It has recently been shown that Fe3+ is stabilised by Al in (Mg,Fe,Al)(Si,Al)O3 perovskite (see Section 3.2g). Since the perovskite phase is likely the most abundant phase within the Earth, its properties have a large influence on the bulk physical and chemical properties of the Earth. The presence of even small amounts of Fe3+ can have significant effects, since Fe3+ affects the electrostatic charge balance and equilibrium defect concentration. Properties highly sensitive to such effects include sub- and supersolidus phase relations, electrical conductivity, diffusivity, the nature of degassed volatile species and mechanical behaviour.
To characterise the crystal chemistry of Fe3+ in the perovskite phase, in particular its concentration and site distribution in the crystal structure, we are synthesising samples of (Mg,Fe,Al)(Si,Al)O3 perovskite in the multianvil press as a function of oxygen fugacity and bulk Fe and Al content. The crystal chemistry of iron will be studied using X-ray diffraction and variable-temperature Mössbauer spectroscopy. The latter technique allows a quantitative determination of the relative Fe3+ content through recoil-free fraction corrections, and enables the location of Fe3+ in the crystal structure to be identified.
The samples will also be used as standards to calibrate Electron Energy-Loss Near-Edge Structure (ELNES) spectra as a complementary method for determination of Fe3+/Fe. Recent work by Van Aken has produced a calibration curve based on known mineral standards, and we plan to investigate the application of this method to the perovskite phase, including limits of the technique (e.g. minimum iron content, Fe3+ detection limit). This research builds on previous success at Bayerisches Geoinstitut in applying ELNES spectra to the determination of Si coordination in high-pressure phases (Annual Reports 1994 and 1995). One of the ultimate goals of the project is to improve significantly the spatial resolution of Fe3+/Fe determination, which is currently limited to approximately 50 µm using the Mössbauer milliprobe (Annual Report 1993).