##### U.S. Geological Survey     Fact Sheet 087-02

## **Rare Earth   Elements—Critical Resources for High Technology**

| High-technology       and environmental applications of the rare earth elements (REE) have grown       dramatically in diversity and importance over the past four decades. As       many of these applications are highly specific, in that substitutes for       the REE are inferior or unknown, the REE have acquired a level of technological       significance much greater than expected from their relative obscurity. Although       actually more abundant than many familiar industrial metals, the REE have       much less tendency to become concentrated in exploitable ore deposits. Consequently,       most of the world’s supply comes from only a few sources. The United       States once was largely self-sufficient in REE, but in the past decade has       become dependent upon imports from China. |
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The rare earth   elements (REE) form the largest chemically coherent group in the periodic table.   Though generally unfamiliar, the REE are essential for many hundreds of applications.   The versatility and specificity of the REE has given them a level of technological,   environmental, and economic importance considerably greater than might be expected   from their relative obscurity. The United States once was largely self-sufficient   in these critical materials, but over the past decade has become dependent upon   imports (fig. 1). In 1999 and 2000, more than 90% of REE required by U.S. industry   came from deposits in China.

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| **Figure       1.** Global rare earth element production (1 kt=106 kg) from 1950 through       2000, in four categories: United States, almost entirely from Mountain Pass,       California; China, from several deposits; all other countries combined,       largely from monazite-bearing placers; and global total. Four periods of       production are evident: the monazite-placer era, starting in the late 1800s       and ending abruptly in 1964; the Mountain Pass era, starting in 1965 and       ending about 1984; a transitional period from about 1984 to 1991; and the       Chinese era, beginning about 1991. |

Although the   15 naturally occurring REE (table 1; fig. 2) are generally similar in their   geochemical properties, their individual abundances in the Earth are by no means   equal. In the continental crust and its REE ore deposits, concentrations of   the most and least abundant REE typically differ by two to five orders of magnitude   (fig. 3). As technological applications of REE have multiplied over the past   several decades, demand for several of the less abundant (and formerly quite   obscure) REE has increased dramatically.

The diverse   nuclear, metallurgical, chemical, catalytic, electrical, magnetic, and optical   properties of the REE have led to an ever increasing variety of applications.   These uses range from mundane (lighter flints, glass polishing) to high-tech   (phosphors, lasers, magnets, batteries, magnetic refrigeration) to futuristic   (high-temperature superconductivity, safe storage and transport of hydrogen   for a post-hydrocarbon economy).

**Some Applications   of the Rare Earth Elements**

Many applications   of REE are characterized by high specificity and high unit value. For example,   color cathode-ray tubes and liquid-crystal displays used in computer monitors   and televisions employ europium as the red phosphor; no substitute is known.   Owing to relatively low abundance and high demand, Eu is quite valuable—$250   to $1,700/kg (for Eu2O3) over the past decade.

Fiber-optic   telecommunication cables provide much greater bandwidth than the copper wires   and cables they have largely replaced. Fiber-optic cables can transmit signals   over long distances because they incorporate periodically spaced lengths of   erbium-doped fiber that function as laser amplifiers. Er is used in these laser   repeaters, despite its high cost (~$700/kg), because it alone possesses the   required optical properties.

Specificity   is not limited to the more exotic REE, such as Eu or Er. Cerium, the most abundant   and least expensive REE, has dozens of applications, some highly specific. For   example, Ce oxide is uniquely suited as a polishing agent for glass. The polishing   action of CeO2 depends on both its physical and chemical properties, including   the two accessible oxidation states of cerium, Ce,3+ and Ce4+, in aqueous solution.   Virtually all polished glass products, from ordinary mirrors and eyeglasses   to precision lenses, are finished with CeO2.

| Table 1.       Names and symbols of the REE |
|----------------------------------------------|
| La                                           | lanthanum |  | Tb | terbium |
| Ce                                           | cerium |  | Dy | dysprosium |
| Pr                                           | praseodymium |  | Ho | holmium |
| Nd                                           | neodymium |  | Er | erbium |
| Pm                                           | promethium |  | Tm | thulium |
| Sm                                           | samarium |  | Yb | ytterbium |
| Eu                                           | europium |  | Lu | lutetium |
| Gd                                           | gadolinium |  | Y | yttrium |

