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

Roberta Oberti¹, Simona Quartieri^2,*, Maria Chiara Dalconi^2,, Federico Boscherini³, Gianluca Iezzi⁴, Massimo Boiocchi⁵ and Sigrid Griet Eeckhout⁶

¹ CNR, Istituto di Geoscienze e Georisorse, Università di Pavia, via Ferrata 1, I-27100 Pavia, Italy
² Dipartimento di Scienze della Terra,Università di Messina, Salita Sperone 31, I-98166 Messina S. Agata, Italy
³ Dipartimento di Fisica and CNISM, Università di Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy
⁴ Dipartimento di Scienze della Terra, Università di Chieti, via dei Vestini 30, I-66013 Chieti Scalo, Italy
⁵ Centro Grandi Strumenti, Università di Pavia, via Bassi 21, I-27100 Pavia, Italy
⁶ European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex, France

Correspondence: ^* E-mail: simonaq{at}unimo.it

We applied different independent techniques (electron microprobe analysis, structure refinement, 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 fit 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 averaged 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 D_Sc can be treated together with D_REE in models based on the elastic strain theory in garnets close to the pyrope composition, but deviate from the parabolic fit in grossular-rich garnets.

Key Words: Crystal structure • garnet • trace elements and REE • scandium • XAS • XRD data • crystal synthesis

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