Rare-earth elementsin the periodic table Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Rare-earth ore (shown with a 19 mm diameter US 1 cent coin for size comparison)Refined rare-earth oxides are hey, gritty powders usually brown or black, but can be lighter colors as shown here.Back row: gadolinium · praseodymium · ceriumMiddle row: samarium · lanthanum Front: neodymium
The rare-earth elements (REE), also called rare-earth metals, or rare earths, are a set of 17 nearly indistinguishable lustrous silvery-white soft hey metals. The 15 lanthanides (or lanthanoids),[a] along with scandium and yttrium, are usually included as rare earths. Compounds containing rare-earths he 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,[b] 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 (cerium being the 25th-most-abundant element at 68 parts per million, more abundant than copper), 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 he 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 (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 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" (REOs) containing mixtures of various rare earth elements in oxide compounds.
The uses, applications, and demand for rare-earth elements he 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 (EVs) 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. As of 2025[update], the US and Myanmar produce the second- and third-highest amounts of REEs, but Brazil and India he the second- and third-largest reserves of the metals.
History[edit] This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources in this section. Unsourced material may be challenged and removed.Find sources: "Rare-earth element" – news · newspapers · books · scholar · JSTOR (November 2025) (Learn how and when to remove this message) 1787: Discovery[edit]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.[2]: 5 The adjective "rare" may also mean strange or extraordinary.[3]: 12
In 1787, a mineral discovered by Lieutenant Carl Axel Arrhenius at a quarry in the village of Ytterby, Sweden,[2]: 9 reached Johan Gadolin, a Royal Academy of Turku professor, and his analysis yielded an unknown oxide which he called yttria.[4]
1794–1878: Chemical isolation[edit]Anders Gust Ekeberg, 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 Gust 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.[5] 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,[6][7] with some putting the total number of discoveries at over a hundred.[8]
1879–1930s: Spectroscopic identification[edit]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 ailable. 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 Gust 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 (columbium) 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.[9] 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.[10]
Using these facts about atomic numbers from X-ray crystallography, Moseley also showed that hafnium (element 72) 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 he very similar chemical and physical properties.
1940s onwards: Purification[edit]In the 1940s, Frank Spedding and others in the United States, during the Manhattan Project, developed chemical ion-exchange procedures for separating and purifying rare-earth elements. This method was first applied to the actinides for separating plutonium-239 and neptunium from uranium, thorium, actinium, and the other actinides in the materials produced in nuclear reactors. Plutonium-239 was very desirable because it is a fissile material.
2022: Flash heating isolation method[edit]A 2022 study mixed fly ash with carbon black and then sent a 1-second current pulse through the mixture, heating it to 3,000 °C (5,430 °F). The fly ash contains microscopic bits of glass that encapsulate the metals. The heat shatters the glass, exposing the rare earths. Flash heating also converts phosphates into oxides, which are more soluble and extractable. Using hydrochloric acid at concentrations less than 1% of conventional methods, the process extracted twice as much material.[11]
Etymology[edit]The term "rare" in "rare-earth" is a misnomer because they are not actually scarce, but rather because they are only found in compounds, not as pure metals, or perhaps because they were considered exotic at the time of their discovery. The "earth" part refers to an old term for minerals that dissolve in acids and thus are stable to oxidation.[12][13] They are never found in highly concentrated form, usually being mixed together with one another, or with radioactive elements such as uranium and thorium, and can only be separated from other materials or one another with difficulty. This makes them difficult to purify.[14]
List of rare-earth elements[edit]Rare-earth elements or minerals are distinct from minerals or materials described as critical minerals or raw materials, which refers to materials that are considered to be of strategic or economic importance to a country. There is no single list, but individual governments compile lists of materials that are critical for their own economies.[15]
A table listing the 17 rare-earth elements, their atomic number and symbol, the etymology of their names, and their main uses (see also Applications of lanthanides) is provided here. Some of the rare-earth elements are named after the scientists who discovered them, or elucidated their elemental properties, and some after the geographical locations where discovered.
