Ionic radii of the REE, 1: A novel method to refine ionic radii shows Y3+ is smaller than assumed in common minerals

The ionic radii of rare-earth elements (REE: Sc, Y, and the lanthanides) are key to interpreting partitioning behavior among minerals and melts for use as petrogenetic tracers and thermometry. Here, we show that published experimental data on mineral-melt partitioning of REE imply the ionic radius of Y3+ is smaller than commonly assumed by 0.002Å for 6-fold coordination to 0.004Å for 8- and 9-fold coordination. This difference reconciles the partitioning behavior of Y3+ compared to other REE and improves reference states for interpretations of trace element systematics in rocks. Thermobarometry can be highly sensitive to assumed ionic radii, and downward correction of the radius for Y3+ improves some REE-based temperatures by hundreds of degrees. Future studies that employ the common tabulation of ionic radii from Shannon (1976; Acta Crystallographica, A32, 751-767) should use an ionic radius of Y3+ of 0.898, 1.015, and 1.071Å for 6-, 8-, and 9-fold coordination; alternatively, the ionic radius of Y can be scaled to that of Ho3+ x 0.999. More generally, trace element partitioning data coupled with theoretical models provide a novel method to critically evaluate and refine effective relative ionic radii.