Trivalent geochemical behaviour

The rare earth elements (REE) form a group of elements that have coherent geochemical behaviour due to their trivalent charge and similar ionic radii. They can, however, be fractionated from one another as a result of geochemical processes operating under specific physico-chemical conditions. In order to outline general trends within and differences between the individual REE, concentrations are usually normalized to a reference system (e.g. to shale). Deviations of individual elements from the generally smooth trend are referred to as anomalies. 

Our results show that coprecipitation of the REE with phosphate removed Ce, Pr, Nd, Sm, and Eu more easily from the brine than other REE. This finding might be of importance for the mobility of trivalent Am and Cm in a radioactive waste salt repository, because for these elements, owing to their almost identical ionic radii, an almost analogous geochemical behaviour is expected as for Sm and Nd (Choppin 1983 Krauskopf 1986). These radionuclides would, in the case of a leaking HLW salt repository, probably be retained when phosphate minerals are present in the backfill material. 

Fig. 1. The relationship between valency and radius for the rare earth elements (in larger font) and a number of other geochemically important species. Note the relative isolation of the trivalent lanthanides and yttrium from most common cations, which accounts for much of their geochemical behaviour. Sc on the other hand lies close to many other common trivalent cations and enters major rock-forming mineral phases. The increase in radius of divalent Eu brings it close to Sr, whose geochemical behaviour it mimics. Note the decrease in the radius of Ce" relative to Ce which brings it close both in size and valency to U and Th. (Data are from tables 1 and 2.)...

The subtle variations in ionic radius among the trivalent lanthanides, coupled with the variable oxidation state of Eu have made these elements of unique interest to students of terrestrial igneous rocks. A wide variety of patterns, which are in general quite distinctive for differing rock types, are found. In all these cases, there is a clear separation between the geochemical behaviour of Sc, which typically enters early crystallising pyroxene phases, and that of yttrium and the lanthanides. The discussion in the rest of this section concentrates on the latter. 

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