Single-crystal X-ray diffraction and temperature dependent 57Fe Mössbauer spectroscopy on the hedenbergite-aegirine (Ca,Na)(Fe2+,Fe3+)Si^sub 2^O^sub 6^ solid solution

• Published in: The American mineralogist Vol. 91; no. 8/9; p. 1271
• Main Authors: Redhammer, Günther J, Amthauer, Georg, Roth, Georg, Tippelt, Gerold, Lottermoser, Werner
• Format: Journal Article
• Language: English
• Published: Washington Walter de Gruyter GmbH 01.08.2006
• Subjects: Absorption; Chemical compounds; Chemicals; Crystals; Diffraction; Electrons; High temperature; Low temperature; Resonance; Spectroscopy; Spectrum analysis; X-ray diffraction; X-rays
• ISSN: 0003-004X; 1945-3027

Synthetic samples with different chemical compositions along the hedenbergite-aegirine (CaFe^sup 2+^Si^sub 2^O^sub 6^-NaFe^sup 3+^Si^sub 2^O^sub 6^) 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 (NINN) 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. [PUBLICATION ABSTRACT]