1. Introduction

Over the past few years, more and more people have begun to pay attention to a rather special class of metals—rare-earth elements. Many have never heard of them at all, or perhaps only know that they belong to the upper row of the two rows of elements located below the main body of the periodic table. In fact, despite the astonishingly wide range of applications for these metals, the public knew very little about them just over a decade ago.

Rare-earth elements go by several names: rare-earth metals, rare earths, or simply REE. They constitute a closely related group of heavy elements, totaling 17 in all, including scandium (Sc), yttrium (Y), and the lanthanide series. In the periodic table shown in Figure 1, these elements are highlighted with red outlines. Among them, the lanthanides should actually be located between barium (Ba) and hafnium (Hf).

Figure 1: Periodic Table of the Elements

The 17 rare-earth elements specifically include: scandium (Sc) at atomic number 21, yttrium (Y) at atomic number 39, lanthanum (La) at atomic number 57, cerium (Ce) at atomic number 58, praseodymium (Pr) at atomic number 59, neodymium (Nd) at atomic number 60, promethium (Pm) at atomic number 61, samarium (Sm) at atomic number 62, europium (Eu) at atomic number 63, gadolinium (Gd) at atomic number 64, terbium (Tb) at atomic number 65, dysprosium (Dy) at atomic number 66, holmium (Ho) at atomic number 67, erbium (Er) at atomic number 68, thulium (Tm) at atomic number 69, ytterbium (Yb) at atomic number 70, and lutetium (Lu) at atomic number 71. In addition, based on the differences and similarities in the physicochemical properties of these rare-earth elements, as well as their coexistence in minerals and the varying ionic radii that give rise to distinct characteristics, the 17 rare-earth elements are typically divided into three groups: the light rare-earth group—lanthanum, cerium, praseodymium, neodymium, and promethium; the medium rare-earth group—samarium, europium, gadolinium, terbium, and dysprosium; and the heavy rare-earth group—holmium, erbium, thulium, ytterbium, lutetium, scandium, and yttrium.

2. Name

The term “rare earth elements” is actually a misnomer. First, they are not alkaline earth metals—alkaline earth metals refer to the elements in Group 2 of the periodic table, including beryllium, magnesium, calcium, strontium, barium, and radium. On the contrary, as shown in Figure 1, rare earth elements belong to the transition metals (Group 3B). Second, rare earth elements are also not “rare.”

The origin of the term “rare earth elements” is closely tied to their discovery history. Most rare earth elements were discovered in the 19th century, with the exceptions of yttrium (1794), lutetium (1907), and promethium (1943). In the 19th century, only one rare earth deposit was known—the quarry near the Swedish town of Ytterby—leading people at the time to believe that these elements were exceedingly rare. As for the origin of the word “earth,” most rare earth elements were initially extracted in the form of oxides. In French, the dominant scientific language of the 19th century, an element’s oxide was referred to as its “terre,” meaning “earth.” Similarly, in German, another major scientific language of the time, an element’s oxide was also called its “Erde,” also meaning “earth.”

As mentioned earlier, rare-earth elements are actually not rare. Although the number of rare-earth deposits is relatively limited, the abundance of these elements is quite high. The most common rare-earth element is cerium (Ce), with a crustal abundance of 60 parts per million, ranking it 27th among Earth’s crustal elements in terms of abundance—higher than lead (Pb), which ranks 37th with a crustal abundance of 10 parts per million. Even lutetium, one of the least abundant rare-earth elements (with a crustal abundance of 0.5 parts per million), has a crustal abundance roughly 200 times greater than that of gold (0.0031 parts per million).

Within the rare-earth element series, their abundance exhibits a sawtooth distribution (Figure 2). This phenomenon originates from Odo- Hawkins' Law. This law... It is pointed out that the abundance of elements with even atomic numbers is higher than that of elements with odd atomic numbers.
 

Figure 2: Curve showing the variation of elemental abundance (expressed as atomic fraction) with atomic number.