Overview of rare-earth metal properties Z Symbol Name Etymology Selected applications Abundance[16][17](ppm[c]) 21 Sc Scandium from Latin Scandia (Scandinia). Light aluminium-scandium alloys for aerospace components, additive in metal-halide lamps and mercury-vapor lamps,[18] radioactive tracing agent in oil refineries 022 39 Y Yttrium after the village of Ytterby, Sweden, where the first rare-earth ore was discovered. Yttrium aluminium garnet (YAG) laser, yttrium vanadate (YVO4) as host for europium in television red phosphor, YBCO high-temperature superconductors, yttria-stabilized zirconia (YSZ) (used in tooth crowns; as refractory material - in metal alloys used in jet engines, and coatings of engines and industrial gas turbines; electroceramics - for measuring oxygen and pH of hot water solutions, i.e. in fuel cells; ceramic electrolyte - used in solid oxide fuel cell; jewelry - for its hardness and optical properties; do-it-yourself high temperature ceramics and cements based on water), yttrium iron garnet (YIG) microwe filters,[18] energy-efficient light bulbs (part of triphosphor white phosphor coating in fluorescent tubes, CFLs and CCFLs, and yellow phosphor coating in white LEDs),[19] spark plugs, gas mantles, additive to steel, aluminium and magnesium alloys, cancer treatments, camera and refractive telescope lenses (due to high refractive index and very low thermal expansion), battery cathodes (LYP) 033 57 La Lanthanum from the Greek "lanthanein", meaning to be hidden. High refractive index and alkali-resistant glass, flint, hydrogen storage, battery-electrodes, camera and refractive telescope lenses, fluid catalytic cracking catalyst for oil refineries 039 58 Ce Cerium after the dwarf planet Ceres, named after the Roman goddess of agriculture. Chemical oxidizing agent, polishing powder, yellow colors in glass and ceramics, catalyst for self-cleaning ovens, fluid catalytic cracking catalyst for oil refineries, ferrocerium flints for lighters, robust intrinsically hydrophobic coatings for turbine blades[20] 066.5 59 Pr Praseodymium from the Greek "prasios", meaning leek-green, and "didymos", meaning twin. Rare-earth magnets, lasers, core material for carbon arc lighting, colorant in glasses and enamels, additive in didymium glass used in welding goggles,[18] ferrocerium firesteel (flint) products, single-mode fiber optical amplifiers (as a dopant of fluoride glass) 009.2 60 Nd Neodymium from the Greek "neos", meaning new, and "didymos", meaning twin. Rare-earth magnets, lasers, violet colors in glass and ceramics, didymium glass, ceramic capacitors, electric motors in electric automobiles 041.5 61 Pm Promethium after the Titan Prometheus, who brought fire to mortals. Nuclear batteries, luminous paint 01×10−15[21][d] 62 Sm Samarium after mine official, Vasili Samarsky-Bykhovets. Rare-earth magnets, lasers, neutron capture, masers, control rods of nuclear reactors 007.05 63 Eu Europium after the continent of Europe. Red and blue phosphors, lasers, mercury-vapor lamps, fluorescent lamps, NMR relaxation agent 002 64 Gd Gadolinium after Johan Gadolin (1760–1852), to honor his investigation of rare earths. High refractive index glass or garnets, lasers, X-ray tubes, computer bubble memories, neutron capture, MRI contrast agent, NMR relaxation agent, steel and chromium alloys additive, magnetic refrigeration (using significant magnetocaloric effect), positron emission tomography scintillator detectors, a substrate for magneto-optical films, high performance high-temperature superconductors, ceramic electrolyte used in solid oxide fuel cells, oxygen detectors, possibly in catalytic conversion of automobile fumes. 006.2 65 Tb Terbium after the village of Ytterby, Sweden. Additive in neodymium based magnets, green phosphors, lasers, fluorescent lamps (as part of the white triband phosphor coating), magnetostrictive alloys such as terfenol-D, nal sonar systems, stabilizer of fuel cells 001.2 66 Dy Dysprosium from the Greek "dysprositos", meaning hard to get. Additive in neodymium based magnets, lasers, magnetostrictive alloys such as terfenol-D, hard disk drives 005.2 67 Ho Holmium after Stockholm (in Latin, "Holmia"), the native city of one of its discoverers. Lasers, welength calibration standards for optical spectrophotometers, magnetic fields,permanent magnets. 