The Middle Rare Earth Weight Debate
Rare earth metals or rare earth minerals comprise of 15 elements in the periodic table plus scandium and yttrium sharing similar chemical characteristics. Their ‘rare’ nomenclature has little to do with the way one might order them at a steakhouse restaurant or with an actual geological scarcity. Nevertheless, they tend to occur in dispersed ore deposits that are difficult to mine commercially. The fact that they are very similar in atomic number, featuring a steady progression, also makes them hard to separate. Commercial viability of rare earth ores depends on the extent of concentration of the various elements. Typically, rare earths have been classified as “light” or “heavy”.
The ‘Light’ elements are closer to the left side of the Periodic Table starting at the atomic number 57 with lanthanum. As the elements progress toward samarium (atomic no. 62), they are known as Light Rare Earths (LREE). The next elements in the progression, starting with Europium and ending with Lutetium, are generally known as Heavy Rare Earths (HREE). The ‘Heavies’ are found in far lower concentrations, making them more valuable. Under this basic separation of Light from Heavy, Dysprosium, one of the rare earths with the highest demand thanks to its heat resistance and magnetic strength. Dysprosium’s classification as an HREE and its rarity also helps to explain the higher value if HREE in a given deposit. Nevertheless, rare metals have also been classified as “middle”, the middleweights, in which case the lights are classified as Lanthanum to Promethium, the ‘middles’ from Samarium to Dysprosium and the Heavies from Holmium to Lutetium.
The heavies also include, because of their chemical properties, if not their atomic number, Yttrium. The addition of a Middle Rare Earth Element (MREE) has added some confusion. Indeed, the separation of Heavy and Light REE’s is based, as noted above, on the atomic number; however, it is also based on the direction of spin chosen by their respective electrons. It is the configuration of the electrons in the rare earth elements that ultimately decides whether they are to be classified as ‘heavy’ or ‘light’. Indeed, Yttrium and Scandium have atomic numbers lower than Lanthanum (39 and 21 respectively) yet these are included in the ‘cool’ group of HREE’s. HREE’s have ‘paired’ electrons (in scientific terms represented by the combination of a clockwise and counter-clockwise spinning electron), whereas the LREE have no pairing. Considered from this logic, therefore, there is no need to create the additional category of Middle REE’s. Terbium to Lutetium feature paired electrons; Lanthanum to Gadolinium do not.
The addition of the ‘Middle’ category, nevertheless, may help in better defining the commercial value of a resource, even if their validity under geological certifications such as NI 43-101 or JORC is questionable, given the lack of a valid scientific basis for their categorization as such. A mining exploration Company may wish to use the ‘Middle’ classification to help distinguish the lower end of the LREE such as Lanthanum or Cerium, which are abundant, with ‘rarer’ LREE’s such as Europium or Samarium, which command higher prices on the market once processed. The rare earth market is highly complex because of the difficulties associated with the related extraction processes. It is not easy to separate the various minerals into individual oxides and the development of rare earth minerals requires the availability of advanced processes as much as it does rich deposits.
This is because a deposit may contain all the REE’s but not all minerals can be separated sometimes. Nevertheless, even this consideration cannot obscure the fact that the U.S. Department of the Interior-U.S. Geological Survey, among others, strictly defines rare earths elements as being either ‘Light’ or ‘Heavy’. The electron configuration at the heart of the distinction between heavy and light rare earths does not leave any room for the ‘middle’ category. This is not a mere matter of semantics; there is a clear scientific reasoning behind the two main REE ‘weight’ classes. By all means, the ‘Mid-weight’ categorization can work, but it would require a re-definition of the Heavy and Light to be based exclusively on the atomic number. Of course, in that optic, any number of weight categories could be added to further redefine a resource; there could be featherweights or medium lights and medium heavy just like in boxing or wrestling. The point is consistency and to that end maintaining the standard and government recognized definitions would be the wise course to take, avoiding confusion and any potential legal issues.
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