Permanent magnet   technology has been revolutionized by alloys containing Nd, Sm, Gd, Dy, or Pr.   Small, lightweight, high-strength REE magnets have allowed miniaturization of   numerous electrical and electronic components used in appliances, audio and   video equipment, computers, automobiles, communications systems, and military   gear. Many recent technological innovations already taken for granted (for example,   miniaturized multi-gigabyte portable disk drives and DVD drives) would not be   possible without REE magnets.

Environmental   applications of REE have increased markedly over the past three decades. This   trend will undoubtedly continue, given growing concerns about global warming   and energy efficiency. Several REE are essential constituents of both petroleum   fluid cracking catalysts and automotive pollution-control catalytic converters.   Use of REE magnets reduces the weight of automobiles. Widespread adoption of   new energy-efficient fluorescent lamps (using Y, La, Ce, Eu, Gd, and Tb) for   institutional lighting could potentially achieve reductions in U.S. carbon dioxide   emissions equivalent to removing one-third of the automobiles currently on the   road. Large-scale application of magnetic-refrigeration technology (described   below) also could significantly reduce energy consumption and CO2 emissions.

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| **Figure       2.** Chemical periodic table delineating the 16 rare earth elements (REE):       the lanthanides, La through Lu, plus Y, whose geochemical behavior is virtually       identical to that of the heavier lanthanides. Promethium has no long-lived       isotopes and occurs naturally on Earth only in vanishingly small quantities.       An represents the first 14 actinide elements; Lr is the last actinide. |

In many applications,   REE are advantageous because of their relatively low toxicity. For example,   the most common types of rechargeable batteries contain either cadmium (Cd)   or lead. Rechargeable lanthanum-nickel-hydride (La-Ni-H) batteries are gradually   replacing Ni-Cd batteries in computer and communications applications and could   eventually replace lead-acid batteries in automobiles. Although more expensive,   La-Ni-H batteries offer greater energy density, better charge-discharge characteristics,   and fewer environmental problems upon disposal or recycling. As another example,   red and red-orange pigments made with La or Ce are superseding traditional commercial   pigments containing Cd or other toxic heavy metals.

The next high-technology   application of the REE to achieve maturity may be magnetic refrigeration. The   six REE ions Gd3+ through Tm3+ have unusually large magnetic moments, owing   to their several unpaired electrons. A newly developed alloy, Gd5(Si2Ge2), with   a “giant magne-tocaloric effect” near room temperature reportedly   will allow magnetic refrigeration to become competitive with conventional gas-compression   refrigeration. This new technology could be employed in refrigerators, freezers,   and residential, commercial, and automotive air conditioners. Magnetic refrigeration   is considerably more efficient than gas-compression refrigeration and does not   require refrigerants that are flammable or toxic, deplete the Earth’s ozone   layer, or contribute to global warming.

#### Rare Earth   Element Abundances

“Rare”   earth elements is a historical misnomer; persistence of the term reflects unfamiliarity   rather than true rarity. The more abundant REE are each similar in crustal concentration   to commonplace industrial metals such as chromium, nickel, copper, zinc, molybdenum,   tin, tungsten, or lead (fig. 4). Even the two least abundant REE (Tm, Lu) are   nearly 200 times more common than gold. However, in contrast to ordinary base   and precious metals, REE have very little tendency to become concentrated in   exploitable ore deposits. Consequently, most of the world’s supply of REE   comes from only a handful of sources.

Differences   in abundances of individual REE in the upper continental crust of the Earth   (figs. 3, 4) represent the super-position of two effects, one nuclear and one   geochemical. First, REE with even atomic numbers (58Ce, 60Nd, …) have greater   cosmic and terrestrial abundances than adjacent REE with odd atomic numbers   (57La, 59Pr, …). Second, the lighter REE are more incompatible (because   they have larger ionic radii) and therefore more strongly concentrated in the   continental crust than the heavier REE. In most rare earth deposits, the first   four REE—La, Ce, Pr, and Nd—constitute 80 to 99% of the total. Therefore,   deposits containing relatively high grades of the scarcer and more valuable   heavy REE (HREE: Gd to Lu, Y) and Eu are particularly desirable.

