As I mentioned in the introduction, the Rare Earth Elements (known to chemists as lanthanides) are an esoteric yet commonly studied group of elements. The reason they are studied is that both their behavior as a group, and the more subtle change in behavior between the different rare earth elements can reveal information about the system in which they are observed.
The REE are refactory lithophile elements, meaning that for the most part they condensed at high temperature in the solar nebula as oxides*, and thus have similar behavior to calcium and aluminum during the planet-forming process.
In nature, the REE on earth generally form large, 3+ cations in a variety of complex oxides (figure 1). They are most commonly found as trace elements in silicates, but are readily concentrated in phosphate minerals. They rarely occur as carbonates.
Figure 1. ionic radii of the REE and selected other elements. Data from Shannon & Prewitt (1969), via the web. REE are light blue, comparison trivalent cations are dark blue. Divalent ions are green, with light green for Eu. Tetravalent cations are red, with pink for Ce. pdf available on request.
The ionic radius of the REE decreases with increasing atomic number, so that lutetium is about 20% smaller than lanthanum. The heavy rare earth elements (HREE) are similar to yttrium, but are still substantially larger than other common rock-forming trivalent elements (figure 1).
Although all REE are generally trivalent, two of them have other valence states that occur in nature. Europium can have a +2 valence under moderately reducing conditions, which makes it behave much like the element strontium (figure 1). Under oxidizing conditions found in surface processes on the modern Earth, cerium can be tetravalent, and Ce+4 has a size intermediate between zirconium and uranium.
In general, the large ionic radius makes the REE incompatible in most mantle mineral lattices (which are comprised mostly of Mg, Si, Al, and Ca). So mantle melts are enriched in REE relative to the residual mantle. The continental crust in enriched further still relative to the oceanic crust. However, the larger light rare earth elements are more incompatible than the more compact heavy rare earths. The degree of incompatibility is related to the minerals present in the mantle when it melts, so the pattern of REE in igneous rock at the surface can give us a clue as to what minerals are present deep in the inaccessible part of the Earth where the melting occurs.
R. D. Shannon and C. T. Prewitt, Acta Cryst., 1969, B25, 925
Rare Earth Revelry
* We’re ignoring enstatite chondrites for now. I’ll come back to them another time.