001.3 68 Er Erbium after the village of Ytterby, Sweden. Infrared lasers, vanadium steel, fiber-optic technology 003.5 69 Tm Thulium after the mythological northern land of Thule. Portable X-ray machines, metal-halide lamps, lasers 000.52 70 Yb Ytterbium after the village of Ytterby, Sweden. Infrared lasers, chemical reducing agent, decoy flares, stainless steel, strain gauges, nuclear medicine, earthquake monitoring 003.2 71 Lu Lutetium after Lutetia, the city that later became Paris. Positron emission tomography – PET scan detectors, high-refractive-index glass, lutetium tantalate hosts for phosphors, catalyst used in refineries, LED light bulb 000.8 ^ The 1985 International Union of Pure and Applied Chemistry "Red Book" (p. 45) recommends that lanthanoid is used rather than lanthanide. The ending "-ide" normally indicates a negative ion. However, owing to wide current usage, "lanthanide" is still allowed and is roughly analogous to rare-earth element.[1] ^ However many countries, including the United States, designate REEs as critical minerals. ^ Parts per million in Earth's crust, e.g. Pb=13 ppm ^ Promethium has no stable isotopes or primordial radioisotopes; trace quantities occur in nature as fission products. Classification[edit]Before the time that ion exchange methods and elution were ailable, the separation of the rare earths was primarily achieved by repeated precipitation or crystallization. In those days, the first separation was into two main groups, the cerium earths (lanthanum, cerium, praseodymium, neodymium, and samarium) and the yttrium earths (scandium, yttrium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium).
Europium, gadolinium, and terbium were either considered as a separate group of rare-earth elements (the terbium group), or europium was included in the cerium group, and gadolinium and terbium were included in the yttrium group. In the latter case, the f-block elements are split into half: the first half (La–Eu) form the cerium group, and the second half (Gd–Yb) together with group 3 (Sc, Y, Lu) form the yttrium group.
The reason for this division arose from the difference in solubility of rare-earth double sulfates with sodium and potassium. The sodium double sulfates of the cerium group are poorly soluble, those of the terbium group slightly, and those of the yttrium group are very soluble.[22] Sometimes, the yttrium group was further split into the erbium group (dysprosium, holmium, erbium, and thulium) and the ytterbium group (ytterbium and lutetium), but today the main grouping is between the cerium and the yttrium groups.[23] Today, the rare-earth elements are classified as light or hey rare-earth elements, rather than in cerium and yttrium groups.
Light versus hey classification[edit]The classification of rare-earth elements is inconsistent between authors.[3] The most common distinction between rare-earth elements is made by atomic numbers. Those with low atomic numbers are referred to as light rare-earth elements (LREE), those with high atomic numbers are the hey rare-earth elements (HREE), and those that fall in between are typically referred to as the middle rare-earth elements (MREE).[24] Commonly, rare-earth elements with atomic numbers 57 to 61 (lanthanum to promethium) are classified as light and those with atomic numbers 62 and greater are classified as hey rare-earth elements.[25]
Increasing atomic numbers between light and hey rare-earth elements and decreasing atomic radii throughout the series causes chemical variations.[25] Europium is exempt of this classification as it has two valence states: Eu2+ and Eu3+.[25] Yttrium is grouped as a hey rare-earth element due to chemical similarities.[26] The break between the two groups is sometimes put elsewhere, such as between elements 63 (europium) and 64 (gadolinium).[27] The actual metallic densities of these two groups overlap, with the "light" group hing densities from 6.145 (lanthanum) to 7.26 (promethium) or 7.52 (samarium) g/cc, and the "hey" group from 6.965 (ytterbium) to 9.32 (thulium), as well as including yttrium at 4.47. Europium has a density of 5.24.
