Al, Fe substitution in the MgSiO3 perovskite structure: A single-crystal X-ray diffraction study

We have determined by single-crystal X-ray diffraction the crystal structure of three Fe–Al–MgSiO3 perovksite samples containing up to 9.5 wt% of Al2O3 and 19 wt% of FeO. We find that there is no evidence for Fe (Fe3+ or Fe2+) on the octahedral site. Therefore, we deduce that the two dominant substitution mechanisms for the combined substitution of Al and Fe into the perovskite structure are: (i) MgA2+ + SiB4+ ⇔ FeA3+ + AlB3+, where the excess of Fe is accommodated by (ii) MgA2+ ⇔ FeA2+. This is in agreement with all past theoretical and experimental work and solves the long-debated issue of Fe3+ occupancy in the perovskite structure.