Optical absorption spectra of ferropericlase to 84 GPa

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 Fe 21 at 8922 and 12533 cm(-1), which shift to higher frequencies with increasing pressure (dv/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 14592 cm(-1) with frequency shifts at higher pressures of dv/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 Delta = 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 Delta, 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.