Fe spin transition in the Earth's mantle: insight from X-ray Raman scattering and X-ray absorption spectroscopy

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Both, Fe2+ (3d6-electronic configuration) and Fe3+ (3d5) in minerals are in high spin electronic state at ambient conditions and may undergo transitions from the high spin (HS) to the low spin (LS) state at pressures and temperatures expected in the Earth mantle. The change in spin state has considerable impact on structural, physical and chemical properties of Fe-Mg minerals as well as on geodynamic processes in the Earth mantle that depend on these. The aim of this project is to provide new experimental insights to the electronic changes during the HS-LS transition and their relationship to changes in the local and crystal structure. New insights will be gained by application of X-ray Raman scattering (XRS) and X-ray absorption spectroscopy (incl. Resonant X-ray emission spectroscopy, RXES). Both techniques provide valuable insights to the electronic and local structure by measurements of the Fe M, L and K-edges. XRS also allows for investigation of the, e.g. O K-edge and Si, Mg L-edge. Feasibility of XRS measurements at pressures up to 50 GPa has been shown on samples of siderite and magnesio-siderite. During the project, the work on the magnesite-siderite solid solution will be completed. Final goal is the investigation of the more complex mineral bridgmanite, which is one of the most abundant phases of the lower mantle. The newly obtained data will be used to solve the controversy regarding the mechanism of the spin transition in this mineral phase. From the measured data, structural models will be developed by combination with DFT-based computer simulations. Finally, the implications of the results for the lower Earth's mantle will be assessed.