X-ray absorption study at the Fe K-edge of garnets from the Ivrea-Verbano zone

Abstract

AbstractK-edge X-ray absorption spectra of Fe in garnet samples from the Ivrea-Verbano zone were collected using synchrotron radiation. From XANES analysis, the prevalent oxidation state of Fe has been determined as 2+ in all studied samples. Coordination numbers and Fe-O bond lengths derived from the EXAFS analysis are compatible with a dodecahedral environment of Fe atoms and seem to be nearly independent of the variable Fe/Ca ratio of the cations sharing the dodecahedral site in these garnets. This suggests that, since at least up to 0.5 Ca atoms p.f.u, no sensible deformation of the dodecahedron geometry is sensed by the Fe cations, iron might strongly compete with Ca atoms in controlling the entry of rare earth elements in the dodecahedral site of garnets falling within this compositional range. Comparison of the EXAFS results with the data from single crystal X-ray diffraction structure refinements indicates a first shell neighbour distance accuracy of ±0.02 Å, using theoretical EXAFS phases and amplitudes. The Debye-Waller factors derived from the EXAFS analysis indicate a higher degree of disorder on the four longer Fe-O bond distances, in comparsion with the other four shorter distances of the height-coordinated cation; this could be related to the nonrigid polyhedral behaviour of the dodecahedral site.