6.4 Rare earth elements
12 min read•august 21, 2024
Rare earth elements (REEs) are crucial in geochemistry, offering insights into Earth's formation and evolution. Their unique properties make them valuable tracers in geological studies, helping scientists unravel complex histories and processes.
REEs exhibit similar chemical behavior due to their electronic configuration, with varying ionic radii influencing their geochemical properties. They're classified into light and heavy groups, each with distinct characteristics that shape their distribution and fractionation in geological settings.
Characteristics of rare earth elements
- Rare earth elements (REEs) play a crucial role in geochemistry due to their unique properties and behavior in geological systems
- Understanding REE characteristics provides insights into Earth's formation, evolution, and various geological processes
- REEs serve as important tracers and indicators in geochemical studies, allowing scientists to unravel complex geological histories
Chemical properties
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- Highly electropositive elements with a strong affinity for forming stable compounds with oxygen
- Exhibit similar chemical behavior due to their electronic configuration, with 4f orbitals gradually filled across the series
- Lanthanide contraction causes a decrease in ionic radii across the series, influencing their geochemical behavior
- Predominantly occur in the +3 oxidation state, with exceptions like Ce4+ and Eu2+ under specific conditions
- Form strong complexes with ligands such as fluoride, carbonate, and phosphate in geological fluids
Physical properties
- Silvery-white metals with high luster, tarnishing rapidly when exposed to air
- Relatively soft and malleable, with increasing hardness across the lanthanide series
- High melting and boiling points, ranging from 798°C (Cerium) to 1663°C (Lutetium)
- Strong paramagnetic properties due to unpaired electrons in the 4f orbitals
- Exhibit unique spectroscopic characteristics, including sharp absorption and emission lines in the visible and near-infrared regions
Abundance in Earth's crust
- Despite the name "rare," REEs are relatively abundant in the Earth's crust
- Total REE abundance ranges from 150 to 220 ppm, comparable to common metals (copper, zinc)
- Light REEs (La to Eu) are more abundant than heavy REEs (Gd to Lu)
- Cerium is the most abundant REE, with an average crustal concentration of 60-70 ppm
- Promethium, a radioactive element, is the rarest naturally occurring REE
- Odd-numbered atomic REEs are less abundant than even-numbered ones due to the Oddo-Harkins effect
Classification of rare earth elements
- REEs are categorized based on their atomic numbers, chemical properties, and geological behavior
- Understanding REE classification helps geochemists interpret their distribution and fractionation in various geological settings
- Classification schemes provide a framework for studying REE patterns and anomalies in geochemical systems
Light vs heavy REEs
- (LREEs) include elements from (La) to Europium (Eu)
- (HREEs) comprise elements from Gadolinium (Gd) to Lutetium (Lu)
- LREEs have larger ionic radii and are generally more incompatible in geological processes
- HREEs have smaller ionic radii and tend to be more compatible in mineral structures
- Yttrium (Y) is often grouped with HREEs due to its similar ionic radius and geochemical behavior
- The LREE/HREE ratio is used as an indicator of fractionation processes in igneous and metamorphic systems
Lanthanide series
- Consists of 15 elements from Lanthanum (La, atomic number 57) to Lutetium (Lu, atomic number 71)
- Characterized by the progressive filling of the 4f electron orbital across the series
- Exhibit similar chemical properties due to their electronic configuration, with minor variations
- Lanthanide contraction causes a decrease in ionic radii from La to Lu, influencing their geochemical behavior
- Often divided into three subgroups: light (La-Nd), middle (Sm-Dy), and heavy (Ho-Lu) lanthanides
- Cerium and Europium can exist in multiple oxidation states, leading to unique geochemical signatures
Promethium anomaly
- Promethium (Pm) is the only lanthanide without stable isotopes in nature
- Exists primarily as a short-lived radioactive element with a half-life of 17.7 years (147Pm)
- Produced through the spontaneous fission of uranium and thorium in trace amounts
- Absence of stable Pm isotopes creates a gap in the REE series, affecting geochemical patterns
- Synthetic Pm is used in various applications, including nuclear batteries and luminous paints
- Study of Pm in extraterrestrial materials provides insights into nucleosynthesis processes
