Thematic Articles

The occurrences of rare earth elements (REEs) in North America are abundant and diverse in mineralogy and geology. The Mountain Pass carbonatite in California historically has been a major world source for the light REEs. Monazite sands have also been mined on a moderate level in the southeastern United States. Fluids released from the mining of uraninite at Elliot Lake, Ontario, were intermittently a source for yttrium. Peralkaline igneous rocks in several areas of North America are currently under exploration for the entire REE spectrum, with emphasis on the heavy REEs. Although many REE occurrences contain a substantial tonnage of REEs, amenability to mineral processing and extraction of the REEs must be definitively established in each case.

As a source of strategic commodities for high technologies, the deposits of rare earth elements (REEs) in China are a world-class phenomenon. The combination of the world’s largest accumulation of REEs in the Bayan Obo deposit and the low cost of mining the extremely valuable heavy REEs from residual deposits makes China almost a monopoly producer. Research on a range of Chinese deposits shows that not only hypogene but also secondary processes create economic REE deposits. These deposits have characteristic REE distribution patterns, which range from primary light REE enrichment in carbonatites from the Himalayan Mianning–Dechang orogenic belt and in metamorphosed carbonatite and polyphase mineralization at Bayan Obo, through unusual flat REE patterns in carbonatites from the Qinling orogenic belt, to strong secondary heavy REE enrichment in residual clays from southern China.

Although the rare earth elements have been thought by many to be immobile in hydrothermal fluids, we have known since the first attempts to separate them in the early nineteenth century that they are soluble in aqueous solutions. Driven by a need to isolate individual REEs for industrial applications, and more recently to explore for them, we have started to develop an understanding of their solubility and speciation in hydrothermal fluids. This knowledge is allowing us to understand the processes that promote their transport in the Earth’s crust, their concentration, and their fractionation.

Deposits of rare earth elements (REEs) in igneous rocks have played an instrumental role in meeting the growing industrial demand for these elements since the 1960s. Among the many different igneous rocks containing appreciable concentrations of REEs, carbonatites and peralkaline silicate rocks are the most important sources of these elements, both historically and for meeting the anticipated growth in REE demand. The contrasting geochemical and mineralogical characteristics of REE mineralization in carbonatites, peralkaline feldspathoid rocks, and peralkaline granites reflect different sources and evolutionary pathways of their parental magmas, as well as differences in the extent of postmagmatic reworking of primary REE minerals by hydrothermal fluids.

The long-term growth of numerous industries will depend on the ability to secure stable and diverse sources of rare earths. Recent years have seen unprecedented volatility in this sector, with the rare earths being increasingly considered as strategic and critical to a wide range of technologies. During the next few years, demand for some of the rare earths is expected to exceed supply. Chinese export-quota policies have had a severe impact on the market. Worldwide exploration efforts are now leading to the deployment of a rare earth supply chain based outside China.

The rare earth elements (REEs) are all around us, not only in nature but in our everyday lives. They are in every car, computer, smartphone, energy-efficient fluorescent lamp, and color TV, as well as in lasers, lenses, ceramics, and more. Scientific applications of these elements range from tracing the provenance of magmas and sediments to studying body structures with magnetic resonance imaging. The realization that we need rare earths for so many applications, but that their supply is effectively restricted to several mining districts in China, has brought these elements to the headlines and created a critical-metals agenda. Here we introduce the REE family: their properties, minerals, practical uses, and deposits. Potential sources of these elements are diverse and abundant if we can overcome the technical challenges of rare earth mining and extraction in an environmentally and socially responsible way.