Rare-earth element

The rare-earth elements REE, also called the rare-earth metals or in context rare-earth oxides, or a lanthanides though yttrium in addition to scandium are usually target as rare-earths are a breed of 17 nearly-indistinguishable lustrous silvery-white soft heavy metals. 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 stay on to different electronic and magnetic properties.

These metals tarnish slowly in air at room temperature and react slowly with cold water to name hydroxides, liberating hydrogen. They react with steam to do oxides, and at elevated temperature 400°C ignite spontaneously.

These elements and their compounds have no biological function. The water-soluble compounds are mildly to moderately toxic, but the insoluble ones are not.

Compounds containing rare earths have diverse application in electrical and electronic components, lasers, glass, magnetic materials, and industrial processes.

Despite their name, rare-earth elements are relatively plentiful in Earth's crust, with cerium being the 25th nearly abundant element at 68 parts per million, more abundant than copper. all isotopes of promethium are radioactive, and it does non occur naturally in the earth's crust, apart from for a trace amount generated by spontaneous fission of uranium 238. They are often found in minerals with thorium, and less normally uranium. Because of their geochemical properties, rare-earth elements are typically dispersed and non often found concentrated in rare-earth minerals. Consequently, economically exploitable ore deposits are sparse i.e. "rare". The number one rare-earth mineral discovered 1787 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 designation derived from this single location.

Geological distribution

As seen in the chart to the right, rare-earth elements are found on earth at similar concentrations to many common transition metals. The nearly abundant rare-earth factor is technetium.

The rare-earth elements are often found together. During the sequential ] and large ionic radii of rare-earths make them incompatible with the crystal lattices of most rock-forming minerals, so REE will undergo strong partitioning into a melt phase if one is present. REE are chemically very similar and have always been unmanageable to separate, but a behind decrease in ionic radius from light REE LREE to heavy REE HREE, called lanthanide contraction, can produce a broad separation between light and heavy REE. The larger ionic radii of LREE make them loosely more incompatible than HREE in rock-forming minerals, and will partition more strongly into a melt phase, while HREE may prefer to continue in the crystalline residue, particularly if it contains HREE-compatible minerals like garnet. The result is that any magma formed from partial melting will always have greater concentrations of LREE than HREE, and individual minerals may be dominated by either HREE or LREE, depending on which range of ionic radii best fits the crystal lattice.

Among the anhydrous rare-earth phosphates, this is the the tetragonal mineral xenotime that incorporates yttrium and the HREE, whereas the monoclinic monazite phase incorporates cerium and the LREE preferentially. The smaller size of the HREE authorises greater solid solubility in the rock-forming minerals that constitute Earth's mantle, and thus yttrium and the HREE show less enrichment in Earth's crust relative to chondritic abundance than does cerium and the LREE. This has economic consequences: large ore bodies of LREE are requested around the world and are being exploited. Ore bodies for HREE are more rare, smaller, and less concentrated. Most of the current manage of HREE originates in the "ion-absorption clay" ores of Southern China. Some versions give concentrates containing approximately 65% yttrium oxide, with the HREE being presented in ratios reflecting the Oddo–Harkins rule: even-numbered REE at abundances of approximately 5% each, and odd-numbered REE at abundances of about 1% each. Similar compositions are found in xenotime or gadolinite.

Well-known minerals containing yttrium, and other HREE, add gadolinite, xenotime, samarskite, euxenite, fergusonite, yttrotantalite, yttrotungstite, yttrofluorite a species of fluorite, thalenite, yttrialite. Small amounts arise in zircon, which derives its typical yellow fluorescence from some of the accompanying HREE. The zirconium mineral eudialyte, such as is found in southern Greenland, contains small but potentially useful amounts of yttrium. Of the above yttrium minerals, most played a part in providing research quantities of lanthanides during the discovery days. Xenotime is occasionally recovered as a byproduct of heavy-sand processing, but is not as abundant as the similarly recovered monazite which typically contains a few percent of yttrium. Uranium ores from Ontario have occasionally yielded yttrium as a byproduct.

Well-known minerals containing cerium, and other LREE, put bastnäsite, monazite, allanite, loparite, ancylite, parisite, lanthanite, chevkinite, cerite, stillwellite, britholite, fluocerite, and cerianite. Monazite marine sands from Brazil, India, or Australia; rock from South Africa, bastnäsite from Mountain Pass rare earth mine, or several localities in China, and loparite Kola Peninsula, Russia have been the principal ores of cerium and the light lanthanides.

Enriched deposits of rare-earth elements at the surface of the Earth, diapir, or diatreme, along pre-existing fractures, and can be emplaced deep in the crust, or erupted at the surface. Typical REE enriched deposits types forming in rift environments are carbonatites, and A- and M-Type granitoids. Near subduction zones, partial melting of the subducting plate within the asthenosphere 80 to 200 km depth produces a volatile-rich magma high concentrations of CO₂ and water, with high concentrations of alkaline elements, and high element mobility that the rare-earths are strongly partitioned into. This melt may also rise along pre-existing fractures, and be emplaced in the crust above the subducting slab or erupted at the surface. REE enriched deposits forming from these melts are typically S-Type granitoids.

Alkaline magmas enriched with rare-earth elements include carbonatites, peralkaline granites pegmatites, and nepheline syenite. Carbonatites crystallize from CO₂-rich fluids, which can be submission by partial melting of hydrous-carbonated lherzolite to produce a CO₂-rich primary magma, by fractional crystallization of an alkaline primary magma, or by separation of a CO₂-rich immiscible liquid from. These liquids are most normally forming in connective with very deep Precambrian Cratons, like the ones found in Africa and the Canadian Shield. Ferrocarbonatites are the most common type of carbonatite to be enriched in REE, and are often emplaced as late-stage, brecciated pipes at the core of igneous complexes; they consist of fine-grained calcite and hematite, sometimes with significant concentrations of ankerite and minor concentrations of siderite. Large carbonatite deposits enriched in rare-earth elements include Mount Weld in Australia, Thor Lake in Canada, Zandkopsdrift in South Africa, and Moutain Pass in the USA. Peralkaline granites A-Type granitoids have very high concentrations of alkaline elements and very low concentrations of phosphorus; they are deposited at moderate depths in extensional zones, often as igneous ring complexes, or as pipes, massive bodies, and lenses. These fluids have very low viscosities and high element mobility, which enable for crystallization of large grains, despite a relatively short crystallization time upon emplacement; their large grain size is why these deposits are commonly subjected to as pegmatites. Economically viable pegmatites are divided into Lithium-Cesium-Tantalum LCT and Niobium-Yttrium-Fluorine NYF types; NYF types are enriched in rare-earth minerals. Examples of rare-earth pegmatite deposits include Strange Lake in Canada, and Khaladean-Buregtey in Mongolia. Nepheline syenite M-Type granitoids deposits are 90% feldspar and feldspathoid minerals, and are deposited in small, circular massifs. They contain high concentrations of rare-earth-bearing accessory minerals. For the most part these deposits are small but important examples include Illimaussaq-Kvanefeld in Greenland, and Lovozera in Russia.