American Mineralogist, Volume 91, pages 12711292, 2006 0003-004X/06/08091271$05.00/DOI: 10.2138/am.2006.2173 1271 INTRODUCTION Both hedenbergite (CaFe 2+ Si 2 O 6 ) and aegirine (NaFe 3+ Si 2 O 6 ) are clinopyroxenes that are monoclinic with C2/c symmetry at room temperature (Clark et al. 1969; Cameron et al. 1973; Redhammer et al. 2000). A topological description and repre- sentations of the crystal structure of clinopyroxenes including the atomic nomenclature of Burnham et al. (1967) can be found elsewhere (e.g., Cameron and Papike 1981; Redhammer et al. 2000) and will not be repeated here. Both end-members have been investigated structurally several times. Redhammer et al. (2000) were the rst to give structural data for synthetic samples on this join. They rened the crystal structures of 7 different compositions of the heden- bergiteaegirine solid-solution series by means of the Rietveld method from powder data. However, they were not able to extract detailed information on structural changes (variation of individual bond lengths, bond angles, distortion parameters) as a function of chemical composition. Furthermore, information on the low-temperature crystal structure is not yet available. This information, however, is of basic importance for understanding and interpreting trends in 57 Fe hyperne parameters as derived from Mssbauer spectroscopy. Also, atomic positional param- eters are the basic input data for any theoretical calculation of the electronic structure of the clinopyroxenes under investigation. 57 Fe Mssbauer spectroscopy has been applied in several instances to the study of clinopyroxenes (Bancroft and Williams 1969; Aldridge et al. 1978; Williams et al. 1971). Dollase and Gustafson (1982) investigated 4 different synthetic samples of the hedenbergiteaegirine solid-solution series by Mssbauer spectroscopy at 77 and 298 K and found substantial, asymmetric Fe 2+ resonance absorption line broadening. Similar observations were made by Aldridge et al. (1978) for a natural C2/c omphacite and by Amthauer and Rossman (1984) for a natural aegirine. Two opposing interpretations have been given for this behavior. Aldridge et al. (1978) assigned this effect to next-nearest-neigh- bor (NNN) inuences arising from different occupation of the M2 site by Ca 2+ and Na + , respectively. This interpretation also was favored by Dowty and Lindsley (1973) to explain Fe 2+ line broadening along the hedenbergiteferrosilite Fe 2 Si 2 O 6 solid solution. In contrast, Amthauer and Rossman (1984) suggested that an inner Fe 2+ doublet arises from thermally activated electron charge transfer between Fe 2+ and Fe 3+ , whereas the outer Fe 2+ doublet was assigned to pure ionic Fe 2+ on the M1 site. Dollase * E-mail: guenther.redhammer@sbg.ac.at Single-crystal X-ray diffraction and temperature dependent 57 Fe Mssbauer spectroscopy on the hedenbergite-aegirine (Ca,Na)(Fe 2+ ,Fe 3+ )Si 2 O 6 solid solution GNTHER J. REDHAMMER, 1,2, * GEORG AMTHAUER, 1 GEORG ROTH, 2 GEROLD TIPPELT, 1 AND WERNER LOTTERMOSER 1 1 Department of Material Science, Division of Mineralogy, University Salzburg, Hellbrunnerstr. 34, A-5020, Salzburg, Austria 2 Institute of Crystallography, University of Technology, RheinischWestflische Technische Hochschule Aachen, Jgerstrasse 17/19, D-52056, Aachen, Germany ABSTRACT Synthetic samples with different chemical compositions along the hedenbergiteaegirine (CaFe 2+ Si 2 O 6 NaFe 3+ Si 2 O 6 ) solid-solution series have been investigated by single-crystal X-ray diffraction and 57 Fe Mssbauer spectroscopy. All compounds show C2/c symmetry, both at 298 K and at low temperature (100 K). The structural changes within the hedenbergiteaegirine series are dominated by the M1 site while the M2 site plays a minor role. Replacement of Fe 2+ by Fe 3+ 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 57 Fe Mssbauer spectroscopy at 298 K and, for three selected compositions, between 80 and 700 K. Here, substantial line broadening of the Fe 2+ resonance absorption was observed as an aegirine component is substituted. Two different groups of local distortion environments were observed for Fe 2+ 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) congurations 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 57 Fe hyperne parameters approach values intermediate between Fe 2+ and Fe 3+ . This effect is ascribed to fast electron delocalization between Fe 2+ and Fe 3+ at elevated temperature. Keywords: Synthesis, crystal structure, Mssbauer spectroscopy, hedenbergiteaegirine series, clinopyroxenes