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Rare earth elements are commonly defined as 17 chemical elements: the 15 lanthanides, plus scandium and yttrium. The name can mislead: several are relatively abundant in Earth’s crust, but deposits concentrated enough to mine economically are limited.
Which elements are rare earth elements?
The broad, commonly used definition includes scandium, yttrium and all 15 lanthanides. The lanthanides are the elements from lanthanum (atomic number 57) through lutetium (atomic number 71).
| Group | Elements | How it is used in definitions |
|---|---|---|
| Lanthanides | Lanthanum through lutetium (15 elements) | The 15-element group at the core of rare-earth definitions. |
| Yttrium | Yttrium | Commonly grouped with the lanthanides because of chemical and physical similarities and natural affinities. |
| Scandium | Scandium | Included in the commonly used broad 17-element definition. |
Promethium is one of the 15 lanthanides and therefore part of the 17-element grouping. However, a 2019 USGS fact sheet says it does not occur in Earth’s crust: USGS rare-earth elements information.
Why do some sources define the group differently?
The membership wording depends on the source’s context. The USGS National Minerals Information Center describes rare earths as a group of 17 elements—scandium, yttrium and the lanthanides: Rare Earths Statistics and Information. Another USGS publication describes the 15 lanthanides as rare-earth elements and says yttrium is commonly regarded as one because of its similarities and affinities: Rare-earth elements. USGS thesaurus wording also says yttrium and scandium are often included: USGS Thesaurus: rare earth elements.
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These are variations in how sources frame the group, not disagreement about which elements are lanthanides or their atomic numbers. When a definition matters, check whether it means the 15 lanthanides, the lanthanides plus yttrium, or the broader group of 17.
Are rare earth elements actually rare?
Not uniformly. “Rare” does not mean that every element in the group is scarce in average crustal abundance. USGS says the elements are relatively abundant overall, while deposits concentrated enough to exploit are limited: USGS, December 19, 2017.
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As examples of crustal abundance, USGS lists cerium at 60 parts per million and thulium and lutetium at about 0.5 parts per million each on its overview page, accessed in 2026: USGS Rare Earths Statistics and Information. These figures describe abundance in the crust; they do not indicate how much can be economically extracted from a particular deposit.
Where are rare earth elements found?
Rare earth elements occur in minerals and deposits such as bastnaesite, monazite, loparite and lateritic ion-adsorption clays, identified by the USGS: Rare Earths Statistics and Information. These names refer to minerals or deposit types that contain rare earths, not additional elements in the chemical group.
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What are rare earth elements used for?
Uses vary by element and material; the elements are not interchangeable. USGS examples include glass polishing and glass additives for color or optical properties, lanthanum-based catalysts in petroleum refining, cerium-based catalysts in automotive catalytic converters, and magnets as a growing application: USGS Rare Earths Statistics and Information. These examples describe particular uses, not a claim that every rare earth is used in each application.
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