Geochemical classification[edit]The REE geochemical classification is usually done on the basis of their atomic weight. One of the most common classifications divides REE into 3 groups: light rare earths (LREE - from 57La to 60Nd), intermediate (MREE - from 62Sm to 67Ho) and hey (HREE - from 68Er to 71Lu). REE usually appear as trivalent ions, except for Ce and Eu which can take the form of Ce4+ and Eu2+ depending on the redox conditions of the system. Consequentially, REE are characterized by a substantial identity in their chemical reactivity, which results in a serial behiour during geochemical processes rather than being characteristic of a single element of the series. Sc, Y, and Lu can be electronically distinguished from the other rare earths because they do not he f valence electrons, whereas the others do, but the chemical behiour is almost the same.
A distinguishing factor in the geochemical behiour of the REE is linked to the so-called "lanthanide contraction" which represents a higher-than-expected decrease in the atomic/ionic radius of the elements along the series. This is determined by the variation of the shielding effect towards the nuclear charge due to the progressive filling of the 4f orbital which acts against the electrons of the 6s and 5d orbitals. The lanthanide contraction has a direct effect on the geochemistry of the lanthanides, which show a different behiour depending on the systems and processes in which they are involved.[28]
The effect of the lanthanide contraction can be observed in the REE behiour both in a CHARAC-type geochemical system (CHArge-and-RAdius-Controlled[28]) where elements with similar charge and radius should show coherent geochemical behiour, and in non-CHARAC systems, such as aqueous solutions, where the electron structure is also an important parameter to consider as the lanthanide contraction affects the ionic potential. A direct consequence is that, during the formation of coordination bonds, the REE behiour gradually changes along the series. Furthermore, the lanthanide contraction causes the ionic radius of Ho3+ (0.901 Å) to be almost identical to that of Y3+ (0.9 Å), justifying the inclusion of the latter among the REE.
Origin of rare-earth elements[edit]Rare-earth elements, except scandium, are heier than iron and thus are produced by supernova nucleosynthesis or by the s-process in asymptotic giant branch stars. In nature, spontaneous fission of uranium-238 produces trace amounts of radioactive promethium, but most promethium is synthetically produced in nuclear reactors. Due to their chemical similarity, the concentrations of rare earths in rocks are only slowly changed by geochemical processes, making their proportions useful for geochronology and dating fossils.
The principal sources of rare-earth elements are the minerals bastnäsite (RCO3F, where R is a mixture of rare-earth elements), monazite (XPO4, where X is a mixture of rare-earth elements and sometimes thorium), and loparite ((Ce,Na,Ca)(Ti,Nb)O3), and the lateritic ion-adsorption clays. Despite their high relative abundance, rare-earth minerals are more difficult to mine and extract than equivalent sources of transition metals, due in part to their similar chemical properties, making the rare-earth elements relatively expensive. Their industrial use was very limited until efficient separation techniques were developed, such as ion exchange, fractional crystallization, and liquid–liquid extraction in the late 1950s and early 1960s.[29]
Some ilmenite concentrates contain small amounts of scandium and other rare-earth elements, which could be analysed by X-ray fluorescence (XRF).[30]
Properties[edit]According to chemist Andrea Sella in 2016, rare-earth elements differ from other elements, in that when looked at analytically, they are virtually inseparable, hing almost the same chemical properties. However, in terms of their electronic and magnetic properties, each one occupies a unique technological niche that nothing else can.[31] For example, "the rare-earth elements praseodymium (Pr) and neodymium (Nd) can both be embedded inside glass and they completely cut out the glare from the flame when one is doing glass-blowing."[31]
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 he different electrical and magnetic properties.[31][32]
Rare-earth metals tarnish slowly in air at room temperature and react slowly with cold water to form hydroxides, liberating hydrogen. They react with steam to form oxides and ignite spontaneously at a temperature of 400 °C (752 °F). These elements and their compounds he no biological function other than in several specialized enzymes, such as in lanthanide-dependent methanol dehydrogenases in bacteria.[33] The water-soluble compounds are mildly to moderately toxic, but the insoluble ones are not.[34] 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.
