Light vs. Heavy Rare Earths: What Is the Difference?
The energy transition relies increasingly on a limited group of materials with exceptional properties. Among them, rare earths hold a strategic place because of their role in manufacturing the permanent magnets used in electric vehicles and wind turbines. Demand for these elements, particularly those associated with magnets, has risen sharply over the past decade, driven by the electrification of uses and the deployment of low-carbon technologies.
Behind this label, however, lies a more nuanced reality. Rare earths comprise 17 chemical elements with similar properties, but whose geological behavior, availability, and industrial uses vary significantly. To better understand these differences, scientists and industry players generally classify them into two or three groups depending on the discipline: light (LREE), intermediate (MREE), and heavy (HREE). This distinction plays a decisive role in resource assessment, technological choices, and supply strategies.
Although their abundance in the Earth's crust is relatively high, it is their concentrated deposits that remain limited on a global scale. Understanding the differences between light and heavy rare earths therefore helps to grasp the economic and geopolitical dynamics that structure this key sector of critical materials today.
What Is a Rare Earth and How Are They Classified?
Rare earth elements (REE) designate a group of 17 chemical elements comprising the 15 lanthanides (from lanthanum to lutetium), to which scandium (Sc) and yttrium (Y) are added. Although these last two are not strictly part of the lanthanide series, they are generally included in this family because of their similar chemical properties and geological behavior [2].
Contrary to what their name might suggest, these elements are not all particularly rare in the Earth's crust. Several of them are even more abundant than certain common metals. What distinguishes them, on the other hand, is the rarity of deposits sufficiently concentrated to be exploited in an economically viable way. This characteristic largely explains the challenges associated with their production and supply on a global scale [2].
Beyond their definition, understanding rare earths rests on their classification into subgroups, which helps to explain their differences in behavior, geological distribution, and industrial use. This classification constitutes an essential analytical tool for geoscientists, engineers, and industry players.
The Classification Principle: Atomic Weight and Lanthanide Contraction
In most geoscientific contexts, the distinction between light and heavy rare earths rests first on criteria linked to atomic structure. As one progresses from lanthanum (La) toward lutetium (Lu), the atomic number increases, but the atomic radius decreases progressively. This phenomenon, called lanthanide contraction, directly influences the chemical properties of the elements, notably their behavior in solution and their affinity for certain minerals [3].
On this basis, rare earths are generally separated into two main groups:
The light rare earths (LREE) generally comprise the elements of lower atomic mass, namely lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm) and, according to several classifications, europium (Eu). These elements concentrate more frequently in minerals such as bastnäsite and monazite, which explains their significant presence in several commercially exploited deposits.
The heavy rare earths (HREE) generally comprise the elements of higher atomic mass, namely gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). Yttrium (Y) is also often associated with them, not because of its atomic mass, but because its geochemical behavior is close to that of certain heavy rare earths [1].
Some classifications shift this boundary slightly. Gadolinium (Gd) can sometimes be included among the LREE, while europium (Eu) can be attached to the HREE in certain approaches. Scandium (Sc), for its part, is generally included in the rare earth family because of its chemical similarities, even though it is not strictly part of the lanthanide series [1].
In parallel, an approach drawn from fundamental chemistry offers a complementary reading based on electronic structure, in particular on the organization of electrons in the 4f shell. This perspective helps to explain certain specific physical properties, but it is less directly used in industrial contexts. In practice, these two approaches partly overlap without leading to a single classification [1].
Two or Three Groups? A Boundary That Varies by Discipline
The classification of rare earths is not universal. It varies according to the field of application, whether geochemistry, mineralogy, engineering, or market analysis [1]. This variability does not mean that the classifications are contradictory. Rather, it reflects different analytical objectives:
The industrial classification into two groups is the most widespread in market analyses and sector reports. It groups together the LREE and HREE and helps to distinguish the generally more abundant elements from those associated with more pronounced constraints in terms of supply, separation, or valorization. This approach is particularly useful for analyzing economic dynamics and value chains [1].
The classification of the International Union of Pure and Applied Chemistry rests on a slightly different logic. It includes gadolinium among the light rare earths, which shifts the boundary between the groups and reflects more closely considerations linked to the electronic structure of the elements [1].
The geochemical classification into three groups introduces an intermediate category in order to better describe the behavior of the elements in natural processes. This approach introduces the intermediate rare earths (MREE), located between the LREE and the HREE.
