Cation sites in Al-rich MgSiO3 perovskites

Local structure analysis of Al-containing magnesium silicate perovskite has been carried out with X-ray absorption spectra recorded at the Mg, Al, and Si K-edges using the SA32 beam-line of SuperAco (Orsay, France). The Al-XAFS spectrum of (MgSi)0.85Al0.3O3 perovskite (synthesized in a multi-anvil apparatus) cannot be explained by assuming that Al31 occurs in octahedral or dodecahedral sites only. This conclusion is based on comparison between Al-spectrum and those recorded at the Mg and Si K-edges for the same structure, general trends found for Al-spectra in various atomic sites, and ab-initio calculations using the FEFF-6 code. Thus, Al appears to be partitioned between both octahedral and dodecahedral perovskite sites. However, the structural accommodations needed to stabilize Al31 cation in such different sites are not straightforward. Also, Mg K-edge spectra in enstatite and perovskite were compared with those previously reported at the Fe K-edge for the same structures, confirming that these two elements are located in the same polyhedra in both structures, and thus that Fe21 enters the dodecahedra of the silicate perovskite.