[MSA-talk] REE, lanthanides, lanthanoids, Y and Sc

JOHN PAINE jbp3 at comcast.net
Mon Nov 27 12:39:49 EST 2017


I side with the geochemists on this one: scandium is rarely a significant presence in any lanthanide/yttrium ore, and yttrium and friends only occasionally are significant in scandium minerals. However, yttrium and the heavy lanthanides are inevitably found together, both in ores and at trace levels in other minerals. Geochemistry, after billions of years of operation, (with the exception of the occasional redox separation of cerium) only succeeded in dividing the lanthanides into two broad groups: the light lanthanides as found in monazite and Bastnaes cerite, giving what I would call the "1803 Ceria" composition, a typical example of which might run about 45-48% Ce, 24% La, 17% Nd, 5% Pr, 3% Sm, 2% Gd and traces of the rest. {Alkaline rocks such as carbonatites will have elevated levels of La and Ce relative to the previous.] The heavy lanthanides gravitated into the "1794 Yttria" composition, approximately 65-70% yttrium oxide by weight, circa 5% (+/-2%) even-Z heavies (Gd, Dy,Er,Yb) and circa 1%(+/- 0.5%) odd-Z heavies (Eu, Tb, Ho, Tm, Lu). Typically Dy and Ho would be the most abundant of their respective groups. Compositions rich in Yb have been found, but not in large orebodies. [In case you are wondering who I am to be chiming in on this subject: I am a retired organic chemist, degrees from Harvard, who started playing with rare earths back in 1959 at the age of 13, in his home basement 'playroom". Rare earths came from the Lindsay Chemical Division of American Potash and Chemical corporation (I still keep their catalogs as a memento) - now a Superfund site. I could afford their product at the time: "rare earth chloride" was only 54 cents per pound, back in 1959. The best neodymium oxide I could afford was only the 85% grade, made by fractional crystallization, and cost $3.78 per pound back then. By the time the playroom disbanded, mid 1960's, I had 65 pounds of RE chemicals, some from the newly arrived Molycorp. Circa 1999, I learned how cheap the Chinese REE had become, and with far greater financial clout behind me, made a point of acquiring a minimum of 5 kilograms of each (oxide), not to mention an array of other items, both pure and mixed. (If someone were to need small samples of ionic clay concentrates, I might be able to help them.) I am not in the business of selling anything, it would strictly be pro bono. My goal in acquisition was to prepare displays of rare earths for science museums; so far my only donation was the the Sterling Hill Museum of Mining and Mineralogy.]

> On November 27, 2017 at 11:13 AM Anton Chakhmouradian <Anton.Chakhmouradian at umanitoba.ca> wrote:
>
>
> Hello All,
>
> Indeed, chemists include Sc and Y in the REE family, whereas geochemists do not usually consider Sc as part of that family and are split on the Y issue. The difference is: chemists look at elements mostly from the standpoint of pure elements/compounds, whereas geochemists look at how elements ASSOCIATE with one another in nature. For example, ScPO4 has the same zircon-type structure as YPO4, YbPO4 and other heavy-REE orthophosphates, so it would seem natural to a chemist to just consider Sc as part of the REE family. In reality, Sc3+ is >10% smaller than Lu3+, the smallest lanthanide (or lanthanoid, according to the revised IUPAC nomenclature). This difference in ionic radii will drive Sc partitioning in natural processes, effectively forcing it to either form its own minerals or to associate with trivalent or tetravalent cations that are closer to it in ionic radius (IR). For example, zircon associated with the scandium phosphate pretulite (IR [8]Zr = 0.84 vs. IR [8]Sc = 0.87 A) would contain up to 3.2% Sc2O3, whereas xenotime (YPO4) found in the same association (IR [8]Y = 1.02 A) < 0.7% Sc2O3 (Moelo et al., Can. Mineral., 40, 1657).
>
> The differences in radius do also affect the crystal chemistry and metallogeny of Sc. For instance, kolbeckite (ScPO4.2H2O) is isostructural with metavariscite (AlPO4.2H2O), not churchite YPO4.2H2O (Yang et al., Acta Crystal., C63, i91). In laterites, which are probably the most promising future Sc resource (if there is ever any need for much Sc), where this element substitutes in Fe3+ oxides and hydroxides (Chassé et al., Geochem. Persp. Lett., 3, 105).
>
> Yttrium, on the other hand, is a close match for holmium and usually present in heavy-REE minerals at CI-normalized levels comparable to those of Ho. This is the reason why it is often placed between Dy and Ho in chondrite-normalized REE plots. Some minerals (notably, fluorite) exhibit departure of Y from this predictable behavior, which is reflected in Y anomalies in normalized plots. These anomalies are undoubtedly related to differences in Y and Ho speciation in some types of fluids and melts. Despite this intimate GEOCHEMICAL association between the lanthanides and Y, some geoscientists prefer not to include the latter in the REE family, usually without explaining their reasons for doing so.
>
> To those interested in learning more about REE, their applications and politics behind them, as well as the "rare vs. not-so-rare" debate, I can recommend the special issue of Elements Frances and I edited in 2012:
> http://elementsmagazine.org/get_pdf.php?fn=e8_5.pdf&dr=e8_5
>
> Anton R. Chakhmouradian
> University of Manitoba
>
>
> -----Original Message-----
> From: msa-talk-bounces at minlists.org [mailto:msa-talk-bounces at minlists.org] On Behalf Of Frank Spera
> Sent: November 26, 2017 5:51 PM
> To: Don Halterman <donhalterman at comcast.net>
> Cc: msa-talk at minlists.org
> Subject: Re: [MSA-talk] REE...
>
> well, if i recall the REE include Y and Sc which are NOT rare Lanthanides …..but its been a long time since i took an inorganic chemistry class!!
> fs
>
> -----Original Message-----
> From: msa-talk-bounces at minlists.org [mailto:msa-talk-bounces at minlists.org] On Behalf Of Don Halterman
> Sent: November 26, 2017 6:13 PM
> To: Frank Spera <spera at ucsb.edu>
> Cc: msa-talk at minlists.org
> Subject: Re: [MSA-talk] REE...
>
>
> It has been some time since I read the article deprecating the term, but I recall the phrase "the elements in question are neither rare nor earths," or words to that effect.  Unlike other matters of terminology, this one isn't that critical so I won't pursue the matter further if I seem to be the only one who recalls this.
>
> Don
>
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