Table I — Summary of REE subgroups
| Group | Element | Symbol | Dominant uses | Geological presence |
|---|---|---|---|---|
| LREE (light rare earths) | Lanthanum | La | Catalysts, optical glass | More abundant, present in the main exploited deposits |
| Cerium | Ce | Catalytic converters, polishing | ||
| Praseodymium | Pr | Alloys, magnets | ||
| Neodymium | Nd | Permanent magnets | ||
| Samarium | Sm | Magnets, lasers | ||
| MREE * (medium rare earths) intermediate group | Europium | Eu | Lighting, displays | Less abundant, often associated with more specialized deposits |
| Gadolinium | Gd | Medical imaging (MRI), nuclear | ||
| Terbium | Tb | Phosphors, high-performance magnets | ||
| HREE (heavy rare earths) | Dysprosium | Dy | High-temperature magnets | Rarer and more dispersed |
| Yttrium | Y | Alloys, lasers, phosphors | ||
| Holmium | Ho | Lasers, specialized magnets | ||
| Erbium | Er | Optical fiber, glass | ||
| Thulium | Tm | Lasers, specialized applications | ||
| Ytterbium | Yb | Lasers, alloys | ||
| Lutetium | Lu | Catalysis, specialized applications |
* Intermediate group with a variable boundary: depending on the classification, the MREE are attached to the HREE, or else distributed between LREE and HREE (some sources use only two groups, LREE and HREE).
Thus, the terms LREE, MREE and HREE should be understood as analytical categories rather than as strictly universal boundaries. Depending on the scientific, industrial, or economic context, certain elements, such as gadolinium or europium, may be classified differently.
Ultimately, the classification of rare earths does not constitute merely a theoretical framework. It helps to better understand the fundamental differences between these elements, which then translate into significant gaps in terms of extraction, availability, and industrial applications.
Distinctive Characteristics: Mineralogy, Extraction, and Availability
Beyond the theoretical classifications, the differences between light rare earths (LREE) and heavy rare earths (HREE) manifest concretely in their geological environment, their extraction conditions, and their distribution on a global scale. These elements do not behave the same way in natural systems, which directly influences the types of deposits in which they concentrate, as well as the processes needed to exploit them.
Different Host Minerals Depending on the Group
Rare earths are not distributed uniformly in the Earth's crust. They are integrated into a wide variety of minerals, but each group presents specific affinities with certain types of geological formations.
The light rare earths (LREE) concentrate mainly in minerals such as bastnäsite and monazite. These minerals are relatively widespread on a global scale and constitute today the main industrially exploited sources. They present high contents of elements such as neodymium and praseodymium, which play a central role in the manufacture of permanent magnets. As an indication, neodymium can represent between 10% and 18% of the total rare earth content in these minerals [5].
The heavy rare earths (HREE), on the other hand, are generally associated with distinct and much less frequent minerals. Among these are notably gadolinite, xenotime, samarskite, euxenite, fergusonite, as well as several yttrium-rich minerals such as yttrotantalite or yttrialite [1]. These minerals are often more dispersed and present lower concentrations, which complicates their exploitation.
Certain minerals illustrate this separation between groups particularly well. Monazite is generally enriched in light rare earths, whereas xenotime is more associated with intermediate and heavy rare earths. This distinction reflects different geochemical processes during the formation and alteration of rocks [3].
On a global scale, the diversity of rare earths translates into a wide variety of host minerals. At least 245 minerals containing rare earths have been recorded, distributed among several large families such as carbonates, oxides, silicates, and phosphates [2].
Industrial Uses and Strategic Stakes
The distinction between light rare earths (LREE) and heavy rare earths (HREE) is not limited to a scientific classification. It has direct implications for industrial uses, the economic value of the elements, and the risks associated with their supply. In practice, not all elements play the same role. Some dominate in volume in industrial chains, while others, rarer, are essential to ensure the performance of technologies.
The Main Uses by Group
Light rare earths dominate in volume. Neodymium (Nd) and praseodymium (Pr) hold a central place in the manufacture of NdFeB-type permanent magnets, which are widely used in the motors of electric vehicles and the generators of wind turbines. These elements represent today the main source of demand for rare earths, owing to the growth of technologies linked to electrification.
Beyond magnets, several LREE play a key role in well-established industrial applications:
Lanthanum (La) is used in petroleum refining catalysts, where it helps to improve the efficiency of hydrocarbon conversion processes.
Cerium (Ce) is involved notably in automotive catalytic converters, as well as in applications linked to polishing and surface treatment.
Polishing powders and optical glasses also constitute important outlets for these elements, reflecting their use in varied industries ranging from electronics to optics [2].
Recent data illustrate this predominance of LREE in the volumes consumed. In 2024, permanent magnets represented about 48% of global demand for rare earths, followed by catalysts (15.7%) and polishing powders (10.5%) [6]. These segments rely mostly on light elements.
