Rare-earth element
The rare-earth elements, also called rare-earth metals, or rare earths, are a set of 17 nearly indistinguishable lustrous silvery-white soft heavy metals. The 15 lanthanides, along with scandium, and yttrium, are usually included as rare earths. Compounds containing rare-earths have diverse applications in electrical and electronic components, lasers, glass, magnetic materials, and industrial processes. Rare-earths are to be distinguished from critical minerals, which are materials of strategic or economic importance that are defined differently by different countries, and rare-earth minerals, which are minerals that contain one or more rare-earth elements as major metal constituents.
The term "rare-earth" is a misnomer, because they are not actually scarce, but because they are found only in compounds, not as pure metals, and are difficult to isolate and purify. They are relatively plentiful in the entire Earth's crust, but in practice they are spread thinly as trace impurities, so to obtain rare earths at usable purity requires processing enormous amounts of raw ore at great expense.
Scandium and yttrium are considered rare-earth elements because they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties, but have different electrical and magnetic properties. All isotopes of promethium are radioactive, and it does not occur naturally in the earth's crust, except for a trace amount generated by spontaneous fission of uranium-238. They are often found in minerals with thorium, and less commonly uranium.
Because of their geochemical properties, rare-earth elements are typically dispersed and not often found concentrated in rare-earth minerals. Consequently, economically exploitable ore deposits are sparse. The first rare-earth mineral discovered was gadolinite, a black mineral composed of cerium, yttrium, iron, silicon, and other elements. This mineral was extracted from a mine in the village of Ytterby in Sweden. Four of the rare-earth elements bear names derived from this single location. Commercial production in modern times describes the reserves of the rare-earth elements in terms of "rare-earth oxides" containing mixtures of various rare earth elements in oxide compounds.
The uses, applications, and demand for rare-earth elements have expanded over the years. In 2015, most REEs were being used for catalysts and magnets. The global move towards renewable energy technologies, such as electric vehicles and wind turbines, along with advanced electronics, defence applications, and consumer electronics such as smartphones, has caused increased demand for REEs.
China dominates the rest of the world in terms of REE reserves and production; in 2019, it supplied around 90% of the global demand for the 17 rare-earth powders. The Chinese government has placed restrictions on its supply and sales of REEs since around 2010 for various reasons. After United States president Donald Trump escalated the trade war with China in 2025, China introduced further restrictions, leading other countries with known reserves to step up their exploration and production efforts., the US and Myanmar produce the second- and third-highest amounts of REEs, but Brazil and India have the second- and third-largest reserves of the metals.
History
1787: Discovery
Rare earths were mainly discovered as components of minerals. The term "rare" refers to these rarely found minerals and "earth" comes from an old name for oxides, the chemical form for these elements in the mineral. The adjective "rare" may also mean strange or extraordinary.In 1787, a mineral discovered by Lieutenant Carl Axel Arrhenius at a quarry in the village of Ytterby, Sweden, reached Johan Gadolin, a Royal Academy of Turku professor, and his analysis yielded an unknown oxide which he called yttria.
1794–1878: Chemical isolation
, Swedish analytical chemist, chemically isolated the beryllium from the gadolinite but failed to recognize other elements in the ore. After this discovery in 1794, a mineral from Bastnäs near Riddarhyttan, Sweden, which was believed to be an iron–tungsten mineral, was re-examined by Jöns Jacob Berzelius and Wilhelm Hisinger. In 1803, they obtained a white oxide and called it ceria. Martin Heinrich Klaproth independently discovered the same oxide and called it ochroia. It took another 30 years for researchers to determine that other elements were contained in the two ores ceria and yttria. The similarity of the rare-earth metals' chemical properties made their separation difficult.In 1839, Carl Gustav Mosander, an assistant of Berzelius, separated ceria by heating the nitrate and dissolving the product in nitric acid. He called the oxide of the soluble salt lanthana. It took him three more years to separate the lanthana further into didymia and pure lanthana. Didymia, although not further separable by Mosander's techniques, was in fact still a mixture of oxides.
In 1842, Mosander separated the yttria into three oxides: pure yttria, terbia, and erbia. All the names are derived from the town name "Ytterby". The earth giving pink salts he called terbium. The one that yielded yellow peroxide he called erbium.
By then the number of known rare-earth elements had reached six: yttrium, cerium, lanthanum, didymium, erbium, and terbium.
Nils Johan Berlin and Marc Delafontaine tried also to separate the crude yttria and found the same substances that Mosander obtained. In 1860, Berlin named the substance giving pink salts erbium. Delafontaine named the substance with the yellow peroxide, terbium. This confusion led to several false claims of new elements, such as the mosandrium of J. Lawrence Smith, or the philippium and decipium of Delafontaine. Due to the difficulty in separating the metals, and determining the separation is complete, the total number of false discoveries was dozens, with some putting the total number of discoveries at over a hundred.
1879–1930s: Spectroscopic identification
There were no further discoveries for 30 years, and the element didymium was listed in the periodic table of elements with a molecular mass of 138. In 1879, Delafontaine used the new physical process of optical flame spectroscopy and found several new spectral lines in didymia. Also in 1879, Paul Émile Lecoq de Boisbaudran isolated the new element samarium from the mineral samarskite.In 1886, the samaria earth was further separated by Lecoq de Boisbaudran. A similar result was obtained by Jean Charles Galissard de Marignac by direct isolation from samarskite. They named the element gadolinium after Johan Gadolin, and its oxide was named "gadolinia".
Further spectroscopic analysis between 1886 and 1901 of samaria, yttria, and samarskite by William Crookes, Lecoq de Boisbaudran and Eugène-Anatole Demarçay yielded several new spectral lines that indicated the existence of an unknown element. In 1901, the fractional crystallization of the oxides yielded europium.
In 1839, the third source for rare earths became available. This is a mineral similar to gadolinite called uranotantalum, now called "samarskite", an oxide of a mixture of elements such as yttrium, ytterbium, iron, uranium, thorium, calcium, niobium, and tantalum. This mineral from Miass in the southern Ural Mountains was documented by Gustav Rose. The Russian chemist R. Harmann proposed that a new element he called "ilmenium" should be present in this mineral, but later, Christian Wilhelm Blomstrand, Galissard de Marignac, and Heinrich Rose found only tantalum and niobium in it.
The exact number of rare-earth elements that existed was highly unclear, and a maximum number of 25 was estimated. Using X-ray spectra Henry Gwyn Jeffreys Moseley confirmed the atomic theory of Niels Bohr and simultaneously developed the theory of atomic numbers for the elements. Moseley found that the exact number of lanthanides had to be 15, revealing a missing element, element 61, a radioactive element with a half-life of 18 years.
Using these facts about atomic numbers from X-ray crystallography, Moseley also showed that hafnium would not be a rare-earth element. Moseley was killed in World War I in 1915, years before hafnium was discovered. Hence, the claim of Georges Urbain that he had discovered element 72 was untrue. Hafnium is an element that lies in the periodic table immediately below zirconium, and hafnium and zirconium have very similar chemical and physical properties.