Rebooting REE3+ radii in common minerals—a practical approach

Abstract

Nearly all geochemical applications of ionic radii appeal to the classic tabulation of Shannon (Acta Crystallographica, A32:751–767, 1976). In that work, smoothing was applied to the crystallographic systematics of lanthanides to ensure consistent decreases in cationic radii with increasing atomic number—the lanthanide contraction. Recent work has now updated preferred radii, based on a much larger database of crystal structures. However, values have not been smoothed, and several average radii violate the lanthanide contraction principle. Here, we propose a set of effective ionic radii for trivalent lanthanides using simple regressions based in part on atomic theory, and verify that these radii satisfy theoretical principles of lattice strain models. We then use prior analysis of partitioning to propose an ionic radius for Y³⁺. The ionic radius of Sc³⁺ cannot be refined to higher precision using partitioning data because its ionic radius is so different from other rare-earth elements (REE) and because it does not necessarily substitute into the same crystallographic sites as other REE. Recommendations are provided for ionic radii for REE-poor silicates and for monazite, xenotime, zircon, and apatite.