Structural changes in the FeAl2O4 - FeCr2O4 solid solution series and their consequences on natural Cr-bearing spinels

The influence of Al - Cr substitution on the spinel structure was studied in synthetic single crystals belonging to the FeCr2O4–FeAl2O4 series produced by flux growth at 1000–1300 °C in controlled atmosphere. Samples were characterized by single-crystal X-ray diffraction, electron microprobe analyses, and Mössbauer spectroscopy. Crystals of sufficient size and quality for single-crystal X-ray diffraction were obtained in the ranges Chr0-0.45 and Chr70-100 but not for intermediate compositions, possibly due to a reduced stability in this range. The increase in chromite component lead to an increase of the cell edge from 8.1534 (6) to 8.3672 (1) Å and a decrease in the u parameter from 0.2645 (2) to 0.2628 (1). Chemical analyses show that Fe2+ is very close to 1 apfu (0.994-1.007), Al is in the range 0.0793-1.981 apfu, Cr between 0 and 1.925 apfu. In some cases Fe3+ is present in amounts up to 0.031 apfu. Spinels with intermediate Cr content (Chr component between 40 and 60) are strongly zoned with Cr-rich cores and Cr-poor rims. Mössbauer analyses on powdered spinels of the runs from which single crystal have been used for X-ray structural data show values of Fe3+/Fetot consistently larger than that calculated by EMPA on single crystals, presumably due to chemical variation between single crystals from the same runs. The synthesis runs ended at a temperature of 1000°C, but it possible that cation ordering continued in the Cr-poor samples towards lower temperatures, possibly down to 700°C