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

K-edge X-ray absorption specrra of Fc in garnct samples from thc Ivrea-Verbano zone were collected using synchrotron radiation. From XANES analysis, the prevalent oxidation state of Fe has been deterrnined as 2+ in ali studied samples. Coordination numbers and Fe-O hond lcngths derivcd from the EXAFS analysis are compatible with a dodecahedral environment of Fc atoms and seem to bc ncarly indepcndent of the variable Fe/Ca ratio of the cations sharing the dodecahedral site in thesc garners. This suggests that, since at least up to 0.5 Ca atoms p.f.u. no sensible dcformation of thc dodccahcdron gcornetry is sensed by the Fe cations, iron might strongly compete with Ca atorns in controlling thc cntry of rare carth elements in the dodecahedral site of garnets falling within this compositional range. Cornparison of the EXAFS results with the data frorn single crystal X-ray diffraction structurc refinerncnts indicates a first shell neighbour distance accuracy of ±0.02 À, using theorctical EXAFS phases and amplitudes, The Debye-Waller factors derived from the EXAFS analysis indicate a higher degree of disorder on the four longcr Fc-O bond distances, in cornparsion with the othcr four shortcr distances of the hcight-coordinatcd cation; this could be related IO the 110nrigid polyhedral behaviour of the dodecahedral site.