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⁵⁷Fe Mössbauer spectroscopy study of the correlation between the Fe³⁺ content and the magnetic properties of natural Cr-spinels

¹ Instituto Tecnológico e Nuclear, Quimica, P-2686–953 Sacavém, Portugal
² Departamento de Geologia e CREMINER, Faculdade de Cincias da Universidade de Lisboa, Departamento de Geologia, P-1749–016 Lisboa, Portugal

^# corresponding author, e-mail: jcarlos{at}itn.pt

Both the Fe oxidation degree and the magnetic properties determined by ⁵⁷Fe Mössbauer spectroscopy were used to characterize the Cr-spinels from the Beja-Acebuches Ophiolite Complex (SE Portugal). Two different types of Cr-spinel were observed. The first one has an average Fe³⁺/(Fe total) 25%, and corresponds to unaltered Cr-spinel grains which remain paramagnetic down to 6K. The second one results from the alteration of the first type, has an average Fe³⁺/(Fe total) 40% and order magnetically between 222 K and 78 K. Fe³⁺/(total Fe) increases with the magnetic ordering temperature. The grain population ordering at the highest temperatures, in the range 202–222K, has the highest Fe³⁺/(total Fe) 58%. Quantitative Fe site distributions can be obtained from room-temperature Mössbauer data if the different recoilless factors for tetrahedral Fe²⁺ and octahedral Fe³⁺ are considered. The observed second-order Doppler shifts are consistent with Mössbauer temperatures of 330K and 605K, reported in the literature for tetrahedral Fe²⁺ and octahedral Fe³⁺ in other oxide spinels. The differences in the magnetic ordering temperatures allowed the characterization of both types of Cr-spinels using a single Mössbauer absorber, thus avoiding the need to separate them physically, which, in this case, would be very difficult. The results are compared to those obtained for other Cr-spinels and, for the first time in the case of natural Cr-spinels, the increase of their magnetic ordering temperatures with their oxidation degree, resulting from natural processes, is discussed in terms of the magnetic exchange interactions between the Fe cations.

Key-words: spinel, chromite, Mössbauer spectroscopy, crystal chemistry, magnetic properties.