Local structural environment of calcium in garnets: A combined structure-refinement and XANES investigation

Comparative crystal-chemical analysis of natural garnets allowed the compositional gap at Gr 45÷70% in the (Py, Alm)-Gr join to be related to the presence of two different structural arrangements around the X site in pyralspitic and ugranditic composition, respectively. A combined investigation (single-crystal X-ray structure-refinement (SREF), Ca K-edge XANES spectroscopy and multiple-scattering calculation of XANES spectra) carried out on a series of natural garnets in the (Py, Alm)-Gr join, allowed the structural modifications of the Ca local environment at increasing grossular content to be followed. The differences in the experimental XANES spectra as a function of the Ca content were shown to depend only on the configuration of the ligand shells surrounding the absorbing atom, and not on the nature of adjacent dodecahedral cations. Both experimental XANES spectra and their theoretical simulations confirm that Ca at the X site, when less abundant than (Fe+Mg), is not in the same structural configuration as in grossular, but adapts itself to that imposed by the dominant (Fe, Mg) cations. In the same way, (Fe, Mg) adapt themselves to the X-site configuration observed in grossular when Ca is the dominant cation. The deformation of the X site due to progressive Ca → (Fe, Mg) substitution is gradual but not linear along the solid solution; it shows different slopes on the two sides of the observed (Py, Alm)-Gr compositional gap. Best performances in reproducing the XANES spectra were obtained in the framework of the one-electron fullmultiple-scattering theory, using the real Hedin-Lundqvist exchange-correlation potential and clusters of 83 atoms (i.e. for a coordination sphere with 7 Å radius from the central Ca absorber); smaller clusters resulted inadequate in simulating some XANES features, indicating that a high number of single and multiple scatteringpaths contribute in garnets to the first part of the absorption spectrum.