The rare-earth elements

What they are

The rare earths are the fifteen elements from lanthanum, atomic number 57, to lutetium, 71 — the lanthanides. Yttrium, atomic number 39, is counted with them because its charge and radius make it behave like one.^[1]

They are conventionally split by atomic weight into light and heavy groups, a division of convenience that tracks the steady contraction in ionic radius across the series.^[1]

Why they travel together

The rare earths occur together because they share a trivalent charge and similar ionic radii — so a site that will take one will usually take its neighbours.^[1]

Two break the pattern. Cerium can be tetravalent and europium divalent, and when they are, their charge and radius change and they separate from the rest. Redox chemistry becomes a fractionation mechanism.^[1,2]

The contraction that makes them separable

Across the lanthanides each added electron enters an inner, incomplete 4f subshell rather than an outer one, so the trivalent ionic radius decreases steadily from lanthanum to lutetium.^[1]

That systematic decrease is what allows the rare earths to be fractionated from one another in geological environments at all — and it is a consequence of electron configuration, not of geology.^[1,2]

The name is misleading

Cerium is the most abundant rare earth, at about 66.5 parts per million in the crust on average — more common there than copper or lead. Every rare earth except promethium is on average more abundant in the crust than silver, gold or platinum.^[1,2,3]

Promethium is the exception that earns the name: it is excluded from discussions of rare-earth deposits because it is extremely rare and unstable in nature.^[1]

So what is limited is not the elements but the deposits. Concentrated, economic accumulations of rare earths are unusual and occur in unusual rock types and uncommon minerals. Presence in the crust and recoverability are entirely different questions.^[1]

Commonly misunderstood