[4]Fe3+-O distance in synthetic kimzeyite garnet

A synthetic kimzeyite analogue (Ca3Zr2[Fe2SiO12]) has been analysed by powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) in order to determine the [4]Fe3+–O distance, for which only few data are available. The XRD-determined cell parameter (a0 = 12.625 ± 0.001 Å) is consistent with those found in the literature for synthetic samples of similar composition. Also interatomic distances (<Ca–O>= 2.515 Å, <Zr–O>= 2.093 Å, <Z–O>= 1.774 Å, as determined by Rietveld refinement) are in keeping with structural data of natural samples when taking into account chemical differences of the samples examined. Due to the large Fe content of the tetrahedral site, the <Z–O> distance is unusually long compared with those of garnets where tetrahedra are occupied solely by Si, it is instead consistent with structural data for natural kimzeyite. The XANES data indicate the presence of trivalent Fe in tetrahedral coordination. The EXAFS-derived <Fe–O>distance (1.85 ± 0.01 Å) is in agreement with the few literature data available for [4]Fe3+, i.e. tetra-ferriphlogopite (<Fe–O> = 1.86 ± 0.01 Å), rodolicoite (<Fe–O> = 1.825 Å) and silicate glasses (Fe–O = 1.85 ± 0.01 Å and 1.84 ± 0.02 for phonolitic and rhyolitic glasses, respectively). In view of the large size difference between FeO4 and SiO4 tetrahedra a further Rietveld structural refinement was performed assuming a splitting of the oxygen position, resulting in two distinct oxygen sites (OA and OB with fractional oxygen occupancies of 2/3 and 1/3) at 1.845 and 1.606 Å distance, respectively, from the tetrahedral cation. Although there are still open questions on the distribution of FeO4 and SiO4 tetrahedra and on how the structure accommodates the size difference of these two tetrahedra, this study provides a direct determination of the [4]Fe3+–O distance for which only few data are available in the literature.