Electronic and magnetic properties of a natural aegirine as observed from its Mossbauer spectra

A natural sample of aegirine, ideally NaFe-Si2O6, has been studied by transmission Mossbauer spectroscopy in the range 4.2-480 K. At selected temperatures, a longitudinal external field of 60 kOe was applied to the absorber. The sample was observed to order magnetically at 11+/-1 K. The paramagnetic Mossbauer spectra (MS) show the presence of similar to 10% Fe2+ in the M1 sites of the clinopyroxene structure; These MS have been decomposed into four quadrupole doublets: two minor ones for Fe2+ on M1 sites, a dominant one due to Fe3+ on M1 sites, and a second ferric component, with a contribution of similar to 3% and attributable to the tetrahedral sites. Two possibilities concerning the origin of the two distinct Fe2+ (M1) doublets are discussed. They are respectively based on inter-valence charge transfer and on the existence of distinct Fe2+ orbital configurations at the two M1 sites. Neither of the two models could be firmly excluded. The asymmetry parameter eta of the electric field gradient at the Fe3+ (M1) sites is close to 1.0 and the quadrupole splitting within 0.34+/-0.01 mm/s at all temperatures. The MS at 4.2 K shows an asymmetric hyperfine-field distribution for Fe3+, with a maximum-probability field of 468 kOe. The maximum-probability field for Fe2+ is found to be 220 kOe. The shape of the applied-field MS at 4.2 K implies a static antiferromagnetic ordering and was successfully interpreted by a bidimensional distribution of the magnitude and orientation of the hyperfine field. Finally, the temperature variations of the respective centre shifts and quadrupole splittings could be explained on the basis of existing theoretical models.