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3.4 e. High-pressure Mössbauer study of FeSiO3 clinopyroxene and the P21/c -> C2/c phase transition (C.A. McCammon, in collaboration with C. Tennant/Christchurch)

Pyroxenes constitute a significant proportion of the Earth's upper mantle. Recent experiments (see Sect. 3.4d) have shown that a C2/c high pressure phase in the MgSiO3 and FeSiO3 systems has a large stability field at high P,T, so it is important to understand the crystal chemistry of this phase. Mössbauer spectroscopy is sensitive to the local electronic environment of the iron atoms, and hence is an excellent tool to study the nature of the low- and high-pressure phases of FeSiO3 and the changes occurring during the transition.

We synthesised 57Fe-enriched FeSiO3 in a multianvil press at 8 GPa and 1150 °C. The resulting sample (P21/c structure) was studied at room temperature between 0 and 3.8 GPa in a diamond anvil cell using Mössbauer spectroscopy. We observed large discontinuities in the hyperfine parameter variation with pressure between 1.3 and 1.7 GPa, which corresponds to the non-quenchable phase transition to the C2/c structure. The electronic structure of Fe2+ in the M2 site changes significantly more than in the M1 site, which is consistent with results from single-crystal X-ray refinements that show larger changes in bond lengths and site distortion for the M2 site compared to the M1 site. Changes in the electronic structure of Fe2+ in the M1 and M2 sites with pressure are more similar in the C2/c phase compared to the P21/c phase. Results are useful for understanding the partitioning behaviour of Fe2+ in (Mg,Fe)SiO3 solid solutions of the C2/c phase (see Annual Report 1994).

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