The K-edge x-ray absorptions spectra of Fe in garnet samples from the Ivrea-Verbano zone were collected using synchrotron radiation. From XANES anal., the prevalent oxidn. state of Fe has been detd. as 2+ in all studied samples. Coordination nos. and Fe-O bond lengths derived from the EXAFS anal. 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 neighbor distance accuracy of ±0.02 Å, using theor. EXAFS phases and amplitudes. The Debye-Waller factors derived from the EXAFS anal. indicate a higher degree of disorder on the four longer Fe-O bond distances, in comparison with the other four shorter distances of the height-coordinated cation; this could be related to the nonrigid polyhedral behavior of the dodecahedral site.
X-ray absorption study at Fe K-edge in garnets from Ivrea-Verbano zone
QUARTIERI, Simona;
1993-01-01
Abstract
The K-edge x-ray absorptions spectra of Fe in garnet samples from the Ivrea-Verbano zone were collected using synchrotron radiation. From XANES anal., the prevalent oxidn. state of Fe has been detd. as 2+ in all studied samples. Coordination nos. and Fe-O bond lengths derived from the EXAFS anal. 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 neighbor distance accuracy of ±0.02 Å, using theor. EXAFS phases and amplitudes. The Debye-Waller factors derived from the EXAFS anal. indicate a higher degree of disorder on the four longer Fe-O bond distances, in comparison with the other four shorter distances of the height-coordinated cation; this could be related to the nonrigid polyhedral behavior of the dodecahedral site.Pubblicazioni consigliate
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