Geochemical behavior of REEs
- REEs serve as powerful tracers in geochemical systems due to their coherent behavior and systematic variations
- Understanding REE behavior helps geochemists interpret magmatic, metamorphic, and sedimentary processes
- REE patterns and anomalies provide valuable information about source compositions and geological environments
Fractionation processes
- Ionic radius and charge control REE fractionation in geological systems
- Crystal-liquid partitioning during magmatic processes leads to REE enrichment or depletion
- causes progressive changes in REE patterns of evolving magmas
- Partial melting of source rocks results in LREE enrichment in initial melts
- Metamorphic reactions can redistribute REEs between minerals, altering bulk rock patterns
- Weathering and sedimentary processes can fractionate REEs based on their mobility and adsorption properties
Partitioning in minerals
- REEs substitute for major elements in mineral structures based on ionic radius and charge compatibility
- Garnets preferentially incorporate HREEs due to their smaller ionic radii
- Plagioclase feldspar shows a strong affinity for Europium, leading to Eu anomalies in magmatic systems
- Zircon concentrates HREEs and exhibits a pronounced positive Ce anomaly
- Apatite and are important hosts for LREEs in igneous and metamorphic rocks
- Clay minerals and Fe-Mn oxides play a crucial role in REE adsorption and transport in sedimentary environments
Mobility in geological systems
- REEs are generally considered immobile under most geological conditions
- However, REE mobility can be enhanced by complexation with ligands (F-, CO32-, PO43-)
- Hydrothermal fluids can mobilize and transport REEs, leading to enrichment in ore deposits
- Weathering processes can mobilize REEs, with LREEs being more mobile than HREEs
- Redox conditions affect the mobility of Ce and Eu due to their multiple oxidation states
- Organic matter and colloids play a role in REE transport in surface and groundwater systems
REE patterns and distributions
- REE patterns provide valuable information about geological processes and source compositions
- Normalized REE plots are widely used to visualize and interpret REE distributions in rocks and minerals
- Understanding REE patterns helps geochemists decipher complex geological histories and environments
Chondrite-normalized patterns
- REE concentrations are typically normalized to chondritic meteorite values to remove the Oddo-Harkins effect
- Chondrite-normalized patterns reveal enrichment or depletion relative to primitive solar system composition
- Flat patterns indicate similar behavior of all REEs during geological processes
- LREE-enriched patterns are common in continental crust and alkaline igneous rocks
- HREE-enriched patterns are characteristic of residual mantle rocks and some oceanic basalts
- Negative slopes in normalized patterns suggest fractionation between LREEs and HREEs
- Positive slopes indicate preferential HREE enrichment, often seen in garnet-bearing rocks
Europium and cerium anomalies
- Europium anomalies result from the ability of Eu to exist in both +2 and +3 oxidation states
- Positive Eu anomalies occur in plagioclase-rich cumulates due to Eu2+ substitution for Ca2+
- Negative Eu anomalies are common in evolved igneous rocks due to plagioclase fractionation
- Cerium anomalies arise from the oxidation of Ce3+ to Ce4+ in oxidizing environments
- Positive Ce anomalies are observed in oxidized sediments and some manganese nodules
- Negative Ce anomalies occur in seawater and marine sediments due to Ce4+ removal by scavenging
- The magnitude of Eu and Ce anomalies provides information about redox conditions and mineral fractionation
Tetrad effect
- Subtle but systematic deviation from smooth chondrite-normalized REE patterns
- Characterized by four segments or "tetrads" in the lanthanide series (La-Nd, Pm-Gd, Gd-Ho, Er-Lu)
- Observed in highly evolved igneous rocks, pegmatites, and some hydrothermal systems
- Attributed to changes in electron configuration and bonding across the lanthanide series
- M-type tetrad effect shows convex patterns within each tetrad
- W-type tetrad effect exhibits concave patterns within each tetrad
- Provides insights into extreme fractionation processes and fluid-rock interactions
Analytical techniques for REEs
- Accurate and precise measurement of REEs is crucial for geochemical studies and interpretations
- Various analytical techniques have been developed to quantify REEs in geological materials
- Each method has its advantages and limitations, suitable for different sample types and research objectives
