Rare earths are a set of related metals close together on the Periodic Table of the Elements. Most are clustered in the lanthanide series (the * in the table at the bottom, beginning with lanthanum), plus scandium and yttrium just above them. Scandium is discussed elsewhere.
The story of the rare earths starts in 1787, when swedish chemist Carl Axel Arrhenius picked up a black mineral he called "ytterbite" from a quarry in the village of Ytterby, Sweden. Over the next sixty years, various chemical separation techniques teased several new elements out of the mineral. In consequence, we have a number of elements (yttrium, ytterbium, erbium, terbium) all named after one small Swedish settlement.
Meanwhile, chemists Jöns Jacob Berzelius and Wilhelm Hisinger in 1803 produced an oxide of what would prove to be the rare earth element cerium. That oxide, ceria, was later found to contain other rare earth elements, including lanthanum.
The use of spectroscopy in the late 19th century revealed other trace elements in both ytterbite and ceria. The eventual number of rare earths would be 17 – 15 lanthanides plus the two elements above them.
Element 61, promethium, is a curiosity. When Henry Moseley in 1914 determined the atomic number of elements, he saw that there was no element 61. In fact, all forms of this element are radioactive and unstable with very short half-lives. The element was actually created synthetically in 1945 at the Oak Ridge National Laboratory. The new element, named promethium after the god Promethius, was one of the decay products of uranium fission. Even now, promethium is created by nuclear reactions and not mined.
Mining and Production
Rare earth metals are not called rare for their scarcity in the Earth's crust(cerium, at least, is moderately common), but because they tend not to occur in concentrated ore bodies. They are also chemically very similar and notoriously hard to separate. Any ore that contains one of the rare earths also contains many others.
Consistent with their history, rare earths tend to occur together in two groups, one with cerium and one with yttrium. The cerium elements are mined in small amounts around the world. The yttrium elements are mined primarily in China, a country not known for generous exports. New applications of these metals may easily become frustrated by lack of supply.
Properties and Uses
Obvious chemistry notwithstanding, the rare earths have highly specific uses in which their performance justifies the cost.
Clean tech uses of the rare earths include lasers and magnets (the powerful so-called "rare earth magnets"). These magnets may be found in wind turbine engines, for example, where they help to generate clean energy. The most powerful laboratory magnets will use holmium. Much more numerous are the magnets in computer hard drives containing neodymium, dysprosium, or terbium.
The (nickel-metal hydride) batteries and regenerative braking of electric cars use trace rare earths. For fossil-fuel cars, cerium and lanthanum are alternatives to platinum as the catalyst in catalytic converters.
Yttrium, europium, and others are used in lasers, such as this one using neodymium-doped yttrium aluminum garnet crystal. Yttrium is used in red LEDs and in the red phosphors of CRT monitors.
Gadolinium is valuable for use in computer memory chips and other high-tech components. Relatively abundant cerium produces a yellow color in glass and ceramics.