Distinct local environments for Ca along the non-ideal pyrope-grossular solid solution: A new model based on crystallographic and EXAFS analysis

A multi-technique approach (based on electron microprobe analysis, structure refinement, and EXAFS analysis at the Ca Kedge) was used to characterise the local geometry of Ca in synthetic and natural garnet compositions referable to the pyrope-grossular solid solution. Multi-shell fits of the EXAFS data indicate that Ca assumes the standard [4+4]-fold coordination (the polyhedral shape being a triangular dodecahedron with Cal-O=2.30-2.31(1) and Ca2-O=2.45-2.46(1) angstrom) when Ca > 1.50 atoms per formula unit (apfu), but assumes a nearly regular [8]-fold coordination with Ca-O=2.35-2.36 (1) A when (Mg, Fe2+, Mn2+)> 1.50 apfu. Therefore, in the pyrope-dominant structure the Cal-O distance lengthens and the Ca2-O distance shortens to converge towards the value observed for the Mg2-O bond in pyrope. This finding is consistent with many distinct structural features observed in solid solution terms with (Mg, Fe2+, Mn2+)> 1.50 apfu or Ca > 1.50 apfu, as well as with the anomalous properties of the intennediate terms observed both in the short-range and in the long-range perspective. The presence of two distinct Ca coordinations in the pyrope (almandine, spessartine)-like and in the grossular-like structure, and thus of an isosymmetric transition at the intennediate composition, can help to explain both the strong and asymmetric non-ideality of the solid solution between pyrope (almandine) and grossular, as well as the differences in the ability to incorporate some trace elements (such as REE and actinides) which are commonly used as process-specific indicators. This feature must be taken into account when building theoretical models of the garnet solid solutions, which are at the moment the most promising approach for calculating thermodynamic properties or for interpreting and predicting trace-element behaviour in this crucial mineral phase.