The BGS indicated in 2011 that the lanthanides are commonly divided into: lower atomic weight elements, lanthanum through to europium,referred to as the light rare earth elements (LREE) and the heavy rare earth elements (HREE) — gadolinium through to lutetium and yttrium (Table 1). Yttrium is usually grouped with the HREE because of its chemical similarity. The division is somewhat arbitrary and the term middle REE (MREE) is sometimes used to refer to those elements between europium to dysprosium (Samson and Wood, 2004). The relative abundance of the REE varies considerably and relates to two main factors: REE with even atomic numbers have greater abundance than their odd numbered neighbors (Oddo-Harkins effect); and
Secondly the lighter REE are more incompatible (because they have larger ionic radii) and consequently more strongly concentrated in the continental crust than the REE
with larger atomic numbers. The REE are trivalent (Ln3+), with the exception of cerium (Ce4+) and europium (Eu2+) in some environments. The chemically similar nature (ionic radii and oxidation states) of the REE means they can substitute for one another in crystal structures. This results in the occurrence of multiple REE within a single mineral and a broad distribution in the Earth’s crust (Castor and Hedrick, 2006). As refined metals the REE are lustrous, iron grey to silvery in appearance. They are characteristically soft, malleable, ductile and typically reactive. The electron structure of REE gives them some unusual magnetic and optical properties.
With a few exceptions melting points of REE increase across the series, from 798°C for cerium to 1,663°C for lutetium. Melting point has direct implications for the
reduction process used in metal production. The REE, with the exception of Sc, Y, La, Yb and Lu, are strongly paramagnetic and have strong magnetic anisotropy
(Gupta and Krishnamurthy, 2005).
Mineralogy
REE do not occur naturally as metallic elements. REE occur in a wide range of mineral types including halides, carbonates, oxides, phosphates and silicates (Table 2). The
REE are largely hosted by rock-forming minerals where they substitute for major ions. Higher concentrations of REE are required to form their own minerals (Möller, 1986). Around 200 minerals are known to contain REE, although a relatively small number are, or may become, commercially significant (Table 2). REE-bearing minerals generally contain most of the REE in varying concentrations, but tend to be biased towards either the LREE or HREE. The most abundant REE in a mineral is usually indicated by chemical symbol, for example in bastnäsite -(Ce), cerium is the dominant REE, but it can also contain lanthanum, neodymium and praseodymium.
The vast majority of resources are associated with just four minerals, bastnäsite, allanite, monazite and xenotime. In some REE minerals the LREE are particularly enriched
relative to the HREE, whilst in others the opposite is the case. Bastnäsite, allanite, and monazite and associated minerals are the primary source of the LREE, mainly Ce, La and Nd. Monazite has a different balance as it contains less La and more Nd and HREE. It is also significant to note that allanite monazite contain the radioactive elements uranium thorium. Xenotime is dominated by the heavier REE including Y, Dy, Er, Yb and Ho (Harben, 2002).
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