A MULTI-TECHNIQUE IN-SITU INVESTIGATION OF SITE PREFERENCE AND LOCAL ENVIRONMENT OF SCANDIUM IN GARNETS

complexity of their crystal chemistry and solid-solution properties and of their stability over a wide range of physico-chemical conditions. In particular, understanding of trace-element behaviour in garnets is crucial for geochemical modelling of petrogenetic processes in the upper-mantle region. Albeit many data are available, some major features still lack of proper interpretation. For instance, the trace-element data reported for natural and synthetic compositions indicate that the behaviour is not linear within the pyrope-grossular solid-solution, and that trace-element incorporation, at least at the X site, is most effective close to 50:50. This issue has been recently addressed by atomistic simulation (van Westrenen et al. 2003).The crucial point at the moment is to understand whether or not a given trace element has the same site preference and local environment in all the garnet compositions, which implies that its steric and energetic properties can or can not be considered invariant within the solid solution.In order to address this issue, a multi-technique project has been designed for the complete characterisation (by electron microprobe analysis, X-ray powder diffraction, single-crystal structure refinement and synchrotron radiation XAFS spectroscopy) of the structural properties and of the local environment of synthetic solid-solution terms doped by minor amounts of the relevant elements. We report here on the results obtained for Scandium. In principle, its ionic radius may allow incorporation in the garnet structure both in octahedral (i.r. = 0.745 Å) and in dodecahedral (i.r. = 0.870 Å) coordination, i.e. both at the Y and the X sites. As a matter of fact, Sc is usually considered as an octahedral constituent in geochemical modelling of garnets.The results obtained with all the independent analytical techniques mentioned above can only be interpreted by considering that Sc is incorporated at the X site in pyrope but at the X and Y sites in grossular. Work is still in progress to accurately determine the geometry of the local environments of Sc and Ca in the pyrope-grossular solid-solution, and to understand whether or not the intermediate terms can be modelled by linear combination of the behaviour of the two end-members.Van Westrenen et al. (2003) Phys. Chem. Minerals, 30, 217-229.