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

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.