Today, let’s take a look at interesting facts about Thulium and answer the following questions: “Why Is Thulium Considered A Rare Earth Element?”, and “Why Is Thulium Considered A Critical Raw Material?”

For more information, check out the light rare earth elements (LREEs) as a group, the heavy rare earth elements (HREEs) as a group, and all rare earth elements (REEs). Be sure to check out all other critical raw materials (CRMs), as well.

Why Is Thulium Considered A Rare Earth Element?

Thulium is considered a rare earth element because it is one of the 15 lanthanide elements that form the core of the rare earth element group. With atomic number 69, thulium is positioned between erbium (68) and ytterbium (70) in the lanthanide series, which extends from lanthanum (atomic number 57) through lutetium (atomic number 71). As a lanthanide, thulium exhibits all the characteristic chemical properties that define this group, including the typical trivalent oxidation state (Tm3+) and an ionic radius that reflects the systematic “lanthanide contraction” – the progressive decrease in ionic radius across the lanthanide series. Despite being one of the two least abundant rare earth elements with a crustal abundance of only 0.52 parts per million, thulium’s fundamental chemistry and behavior align it firmly within the rare earth element family.

Thulium’s classification as a heavy rare earth element (HREE) reflects both its position in the periodic table and its geochemical behavior. The heavy rare earth elements comprise “terbium through lutetium (atomic numbers 65 through 71),” and thulium’s placement near the end of this series means it has one of the smallest ionic radii among the lanthanides due to maximum lanthanide contraction. Even though thulium is one of the rarest REEs, it is still “nearly 200 times more common than gold,” illustrating that “rare” earth elements is indeed a historical misnomer. In rare earth deposits, thulium occurs in very low concentrations – at Mountain Pass, California, the combined content of all heavy rare earth elements from europium through lutetium plus yttrium totals only 0.4% of the rare earth oxides, with thulium representing only a minute fraction of this already small percentage.

Thulium’s geochemical behavior perfectly exemplifies the characteristics of rare earth elements. Like all REEs, thulium possesses a high charge (+3) and ionic radius that impedes its incorporation into common rock-forming minerals. During magmatic processes, thulium remains in the melt phase along with other REEs until specialized rare earth minerals crystallize. Thulium invariably occurs with other rare earth elements in nature, particularly in minerals that can accommodate heavy rare earth elements such as xenotime ((Y,HREE,Th,U)PO4), where the notation specifically indicates that HREEs including thulium can substitute for yttrium in the crystal structure. The principle that “REEs can substitute for one another in crystal structures, and multiple REEs typically occur within a single mineral” applies directly to thulium, which never occurs in isolation but always as part of the rare earth element suite.

From scientific, historical, and practical perspectives, thulium exemplifies the rare earth element group despite its scarcity. Its name derives from Thule, the ancient name for Scandinavia, reflecting its discovery by Swedish chemist Per Teodor Cleve in 1879 through the painstaking process of separating rare earth elements from one another. The fact that “six REE ions Gd3+ through Tm3+ have unusually large magnetic moments, owing to their several unpaired electrons” demonstrates that thulium shares the magnetic properties characteristic of heavy rare earth elements. Industrially, thulium’s applications, though limited by its scarcity, leverage its rare earth properties: it is used in portable X-ray machines and specialized lasers that utilize its unique electronic configuration. 

The combination of thulium’s position in the lanthanide series, its characteristic REE chemistry including the Tm3+ oxidation state and systematic ionic radius, its invariable co-occurrence with other REEs in minerals, and its specialized applications that depend on its rare earth electronic and magnetic properties unequivocally establishes thulium as a rare earth element, albeit one of the rarest members of this group.

Why Is Thulium Considered A Critical Raw Material?

Thulium is considered a critical raw material due to its extreme scarcity, specialized high-technology applications, and severe supply concentration in China. As one of the two least abundant rare earth elements with a crustal abundance of only 0.52 parts per million, thulium is extraordinarily rare even by rare earth standards. In major rare earth deposits, thulium concentrations are vanishingly small – at Mountain Pass, California, the combined content of all heavy rare earth elements from europium through lutetium plus yttrium totals only 0.4% of the rare earth oxides, with thulium representing merely a trace fraction of this percentage. This extreme scarcity means that thulium is only produced as a byproduct of processing vast quantities of other rare earth elements, making its availability entirely dependent on the economics of mining deposits for more abundant REEs.

The criticality of thulium is amplified by the absolute concentration of heavy rare earth element production in China. Between 2011 and 2017, China produced approximately 84% of the world’s rare earth elements overall, but critically, China’s ion-adsorption clay deposits are “the world’s primary source of the heavy REEs.” These deposits in southern China are the only economically viable source of thulium because they are preferentially enriched in heavy rare earth elements and “the REEs can be easily extracted from the clays with weak acids.” When China announced export restrictions in 2010 through quotas, licenses, and taxes, it created extreme vulnerability for thulium supply, as no alternative sources exist at any meaningful scale. The processing infrastructure for separating thulium from other rare earth elements is also concentrated in China, with “nearly all REE separation and refining” occurring within Chinese facilities.

