The role of fluoride complexes in REE geochemistry and the importance of 4f electrons; some examples in minerals

The stability constants of the REE fluoride complexes increase on decreasing the ionic radius, thereby apparently confirming that the geochemical behaviour of REEs (apart from the well known exceptions of Eu and Ce) should depend on the values of the ionic radii almost exclusively. However, for scandium and yttrium, which are usually considered together with the lanthanides, the complexes are markedly more stable than it would be expected according to the values of their ionic radius. In nature, a notable enrichment (or depletion) of Y with respect to lanthanides is indeed often linked to the presence of fluorides or of other complexing agents. Since the ionic radius of Dy is close to that of Y, a useful parameter to evidence such a phenomenon is the Y/Dy abundance ratio, which is subject to extensive variation in minerals, whereas it should remain constant if the chemical properties would depend on the ionic radius only. Besides examples of such a variation observed in the Alpine region, the formation of remarkably unusual Yb-rich minerals in fluorine-rich pegmatites observed in the Kola peninsula or in Colorado can be explained. For scandium, the presence of fluorides emphasizes its difference with respect to most elements, including the REEs.