Rare-earth compounds[edit]Rare-earth elements occur in nature in combination with phosphate (monazite), carbonate-fluoride (bastnäsite), and oxygen anions.
In their oxides, most rare-earth elements only he a valence of 3 and form sesquioxides (cerium forms CeO2). Five different crystal structures are known, depending on the element and the temperature. The X-phase and the H-phase are only stable above 2000 K. At lower temperatures, there are the hexagonal A-phase, the monoclinic B-phase, and the cubic C-phase, which is the stable form at room temperature for most of the elements. The C-phase was once thought to be in space group I213 (no. 199),[35] but is now known to be in space group Ia3 (no. 206).
The structure is similar to that of fluorite or cerium dioxide (in which the cations form a face-centred cubic lattice and the anions sit inside the tetrahedra of cations), except that one-quarter of the anions (oxygen) are missing. The unit cell of these sesquioxides corresponds to eight unit cells of fluorite or cerium dioxide, with 32 cations instead of 4. This is called the bixbyite structure, as it occurs in a mineral of that name ((Mn,Fe)2O3).[36]
Geological distribution[edit] The abundance of elements in Earth's crust per million Si atoms (y axis is logarithmic)The rare-earth elements are found on Earth at similar concentrations to many common transition metals. The most abundant rare-earth element is cerium, which is actually the 25th most abundant element in Earth's crust, hing 68 parts per million (about as common as copper). The exception is the highly unstable and radioactive promethium "rare earth" is quite scarce. The longest-lived isotope of promethium has a half-life of 17.7 years, so the element exists in nature in only negligible amounts (approximately 572 g in the entire Earth's crust).[37] Promethium is one of the two elements that do not he stable (non-radioactive) isotopes and are followed by (i.e. with higher atomic number) stable elements (the other being technetium).
The rare-earth elements are often found together. During the sequential accretion of the Earth, the dense rare-earth elements were incorporated into the deeper portions of the planet. Early differentiation of molten material largely incorporated the rare earths into mantle rocks.[38] The high field strength[clarification needed] 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.[38]
REE are chemically very similar and he always been difficult to separate, but the gradual decrease in ionic radius from light REE (LREE) to hey REE (HREE), called the lanthanide contraction, can produce a broad separation between light and hey REE. The larger ionic radii of LREE make them generally more incompatible than HREE in rock-forming minerals, and will partition more strongly into a melt phase, while HREE may prefer to remain in the crystalline residue, particularly if it contains HREE-compatible minerals like garnet.[38][39] The result is that all magma formed from partial melting will always he 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.[38]
Among the anhydrous rare-earth phosphates, it is 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 allows greater solid solubility in the rock-forming minerals that make up 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.[40]
This has economic consequences: large ore bodies of LREE are known around the world and are being exploited. Ore bodies for HREE are more rare, smaller, and less concentrated. Most of the current supply of HREE originates in the "ion-absorption clay" ores of Southern China. Some versions provide concentrates containing about 65% yttrium oxide, with the HREE being present in ratios reflecting the Oddo–Harkins rule: even-numbered REE at abundances of about 5% each, and odd-numbered REE at abundances of about 1% each. Similar compositions are found in xenotime or gadolinite.[40]
Well-known minerals containing yttrium, and other HREE, include gadolinite, xenotime, samarskite, euxenite, fergusonite, yttrotantalite, yttrotungstite, yttrofluorite (a variety of fluorite), thalenite, and yttrialite. Small amounts occur 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 (an autonomous territory of Denmark), 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 hey-sand processing, but is not as abundant as the similarly recovered monazite (which typically contains a few percent of yttrium). Uranium ores from Ontario he occasionally yielded yttrium as a byproduct.[40]
Well-known minerals containing cerium, and other LREE, include 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) he been the principal ores of cerium and the light lanthanides.[40]
Enriched deposits of rare-earth elements at the surface of the Earth, carbonatites and pegmatites, are related to alkaline plutonism, an uncommon kind of magmatism that occurs in tectonic settings where there is rifting or that are near subduction zones.[39] In a rift setting, the alkaline magma is produced by very small degrees of partial melting (