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1 Department of Geology, University of Illinois at Urbana-Champaign, Illinois 61801, U.S.A.
2 Advanced Photon Source, Argonne National Laboratory, Illinois 60439, U.S.A.
3 Consortium for Advanced Radiation Sources, University of Chicago, Illinois 60637, U.S.A.
4 HP-CAT, Advanced Photon Source, Argonne National Laboratory, Illinois 60439, U.S.A.
5 Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, D.C. 20015, U.S.A.
Correspondence: * E-mail: jmjackso{at}uiuc.edu
The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe0.05Mg0.95SiO3 (Pv05) and Fe0.1Mg0.9SiO3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mössbauer spectroscopy (SMS). Such investigations of extremely small and dilute 57Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the "three-doublet" model, which assumes two Fe2+-like sites and one Fe3+-like site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe3+/
Fe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fe3+ remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe3+ from a high-spin to a low-spin state.
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