What are Rare Earth Elements?

Rare Earth Elements (REEs) are a group of 17 metallic elements with very similar physical and chemical properties that make them nearly indistinguishable from one other. These elements occur together in the periodic table and include the 15 lanthanides (atomic numbers 57–71) in addition to scandium and yttrium (atomic numbers 21 and 39, respectively). Scandium and yttrium have very similar chemical properties to the lanthanides, and yttrium is often found in the same deposits as the other rare earth elements, so they are typically considered part of the rare earth element group. Promethium (atomic number 61) does not occur as a stable element in nature, unlike the others.

The REEs are often classified into “heavy” and “light” groups; this refers to the atomic number of each element. The light REEs typically include lanthanum to gadolinium plus scandium, while the heavy REEs include terbium to lutetium plus yttrium, based on similarities in chemical behavior (USGS, 2019). However, there are no “set” rules that define how the two groups are divided and other scientists may make different distinctions. Despite these minor inconsistencies, REEs are divided into heavy and light groups because it is common to see preferential concentration of one group or the other in specific minerals. Therefore, REE-enriched minerals may have all of the REEs present, but often only contain highly elevated quantities of the heavy or the light rare earths.

REEs are not actually rare, despite what their name may lead you to believe. REEs are found all over the Earth and are in fact quite common; there is more naturally occurring cerium (atomic number 58) than copper, and more of all of the other REEs (except promethium) than precious metals like gold and silver! REEs don’t occur in high concentrations in most minerals though, and REE-enriched minerals don’t tend to occur in high concentrations either. Further, rocks with REE-enriched minerals tend to occur in unusual geologic settings (USGS, 2014). This makes finding deposits of REEs that are concentrated enough to be economically minable very unusual.

World Deposits and Reserves

The largest known REE deposits occur in China, Australia, and North America, with much smaller reserves found in India, Brazil, Malaysia, and South Africa. Production has been dominated by China, with additional production from Australia, India, Malaysia, Russia, and Thailand. Estimates project that China has the largest percentage worldwide of REE reserves at about 36 percent, compared to the United States at about 13 percent. The Commonwealth of Independent States (Azerbaijan, Armenia, Belarus, Georgia, Kazakhstan, Kyrgyzstan, Moldova, Russia, Tajikistan, Turkmenistan, Uzbekistan, and Ukraine) controls about 19 percent, followed by Australia with more than 5 percent and India with 3 percent.

REEs Uses

REEs are essential to manufacturing a wide range of products, from everyday consumer items like cellphones to important defense applications like precision-guided munitions. Because of their unique chemical properties, REEs often are the only option for specific applications and many devices could not function without them.

Today’s technologically reliant society requires a steadily increasing amount of REEs to function/operate. For example, the average smart phone or laptop requires a number of different REEs for the camera lens, screen, rechargeable battery, phosphors (lighting), hard drive, disk drives, and speakers. LED and fluorescent lighting are also reliant on a combination of REEs.

Our need for REEs will continue to grow as the world transitions towards renewable energy sources and vehicle electrification, which will depend in part on what are known as rare earth permanent magnets. These magnets, which are permanently magnetized, are usually made of praseodymium, neodymium, samarium, and dysprosium, and can be very lightweight, small, and yet are stronger than any other type of magnet. Permanent magnets are used in wind turbines and electric vehicle motors, in addition to hundreds of other common applications. Large volumes of REEs, particularly lanthanum, are also essential for rechargeable batteries in electric or hybrid vehicles. Several REEs are also important for manufacturing nuclear fuel and control rods, while others are needed to produce petroleum.

REEs are also critical from a national security perspective, as they are used in lasers, communication and radar systems, night-vision equipment, range finders, precision-guided weapons, stealth technology, and satellites.

Awareness of the diversity of REE deposit types is beneficial to exploration efforts. Economically, exploitable concentrations of REE are primarily derived from crystalline rocks. REEs are more common in alkaline igneous rocks and carbonatites than in mafic rocks. Rare earths typically occur as trivalent cations in rock-forming minerals in carbonates, oxides, phosphates, and silicates. REEs are chemically similar to thorium and are often found in minerals and rocks in association with this element. Economic concentrations of REE-host minerals are known from alkaline igneous rocks, carbonatites, and from a wide variety of dikes and veins that cross-cut alkaline intrusions and surrounding rocks.

REEs occur in a variety of rock units across Wyoming. Reported Precambrian REE occurrences are hosted by pegmatites, veins and dikes, faults and shear zones, metacarbonate rocks, disseminated minerals in generally alkalic igneous rocks, and metasediments. Sedimentary occurrences in paleoplacers are found primarily in the Cambrian Flathead Sandstone and the Cretaceous Mesaverde Formation as well as local, smaller deposits in numerous units. Disseminations of REEs are also reported within phosphate-rich rock of the Permian Phosphoria Formation. Tertiary-aged hosts include alkaline igneous rocks, carbonatite veins, dikes, hydrothermal zones, paleoplacers, and apparent depositions from solution movement within sandstones. Quaternary REE hosts can be found in alluvial placers and paleoplacers (King and Harris, 2002; Sutherland and others, 2013).

Numerous minerals are known to contain REEs as essential constituents, and a greater number contain REEs as accessory elements. Only a few of these minerals host large enough concentrations of REEs to be considered ore minerals. Allanite and monazite occur as relatively common accessory minerals within many types of felsic igneous rocks. Other REE-bearing minerals can be uncommon and are often hosted in generally uncommon rock types. Worldwide, the principal commercial sources of REE are the minerals bastnasite, loparite, monazite, and xenotime, and rare earth ion-adsorption clays. When discussing principal REE-bearing minerals specific to the United States, Long and others (2010) omit loparite and ion-adsorption clays but add euxenite and allanite, although other REE-bearing minerals are also present.

Of the principal REE-bearing minerals, allanite, bastnasite, euxenite, monazite, and xenotime are known to occur in Wyoming. Although not primary REE sources, ewaldite and mckelveyite were actually discovered with association to trona beds in the Green River Formation in Sweetwater County. The table below lists minerals reported in Wyoming that contain REEs. Mineral formulas are variable depending on the source cited and on sub-varieties that may contain differing elemental substitutions.

REE-bearing minerals that occur in Wyoming.

Consumer products that rely on integral use of REEs include catalytic converters, cell phones, digital cameras, and computers. Additionally, neodymium, samarium, gadolinium, dysprosium, and praseodymium are used in high-power permanent magnets where weight and space are of concern. The table below shows typical uses of REEs. These applications range from computer hard drive motors and MRI scanners to wind generators. A typical hybrid car, such as a Toyota Prius, contains around 11.3 kg (25 lbs) of REEs, mostly lanthanum, in its rechargeable battery, and about 2 kg (4.5 lbs) of neodymium and dysprosium in its drive motor. Smart phones and LED lights each contain less than a gram of REEs (Molycorp, 2015a).

REEs are also used for national security purposes. The U.S. Department of Defense uses REEs in lasers, precision-guided munitions, communications and radar systems, avionics, night-vision equipment, satellites, range finders, and control systems.

Typical uses of the rare earth elements.