Rietveld refinement of high-pressure CaAl₄Si₂O₁₁ with the R-type ferrite structure

The natural counterpart of the newly-discovered phase CaAl₄Si₂O₁₁ might well play a modest role in the dynamic and isotopic evolution of the mantle. CaAl₄Si₂O₁₁ synthesised at 14 GPa and 1,600 °C is found to be isostructural with the R-type hexagonal ferrite, BaFe₄Ti₂O₁₁, with hexagonal symmetry, P6₃/mmc, and a = 5.4228(1), c = 12.6933(1) Å Rietveld refinement of powder X-ray intensity data (R_wp = 0.10, R_B = 0.022) reveals that aluminium is preferentially ordered in a split tetrahedral site, M1, and in face-shared octahedral sites M2. The displacement of M1 from the centre of a trigonal bipyramidal site, so as to adopt essentially tetrahedral coordination instead, is attributed to the structure's minimisation of non-bonded Ca-M1 repulsions.