The rare earth elements, lanthanum to lutetium (atomic numbers 57—71), are members of Group IIIA in the periodic table (Fig. 1.1) and all have very similar chemical and physical properties. This uniformity arises from the nature of their electronic configurations, leading to a particularly stable 3+ oxidation state and a small but steady decrease in ionic radius with increase in atomic number for a given co-ordination number. Despite the similarity in their chemical behaviour, these elements can be partially fractionated, one from the other, by several petrological and mineralogical processes. The wide variety of types and sizes of the cation co-ordination polyhedra in rock-forming minerals provides the means for this chemical fractionation: it is this phenomenon which has important consequences in geochemistry <...>