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| **Figure       3.** Prices and abundances of the rare earth elements. Prices are for       1999 or 2000 in U.S. dollars per kilogram of REE metal, in two forms: (1)       as oxides, in 2 to 25 kg packages, at 95 to 99.99% purity; (2) as 0.1 to       0.45 kg metal ingot, at 99.9% purity. Two representative REE ores—high-grade       carbonatite ore from Mountain Pass, California, and lateritic ion-adsorption       ore from southern China—are compared with Earth’s upper continental       crust. Z, atomic number. |

 

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| **Figure       4.** Abundance (atom fraction) of the chemical elements in Earth’s       upper continental crust as a function of atomic number. Many of the elements       are classified into (partially overlapping) categories: (1) rock-forming       elements (major elements in green field and minor elements in light green       field); (2) rare earth elements (lanthanides, La–Lu, and Y; labeled       in blue); (3) major industrial metals (global production >~3x107 kg/year;       labeled in bold); (4) precious metals (italic); and (5) the nine rarest       “metals”—the six platinum group elements plus Au, Re, and       Te (a metalloid). |

#### Rare Earth   Element Resources

From the discovery   of the REE (during the period 1794–1907) through the mid-1950s, a few of   the REE were produced in modest amounts from monazite-bearing placers and veins,   from pegmatites and carbonatites, and as minor byproducts of uranium and niobium   extraction. During this time, the middle and heavy REE generally were available   in pure form only in sub-kilogram quantities and were chiefly chemical curiosities.

In 1949, a   carbonatite intrusion with extraordinary contents of light REE (8 to 12% rare   earth oxides [REO]), was discovered at Mountain Pass, in the upper Mojave Desert,   California (fig. 5). The REE at Mountain Pass are hosted chiefly by bastnäsite,   (Ce,La,Nd,…)CO3F, and related minerals. By 1966, this single, world-class   deposit (owned and operated by Molycorp, Inc.) had become the paramount source   of REE. Early development was supported largely by the sudden demand for Eu   created by the commercialization of color television. Mountain Pass, with an   average grade of 9.3% and reserves of 20 million metric tons (Mt) REO (at 5%   cutoff), remains the only large ore deposit mined solely for its REE content.   Mountain Pass ore is very strongly dominated by the light REE (figs. 3, 6).   Nonetheless, the large quantities of ore processed and development of solvent-extraction   techniques for large-scale separation of individual REE from one another allowed   recovery of several of the middle REE. Increased availability led in turn to   applications for these formerly exotic elements.

From 1965 through   the mid-1980s, Mountain Pass was the dominant source of REE, and the United   States was largely self-sufficient in REE. Since 1985, production of REE in   China has increased dramatically (fig. 1). Chinese REE production comes chiefly   from two sources. The most important is the Bayan Obo iron-niobium-REE deposit,   Inner Mongolia. This deposit has geological affinities both to carbonatite REE   deposits and to hydrothermal iron-oxide (-Cu-Au-REE) deposits, such as Olympic   Dam, Australia, and Kiruna, Sweden. Grades at Bayan Obo are 3 to 6% REO; reserves   are at least 40 Mt, possibly considerably more. The second major source of Chinese   REE is ion-adsorption ores in lateritic weathering crusts developed on granitic   and syenitic rocks in tropical southern China. These oxide ores are advantageous   in their relatively high proportions of HREE (fig. 6) and, especially, in the   ease with which they can be mined and the REE extracted.

The number   of workable REE deposits, already severely limited by the geochemical properties   of the REE, has in recent years also been affected by environmental and regulatory   factors. Monazite, the single most common REE mineral, generally contains elevated   levels of thorium. Although Th itself is only weakly radioactive, it is accompanied   by highly radioactive intermediate daughter products, particularly radium, that   can accumulate during processing. Concern about radioactivity hazards has now   largely eliminated monazite as a significant source of REE and focused atten-tion   on those few deposits where the REE occur in other, low-Th minerals, particularly   bastnäsite.

