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The Rare Earth Elements in Geology

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Rare earths are an interesting group of metals that have recently become quite useful in high tech, and today they are strategic materials in the world economy. To geologists, the rare earths are a major research tool in studying igneous rocks and the processes that create them.

Defining the Rare Earth Elements

The rare earths are defined slightly differently by technologists, chemists and geologists.

Chemists and technologists include 17 elements in the rare earths. These basically consist of that extra row of elements beneath the rest of the periodic table. (That row is actually meant to be inserted between lanthanum (La) and hafnium (Hf), but such an arrangement is rarely shown.) These are also known as the lanthanides or lanthanoids after the element lanthanum, which is the first of the series. The lanthanides include the 15 elements from lanthanum (atomic number 57) through lutetium (71). One of the lanthanides, promethium, is too unstable to exist in nature, but it can be manufactured in nuclear reactors. Two other very similar elements, scandium and yttrium (elements 21 and 39), are also included in the official definition. These 17 elements have important roles in metallurgy, electronics and magnetic applications.

Geologists care about actual minerals, which means the 14 natural lanthanides, but promethium is included for completeness. Geochemists usually include yttrium, for a total of 16, and sometimes scandium and thorium, to make 18 elements. Geologists abbreviate the rare earth elements as REEs.

Why are these elements lumped together? To speak chemicalese, the lanthanide series has an electron shell structure that keeps their ions at the same size within 11 percent. All of them have trivalent ions (trading three electrons to other elements), with two interesting exceptions. They all bond strongly to oxygen. So they all tend to flock together in the structure of various minerals. This is also what makes them hard for refiners to separate.

Rare Earths: Not Rare, Not Earths

The rare earths got their name starting in the 1700s, but the label stuck even when they were found to be metallic elements and not especially rare. Chemists first discovered them as oxide compounds (thus "earths"), difficult to reduce to metal, in obscure minerals (thus "rare"). It took a century to isolate all of them. Nowadays the historic term is still the handiest way to refer to them.

Rare earths are not as rare as the name implies. In cosmic terms, they're pretty rare, like most elements. Only one of them, cerium (Ce), is more abundant than 1 part per million in the whole Earth (core, mantle and crust). However, they tend to rise to the top because their atoms don't fit into the most common minerals—thus geochemists list them in the incompatible elements. In the upper continental crust, total REEs amount to around 200 parts per million and are more common there than copper, chromium, lead, zinc and tin.

Rare Earth Minerals and Mines

The REEs can tuck themselves away to a small extent in garnet, zircon and titanite. The phosphate minerals apatite, monazite and xenotime can have high REE contents; so can the silicate minerals allanite and eudialyte.

If conditions concentrate them enough, the REEs prefer their own peculiar minerals like bastnasite (a carbonate), euxinite and samarskite (both titanates). One of those conditions is met in pegmatites, the last liquid dregs of granite intrusions. That's where the REEs were first found and first mined. Another is in highly alkaline (sodium/potassium-rich) magmas and carbonatites. Both of these rare rock types form by repeated partial melting of deep crustal rocks, the geochemical equivalent of separating cream from milk. Until recently, most REEs were mined from these kinds of rocks.

Another way that REEs become concentrated is by removing everything else. This appears to have happened in a large area of southern China where granite was weathered into thick deposits of clay: laterites. The clays there have absorbed minable amounts of rare earths, and today the Chinese "ion-adsorption deposits" are the world's largest source of REEs. Between them and a large iron mine in northern China where REEs are a byproduct, China accounts for more than 90 percent of world production. (This government report from 2011 has more about the Chinese industry, and this state report from 2014 reviews REE laterites around the world.)

Rare Earths in Geology

Geologists use REEs to trace the histories of rocks and magmas. The sizes of REE ions are slightly larger in the light REEs (elements 57–62) and slightly smaller in the heavy REEs (elements 63–71). This difference allows them to slowly become enriched or excluded as they enter melts of various compositions. When you make graphs for various rocks showing the levels of all the REEs, some graphs have a line sloping up to the right (heavy) while others slope up to the left (light).

Two of the REEs offer extra information. Cerium (Ce, element 58) usually has a +3 valence, like the other REEs, but can also be oxidized to a valence of +4. In that case it behaves differently under oxidizing and reducing conditions, and it also substitutes for zirconium in the widespread mineral zircon. Both of these may leave a record in REE graphs called a "cerium anomaly."

Europium (Eu, element 63) has the option of a +2 valence, in which case it can swap places with calcium in feldspar minerals. At the same time, divalent Eu is excluded to a lesser extent from other minerals. A "europium anomaly" is typically a sign that a magma has lost or gained crystals of plagioclase feldspar. On REE graphs, these anomalies show up as nicks or peaks in an otherwise straight line. They are considered useful for distinguishing rocks of mantle and crustal origins.

Although mining and refining REEs is still a difficult undertaking, modern lab equipment and techniques have made measuring REEs in rocks almost as easy as sorting marbles. The elements that once perplexed chemists and still frustrate refiners are enlightening geologists today.

For much more information, see the U.S. Geological Survey's Scientific Investigations Report 2010-5220, "The Principal Rare Earth Elements Deposits of the United States," my source for much of this article.

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