Mass spectrometry methods
- Inductively Coupled Plasma (ICP-MS) offers high sensitivity and multi-element capability
- Laser Ablation ICP-MS (LA-ICP-MS) allows for in-situ analysis of minerals with high spatial resolution
- Thermal Ionization Mass Spectrometry (TIMS) provides high precision for isotope ratio measurements
- Secondary Ion Mass Spectrometry (SIMS) enables microanalysis of REEs in minerals with excellent spatial resolution
- Accelerator Mass Spectrometry (AMS) is used for ultra-trace analysis and long-lived radioisotope measurements
- Sample preparation involves acid digestion, fusion, or laser ablation depending on the technique and sample type
X-ray fluorescence spectroscopy
- Energy Dispersive (ED-XRF) provides rapid, non-destructive analysis of bulk samples
- Wavelength Dispersive X-ray Fluorescence (WD-XRF) offers higher resolution and sensitivity for REE analysis
- Synchrotron XRF enables high-resolution mapping of REE distributions in geological materials
- Sample preparation typically involves fusion or pressing of powdered samples into pellets
- XRF techniques are particularly useful for analyzing high concentration REE samples and ores
- Limited sensitivity for some REEs compared to mass spectrometry methods
Neutron activation analysis
- Instrumental Neutron Activation Analysis (INAA) offers high sensitivity for many REEs
- Non-destructive technique based on the measurement of induced radioactivity in samples
- Particularly useful for analyzing geological materials with complex matrices
- Provides accurate results for REEs that are difficult to measure by other techniques (Sm, Eu, Tb, Ho, Tm)
- Requires access to a nuclear reactor for sample irradiation
- Long count times may be necessary for precise measurement of less abundant REEs
Applications in geochemistry
- REEs serve as powerful tools in various geochemical applications due to their unique properties
- Understanding REE behavior helps geologists unravel complex geological processes and histories
- REE studies contribute to our knowledge of Earth's formation, evolution, and resource potential
Igneous petrogenesis
- REE patterns provide insights into magma source compositions and melting processes
- Fractional crystallization trends can be traced using REE variations in igneous rock suites
- Eu anomalies indicate the role of plagioclase in magmatic differentiation
- Ce anomalies in volcanic rocks can reveal magma oxidation states and degassing processes
- REE partitioning between minerals and melts constrains crystallization conditions
- Isotopic ratios of Nd and Sm are used to determine magma source ages and mantle heterogeneity
Sedimentary provenance studies
- REE signatures in sedimentary rocks reflect the composition of source terranes
- Chondrite-normalized patterns help distinguish between felsic and mafic source rocks
- Heavy mineral concentrates (zircon, monazite) provide insights into sediment provenance
- Ce anomalies in marine sediments indicate paleoredox conditions during deposition
- REE patterns in phosphorites and carbonates reveal information about depositional environments
- Diagenetic processes and their effects on REE distributions can be traced using careful analysis
Hydrothermal systems
- REEs are mobilized and fractionated in hydrothermal fluids, providing information about fluid sources and evolution
- Eu anomalies in hydrothermal minerals indicate fluid temperature and redox conditions
- REE patterns in hydrothermal veins help distinguish between magmatic and metamorphic fluid sources
- Alteration halos around ore deposits show characteristic REE signatures related to fluid-rock interaction
- REE behavior in geothermal systems provides insights into element transport and mineral precipitation
- Hydrothermal REE deposits (carbonatites, alkaline complexes) are important economic resources
Environmental geochemistry of REEs
- REEs play an increasingly important role in environmental studies due to their growing industrial use
- Understanding REE behavior in natural and anthropogenic systems is crucial for assessing environmental impacts
- REEs serve as tracers for various environmental processes and contaminant transport mechanisms
Natural vs anthropogenic sources
- Natural sources include weathering of REE-bearing rocks and minerals (monazite, )
- Volcanic emissions and hydrothermal vents contribute to natural REE fluxes in the environment
- Anthropogenic sources include mining and processing activities, industrial emissions, and electronic waste
- Agricultural use of phosphate fertilizers can introduce REEs into soils and water systems
- Coal combustion and oil refining release REEs as byproducts in fly ash and petroleum coke
