Non-ideal solid-solution in garnet: crystal-structure evidence and modelling

Crystal-chemical characterization of 281 garnets, representative of most geologic environments, has allowed distortion of the X, Y and Z polyhedra to be examined as a function of chemical composition, allowing us to understand the reasons for compositional preferences in nature. Pyralspites and ugrandites are separated by a significant compositional gap (1.5 to 2.1 Ca apfu). The existence and width of this gap have been explained on the basis of two different configurations for the X and Y polyhedra, corresponding to mainly ionic and mainly covalent bonding schemes, respectively. The unit-cell edge and the fractional coordinates of the oxygen atom have been modelled along the PYR-ALM, ALM-GRO and PYR-GRO joins. Positive excess volumes of mixing (?V) are present along all three solid-solutions. The ?V value increases, and its compositional dependence is progressively more symmetric, with increasing size difference between the component X cations. -from Authors