What are Rare Earth Elements?

Rare Earth Elements (REEs) are a group of 17 metallic elements with very similar physical and chemical properties that make them nearly indistinguishable from one other. These elements occur together in the periodic table and include the 15 lanthanides (atomic numbers 57-71) in addition to scandium and yttrium (atomic numbers 21 and 39, respectively). Scandium and yttrium have very similar chemical properties to the lanthanides, and yttrium is often found in the same deposits as the other rare earth elements, so they are typically considered part of the rare earth element group. Promethium (atomic number 61) does not occur as a stable element in nature, unlike the others. 

The REEs are often classified into “heavy” and “light” groups; this refers to the atomic number of each element. The light REEs typically include lanthanum to gadolinium plus scandium, while the heavy REEs include terbium to lutetium plus yttrium, based on similarities in chemical behavior (USGS, 2019). However, there are no “set” rules that define how the two groups are divided and other scientists may make different distinctions. Despite these minor inconsistencies, REEs are divided into heavy and light groups because it is common to see preferential concentration of one group or the other in specific minerals. Therefore, REE-enriched minerals may have all of the REEs present, but often only contain highly elevated quantities of just the heavy or just the light rare earths. 

REEs are not actually rare, despite what their name may lead you to believe. REEs are found all over the Earth and are in fact quite common; there is more naturally occurring cerium (atomic number 58) than copper, and more of all of the other REEs (except promethium) than precious metals like gold and silver! REEs don’t occur in high concentrations in most minerals though, and REE-enriched minerals don’t tend to occur in high concentrations either. Further, rocks with REE-enriched minerals tend to occur in unusual geologic settings (USGS, 2014). This makes finding deposits of REEs that are concentrated enough to be economically minable very unusual.

WSGS Publication, Reports, and Datasets

The WSGS has published several statewide investigations of REEs in Wyoming: Rare Earth Elements in Wyoming and A Comprehensive Report on Rare Earth Elements in Wyoming. Recent and ongoing investigations around Wyoming include REEs in wide variety of geologic settings, from heavy mineral sandstones and coal to granitic pegmatites.