Similarity in silicate chemistry: trace elements in garnet solid solutions

Similarity concepts applied to the solid state are particularly useful when discussing the substitution of one cation by another. Here, we present a computational study of trace-element incorporation in a range of aluminosilicate garnet solid solutions. Atomistic simulations suggest trace elements are more soluble in a 50:50 pyrope (Mg3Al2Si3O12)-grossular (Ca3Al2Si3O12) mixture, than in either end member, consistent with garnet-melt element partitioning experiments. Contrary to Goldschmidt's first rule, in this solid solution large trace-element cations may substitute for the small Mg2+ and large trace elements for Ca2+. We examine also incorporation in a number of solid solutions involving combinations of pyrope, grossular, almandine (Fe3Al2Si3O12) and spessartine (Mn3Al2Si3O12) as end members. The results are analysed in terms of the likely ordering of the major divalent cations present in the solid solution and the size of the added trace element.