3. Major ore minerals

Currently, the primary ore minerals of rare earth elements are: Monazite Fluorocarbon cerite and xenotime. The earliest rare-earth mineral to be exploited was gadolinite—several rare-earth elements were first isolated from this mineral, yet it was never put into industrial application. The first ore mineral used for the industrial extraction of rare-earth elements, however, was monazite.

Monazite

The general chemical formula for monazite is CePO₄. 4 Its name derives from the Greek word “monazeis” (meaning “solitude”). On the one hand, this is because its crystals often appear isolated; on the other hand, it’s also due to the mineral’s relative rarity at the time of its initial discovery (see Figure 3). In addition to cerium (Ce), monazite also contains other rare-earth elements, with light rare-earth elements (LREEs: lanthanum La, cerium Ce, praseodymium Pr, neodymium Nd, and samarium Sm) being predominant. Moreover, monazite invariably exhibits a mixed state involving multiple rare-earth elements. When naming these minerals, the suffixes “-Ce,” “-La,” “-Nd,” or “-Pr” are added to indicate the rare-earth element present in the highest concentration. Monazite typically also contains thorium (Th) or uranium (U), but the concentrations of these elements are generally too low to be economically viable as valuable by-products.

Monazite typically occurs as a minor mineral in granites, granodiorites, and their associated pegmatites, and is also found in a variety of metamorphic rocks. The reason why monazite can form concentrated deposits lies primarily in its two key characteristics: (1) It is a heavy mineral with a specific gravity ranging from 4.8 to 5.5, with an average value of 5.15; (2) It exhibits extremely high resistance to weathering. Consequently, after the weathering of igneous or metamorphic parent rocks and subsequent transport processes, monazite tends to concentrate in placer deposits and heavy-mineral sands.

Figure 3: Monazite

Fluorocarbon cerite

Fluorocarboncerite was first described by the Swedish chemist Wilhelm Hisinger, who at the time named it "basic fluorocerite." The mineral was discovered at the Bäshnas mine near Riddarhyttan in Västmanland County, Sweden. The general chemical formula for fluorocarboncerite is Ce(CO 3 ) F 。

As another major rare-earth mineral, bastnäsite is primarily composed of light rare-earth elements, with its main constituents being cerium (Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd). Among the heavy rare-earth elements, only yttrium (Y) is relatively common. When naming the mineral, the suffixes “-Ce,” “-La,” “-Nd,” or “-Y” must be added before the mineral name to indicate the predominant rare-earth element present. In addition, this mineral also contains trace amounts of other heavy rare-earth elements. Furthermore, hydroxyl-bearing varieties—hydroxyl-fluorocarboncerite-(Ce) and hydroxyl-fluorocarboncerite-(Nd)—are also found in this mineral.

Because it contains neither uranium (U) nor thorium (Th), fluorocarbonite has replaced monazite as the primary ore mineral for extracting light rare-earth elements. When fluoride ions and carbonate ions undergo substitution, related secondary minerals may form. Although fluorocarbonite is widely distributed, it has never been found in large-scale accumulations. It is commonly found in a variety of igneous rocks, such as carbonatites, vein-type ore deposits, contact metamorphic rocks, and pegmatites. Large fluorocarbon-celestialite deposits are typically associated with carbonatite intrusions, which often occur in association with alkaline intrusive rocks.

Figure 4: Fluorocarbon Cerite

Monazite

Xenotime (Figure 5) was first described by Berzelius, and its sample originated from the island of Hidra in Norway. The mineral’s name derives from the Greek words “xenos” (meaning “foreign”) and “time” (meaning “honor”). The general chemical formula for xenotime is YPO₄. 4 。

Unlike monazite and bastnäsite, xenotime typically contains not only yttrium (Y) but also substantial amounts of heavy rare-earth elements (HREEs: yttrium Y, terbium Tb, dysprosium Dy, holmium Ho, erbium Er, thulium Tm, ytterbium Yb, and lutetium Lu). The content of rare-earth oxides (REO) in xenotime can reach as high as 67%, with heavy rare-earth oxides constituting the majority. Among these, dysprosium, ytterbium, erbium, and gadolinium are the most common heavy rare-earth elements; xenotime also contains small amounts of terbium, holmium, thulium, and lutetium.

