Structural and 57Fe Mossbauer spectroscopic characterization of the synthetic NaFeSi2O6 (aegirine) - CaMgSi2O6 (diopside) solid solution series

Clinopyroxenes along the aegirine - diopside join have been synthesized from flux growth and high pressure experiments. Samples were characterized by Fe-57 Mossbauer spectroscopy and single crystal X-ray diffraction. Coordination, bond lenghts and - angles and polyhedral distortion parameters are discussed as a function of composition. All samples exhibit C2/c space group symmetry and show smooth variation of structural parameters with chemistry. While the lattice parameters a and b increase with increasing diopside component, c decreases, even if the smaller Fe3+ cation is replaced by the larger Mg2+ one. A distinctly decreasing M1 - M1 cationic distance within the octahedral chain from aegirine to diopside suggests, that decreasing repulsive forces between M1 cations with decreasing charge of the M1 cation is responsible for this behaviour. In Fe-57 Mossbauer spectroscopy, small amounts of Fe2+ were detected in the samples of the high pressure experiments. The quadrupole splitting of both, Fe2+ and Fe3+ suggests an increasing distortion of the Fe-octahedra with increasing diopside content, while the overall distortion of the M1 sites decrease. This shows that local structural units may behave quite differently as compared to the average structure.