Crystal-chemical characterization of 281 gamets, 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 [with E(Mg,Fe,Mn) as the dom inant X cationsl and ugrandites (with Ca as the dominant X-cation) 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. In the former configuration, occurring in pyralspitic gamets, the shared X-X and X-Y edges are shorter than the unshared ones; in the latter configuration, occurring in ugrandites, the opposite situation is present. Most structural parameters within (Mg,Fe,Mn)-Ca gamet solid-solutions show non-linear variations: this fact, together with spectroscopic (XANES) data, rules out the presence of domains with either pyralspitic or grossular configurations; thus, the same X cation shows a different bonding behaviour, depending on the gamet structure (pyralspitic or ugranditic) in which it occurs. The unit-cell edge and the fractional coordinates of the oxygen atom have been modelIed along the PYR-ALM, ALM-GRO and PYR-GRO joins. Positive excess volumes of mixing (L'!. V) are present along all three solid-solu tions. The L'!. V value increases, and its compositional dependence is progressively more symmetric, with increasing size difference between the component X cations. The maximum calculated L'!.V of mixing along the PYR-GRO join (-7.5 A3 per unit cell) is mainly due to the excess volume increase of the XOg polyhedron with increasing Ca substitution (-5.5 A3 per unit cell). The Y06 octahedron also shows an excess volume, but its contribution to the L'!. V of mixing is just enough to compensate for the negative L'!. V shown by the Z0 4 tetrahedron (-0.3 A3 per unit ce1I).
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