Why is yttrium found with the heavy lanthanides in mineral deposits?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

Yttrium is found alongside the heavy lanthanides in mineral deposits primarily due to the similarity in their ionic radii.12

In chemistry, the behavior of rare-earth elements is largely dictated by their ionic size and charge. While yttrium is not a lanthanide, its ionic radius (approximately 0.9 Å) is nearly identical to that of holmium (0.901 Å). Because of this close match, yttrium ions can easily substitute for heavy lanthanide ions within the crystal structures of minerals such as xenotime, gadolinite, and euxenite. This chemical similarity makes them difficult to separate and ensures they occur together in nature.21