Monazite is the primary source for extracting heavy rare-earth elements. However, like bastnäsite, it also contains thorium (Th) or uranium (U). Whether these two elements are recoverable byproducts or impurities that need to be treated depends on the location of the ore deposit and the concentrations of these elements in the mineral.

Monazite is a secondary mineral found in pegmatites and other non-basic igneous rocks, and it also commonly occurs in metamorphic rocks. Very similar in properties to monazite, xenotime also has a relatively high specific gravity, ranging from 4.4 to 5.1, with an average value of 4.75; consequently, it tends to concentrate in placer deposits and heavy-mineral sands. However, such xenotime deposits are not widely distributed. In terms of their enrichment characteristics for actinide elements, monazite tends to enrich thorium, whereas xenotime tends to enrich uranium, though it can also adsorb significant amounts of thorium.

Figure 5: Monazite

Heterosexual stone

Heterosite (see Figure 6) was first described by Klaproth in 1810. It is a ring silicate mineral with the general chemical formula Na 4 (Ca, Ce) ₂ (Fe² Plus , Mn² Plus ) ZrSi 8 O 22 (OH, Cl) ₂ Its name derives from the Greek word meaning “easily decomposed,” referring to its property of being readily soluble in acids. In igneous rocks, heterogenite occurs in unsaturated alkaline intrusive rocks and their associated pegmatites.

Figure 6: Heterogeneous stone (a pale-red substance)

4. Minor ore minerals

There are numerous other rare-earth minerals, but most of them are generally of little significance for the extraction processes used in the rare-earth industry. Table 1 lists the other rare-earth-bearing minerals that have been identified to date. Similar to the primary rare-earth minerals, many secondary rare-earth minerals also exhibit multiple varieties—this is because several rare-earth elements can substitute for one another. For these minerals, a suffix is added to their names to indicate the rare-earth element that predominates in their composition; each mineral with such a suffix represents a distinct mineral species. For example, bastnäsite has clearly defined subtypes: cerium bastnäsite, neodymium bastnäsite, and yttrium bastnäsite.

Table 1: Minor Rare-Earth Minerals

5. Rocks containing rare-earth elements

The primary rock types containing rare-earth elements are mainly igneous rocks, with some metamorphic and sedimentary rocks also showing enrichment. The core types and their characteristics are as follows:

I. Core Igneous Rock Types (Primary Hosts of Rare Earth Enrichment)

1. Magmatic carbonatite: Associated with alkaline ultramafic complexes, these rocks are the primary host rocks for light rare-earth elements. Representative minerals include monazite and bastnäsite. China’s Bayan Obo superlarge rare-earth deposit is closely related to such rocks, with total rare-earth concentrations often ranging from hundreds to thousands of parts per million (ppm).

2. Alkaline rocks: These include nepheline syenite, aegirine syenite, and alkaline granites, which are enriched in light rare-earth elements and high-field-strength elements. Such rocks often form composite ore deposits—for example, the Saima Intrusion in Liaoning Province—where the primary minerals are green-phase cerianite-titanite and heterogenite.

3. Porphyry: Shallow-level intrusive rocks (often occurring as dikes) originate from metasomatized lithospheric mantle. Some carbonate-rich lamprophyres exhibit significantly elevated rare-earth element (REE) contents. Such REE-enriched dike bodies have already been discovered in the Wuhai area of Inner Mongolia.

II. Secondary Rock Types (locally enriched or present in lower concentrations)

1. Granitic gneiss (metamorphic rock): Rare-earth minerals commonly found in Proterozoic metamorphic rocks include monazite and xenotime. In rocks of this type from the Bayannur region of Inner Mongolia, the proportion of high-value lanthanum and neodymium can reach 18% to 25%.

