The USGS reports that the REE group is composed of 15 elements that range in atomic number from 57 (lanthanum) to 71 (lutetium) on the periodic table of elements, and are officially referred to as the lanthanoids, although they are commonly referred to as the lanthanides. The rare-earth element promethium (atomic number 61) is not included in discussions of REE deposits because the element is rare and unstable in nature. Yttrium (atomic number 39) is commonly regarded as an REE because of its chemical and physical similarities and affinities with the lanthanoids, and yttrium typically occurs in the same deposits as REEs. Scandium (atomic number 21) is chemically similar to, and thus sometimes included with, the REEs, but it does not occur in economic concentrations in the same geological settings as the lanthanoids and yttrium and will not be discussed further.
Traditionally, the REEs are divided into two groups on the basis of atomic weight ... (1) the light REEs are lanthanum through gadolinium (atomic numbers 57 through 64), and (2) the heavy REEs comprise terbium through lutetium (atomic numbers 65 through 71). Note ... Some authorities include europium and gadolinium within the group of heavy REEs. Yttrium, although light (atomic number 39), is included with the heavy REE group because of its similar chemical and physical properties. REEs generally occur in uncommon geologic rock types and settings. As mentioned earlier, REEs are common in the Earth crust but rarely in economic concentrations. Economic REE deposits occur primarily in four geologic environments: carbonatites, alkaline igneous systems, ion-absorption clay deposits, and monazite-xenotime-bearing placer deposits. Even within these deposit types, minable (economic) concentrations of REEs are rare. For example, globally there are more than 500 known carbonatites but only 6 are currently mined for REEs.
Original Working URL: https://pubs.usgs.gov/fs/2014/3078/pdf/fs2014-3078.pdf
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