Site preference and local geometry of Sc in garnets: I. Multifarious mechanisms in the pyrope-grossular join

We applied different independent techniques (electron microprobe analysis, structure reÞ nement, and X-ray absorption spectroscopy) to unravel the possible mechanisms of Sc incorporation in the pyrope-grossular join. Samples were synthesized at elevated pressure and temperature by adding 5 wt% of Sc2O3 to selected nominal compositions (pyrope, pyrope60grossular40, pyrope20grossular80, and grossular). In this way, the site of incorporation was not pre-determined, and only depends on the availability of a mechanism for local charge-balance. The EXAFS spectra of the two end-members could be analyzed by a multi-shell Þ t procedure, whereas the chemical heterogeneity of the Sc-doped solid-solution terms prevented this approach. However, the available information allows detection of different mechanisms of incorporation, which are active as a function of the bulk composition. In pyrope, Sc mainly enters the dodecahedral X site, and the local charge balance is achieved by incorporation of Mg at the adjacent tetrahedral Z site. Local charge-balance requirements suggest that a Z site occupied by Mg bridges two X sites occupied by Sc. When the entrance of Ca provides relaxation of the aver- aged structure, Sc may enter all the three available cation sites via the coupled heterovalent exchange XSc1 ZSc1 XMg–1 ZSi–1 and the homovalent exchange YSc1 YAl–1. In the samples of this work, there is an apparent limit in the Sc incorporation at the Y site, which is in contrast to the favored mechanism of incorporation in Sc-doped andradites. This limit can be explained in terms of relative dimensions of the structural sites when Al is the dominant Y cation. These results must be taken into account when evaluating trace-element behavior in garnets for geochemical purposes. In particular, they explain why DSc can be treated together with DREE in models based on the elastic strain theory in garnets close to the pyrope composition, but deviate from the parabolic Þ t in grossular-rich garnets.