Partitioning and crystal-chemistry of Sc in garnets: multiform behaviour in the pyrope-grossular join

Trace-elements behaviour is a fundamental tool in petrogenetic studies; it is generally used to decipher the sequence of processes which modified the composition of the crystalline or of the fluid phases. Models of trace-elements behaviour are generally obtained via a method based on the elastic-strain theory, where the available partition coefficients (D) of series of homovalent cations occurring at the same (group of) structural sites are treated in terms of pseudo-parabolic trends. These parabolae have a vertex for the "ideal" cation which best fits the dimension of the site, and a width representing the Young's modulus of the site. The reliability of this approach depends on the many assumptions done, the most important of which is the knowledge of the correct site preference and coordination, as well as of the possible site partitioning (and crystal-chemical mechanisms of incorporation). Sc is an interesting element in this regard, because it is a transition element with a fixed ionic charge, and thus can be used as an indicator of the behaviour of the other transition elements for which the calculation of the Ds is troublesome because of the presence of different oxidation states. Four Sc-doped garnet compositions along the pyrope-grossular join (pyr, pyr60grs40, pyr20grs80, grs) have been synthesised and characterised with a multi-analytical experimental-theoretical approach (EMPA, XRPD, SC-XRD, EXAFS, XANES). The XANES spectra clearly show significant differences in the spectral features of grossular and pyrope, suggesting different local environments for Sc. This finding is supported by the results of the multi-shell fit of the EXAFS signals: Sc is incorporated into the dodecahedral X site in the pyrope-type structure, but in the octahedral Y site in the grossular-type structure. Moreover, a different site-partitioning behaviour is observed for Sc in the solid-solutions terms. The first-shells contributions of the Py20Gr80 EXAFS signal could be fitted by a weighted combination of single scattering paths calculated for Sc in both octahedral and tetrahedral (T) (possibly also dodecahedral) coordination, while the first shells fit of Py60Gr40 EXAFS signal could be obtained by a weighted combination of Sc in tetrahedral and dodecahedral coordination. XRPD-Rietveld and SC-XRD provided unit-cell variations and the atomic coordinates used for XAFS fitting. Polyhedral distances, site-scattering values, and atomic displacement parameters derived from structure refinement are in agreement with XAFS results obtained at the Sc K-edge, and show that different crystal chemical mechanisms are active as a function of composition. In the very compact pyrope and pyrope-type structures, Sc enters only the X site, and electroneutrality is achieved by the entrance of Mg in a T site adjacent to two X sites occupied by Sc. In the looser grossular structure, Sc incorporation at the Y site implies a further passive enlargement of the T site. In the pyr20grs80 structure, the larger size of the tetrahedron allows Sc incorporation at all the three structural sites.