Heavy rare earths are essential to performance and reliability. The HREE generally intervene in smaller quantities, but their role is often decisive. Dysprosium (Dy) and terbium (Tb), for example, are added to NdFeB magnets in order to improve their coercivity, that is, their capacity to resist demagnetization at high temperature. This property is essential for applications subject to high thermal conditions, such as electric motors or certain industrial installations [5].
Other heavy elements intervene in more specialized applications:
Yttrium (Y), often associated with the HREE, is used in advanced materials, notably in certain lighting systems and in specific energy technologies, including some types of electrolyzers.
Europium (Eu) and terbium (Tb) are essential for the phosphors used in fluorescent lighting and certain display technologies, where they help to produce precise and efficient colors.
Scandium (Sc), although sometimes treated separately, is sought after for its properties in certain advanced alloys and emerging energy applications [5].
This strategic importance is recognized by several institutions. The United States Department of Energy identifies notably five elements as critical for the development of clean energy technologies: dysprosium, neodymium, terbium, europium, and yttrium. This selection illustrates well the interdependence between light and heavy rare earths in modern industrial systems [1].
The Price Gap andthe Supply Imbalance
Heavy rare earths (HREE) are generally less abundant, more dispersed in deposits, and more complex to produce than light rare earths (LREE). This relative rarity does not mean that all HREE automatically have a high value, but it helps to explain why certain heavy elements, such as dysprosium (Dy) and terbium (Tb), reach prices clearly higher than those of more abundant elements such as lanthanum (La) or cerium (Ce).
The price data published by IRENA as of December 24, 2021 illustrate well this gap between widely available light elements and heavy elements that are more constrained in terms of supply [5]:
| Element | Group | Price (USD/kg) |
|---|---|---|
| Lanthanum (La) | LREE | 2 |
| Cerium (Ce) | LREE | 1.5 |
| Neodymium (Nd) | LREE | 143 |
| Dysprosium (Dy) | HREE | 452 |
| Terbium (Tb) | HREE | 1,720 |
This price difference reflects several combined factors. First, certain HREE are present in much more limited quantities in the Earth's crust. IRENA indicates, for example, that dysprosium represents less than 1% of all rare earths in relative abundance, which limits the capacity to rapidly increase supply when demand rises [5].
Next, the sources of HREE are more geographically concentrated and often rely on particular types of deposits, notably ion-adsorption clays. This situation accentuates the vulnerability of supply chains, especially when certain mining activities are suspended for environmental or social reasons [5].
Finally, the stakes do not concern only extraction. The transformation and refining of rare earths remain heavily concentrated. In 2024, China represented about 69% of global mining production and 90% of refined production of rare earths [7]. This concentration reinforces the strategic importance of the elements whose supply is already more limited, in particular certain HREE used in high-performance applications.
Thus, the price gap between LREE and HREE does not stem only from the market value at a given moment. It also reflects a deeper geological and industrial reality: the heavy elements are generally less abundant, more difficult to produce, and more sensitive to supply constraints.
Conclusion
The LREE / MREE / HREE distinction highlights a reality that is often underestimated: these elements, although grouped under a single label, respond to profoundly different geological, industrial, and economic logics. The gaps in availability, complexity of extraction, and value reflect structural constraints that directly influence global supply chains.
In this context, securing access to certain elements, in particular among the heavy rare earths, represents a growing strategic stake. It requires not only a better understanding of geological systems, but also the development of new approaches in exploration, in ore processing, and in the diversification of supply sources. The dependence on specific types of deposits and on a limited number of producing regions underscores the importance of a long-term vision in the management of these resources.
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References
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2 Van Gosen, B.S., Verplanck, P.L., Seal, R.R. II, Long, K.R., Gambogi, J. "Rare-earth elements, chap. O of Critical mineral resources of the United States." U.S. Geological Survey Professional Paper 1802, 2017, p. O1–O31. USGS, https://doi.org/10.3133/pp1802O.
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5 Gielen, D. et Lyons, M. Critical materials for the energy transition: Rare earth elements. International Renewable Energy Agency (IRENA), 2022, ISBN : 978-92-9260-437-0. IRENA, https://atf.asso.fr/media/technews/39/tnf39-prof3-irena-rare-earth-elements-2022.pdf.
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[7] Firestone, Melissa D., and Jada Garofalo. An Analysis of the Current Global Market for Rare Earth Elements. University of Wyoming School of Energy Resources, Center for Energy Regulation & Policy Analysis, Jan. 2022, https://www.uwyo.edu/ser/research/centers-of-excellence/energy-regulation-policy/_files/ree-econ-policy.pdf.
