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

Holmium is considered a rare earth element because it is one of the 15 lanthanide elements that constitute the core group of rare earth elements. With atomic number 67, holmium is positioned between dysprosium (66) and erbium (68) in the lanthanide series, which extends from lanthanum (atomic number 57) through lutetium (atomic number 71). As a lanthanide, holmium exhibits all the characteristic chemical properties that define this group, including the typical trivalent oxidation state (Ho3+) and an ionic radius that follows the systematic “lanthanide contraction” – the progressive decrease in ionic radius from lanthanum to lutetium. This places holmium firmly within the chemical family of rare earth elements, sharing their distinctive electronic configuration and chemical behavior.

Holmium’s classification as a heavy rare earth element (HREE) reflects both its atomic number and its geochemical behavior. The heavy rare earth elements “comprise terbium through lutetium (atomic numbers 65 through 71),” and holmium’s position in the middle of this range means it has a relatively small ionic radius due to lanthanide contraction. With a crustal abundance of 1.3 parts per million, holmium is less abundant than the light rare earth elements but still more common than silver, gold, or platinum. This abundance level places it between dysprosium (5.2 ppm) and erbium (3.5 ppm), following the general pattern of decreasing abundance with increasing atomic number among the heavy rare earth elements.

Holmium’s geochemical behavior perfectly aligns with the defining characteristics of rare earth elements. Like all REEs, holmium possesses a high charge (+3) and an ionic radius that prevents its incorporation into common rock-forming minerals during crystallization. This incompatibility causes holmium to remain in residual melts along with other rare earth elements until specialized REE minerals form. Holmium 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 holmium 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 holmium, which never occurs in isolation but always as part of the rare earth element suite.

From both scientific and industrial perspectives, holmium exemplifies the rare earth element group. Holmium is one of “six REE ions Gd3+ through Tm3+” that “have unusually large magnetic moments, owing to their several unpaired electrons,” giving it the highest magnetic moment of any naturally occurring element. This exceptional magnetic property, derived from its electronic configuration as a lanthanide, makes holmium valuable for specialized applications including lasers, high-strength magnets, and glass coloring.

The combination of holmium’s position in the lanthanide series, its characteristic REE chemistry including the Ho3+ oxidation state, its systematic ionic radius following lanthanide contraction, its invariable co-occurrence with other REEs in minerals, and its exceptional magnetic properties shared with other heavy lanthanides unequivocally establishes holmium as a rare earth element.

Why Is Holmium Considered A Critical Raw Material?

Holmium is considered a critical raw material due to its unique magnetic properties essential for specialized technologies, combined with extreme supply concentration in China and limited global availability. Holmium possesses the highest magnetic moment of any naturally occurring element, as it is one of “six REE ions Gd3+ through Tm3+” that “have unusually large magnetic moments, owing to their several unpaired electrons.” This exceptional magnetic property makes holmium irreplaceable in applications including high-strength magnets for medical and scientific equipment, specialized lasers for medical procedures and materials processing, and glass coloring for optical filters. These applications have no adequate substitutes because they depend specifically on holmium’s unique electronic configuration and resulting magnetic properties.

The criticality of holmium is severely amplified by its scarcity and the absolute concentration of heavy rare earth element production in China. With a crustal abundance of only 1.3 parts per million, holmium is among the less common rare earth elements. Between 2011 and 2017, China produced approximately 84% of the world’s rare earth elements overall, but critically for holmium, China’s ion-adsorption clay deposits in southern provinces are “the world’s primary source of the heavy REEs.” These clay deposits are the only economically viable source of holmium because, despite modest concentrations of 0.03 to 0.2 percent REEs, they are enriched in heavy rare earth elements and “the REEs can be easily extracted from the clays with weak acids.” At Mountain Pass, California, the combined content of all heavy REEs from europium through lutetium plus yttrium totals only 0.4% of the rare earth oxides, making Western sources essentially incapable of producing meaningful quantities of holmium.

