Incorporation and site preference of Sc (~ 5%) in four synthetic garnet compositions (Py, Py60Gr40, Py20Gr80, and Gr) along the join pyrope-grossular were investigated by a multi-disciplinary experimental-theoretical approach, which combines EMPA, single-crystal and powder XRD, and Sc K-edge XAFS. The XAFS experiments were performed at ID26(ESRF). The XANES spectra clearly show significant differences in the spectral features of grossular and pyrope, suggesting different local environments for Sc. A multi-shell fit of the EXAFS signals shows that Sc is incorporated into the dodecahedral X site in the pyrope-type structure, but in the octahedral Y site in the grossular-type structure. Investigation of the solid-solution terms showed distinct site-partitioning behaviours for Sc. The first shells contributions of the Py60Gr40 EXAFS signal could be obtained by a weighted combination of Sc in both the tetrahedral (T) and the X site, whereas that of the Py20Gr80 EXAFS signal could be fitted by a weighted combination of single scattering paths calculated for Sc in both the Y and the T site. In this latter case, however, a minor occurrence of Sc at the X site cannot be ruled out. These results are fully compatible with those of the structure refinement of the end-members, which showed (i) higher site-scattering (ss) and adp values at the X site and lower ss at T in Py; (ii) higher ss and longer mbl at Y (and T) in Gr. The site partitioning and preference of Sc in garnets is thus dramatically determined by the matrix.
Incorporation and site preference of Sc in garnets: the join pyrope-grossular
2004
Abstract
Incorporation and site preference of Sc (~ 5%) in four synthetic garnet compositions (Py, Py60Gr40, Py20Gr80, and Gr) along the join pyrope-grossular were investigated by a multi-disciplinary experimental-theoretical approach, which combines EMPA, single-crystal and powder XRD, and Sc K-edge XAFS. The XAFS experiments were performed at ID26(ESRF). The XANES spectra clearly show significant differences in the spectral features of grossular and pyrope, suggesting different local environments for Sc. A multi-shell fit of the EXAFS signals shows that Sc is incorporated into the dodecahedral X site in the pyrope-type structure, but in the octahedral Y site in the grossular-type structure. Investigation of the solid-solution terms showed distinct site-partitioning behaviours for Sc. The first shells contributions of the Py60Gr40 EXAFS signal could be obtained by a weighted combination of Sc in both the tetrahedral (T) and the X site, whereas that of the Py20Gr80 EXAFS signal could be fitted by a weighted combination of single scattering paths calculated for Sc in both the Y and the T site. In this latter case, however, a minor occurrence of Sc at the X site cannot be ruled out. These results are fully compatible with those of the structure refinement of the end-members, which showed (i) higher site-scattering (ss) and adp values at the X site and lower ss at T in Py; (ii) higher ss and longer mbl at Y (and T) in Gr. The site partitioning and preference of Sc in garnets is thus dramatically determined by the matrix.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