#### Rare Earth   Element Supply Issues

Over the past   several years the only domestic source of REE, the mine at Mountain Pass, California,   has operated below capacity and only intermittently. Following environmental   and regulatory problems with the main wastewater pipeline, the REE separation   (solvent extraction) plant was shut down. Mountain Pass currently produces only   bastnäsite concentrates and sells separated REE only from stockpiles produced   before the shutdown. Even after the regulatory situation has been resolved,   however, the long-term viability of Mountain Pass as a supplier of separated   REE for high-technology applications is threatened by market factors. 

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| **Figure       5**. The Mountain Pass rare earth element mine, Mojave Desert, California,       with Clark Mountain (part of the Mojave National Preserve) in the background       and Interstate 15 just beyond the hills in the foreground. |

In 1999 and   2000, nearly all (more than 90%) of the separated REE used in the United States   was imported either directly from China or from countries that imported their   plant feed materials from China. The surprisingly rapid progression from self-sufficiency   prior to about 1990 to nearly complete dependence on imports from a single country   today involves a number of causative factors. These include much lower labor   and regulatory costs in China than in the United States; continued expansion   of electronics and other manufacturing in Asia; the favorable number, size,   and HREE content of Chinese deposits; and the ongoing environmental and regulatory   problems at Mountain Pass. China now dominates world REE markets (fig. 1), raising   several important issues of REE supply for the United States:

> 
    (1) The United States is in danger of losing its longstanding leadership in     many areas of REE technology. Transfer of expertise in REE processing technology     and REE applications from the United States and Europe to Asia has allowed     China to develop a major REE industry, eclipsing all other countries in production     of both ore and refined products. The Chinese Ministry of Science and Technology     recently announced a new national basic research program. Among the first     group of 15 high-priority projects to be funded was “Basic research in     rare earth materials” (Science, Dec. 18, 1998, p. 2171).

(2) United States dependence     on imports from China comes at a time when REE have become increasingly important     in defense applications, including jet fighter engines and other aircraft     components, missile guidance systems, electronic countermeasures, underwater     mine detection, antimissile defense, range finding, and space-based satellite     power and communication systems.

(3) Availability of Chinese     REE to U.S. markets depends on continued stability in China’s internal     politics and economy, and its relations to other countries.

(4) Although the present     low REE prices caused by abundant supply from China are stressing producers,     particularly Mountain Pass, low prices will also stimulate development of     new applications. For example, a recent chemistry text notes that “…     for many years the main use of lutetium was the study of the behavior of lutetium     …” Several promising applications for Lu are known, but most are     precluded by high cost. If the price of Lu were to decrease from many thousand     to a few thousand dollars per kilogram (fig. 3), additional high-technology     applications of even this least abundant of the REE undoubtedly would follow.     How large a role the United States will play in future expansion of REE technology     and markets remains an important, but open, question.

 

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| **Figure       6.**Proportions of individual REE in two representative ores: bastnäsite,       dominated by La, Ce, and Nd, with Eu through Lu plus Y totaling only 0.4%;       and lateritic ion-adsorption ore, Y-dominated. Dark blue and light blue       sectors represent lanthanides of even and odd atomic number, respectively       (see figs. 2, 3). Yttrium is indicated by green. |

The rare earth   elements are essential for a diverse and expanding array of high-technology   applications, which constitute an important part of the industrial economy of   the United States. Long-term shortage or unavailability of REE would force significant   changes in many technological aspects of American society. Domestic REE sources,   known and potential, may therefore become an increasingly important issue for   scientists and policymakers in both the public and private sectors.

***By   Gordon B. Haxel, James B. Hedrick, and Greta J. Orris***

*Edited by   Peter H. Stauffer and James W. Hendley II*

*Graphic   design by Gordon B. Haxel, Sara Boore, and Susan Mayfield; Web layout by Carolyn Donlin*

For questions about the   content of this report, contact Gordon Haxel

References

For more information see:   http://minerals.usgs.gov/minerals/pubs/commodity/rare_earths/

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