2. Basalt (basic igneous rock): Widely distributed, the rare earth element content is generally low. Among them, light rare earth elements are relatively enriched in alkaline basalts, and their distribution patterns are closely related to the rock’s origin (oceanic vs. continental).

3 Sedimentary rock: A few sedimentary rocks, such as phosphorite and claystone, can adsorb rare-earth elements through sedimentary processes; however, their industrial value is far lower than that of the aforementioned igneous rock types.

6. Rare-earth deposits

Rare-earth deposits can be broadly classified into two categories: primary deposits formed by hydrothermal and magmatic processes, and secondary deposits formed by weathering and sedimentation. Previous researchers have classified ore deposits based on their mineral assemblages and the geological processes involved in their formation. The specific categories include: 1. Carbonate-related deposits; 2. Alkaline igneous rock-related deposits; 3. Magmatic-hydrothermal deposits; 4. Weathering residual deposits; 5. Placer deposits; 6. Marine sedimentary deposits; 7. Rare-earth deposits in coal.

Rare-earth mineral deposits have now been discovered in more than 34 countries. Based on the content of rare-earth oxides (REO), global reserves of rare earths are estimated at approximately 130 million tons. The countries with the largest reserves include China (about 33.8%), Vietnam (about 16.9%), Brazil (about 16.2%), Russia (about 16.2%), India (about 5.3%), Australia (about 3.2%), and the United States (about 1.8%).

Since the late 1990s, China has dominated the rare-earth market, both in terms of supply of raw ores and processed/purified products. From the 2000s to the early 2010s, China’s rare-earth production accounted for more than 90% of the global total. However, in recent years, influenced by adjustments in the international rare-earth market, the production of rare-earth oxides outside China has increased somewhat. As a result, China’s share of global rare-earth production has dropped to between 55% and 70%, while the output of the United States, Vietnam, and Thailand has been rising year by year.

Figure 7 Globally Known Rare Earth Element Mines and Deposits (2024)

7. China’s Major Rare Earth Deposits

BaYun Ebo Rare Earth Mine

The Baiyun Ebo iron-rare earth-niobium deposit is located in the northwest of Baotou City, Inner Mongolia Autonomous Region, and is hosted within the Baiyun Ebo Group. This group consists of nine sedimentary rock formations dating from the Paleoproterozoic to Mesoproterozoic periods, sequentially named H1 through H9 from bottom to top; however, the lithological classification of Formation H8 remains controversial. These formations are predominantly composed of slates and metamorphic sandstones, whereas Formation H8 is characterized mainly by dolomitic marbles exhibiting both fine- and coarse-grained textures. Formation H8 forms a spindle-shaped, layered body that extends in an east-west direction, with a length exceeding 18 kilometers and a width exceeding 1 kilometer; it serves as the primary host rock for most of the ore deposits. The deposit also features widespread occurrences of various types of carbonate veins.

Due to the complexity of the mineralization process—primarily attributable to extensive and intense hydrothermal alteration and modification during the late stage of mineralization and metamorphism—the genesis of this deposit has long been a subject of debate. Figures 8a–d illustrate a possible model for the genesis of this ore deposit: the evolution of the carbonatite rock has proceeded through three distinct stages—the magmatic stage, characterized by magnesian-ferrocarbonatites (primarily ore-bearing dolomites); the carbonothermal stage, driven by calcium-carbonate fluids; and the hydrothermal stage, marked by abundant fluorite deposition and alkaline alteration. Subsequently, intense tectonic movements led to local remobilization and enrichment in rare-earth elements and niobium.

The complex, multi-stage rare-earth mineralization is primarily attributable to metasomatic alteration between calcic carbonate fluids and earlier-intruded magnesio-carbonate rocks. Furthermore, it is important to note that although the magmas of magnesio-carbonate rocks were already enriched in rare-earth elements, the substantial enrichment of these elements in the ore-bearing dolomites (i.e., the mined ore bodies) is mainly due to post-magmatic fluids associated with calcic carbonates.

