Intermediate-spin ferrous iron in lowermost mantle post-perovskite and perovskite

Iron-bearing silicate post-perovskite and perovskite are believed to be the dominant minerals of the lowermost mantle and the lower mantle, respectively. The electronic spin state of iron-a quantum property of every electron associated with its angular momentum-can strongly influence the properties of these mineral phases and thereby the nature of the Earth's interior(1-15). However, the spin state of iron at lowermost-mantle pressure/temperature conditions is poorly known(16-27). Here we use in situ X-ray emission, X-ray diffraction and synchrotron Mossbauer spectroscopic techniques to measure the spin and valence states of iron in post-perovskite and perovskite at conditions relevant to the lowermost mantle(25,28). We find that Fe2+ exists predominantly in the intermediate-spin state with a total spin number of one in both phases. We conclude that changes in the radiative thermal conductivity and iron partitioning in the lowermost mantle would thus be controlled by the structural transition from perovskite to post-perovskite, rather than the electronic transition of Fe2+.