Abstract

Optical and near infrared absorption spectra of ferropericlase Mg0.88Fe0.12O have been measured to 84 GPa. Under ambient conditions, the spectrum shows two crystal field bands of high-spin Fe2+ at 8922 and 12 533 cm−1, which shift to higher frequencies with increasing pressure (dν/dP = 50.7 and 85.5 cm−1/GPa). Simultaneously, the intensity of the high-frequency band continuously decreases until it vanishes around 40 GPa, suggesting a quenching of the Jahn-Teller effect. Between 51 and 60 GPa, the absorption spectrum changes drastically. Two new bands appear at 60 GPa at 9728 and 14 592 cm−1 with frequency shifts at higher pressures of dν/dP = 23.8 and 21.0 cm−1/GPa, respectively. If the change in optical spectra between 51 and 60 GPa were interpreted as being due to spin-pairing, the crystal field parameters of low-spin Fe2+ at 60 GPa would be Δ = 10 546 cm−1 and B = 377 cm−1. This would imply that the main cause of spin-pairing is not the increase in crystal field splitting Δ, but the stronger covalency of the Fe-O bond as seen in the reduction of the Racah parameter B. Even at 84 GPa, ferropericlase is by no means opaque. In particular, the inferred spin-pairing transition between 51 and 60 GPa reduces radiative thermal conductivity only by about 15%. Spin-pairing in ferropericlase is therefore unlikely to have major consequences for the temperature distribution or the mode of convection in the lower mantle. The absorption edge of the high-pressure phase appears to be deeper in the UV than for the low-pressure phase, which could imply a reduced electrical (polaron) conductivity.

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