Previous page     Contents     Next page

Silicate perovskite is the main phase of the lower mantle and thus, given the volume of the lower mantle, the main phase of the Earth. Properties of the perovskite phase determine the bulk physical and chemical properties of the Earth. Recent experiments (Annual Report 1996) indicate that Al can be incorporated in the perovskite structure. Al-bearing perovskites, which are believed to be formed under lower mantle conditions, can be found in nature as diamond inclusions. Examination of these synthesised and natural perovskites demonstrates that incorporation of Al in the perovskite structure affects the Fe³⁺/ Fe ratio. The presence of Fe³⁺ in the perovskite phase has effects on electrostatic charge balance and equilibrium defect concentration. Electrical conductivity, diffusivity characteristics and mechanical behaviour for example, are sensitive to such changes.

To investigate the effect of Al content on Fe³⁺/ Fe ratio in perovskites, we synthesised samples in a multi-anvil press at conditions relevant to the lower mantle (26 GPa, ~1700°C, runtime 2 h). By using two different capsule materials (iron and rhenium) we produced relatively reducing and oxidising conditions respectively during the synthesis. The samples were examined by X-ray diffraction, electron microprobe and variable-temperature Mössbauer spectroscopy. Using Mössbauer spectroscopy, which is sensitive to iron in its different oxidation states, we have studied the concentration and site distribution of Fe³⁺ in the crystal structure. The minimal spatial resolution is about 50µm. A recently developed technique based on Electron Energy Loss Spectroscopy (EELS) enables determination of Fe³⁺/ Fe with the transmission electron microscope. The results from the Mössbauer experiments will be used to calibrate EELS-spectra as a complementary method, allowing the determination of the Fe³⁺/ Fe ratio on a nanometer scale. EELS can also be performed on pervoskites with low Fe concentrations.

The Mössbauer spectra (Fig. 3.3-1) demonstrate that the presence of Al³⁺ in the perovskite allows significantly higher amounts of Fe³⁺ to be incorporated compared to the Al-free material. Further experiments shows that the influence of oxygen fugacity on this substitution is secondary. Therefore in the Earth´s lower mantle, which is believed to contain some 4 wt% Al₂O₃, the perovskite will contain highly oxidised Fe.

Fig. 3.3-1: Room temperature Mössbauer spectra of perovskites without Al (a) and with 10 atom% Al (b). Subspectra corresponding to [VI]Fe3+ are shaded. The remainder of the envelope corresponds to Fe2+. The relative amount of Fe3+ (Fe3+/ Fe) is 12 % for the Al-free perovskite and 64 % for the perovskite containing Al.