The formation of scandium minerals as an example of the role of complexes in the geochemistry of rare earths and HFS elements

The stability constants of the fluoride complexes of scandium are much higher than those of most other elements; in particular, with respect to the REE, these complexes are in general much more stable than it would be expected on the basis of the corresponding ionic radii only. Accordingly, the presence of fluorides should easily lead to differentiation between scandium and other elements, including the REE, at times giving origin to specific minerals; a similar behaviour can be expected for zirconium or other HFS elements, whose complexes with the fluoride ion are also very stable, and in general for other ligands, such as for instance the phosphate and carbonate ions. Therefore, rather than the specific properties of sites in crystal structures, the formation of complexes of different stability in the depositing solutions and their disruption may become instead fundamental in explaining the enrichment of these elements, and the consequent formation of particular minerals of unusual composition. The practical consequences of this possibility are supported by crystal-chemical considerations and by a number of examples observed in Alpine veins and in pegmatites.