The economic geology of scandium, the runt of the rare earth element litter

Scandium is currently in high demand because of a number of technological advances in the aerospace and automotive sectors of the global economy. In this paper, we review the properties of scandium, the geology of the major economic and potentially economic scandium deposits, and the processes that may concentrate scandium to exploitable levels. We also show that, although scandium is classified as a rare earth element (REE), it behaves very differently from the rest of its family. The reason for this is that it has an ionic radius very similar to that of iron and magnesium and consequently concentrates easily in major ferromagnesian rock-forming minerals, notably clinopyroxene. Unlike the other REEs, it is therefore a compatible element. In many scandium deposits, clinopyroxene is the main ore mineral, although in some deposits scandium is hosted by minerals that also concentrate the other REEs. As is the case for these other REEs, the main source of scandium is the mantle and the conveyors of scandium are alkaline igneous rocks (and carbonatites), including Alaskan-type ultramafic rocks. The main magmatic processes involved in scandium concentration are partial melting and fractional crystallization. We model the fractional crystallization of clinopyroxene to predict the scandium content of Alaskan-type ultramafic rocks and use this information in conjunction with a simple model of fluid-assisted partial melting to explain the genesis of scandium-rich pegmatites. In addition to magmatic processes, aqueous fluids may play an important or even essential role in scandium ore formation. The lack of reliable high-temperature thermodynamic data for the aqueous scandium species precludes modeling their transport in hydrothermal fluids. However, the availability of ambient-temperature data allowed us to model scandium concentration by rainwater in laterite developed above an Alaskan-type ultramafic complex. This review is no more than an introduction to the economic geology of scandium and the processes that appear to be responsible for the genesis of scandium ores, but one which we hope will provide a guide to future in-depth studies of scandium deposits and strategies for their successful exploration and exploitation.