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Rare earth element | Rare

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 lanthanoids (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 actinoids. 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 United States Patent and Trademark Office (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.

Rare earth elements became known to the world with the discovery of the the black mineral ytterbite (alternately know as gadolinite) by Lieutenant Karl Arrhenius in the year 1887, in a quarry in the village of Ytterby, Sweden. Many of the rare earths are named in honor of the scientists who discovered or elucidated the elemental properties, geographical discovery, Latin or Greek, or mythology:

Gadolinium Johan Gadolin (1760-1852), to honor his investigation of rare earths.
Samarium V.E. Samarsky, discovered the rare-earth ore called samarskite.
Ytterbium named after the Ytterby, Sweden, where the first rare earth ore was discovered.
Thulium refers to the mythological land of Thule.
Promethium after Prometheus who brought fire to mankind.
Cerium after Greek deity of fertility, Ceres.
Lanthanum from the Greek “lanthanon” meaning I am hidden.
Dysprosium from the Greek “dysprositos” meaning hard to get.
Praseodymium from the Greek “praso” which means leek-green.
Neodymium from a Greek word “neo” which means new-one.

“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 rare-earth 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. Their industrial use was very limited until efficient separation techniques were developed, such as ion exchange, fractional crystallization and liquid-liquid extraction during the late 50’s and early 60’s. Spedding F, Daane AH: “The Rare Earths”, John Wiley & Sons, Inc., 1961

The following abbreviations are often used:

  • REE = rare earth elements
  • LREE = light rare earth elements (La-Sm)
  • HREE = heavy rare earth elements (Eu-Lu)

For more details of the properties and uses of these elements, refer to the lanthanoids article.


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