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Rare earth elements and rare earth metals are a collection of sixteen chemical elements in the periodic table, namely scandium, yttrium, and fourteen of the fifteen lanthanides (excluding promethium), which naturally occur on the Earth. The former two are included as they tend to occur with the latter in the same ore deposits. Some definitions additionally include the actinides. The terms "rare earth" and "rare earth metal" are trivial names that fall outside the official IUPAC nomenclature system. Outside of a strict scientific context, however, the terms retain their usability; for instance, the USPTO includes the term "rare earth" in the classification of metal alloys and other compounds, as well as distinguishing rare-earth magnets from other types of magnet.
"Earth" is an obsolete term for oxide; it is a translation from the French terre as French was the lingua franca when these elements were discovered at the beginning of the 19th century. "Rare" was used because some of these elements were believed to be scarce in abundance as minerals. However, the term "rare earth" is now deprecated by IUPAC, as these elements are (except highly-unstable promethium), in fact relatively abundant in the Earth's crust; the most abundant, cerium, at 68 parts per million, is the 25th most abundant element in the crust, more common than lead, while even the least abundant "rare" earth element, lutetium, is 200 times more abundant than gold.
The principal economic sources of rare earth elements are the minerals bastnasite, monazite, and loparite and the lateritic ion-adsorption clays. Despite their relative abundance, however, these are more difficult to mine and extract than the sources of transition metals (due in part to their very similar chemical properties), making them relatively expensive.
The following abbreviations are often used:
For more details of the properties and uses of these elements, refer to the lanthanides article.