Let me ask what should be a pretty straightforward question:
“How many rare-earth elements are there in the Periodic Table?”
Hmmm – apparently this question is not quite as simple to answer as we might think. Depending on who you ask these days, you could hear a number of different answers, each of them based on not unreasonable assumptions.
Okay – having thrown that first question out there, let me ask a second, which in theory should be just as straightforward to answer:
“Which are the so-called heavy-rare-earth elements?”
Uh-oh – now we’ve veered into what appears to be even trickier territory, judging by the wide range of answers that are out there. And yet, given the significant difference in monetary values of the various rare-earths elements [REEs], shouldn’t there really be a single, consistent definition for the constituent REEs, as well as those that make up the heavy REEs [HREEs]?
Okay – so let’s take a crack at the first of these definitions. Here are the most common numbers that I’ve heard people use, and the reasons why they use them:
a) 15: refers to the number of lanthanide elements sitting in a row towards the bottom of the Periodic Table. This group includes La-Ce-Pr-Nd-Pm-Sm-Eu-Gd-Tb-Dy-Ho-Er-Tm-Yb-Lu;
b) 16: refers to the lanthanide series, with the addition of yttrium [Y];
c) 17: refers to the lanthanide series + Y, and the addition of scandium [Sc];
d) 30: refers to the lanthanide series with the addition of the actinide series, which sits below the lanthanides in the Periodic Table. The actinide group includes Ac-Th-Pa-U-Np-Pu-Am-Cu-Bk-Cf-Es-Fm-Md-No-Lr.
My non-scientific survey of usage online by rare-earth exploration and mining companies, indicates that option b) 16 above appears to be the most commonly used definition. However, with that said, things get somewhat muddied when companies use the term “TREO + Y” [i.e. total rare-earth oxide plus yttrium] when providing quantitative data on their deposits. Does this mean that they don’t actually consider Y to be a rare earth, but, thinking that some folks do, they include it anyway?
In giving this issue some thought, I wondered where we might be able to find some sort of definitive answer to this quandry. Since we’re talking about elements of the Periodic Table, it makes sense to me to see what the International Union of Pure and Applied Chemistry [IUPAC] has to say on the matter. IUPAC is the international body that ultimately decides, among other things, what newly discovered elements should be called, and the standard symbols for existing ones. So, what do they have to say on the matter?
In its 2005 “Nomenclature of Inorganic Chemistry”, IUPAC defines the rare-earth metals as “Sc, Y and the lanthanoids”. So by this definition, IUPAC considers there to be 17 rare-earth elements. Thus, it makes sense to me that in the interests of standardization, those of us that are involved with the rare earths, should start using this definition also.
An aside: note here, that IUPAC refers to the lanthanOIDs, and not the lanthanIDEs. Per IUPAC:
Although lanthanoid means ‘like lanthanum’ and so should not include lanthanum, lanthanum has become included by common usage. Similarly, actinoid. The ending ‘ide’ normally indicates a negative ion, and therefore lanthanoid and actinoid are preferred to lanthanide and actinide.
Okay, so that deals with the number of REEs overall. Now, what about the HREEs? Which elements are they? Again, I’ve heard and read a variety of definitions, including:
c) Eu-Gd-Tb-Dy-Ho-Er-Tm-Yb-Lu + Y;
There are numerous other additional variations too – probably even more numerous than the definitions of number of rare-earth elements as a whole. Unfortunately, IUPAC or a similarly august body does not appear to have an opinion on this one. However, I will note that in the world of magnetics, from which I hail, it is generally definition c) Eu-Gd-Tb-Dy-Ho-Er-Tm-Yb-Lu + Y that is used for the HREEs, because of certain magnetic characteristics that they have in common, in comparison to the other REEs.
Since a number of the elements included in the various definitions of the HREEs above, sell at very significant unit prices when compared to the other REEs, I would argue that it is really important that all rare-earth exploration and mining companies get on the same page with their definition of HREEs. This is even more important when the term HREE or heavy-rare-earth oxide [HREO] is used without definition. Reviewing the usage online by these companies, it seems that most [but not all] of them are using the same definition that the magnetics community uses. Consequently, I would propose that this definition be used across the board within the industry. I was chatting with Jack Lifton earlier this week on this subject, and we were in agreement on this point. Going forward, Jack will continue to use this definition when discussing HREEs, and I will be doing the same.
That said – what do you think about the above points and definitions? Which do you use, and why?