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\u2013FeAl2O4 series produced by flux growth at 1000\u20131300 \ub0C in controlled atmosphere. Samples were characterized by single-crystal X-ray diffraction, electron microprobe analyses, and M\uf6ssbauer 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) \uc5 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. M\uf6ssbauer 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\ub0C, but it possible that cation ordering continued in the Cr-poor samples towards lower temperatures, possibly down to 700\ub0