- Medical applications (MRI contrast agents) contribute to REE release in hospital wastewater
Biogeochemical cycling
- REEs are taken up by plants and organisms, with varying degrees of bioaccumulation
- Soil organic matter and clay minerals play a crucial role in REE retention and mobility
- Redox conditions influence the behavior of Ce and Eu in aquatic and soil environments
- Microbial activity can affect REE speciation and bioavailability through various mechanisms
- REEs in marine environments show characteristic vertical profiles influenced by scavenging and remineralization
- Atmospheric transport and deposition of REEs occur through dust particles and aerosols
Environmental impacts
- Increased industrial use of REEs raises concerns about potential ecological and health effects
- REE accumulation in soils can affect plant growth and microbial communities
- Aquatic ecosystems may be impacted by REE contamination from mining and industrial activities
- Bioaccumulation of REEs in food chains poses potential risks to higher trophic levels
- Exposure to high levels of REEs can cause various health effects in humans and animals
- Proper management and recycling of REE-containing products are essential to mitigate environmental impacts
Economic geology of REEs
- REEs have become increasingly important in modern technology and industry
- Understanding the geological processes that concentrate REEs is crucial for exploration and resource assessment
- Economic REE deposits present unique challenges in terms of extraction, processing, and environmental management
Major ore deposits
- Carbonatite-associated deposits (Bayan Obo, China; Mountain Pass, USA) are the primary source of LREEs
- Alkaline igneous complexes (Strange Lake, Canada; Nechalacho, Canada) host significant HREE mineralization
- Ion-adsorption clay deposits in southern China are an important source of HREEs
- Placer deposits contain REE-bearing heavy minerals (monazite, xenotime) concentrated by sedimentary processes
- Phosphorite deposits can contain economically significant REE concentrations as byproducts
- Hydrothermal vein deposits (Steenkampskraal, South Africa) represent another type of REE mineralization
Extraction and processing
- REE ores typically require complex processing due to the chemical similarity of the elements
- Mining methods include open-pit, underground, and in-situ leaching techniques
- Beneficiation processes involve crushing, grinding, and physical separation (magnetic, gravity, flotation)
- Hydrometallurgical extraction uses acid or alkaline leaching to dissolve REE-bearing minerals
- Solvent extraction and ion exchange are employed to separate individual REEs
- Pyrometallurgical methods (molten salt electrolysis) are used for some ore types and metal production
- Environmental challenges include managing and minimizing water and energy consumption
Global production and reserves
- China dominates global REE production, accounting for over 80% of the world's supply
- Other significant producers include Australia, United States, Myanmar, and Russia
- Global REE reserves are estimated at 120 million tonnes, with China holding the largest share
- Increasing demand for REEs in green technologies and electronics drives exploration for new deposits
- Efforts to diversify REE supply chains have led to the development of projects outside China
- Recycling and urban mining of REEs from electronic waste are becoming increasingly important
- Geopolitical factors and trade policies significantly influence the global REE market
REEs in planetary science
- REEs provide valuable insights into the formation and evolution of planetary bodies
- Comparative studies of REE distributions help understand differentiation processes in the solar system
- REE analysis in extraterrestrial materials contributes to our knowledge of cosmic ray exposure and nucleosynthesis
Lunar and Martian geochemistry
- Lunar rocks show distinct REE patterns reflecting their formation processes and source compositions
- KREEP (potassium, rare earth elements, phosphorus) terrain on the Moon represents the last stage of magma ocean crystallization
- Lunar anorthosites exhibit large positive Eu anomalies due to plagioclase accumulation
- Martian meteorites display a range of REE patterns reflecting diverse igneous processes on Mars
- Nakhlites and chassignites show LREE enrichment, while shergottites exhibit varying degrees of LREE depletion
- REE analysis of Martian surface materials by rovers provides insights into weathering and alteration processes
Meteorite studies
- Chondritic meteorites serve as a reference for primitive solar system composition
