Definition of Rare Earths
Rare earths, or rare-earth elements (REEs), are a family of 17 chemically related metallic elements: scandium (Sc), yttrium (Y), and the 15 lanthanides — lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Most are soft, silvery, reactive metals. They are chemically similar because they are usually trivalent (+3) and have closely related ionic sizes, but individual elements also have distinctive magnetic, optical, luminescent, catalytic, and electrochemical properties.
The name “rare earths” is historical and a bit misleading. They were originally identified in oxide-like materials once called “earths,” and they seemed uncommon when first discovered. In reality, many rare earths are not especially rare in the Earth’s crust; what is rare is finding them in highly concentrated, economically mineable deposits. They also usually occur mixed together in the same minerals, which makes separation and refining difficult. Important ore sources include bastnaesite, monazite, loparite, and ion-adsorption clays.
On the periodic table, most rare earths are in the detached lanthanide row below the main body, while scandium and yttrium sit in Group 3. For classification, they are often divided into light REEs (about La–Gd) and heavy REEs (Tb–Lu, with Y often grouped with the heavy side). In mining geology, scandium is sometimes discussed separately because it usually is not found in economic concentrations in the same geologic settings as the lanthanides and yttrium. Promethium is part of the lanthanide series, but it is radioactive and extremely scarce in nature.
Common uses of rare earths:
- Permanent magnets and electric machines. Rare-earth magnets—especially neodymium-iron-boron (Nd-Fe-B) magnets—are among the strongest permanent magnets known. They are used in hard drives, speakers, automotive systems, some electric motors, and some wind-turbine generator designs where strong magnetism is needed in a compact space.
- Catalysts. Lanthanum-based catalysts are used in petroleum refining, while cerium-based catalysts are used in automotive catalytic converters.
- Displays, lighting, lasers, and imaging. Yttrium, europium, and terbium are important phosphors for red, green, and blue light in many displays and lamps. Some rare earths are also used in fiber optics and lasers, and gadolinium compounds are used in MRI contrast agents and other imaging-related materials.
- Glass, optics, and polishing. The glass industry is a major user of rare earths for glass polishing and for giving glass special optical properties. Lanthanum is especially important in camera and other high-index lenses.
- Batteries, alloys, and metallurgy. Lanthanum-based alloys are used in nickel-metal hydride batteries, while mixtures containing cerium, lanthanum, neodymium, and praseodymium are used in steelmaking and in special alloys.
Simple summary
Rare earths are 17 related metals that are not truly rare in the crust, but are hard to find in concentrated deposits and hard to separate from one another. Their value comes from their unusual magnetic, optical, catalytic, and electrochemical behavior, which is why they are important in magnets, electronics, displays, catalysts, batteries, glass, and medical imaging.