The strategic vulnerability of holmium supply extends beyond mining to processing capabilities. The separation of individual rare earth elements requires sophisticated solvent extraction facilities, and “nearly all REE separation and refining” occurs within China. This means that even if rare earth ores containing holmium were mined elsewhere, they would need to be sent to China for processing into pure holmium compounds. When China announced export restrictions in 2010 through quotas, licenses, and taxes, it highlighted the extreme vulnerability of holmium-dependent industries. The complexity of separating holmium from other lanthanides, particularly from its neighbors dysprosium and erbium which have similar chemical properties, requires advanced facilities that exist almost exclusively in China.

The convergence of multiple factors establishes holmium as a critical raw material with severe supply risks. Expert panels from the National Research Council, U.S. Department of Energy, and European Commission have consistently ranked rare earth elements as having the highest “criticality” factor, combining high technological importance with extreme supply vulnerability. For holmium, this criticality is particularly acute because its applications in high-field magnets for MRI machines, lasers for medical procedures, and nuclear control rods have no viable substitutes. The observation that heavy rare earth elements are “less abundant, and harder to source than the LREEs, and thus command a higher price” applies especially to holmium. With viable deposits limited to Chinese ion-adsorption clays, processing capabilities concentrated in China, and irreplaceable applications in medical and scientific equipment, holmium represents a critical vulnerability where supply disruptions could eliminate entire categories of advanced medical and research capabilities. 

The complete dependence on a single country for both raw material and processing, combined with holmium’s unique magnetic properties that cannot be replicated by any other element, firmly establishes holmium as a critical raw material for advanced economies.

Interesting Facts About Holmium

1. Holmium has the highest magnetic permeability of any element, making it ideal for creating the strongest artificially generated magnetic fields when placed within high-strength magnets.
2. Despite being a rare earth element, holmium is actually 20 times more abundant than silver in Earth’s crust, occurring at about 1.3 parts per million.
3. Holmium possesses the highest magnetic moment (10.6 μB) of any naturally occurring element, meaning each atom acts as an exceptionally strong tiny magnet.
4. When exposed to certain wavelengths of light, holmium oxide produces sharp absorption bands that are used as calibration standards for optical spectrophotometers.
5. Holmium can absorb neutrons effectively, making it useful in nuclear reactor control rods, though it’s less commonly used than elements like boron or cadmium.
6. The element exhibits unusual color-changing properties – its oxide appears yellow in daylight but turns pink under fluorescent light due to its unique electronic transitions.
7. Holmium lasers operate at 2.1 micrometers wavelength, which is strongly absorbed by water, making them particularly effective for precise surgical procedures and kidney stone removal.
8. At low temperatures, holmium undergoes a unique magnetic phase transition at 19 K, transforming from a helical antiferromagnetic to a ferromagnetic state.
9. Holmium forms the most paramagnetic compounds known, with holmium(III) compounds showing magnetic susceptibilities exceeding those of all other lanthanide compounds.
10. The element’s name derives from Holmia, the Latin name for Stockholm, making it one of four elements named after the same city (along with yttrium, terbium, and erbium).
11. Holmium has an unusually large magnetocaloric effect, meaning it heats up significantly when magnetized and cools when demagnetized, useful for magnetic refrigeration technologies.
12. The Ho³⁺ ion produces distinctive sharp emission lines in the visible spectrum, particularly a bright red emission, making holmium compounds useful in specialized phosphors.
13. Natural holmium consists of only one stable isotope (¹⁶⁵Ho), making it monoisotopic and giving it very consistent nuclear properties.
14. Holmium exhibits the phenomenon of “giant magnetostriction” – it physically changes shape more than most materials when placed in a magnetic field.
15. The element forms unusual “molecular magnets” when combined with certain organic ligands, creating single-molecule magnets that retain magnetization at the molecular level.
16. Holmium-doped yttrium aluminum garnet (Ho:YAG) crystals can store optical information through spectral hole burning, a quantum mechanical effect useful for data storage.
17. The metal has an unexpectedly high electrical resistivity for a metallic element (94 nΩ·m), about 60 times higher than copper.
18. Holmium compounds can act as quantum bits (qubits) in quantum computing applications due to their well-defined electronic energy levels and long coherence times.
19. When alloyed with yttrium, holmium creates materials with near-zero thermal expansion coefficients, useful for precision instruments that must maintain dimensions across temperature ranges.
20. Holmium has the highest proton-to-neutron ratio (67:98) among stable heavy elements, placing it at the edge of nuclear stability and making it valuable for nuclear physics research.

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