Valence state, coordination and incorporation of Ti and Cr into lower mantle phases : new insight from electron energy-loss spectroscopy
The knowledge of Ti4+/Ti3+ and Cr3+/Cr2+ ratios is particularly important for understanding the incorporation, element partitioning and oxidation state in the Earth’s mantle and other rocky solar system bodies. In addition, both d-elements participate in a number of redox reactions taking place on the Earth’s surface and in the Earth’s crust. Also, the redox reactions are of great importance for the chemistry of materials, material and environmental sciences. Thus, in the presented study, calibration techniques for the determination Ti and Cr oxidation state using electron energy loss spectroscopy (EELS) are developed. In order to explain the signals of Ti and Cr electron energy-loss spectra, L3,2 electron energy loss near edge structures (ELNES) of a number of Ti- and Cr-bearing standard materials have been measured in a transmission electron microscope. The Ti and Cr containing standards represent a set of materials with either pure (Ti2+, Ti3+, Ti4+, Cr2+, Cr3+, Cr4+, Cr5+, Cr6+) or mixed (Ti3+:Ti4+ in various ratios) valence states, known coordination (octahedral and tetrahdedral) and site symmetry. It was found that octahedrally coordinated titanium is very sensitive to the polyhedral distortions. Effects of polyhedral distortions are particularly observed for rutile and anatase (both TiO2) and two oxides with mixed Ti3+-Ti4+ valence state, i.e. the Magnéli phases Ti4O7 and Ti5O9. The effect of valence state manifests itself in a systematic chemical shift of Ti and Cr white lines by 1.5-2.0 eV per valence state. On the basis of collected Ti L3,2 and Cr L3,2 ELNES spectra are developed two new quantification techniques for the determination of Ti4+/Ti3+ and Crn+/ΣCr ratios. One particular application of the EELS technique in mineralogy is the analysis of the local atomic environments in minerals. The geochemical properties of titanium and chromium in the Earth’s lower mantle are still not well constrained. However, understanding the valence state and partitioning of Ti and Cr in the Earth’s lower mantle will shed additional light on the fundamental processes of element partitioning and core-mantle differentiation. Hence, the second issue of this work is to study the effect of pressure, temperature and oxygen fugacity on the valence state and incorporation of Ti and Cr in ferropericlase and silicate perovskite, both most abundant phases in the Earth’s lower mantle. Hence, series of multi-anvil experiments were performed. For determination the titanium and chromium valence state and coordination geometry in ferropericlase and silicate perovskite, electron energy-loss spectroscopy was used. By analyzing the energy dispersive X-ray and electron diffraction data on ferropericlase, is found that at high pressures titanium and chromium were incorporated into the ferropericlase structure and during the quenching of the multi-anvil samples both elements are expelled from the MgO structure by forming nanocrystals of spinel structured phases. Applying the ELNES fingerprinting technique shows that the titanium valence state in the spinel-structured materials depends on the FeO component present in MgO and varies from Ti2+ to Ti4+, whereas Cr is found to be mainly 3+. For the silicate perovskite is found that only titanium (as Ti3+) partitions into the structure by substitution the Si atoms (B site in MgSiO3 perovskite structure). This substitution leads to formation of oxygen vacancies in the MgSiO3 perovskite structure. The energy dispersive X-ray analysis data show that under lower mantle conditions Ti is lithophile (partitions into mantle phases), whereas Cr is mainly siderophile (partitions into the metal phase but also considerably partitions into ferropericlase).