RIETVELD-REFINEMENTS AND TEMPERATURE-DEPENDENT MOESSBAUER SPECTROSCOPIC INVESTIGATIONS OF SYNTHETIC C2/c CLINOPYROXENES. G. Amthauer, G. J. Redhammer, W. Lottermoser, Institute of Mineralogy, University of Salzburg, Austria

C2/c clinopyroxenes on the binary join hedenbergite ^[M2]Ca^[M1]Fe²⁺Si₂O₆ - acmite Na⁺Fe³⁺Si₂O₆ were synthesized using standard hydrothermal techniques and investigated by means of temperature-dependent Moessbauer spectroscopy between 80 and 700K, revealing Fe²⁺ and Fe³⁺ on the octahedral M1 position only and showing additional resonant absorption with hyperfine parameters intermediate between Fe²⁺ and Fe³⁺ above 250K within the solid solution series, which gains intensity with increasing temperature. This is interpreted as thermally activated electron transfer between Fe²⁺ and Fe³⁺ statistically occupying the M1 sites. Syntheses were done at 4 kbar, 973K and the redox conditions of the nickel/ nickeloxid solid state buffer. Run products were characterized by microprobe analyses and XRP-diffraction including Rietveld refinements on selected samples in addition to Moessbauer spectroscopy. The dependence of lattice parameters on chemical composition deviates from linearity. Mean M1-O distances decrease with increasing acmite component, whereas mean M2-O and mean T-O distances show no significant variation. Within the M1-zig-zag chain, which is parallel to the crystallographic c-axis and consists of edge-sharing octahedra, the shortest Fe-Fe distance is 3.112(2)Å in hedenbergite and 3.196(2)Å acmite. Room temperature Moessbauer spectra of the end-compositions show one narrow split doublet with hyperfine parameters characteristic for Fe²⁺ and Fe³⁺ respectively, whereas within the solid solution series the Fe²⁺ resonance absorption feature is broadened asymmetrically and can be refined into three doublets due to next nearest neigbour effects.