Original paper
Non-ideal solid-solution in garnet: crystal-structure evidence and modelling
Ungaretti, Luciano; Leona, Marco; Merli, Marcellο; Oberti, Roberta
European Journal of Mineralogy Volume 7 Number 6 (1995), p. 1299 - 1312
35 references
published: Dec 22, 1995
manuscript accepted: Jul 11, 1995
manuscript received: Jan 12, 1995
DOI: 10.1127/ejm/7/6/1299
Abstract
Abstract 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 [with Σ(Mg,Fe,Mn) as the dominant X cations] 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 garnets, 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 garnet 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 garnet structure (pyralspitic or ugranditic) in which it occurs. 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 (ΔV) are present along all three solid-solutions. The ΔV value increases, and its compositional dependence is progressively more symmetric, with increasing size difference between the component X cations. The maximum calculated ΔV of mixing along the PYR-GRO join (~ 7.5 Å3 per unit cell) is mainly due to the excess volume increase of the XO8 polyhedron with increasing Ca substitution (~ 5.5 Å3 per unit cell). The YO6 octahedron also shows an excess volume, but its contribution to the ΔV of mixing is just enough to compensate for the negative ΔV shown by the ZO4 tetrahedron (~ 0.3 Å3 per unit cell)
Keywords
garnet • crystal-chemistry • excess volume of mixing • non-ideal behaviour • structure modelling