Thulium’s strategic importance lies in its irreplaceable applications in medical and laser technologies. The element is specifically used in “portable X-ray machines” where its unique nuclear properties enable compact, efficient X-ray generation for medical diagnostics in remote locations or emergency situations. Thulium is also essential for specialized lasers that leverage its specific electronic energy levels for applications in medicine, telecommunications, and materials processing. The magnetic properties of thulium, as one of “six REE ions Gd3+ through Tm3+” with “unusually large magnetic moments,” suggest potential applications in emerging magnetic technologies. These specialized uses have no adequate substitutes, as each application depends on thulium’s unique atomic and electronic properties.

The extreme criticality of thulium is underscored by economic and availability constraints that create a self-reinforcing scarcity cycle. The historical observation that “for many years the main use of lutetium was the study of the behavior of lutetium” applies equally to thulium, suggesting that applications have been severely constrained by availability and cost rather than lack of potential uses. With thulium being one of the least abundant REEs and occurring only in trace amounts even in HREE-enriched deposits, any expansion of applications faces immediate supply constraints. Expert panels from the National Research Council, U.S. Department of Energy, and European Commission have ranked rare earth elements as having high “criticality” ratings, with heavy rare earth elements like thulium representing the extreme end of supply risk. 

The combination of thulium’s extreme natural scarcity, its irreplaceable role in portable X-ray devices and specialized lasers, the complete concentration of viable deposits and processing in China, and the absence of any substitute materials or alternative supply sources establishes thulium as one of the most critical raw materials, where even minor supply disruptions could eliminate entire technology applications.

Interesting Facts About Thulium

1. Thulium is the rarest of all naturally occurring lanthanide elements, with an abundance in Earth’s crust of only 0.5 parts per million – making it less common than gold.
2. It was the last of the stable lanthanides to be discovered (1879 by Per Teodor Cleve), primarily because its spectral lines are extremely faint and difficult to detect.
3. Natural thulium consists of only one stable isotope (Tm-169), making it one of only 26 monoisotopic elements – unusual among the lanthanides.
4. Thulium-170, a radioactive isotope with a 128-day half-life, emits unusually low-energy gamma rays (84 keV), making it ideal for portable X-ray devices that don’t require electricity.
5. Pure thulium metal is so soft it can be cut with a knife, yet it becomes significantly harder and more workable when contaminated with other lanthanides – an unusual property among metals.
6. Thulium has the second-lowest melting point (1545°C) among all lanthanides, surpassed only by cerium.
7. Unlike most rare earth elements, thulium forms only one oxidation state (+3) in all its compounds, showing remarkable chemical simplicity.
8. Thulium-doped fiber amplifiers operate at 2 μm wavelength – a “eye-safe” region that doesn’t damage human retinas, unlike conventional 1.5 μm amplifiers.
9. The element exhibits unusual magnetic properties – it’s ferromagnetic below 32 K and antiferromagnetic between 32-56 K, showing complex magnetic phase transitions.
10. Thulium ions produce the purest green emission among all lanthanides when excited, making them valuable for green phosphors despite their rarity.
11. In thulium-doped YAG lasers, the element achieves remarkably high quantum efficiency (approaching 200%) through a process called cross-relaxation.
12. Thulium has one of the highest vapor pressures among the lanthanides, subliming at relatively low temperatures compared to its neighbors.
13. The element’s neutron capture cross-section is unusually high (105 barns), making it an effective neutron absorber in nuclear applications.
14. Thulium metal reacts with water more slowly than other lanthanides, requiring hot water or steam to show appreciable reaction rates.
15. Its ionic radius contracts more than expected across the lanthanide series (lanthanide contraction), contributing significantly to the separation difficulties of heavy rare earths.
16. Thulium sesquioxide (Tm₂O₃) changes color from greenish-white to reddish upon heating – a rare thermochromic property among lanthanide oxides.
17. The element shows anomalously low electrical resistivity compared to neighboring lanthanides, attributed to its unique electronic band structure.
18. Thulium-doped materials exhibit highly efficient up-conversion luminescence, converting infrared light to visible light better than most other lanthanide dopants.
19. Despite being paramagnetic at room temperature, thulium has one of the lowest magnetic susceptibilities among the lanthanides due to its nearly filled 4f electron shell.
20. Thulium’s crystal structure is unique among early discovered elements – it wasn’t correctly determined until 1953, over 70 years after discovery, due to its scarcity and difficulty in obtaining pure samples.

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