Rare Earths: Not All That Rare, and They’re Metals, to Boot
The term ‘rare earths’ is invariably described as misleading, since the elements themselves are neither particularly rare and are all metallic. The expression usually covers the 15 elements in the lanthanide series in the periodic table, plus yttrium and scandium. Although these are not in the same periodic group, they occur with the lanthanides in rare-earth deposits, and have comparable properties.
Rare-earth elements are classified as being ‘light’ or ‘heavy’, the difference stemming from ionic compaction within the atom. Light rare earths include the lanthanides from lanthanum (element 57 in the periodic table) up to europium (63), while the heavy rare earths are those from gadolinium (64) to lutetium (71), plus yttrium.
The principal rare-earth ores, the minerals bastnaesite and monazite, have formed the basis for historical production, with minor contributions from deposits containing xenotime and loparite. A significant proportion of Chinese rare-earth production is sourced from ion absorption clays, which themselves appear to have been derived from the deep weathering of source rocks containing xenotime.
Until the discovery of carbonatite-hosted rare-earth deposits, such as Mountain Pass, all rare-earth production came from monazite, with beach-sand operations in India and Brazil the leading producers. A phosphate mineral, monazite is known to exist in at least four forms, depending on whether Ce, La, Nd or Pr is the principal rare-earth constituent. Its main drawback is its thorium content, with concerns over the potential radioactivity of tailings having effectively rendered it unacceptable as a commercial ore in most parts of the world. Minor monazite production resumed in Brazil in 2004, according to British Geological Survey data, while Indian production has tailed off completely. Malaysian monazite production comes as a by-product of alluvial tin mining.
A carbonate-fluoride mineral, bastnaesite also has more than one composition, with Ce, La or Y forming the main rare-earth component. Typically hosted in carbonatite deposits, this is now the main source of world production. It is also present with monazite at Bayan Obo in China, although this is not a carbonatite-type deposit. Bastnaesite won from Mountain Pass supplied the U.S. market with rare earths for most of the last 60 years, with small-scale production having resumed in 2008 after a six-year hiatus. Bastnaesite is typically richer in the ‘light’ rare-earth metals than is monazite.
Of the other source minerals, xenotime is also a rare-earth phosphate in which yttrium is the major component; a number of heavy rare earth elements can replace some of the yttrium in the atomic structure, as can thorium and uranium. Virtually the only source of xenotime is now as a tin-mining by-product in Malaysia.
Individual rare-earth elements can vary widely in their relative natural abundance, ranging from cerium, the most abundant, to promethium which, being subject to radioactive decay, is virtually unknown in ore deposits. One interesting feature of the lathanides is that the Oddo-Harkins rule applies to their occurrence in nature, in that the odd-numbered elements occur less extensively than the even-numbered ones.
In terms of physical properties, there is a general increase in rare-earth metal hardness, density and melting point from cerium to lutetium. There is also widespread readiness for the metals to oxidize at relatively low temperatures, with ignition in air in the temperature range 150°C–180°C.