Why is scandium usually absent from common rare-earth minerals?

Scandium is typically absent from common rare-earth minerals primarily due to its ionic radius. At 0.745 Å, the scandium ion ($S{c}^{3+}$) is significantly smaller than that of the smallest lanthanide, lutetium, which has an ionic radius of 0.861 Å.1

Because of this size discrepancy, scandium does not readily substitute into the crystal structures of standard rare-earth minerals, where it is generally found in concentrations of less than 0.01 percent by weight. Instead, its ionic radius is closer to that of common metal ions such as $F{e}^{3+}$, $N{b}^{5+}$, $U^{5+}$, and $W^{5+}$, which explains why it is more frequently recovered as a by-product from the processing of uranium or tungsten ores rather than from traditional rare-earth deposits. In contrast, yttrium is commonly found in heavy lanthanide minerals because its ionic radius of 0.9 Å is nearly identical to that of holmium.21