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

Investigation of scandium incorporation in garnets along the synthetic Ca3Fe2 3+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 Å in CaSc10 vs. 2.10 Å 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) Å in CaSc10 and 2.36(2) and 2.50(2) Å in CaSc90, with ∆ Þ xed at 0.14 Å 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 (α rotation), and cannot be modeled from extrapolation of the behavior observed along the Ca3Al2Si3O12–Ca3Fe2 3+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 α values observed to date in calcium silicate garnets. Slightly lower α 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 conÞ rm the efÞ ciency of a combined short- and long-range approach for understanding the properties of garnet solid solutions.