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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).