The Rare-Earth Elements: Vital to Modern Technologies and Lifestyles

A USGS Mineral Resources Program fact sheet from 2014; production and supply figures are as of then.

Until recently the rare-earth elements (REEs) were known mainly to chemists, geologists, materials scientists and engineers. In this century they have drawn wide attention for three reasons: the public has recognised the special properties they give modern technology; China dominates their production and supply; and the rest of the world depends on China for most of it.

Since the late 1990s China has supplied 85 to 95 percent of the world's rare earths. In 2010 it announced it would cut exports, just as use was rising fast. Rare earths are in smartphones, digital cameras, computer hard disks, fluorescent and LED lights, flat-screen televisions, monitors and displays, and large amounts of some go into clean-energy and defence technology. Countries that depend on new technology — Japan, the United States, the members of the European Union — reacted with great concern, and exploration for economic deposits increased.

The USGS studies where and how rare-earth concentrations form in the Earth's crust, where undiscovered or undeveloped resources may lie, trends in supply and demand at home and abroad, and how undisturbed and mined deposits interact with the environment.

Oxides of gadolinium, praseodymium, cerium, samarium, lanthanum and neodymium.

What they are

The group is the 15 elements from lanthanum (atomic number 57) to lutetium (71), officially the "lanthanoids" and commonly the "lanthanides." Promethium (61) is left out of discussions of deposits because it is rare and unstable in nature. Yttrium (39) is usually counted as a rare earth because it behaves like the lanthanoids and occurs in the same deposits. Scandium (21) is chemically similar but does not occur in economic concentrations in the same settings.

For comparison, the crust holds on average 0.004 parts per million of gold, 0.075 of silver, 14 of lead and 60 of copper.

The light rare earths are lanthanum through gadolinium (57–64), the heavy ones terbium through lutetium (65–71); some authorities put europium and gadolinium with the heavy group. Yttrium, though light, is grouped with the heavy ones because it behaves like them.

They are not very rare. They were named in the 18th and 19th centuries, when "earths" meant materials heat could not change further, and they were scarce compared with earths such as lime or magnesia. Cerium, the most abundant, is more common in the crust than copper or lead, and every rare earth except promethium is on average more abundant than silver, gold or platinum. What is unusual is a concentrated, economically minable deposit.

How they are used

Their unusual magnetic and optical properties give them uses across modern life:

• Screens and lights — phosphors, which glow, in displays from smartphones to stadium scoreboards, and in fluorescent and LED lighting; yttrium, europium and terbium make the red, green and blue phosphors in many bulbs, panels and televisions.
• Glass — the largest consumer of rare-earth raw materials, for polishing and for colour and optical properties; lanthanum can make up as much as 50 percent of a digital camera lens, phone cameras included.
• Catalysts — lanthanum-based catalysts refine petroleum; cerium-based ones work in car catalytic converters.
• Magnets — neodymium-iron-boron magnets are the strongest known, used where space and weight are tight: hard disks and CD and DVD drives (they keep the spindle spinning steadily), and car power steering, electric windows, power seats and speakers. Rare-earth magnets are stronger for their weight and volume than any other type; large wind turbines and electric vehicles use permanent magnets that usually contain praseodymium, neodymium, samarium and dysprosium.
• Batteries — nickel-metal hydride batteries use lanthanum-based alloys; in a hybrid car they can need 10 to 15 kilograms of lanthanum.
• Steel and alloys — cerium, lanthanum, neodymium and praseodymium, often as a mixed oxide called mischmetal, remove impurities in steelmaking and go into special alloys.

In the 1940s, as part of the Manhattan Project, Frank Spedding and others developed chemical ion-exchange methods that could separate and purify individual rare earths — first used to separate plutonium-239 and neptunium from uranium, thorium and actinium in reactor materials.

Where they come from

Rare earths occur together because they share a +3 charge and similar ionic sizes. They never occur on their own the way gold or copper can, only as minor or major parts of minerals, usually dominated by either light or heavy rare earths. In cooling magma their large ions do not fit into common minerals such as feldspar, pyroxene, olivine and amphibole, so they stay in the melt and become more concentrated with each round of crystallisation, until rare-earth minerals form. One element can substitute for another, so a single mineral usually holds several.

