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

The natural counterpart of the newly-discovered phase CaAl4Si2O11 might wel l play a modest role in the dynamic and isotopic evolution of the mantle. C aAl4Si2O11 synthesised at 14 GPa and 1,600 degrees C is found to be isostru ctural with the R-type hexagonal ferrite, BaFe4Ti2O11, with hexagonal symme try, P6(3)/mmc, and a = 5.4228(1), c = 12.6933(1) Angstrom Rietveld refinem ent 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, Mi, 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 coordi nation instead, is attributed to the structure's minimisation of non-bonded Ca-M I repulsions.