in otherrock-forming minerals, such as bridgmanite and post-perovskite, remainscontroversial. Multiple valence, spin, and coordination states of iron inbridgmanite and post-perovskite are difficult to resolve with conventionalspin-probing techniques. Optical spectroscopy, on the other hand, is sensitiveto high/low spin ferrous/ferric iron at different sites; thus, it can be apowerful probe for spin transitions. Here we establish the optical signature oflow spin Fe3+O6, a plausible low spin unit in bridgmanite and post-perovskite,by optical absorption experiments in diamond anvil cells. We show that theoptical absorption of Fe3+O6 in NAL (new aluminous phase) is very sensitive tothe iron spin state and represents a model behavior of bridgmanite andpost-perovskite in the deep lower mantle across a spin transition.Specifically, an absorption band centered at ~ 19000 cm-1 is characteristic ofthe 2T2g to 2T1g (2A2g) transition in low spin Fe3+ in NAL at 40 GPa. This newspectroscopic information constrains the crystal field splitting energy of lowspin Fe3+ to ~ 22200 cm-1 which we also independently confirm by ourfirst-principles calculations. Together with available information on theelectronic structure of Fe3+O6-compounds, we constrain the spin-pairing energyof Fe3+ in an octahedral field to ~ 20000-23000 cm-1. This implies thatoctahedrally-coordinated Fe3+ in bridgmanite is low spin at P > ~ 40 GPa.

Optical signatures of low spin Fe3+: a new probe for the spin state of bridgmanite and post-perovskite