Single-crystal X-ray diffraction and temperature dependent ⁵⁷Fe Mössbauer spectroscopy on the hedenbergite-aegirine (Ca,Na)(Fe²⁺,Fe³⁺)Si₂O₆ solid solution

Abstract

Synthetic samples with different chemical compositions along the hedenbergite-aegirine (CaFe²⁺Si₂O₆-NaFe³⁺Si₂O₆) solid-solution series have been investigated by single-crystal X-ray diffraction and 57Fe Mössbauer spectroscopy. All compounds show C2/c symmetry, both at 298 K and at low temperature (100 K). The structural changes within the hedenbergite-aegirine series are dominated by the M1 site while the M2 site plays a minor role. Replacement of Fe²⁺ by Fe³⁺ increases the polyhedral distortion of the M1 site and causes an increased repulsion between neighboring M1 sites. The changes in M1-site geometry also induce distinct alterations within the kinking state of the tetrahedral chains, but the changes in tetrahedral bond lengths and angles are small. In addition to the single-crystal X-ray diffraction experiments, a large number of synthetic samples were investigated by ⁵⁷Fe Mössbauer spectroscopy at 298 K and, for three selected compositions, between 80 and 700 K. Here, substantial line broadening of the Fe²⁺ resonance absorption was observed as an aegirine component is substituted. Two different groups of local distortion environments were observed for Fe²⁺ within the solid-solution series, which change relative proportions and numeric value of the quadrupole splitting as a function of chemistry and temperature. This line broadening cannot be ascribed to discrete next-nearest-neighbor (NNN) configurations of Ca and Na as has been done in the literature. Above ~250 K, additional resonance absorption appears in the spectra of samples with aegirine components between 20 and 75 mol%. This absorption gains intensity with increasing temperature, while the ⁵⁷Fe hyperfine parameters approach values intermediate between Fe²⁺ and Fe³⁺. This effect is ascribed to fast electron delocalization between Fe²⁺ and Fe³⁺ at elevated temperature.