Economic deposits come mainly from four settings — and even there, minable concentrations are rare: more than 500 carbonatites are known worldwide, but only 6 were being mined for rare earths.

• Carbonatites — unusual igneous rocks from carbonate-rich magma, more than 50 percent carbonate minerals (usually calcite and dolomite), with the highest rare-earth concentrations of any igneous rock, mostly light rare earths. They have been the main source of light rare earths since the 1960s: Mountain Pass in California; Bayan Obo, Maoniuping, Daluxiang and Weishan in China; and Mount Weld in Western Australia, which mines a weathered zone above a carbonatite.
• Alkaline igneous rocks — uncommon rocks low in silica relative to sodium, potassium and calcium. Many advanced exploration projects (2014) target them, some finding 0.3 to 2.6 percent total rare-earth oxide, because they are often rich in the valuable heavy rare earths.
• Ion-adsorption clays of southern China — the world's main source of heavy rare earths. In wet tropical climates groundwater leaches rare earths from granite, thick clay soils form above it, and the rare earths cling weakly to the clay. At only about 0.04 to 0.25 percent rare-earth oxide they still pay, because weak acids extract them easily, they are often rich in heavy rare earths, and labour is cheap. A pilot project in Jamaica is testing recovery from the red-mud waste of bauxite mining, which could count as a similar deposit.
• Monazite-xenotime placers — important before the mid-1960s. Monazite, a rare-earth-thorium phosphate, can be recovered from beach sands as a by-product of mining ilmenite, rutile and zircon, the main sources of titanium for paint pigment. India recovers it from its southern beaches for light rare earths and for thorium, stockpiled for thorium-based nuclear power under development. Xenotime has been recovered as a source of yttrium from tin placers.

Other deposits hold minor rare earths but have not been important sources. Australia's giant Olympic Dam iron oxide-copper-uranium-gold-silver deposit, the world's largest single uranium deposit, is enriched in them, but recovering them has not proved economic.

The Mountain Pass mine of Molycorp, Inc., in southeastern California — in 2014 the only active rare-earth producer in the United States. Its ore body, a carbonatite intrusion, is thought to be the country's largest rare-earth resource.

Supply and demand

In recent years China has produced about 95 percent of the market. Citing the need to keep its limited resources for domestic use and concern about mining's environmental effects, it restricted supply with quotas, licences and taxes. Outside China, industry stockpiled, explored widely and pushed to conserve, recycle and substitute; new production began at Mount Weld and Mountain Pass.

Expert panels — the National Research Council, the U.S. Department of Energy, the European Commission, the American Physical Society and Materials Research Society, and the Resnick Institute — rank rare earths high in "criticality": highly important to technology and the economy, with high risk to supply. More than 400 rare-earth projects were under way in 2012, many at advanced stages with drilled deposit sizes and grades. Processing is a major hurdle: the elements occur together, often in more than one mineral per deposit, and must be concentrated and then separated from each other, usually as oxides.

The future

Global resources are estimated at 110 million metric tons of rare-earth oxide, mainly — in descending order — in China, Russia, the United States, India and Australia, and exploration will likely raise that figure. Supply is limited less by deposits than by the cost and complexity of exploration and of building mines and extraction and separation plants. Recycling, substitute materials and recovering rare earths as co-products of other ores may offset some demand.

The USGS Mineral Resources Program publishes production and consumption data, historical statistics and a report on end uses and recycling.

Sources

• U.S. Geological Survey, 2014, The rare-earth elements—Vital to modern technologies and lifestyles: U.S. Geological Survey Fact Sheet 2014–3078. https://pubs.usgs.gov/publication/fs20143078 — the table and two pictures come from its PDF; its stock and privately credited photographs are left out. Crustal abundances from Lide (2004), CRC Handbook of Chemistry and Physics.
• Long, K.R., Van Gosen, B.S., Foley, N.K., and Cordier, D., 2010, The principal rare earth elements deposits of the United States: U.S. Geological Survey Scientific Investigations Report 2010–5220.