Rare-Earth Terminology – A Quick Refresher On The Basics

I came across a couple of online articles this week, which make reference to some pretty basic terminology with respect to the rare earths. Unfortunately these articles got much of it wrong, propagating inaccuracies that are particularly egregious when they keep appearing within various natural-resource media channels. I’m therefore going to take a little time here to address some of the key errors that keep cropping up.

First, let’s tackle one of my pet peeves. The term ‘rare-earth mineral’ is NOT synonymous with the terms ‘rare-earth elements’ or ‘rare-earth metals’. The International Mineralogical Association defines a mineral as “an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes” [1]. By this definition the element gold, for example, would be considered a mineral; however the rare earths are never found in elemental form and so in their case, the term ‘mineral’ is never synonymous with ‘element’.

Rare-earth elements (REEs) can be found in many different minerals. Only when the occurrence of such REEs is in significant quantities, or the chemical formula for a particular mineral requires the presence of one or more REEs, does the compound become a rare-earth (or rare-earth-bearing) mineral.

As for what constitutes a rare-earth element or metal; sticking with the definition formulated by the International Union of Pure and Applied Chemistry (IUPAC) will generally keep you out of trouble. IUPAC defines the rare-earth metals as the 15 lanthanoid elements (with atomic numbers of 57 through to 71) in addition to scandium (Sc) and yttrium (Y) [2]. The lanthanoid promethium (Pm) is radioactive, with no stable isotopes; it is thus present in the Earth’s crust in vanishingly small quantities and does not occur with the other REEs. Sc exhibits some properties that are similar to other REEs, but is seldom found in the same minerals as them. It does not selectively combine with the common ore-forming anions and thus it is generally (though not exclusively) confined to trace occurrences [3].

REEs are difficult to separate from one another once they have been liberated from REE-bearing minerals. At the atomic level, the lanthanoid elements have a similar outer electronic structure, with this structure shielding the so-called 4f electrons within the atom. The presence and incremental filling of these electron orbitals are key characteristics of the lanthanoids. This unique structure leads to REE ions that are very similar in size to each other. It is this similarity in ionic radii across the group, not the adjacency of their atomic numbers, which gives rise to the similar chemical properties associated with each of the REEs. This makes them difficult to separate, even with the use of intensive processes such as solvent extraction (SX).

Now we come to one of the most contentious issues with respect to rare earths, namely the definitions used to describe the specific sub-groups of REEs known as light (LREE), medium (MREE) and heavy REEs (HREEs). I freely admit that in the past year or so, I have become less forgiving of the inconsistencies that we see in the industry with respect to the use of these terms. It’s time ‘we’ got our act together on this. One of the articles that triggered my own piece today is particularly egregious in getting this whole topic wrong.

Let’s start with a differentiation with which all chemists and geologists would likely agree, even if the non-techies in the industry don’t, namely the separation of the REEs into two groups by virtue of electronic structure. On this basis alone, the LREEs would be those that have no paired 4f electrons, specifically lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), Pm, samarium (Sm), europium (Eu) and gadolinium (Gd). This leaves the HREEs as terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Th), ytterbium (Yb) and lutetium (Lu). Because the ionic radius of Y is very similar to that of the HREE Dy, Y is generally included as a HREE, despite not having any 4f electrons, paired or otherwise. In contrast, the ionic radius of Sc is much smaller than any of the other REEs and is therefore generally NOT classified as being either a LREE or HREE.

There are many in the industry that classify Eu and Gd as HREEs, but there really is no sound basis for doing so, unless you consider the inflation of a project’s HREE numbers as being a “sound basis”…

So now let’s turn our attention to the so-called MREEs. Of the relatively few sources in the industry that acknowledge the existence of this group, most of them get the composition of it wrong as well as its origin. The MREEs specifically refer to Sm, Eu and Gd, and the term arose from the metallurgists, process chemists and others who calculate and design the process flow sheets required to complete the separation of REEs from each other. This definition of the MREEs is the same one that the Chinese authorities use, by the way, when talking about the export quotas allocated to light rare earths, and to medium / heavy rare earths.

The MREE or SEG group as it is also known, arises within the early stages of SX, the main commercial process used to separate and to purify REEs, because of the absence of Pm from the feedstocks used. As we saw earlier, although Pm is a LREE, it does not occur with the other REEs, and so the process chemists take advantage of this ‘blip’ in the sequence of REEs, caused by the resulting ‘gap’ between Nd and Sm, during the separation process.

The figure above shows a summary of the elements of most interest to the REE industry, contained within the LREE, MREE and HREE groups. You can click on it to see an enlarged view.

The MREE group therefore arises for process engineering reasons only and does not require any new definitions, or changes to existing ones. The purists and certain laypeople can argue that these elements should strictly be defined as LREEs; such individuals should simply see the MREEs as a subset of the LREEs and worry themselves no more. I prefer to use all three terms – LREEs, MREEs and HREEs – and have been doing so exclusively for the past 18 months as I get more and more into the processing side of the rare-earth industry. Either way, you now know the origin of the term, and the three REEs to which it correctly refers. Either way, you also now know that Eu and Gd are really NOT HREEs.

Finally, the last term that I use quite a lot is critical REEs (CREEs), in reference to the five REEs that are of critical importance to future demand for sustainable energy sources – specifically Nd, Eu, Tb, Dy and Y.

As I always tell people – just make sure that you understand exactly which definitions a particular company is using, when looking at reported data which use one or more of the group names described above. In the meantime, let’s hope that certain of my fellow commentators on the rare-earth sector start to get the hang of the basic terminology for these materials…

References

1. E. H. Nickel, The Canadian Mineralogist, 1995, Vol. 33, pp. 689-690.
2. N. G. Connelly, T. Damhus, R. M. Hartshorn and A. T. Hutton, 2005, Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005, RSC Publishing, Cambridge, p. 366.
3. J. B. Hedrick, 2000, Minerals Yearbook: Volume I – Metals and Minerals, US Geological Survey, Reston, p62.1.