The main rare-earth-bearing minerals in the Baiyun Obo deposit include fluorocarboncerite-(Ce) and monazite-(Ce), as well as a variety of other rare-earth and niobium minerals such as Huanghe mineral, cerianite, browncerite, and pyrochlore. The deposit holds approximately 800 million tons of rare-earth reserves, with an average rare-earth oxide grade of 6%. The estimated niobium reserves amount to 2.2 million tons, Nb ₂ O ₅ The average grade is 0.13%; the total iron ore reserves amount to at least 1.5 billion tons, with an average grade of 35%. The deposit contains extremely high concentrations of light rare earth elements, accounting for 97% of the total rare earth content.

Figure 8, left: Model of the formation of the Bayan Obo rare-earth deposit ((a) formation of primary ore-bearing dolomite, (b) carbon thermal stage, (c) hydrothermal stage, (d) tectonic movement); right: Evolution of ore-forming fluids and mineralization process of the Yaoniuping rare-earth deposit (2024).

Yaoniuping Rare Earth Mine

The ore deposit is located at the junction between the western Yangtze Plate and the eastern Qinghai-Tibet Plateau, and is controlled by the Panxi Rift Valley. The host rocks hosting the ore deposits consist of Devonian–Permian mudstone clastics, limestones, and Tertiary colluvial deposits. The igneous rocks in the area include Yanshan-period granites, alkaline syenite-carbonatite complexes, and Mesozoic rhyolites, which are widely distributed throughout the ore deposit area.

Rare-earth mineralization is hosted within a vein system, primarily in barite-dolomite pegmatite veins as well as linear vein bodies within the northeast-trending syenite-carbonatite complex at Yaoniuping. This complex extends approximately 1,400 meters in length and 260 to 350 meters in width. The genesis of the ore deposit was investigated through fluid inclusion studies (Fig. 8e). The results indicate that the ore-forming fluids originated from a carbonatite-alkaline syenite complex: in the first stage of hydrothermal activity, neosyenitization occurred; in the second stage, immiscible fluids appeared; and in the final stage, rare-earth mineralization took place.

However, a recent study suggests that sustained mantle-derived magmatic underplating at shallow depths, combined with stable regional thermal anomalies, has created favorable conditions for the formation of rare-earth mineralization. The primary rare-earth-bearing mineral in this deposit is fluorocarboncerite-(Ce), accompanied by minor amounts of carbonatocerite-(Ce) and monazite-(Ce). The rare-earth reserves are characterized by a predominance of light rare earth elements. In addition, the deposit also hosts 3.78 million tons of barite, 330,000 tons of lead, 2.4 million tons of fluorite, and 174 tons of silver reserves.

Ion-adsorption deposit

In southern China, there exists another typical type of rare-earth deposit—the weathered crust eluvial deposit, also known as the "ion-adsorption deposit." These deposits are typically rich in high-value light rare earths, while a few are enriched in heavy rare earths. The compositional differences among ion-adsorption deposits are primarily influenced by the diversity of the chemical composition of their parent rocks.

These ore deposits formed in relatively stable tectonic and geological environments (low uplift and low erosion rates) and are primarily found in parts of southern China. The formation process of ion-adsorption-type rare-earth deposits is as follows: In a warm, humid surface environment, parent rocks—particularly granites, gneisses, and granodiorites—are subjected to weathering and leaching. During this process, dissolved rare-earth ions are adsorbed onto clay minerals as they migrate. Moreover, microorganisms play a positive role in the migration and fractionation of rare-earth elements during weathering.

The weathering zone is rich in rare earth elements and typically ranges in thickness from 3 to 10 meters. Based on its mineralogical characteristics, it can be divided into four layers: the surface soil layer (0–2 meters); the fully weathered layer (5–10 meters), which constitutes the primary rare earth ore body and generally has a rare earth concentration of 0.03% to 0.15%; the partially weathered bedrock layer (3–5 meters); and the bedrock layer or weakly weathered layer (a typical profile is shown in Figure 9).

Figure 9: Typical Model of Ion-Adsorption Clay Deposits in the Western Yunnan Region of China (2024)

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