Site preference and local geometry of Sc in garnets: Part II. The crystal-chemistry of octahedral Sc in the andradite-Ca3Sc2Si3O12 join

Investigation of scandium incorporation in garnets along the synthetic Ca3Fe23+Si3O12-Ca3Sc2Si3O12 (adr-CaSc) join, based on the same multi-technique approach used in the companion paper (Oberti et al. 2006a), shows that (1) Sc is incorporated exclusively at the Y octahedron; (2) the local coordination of Sc is slightly different in Sc-poor than in Sc-rich compositions (Sc-O = 2.06 angstrom in CaSc10 vs. 2.10 angstrom in CaSc30-90); (3) the local coordination of Ca is also slightly different in Sc-poor than in Sc-rich compositions [Ca1,2-O are 2.34(2) and 2.48(2) angstrom in CaSc10 and 2.36(2) and 2.50(2) angstrom in CaSc90, with Delta fixed at 0.14 angstrom in all the samples]; (4) the linear increase of the unit-cell edge along the join derives from multiple changes in the geometry of the different polyhedra and from the rotation of the tetrahedron around the 4 axis (alpha rotation), and cannot be modeled from extrapolation of the behavior observed along the Ca3Al2Si3O12-Ca3Fe23+Si3O12=(grs-adr) join. CaSc-rich garnets, where a large X dodecahedron coexists with a large Y octahedron and a Z tetrahedron occupied by Si, similar to pyrope-grossular garnets, have the highest a values observed to date in calcium silicate garnets. Slightly lower of values are observed in pyrope and almandine, but correspond to a different structural arrangement, where a small X dodecahedron coexists with a small Y octahedron. These results further confirm the efficiency of a combined short- and long-range approach for understanding the properties of garnet solid solutions.