- Achondrites show diverse REE patterns reflecting differentiation processes on their parent bodies
- Refractory inclusions (CAIs) in chondrites exhibit distinctive REE patterns indicative of high-temperature condensation
- Iron meteorites contain trace amounts of REEs, providing information about core formation processes
- REE abundances in presolar grains offer clues about nucleosynthesis in stars
- Short-lived radionuclides (146Sm-142Nd system) in meteorites constrain the timing of early solar system events
Cosmochemical implications
- REE fractionation patterns in planetary materials provide evidence for the heterogeneous accretion of the solar system
- Comparison of terrestrial and extraterrestrial REE abundances helps constrain models of planetary formation
- Anomalous REE isotope ratios in some meteorites suggest input from diverse stellar sources
- REE behavior during core formation influences our understanding of planetary differentiation processes
- Study of REEs in cosmic dust and interplanetary particles provides insights into small body evolution
- REE analysis of returned samples from asteroids and comets will further our knowledge of solar system chemistry
Key Terms to Review (19)
Bastnäsite: Bastnäsite is a rare earth fluorocarbonate mineral, primarily composed of cerium and lanthanum, with the formula (Ce, La)CO3F. This mineral is significant for being one of the primary sources of rare earth elements, which are crucial in various high-tech applications, including electronics and renewable energy technologies. Bastnäsite is often associated with carbonatite and alkaline igneous rocks, making its geological context important for understanding the distribution of rare earth elements.
Catalysts: Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by providing an alternative pathway for the reaction, lowering the activation energy required for the reaction to proceed. In the context of rare earth elements, catalysts play a vital role in various industrial processes, including the production of high-performance materials and clean energy solutions.
Dysprosium: Dysprosium is a silvery-white rare earth element with the chemical symbol Dy and atomic number 66. It is part of the lanthanide series and is known for its high magnetic susceptibility and ability to absorb neutrons, making it valuable in various technological applications, especially in electronics and nuclear reactors.
Fractional Crystallization: Fractional crystallization is a process in which different minerals crystallize from a cooling magma at different temperatures, leading to the separation of various chemical components. This method plays a crucial role in understanding the formation of igneous rocks and magmatic differentiation, as it affects the composition of both the residual melt and the crystallized solids. The distribution of rare earth elements can also be influenced by this process, which is essential for interpreting geothermobarometric data.
Geochemical mobility: Geochemical mobility refers to the ability of elements and compounds to move through geological environments, influenced by various factors such as chemical reactions, physical processes, and environmental conditions. This concept is especially significant when considering the distribution and concentration of elements, including rare earth elements, in the Earth's crust and their potential for recovery or environmental impact.
Heavy rare earth elements: Heavy rare earth elements (HREEs) are a subset of rare earth elements that have higher atomic numbers and are typically less abundant than their light counterparts. This group includes elements such as europium, gadolinium, dysprosium, and lutetium, which possess unique chemical properties that make them valuable in various high-tech applications, including electronics, magnets, and catalysts.
Hydrothermal processes: Hydrothermal processes refer to the geological phenomena involving the interaction of heated water with minerals and rocks within the Earth's crust. These processes play a crucial role in the formation and concentration of various mineral resources, particularly rare earth elements, as the hot water can dissolve minerals and transport them to different locations where they can precipitate and form deposits.
Lanthanum: Lanthanum is a chemical element with the symbol La and atomic number 57. It is part of the rare earth elements group, which are known for their unique electronic and magnetic properties. Lanthanum plays a crucial role in various technological applications, including catalysts, phosphors, and rechargeable batteries, linking it closely to advancements in modern materials science.
Light rare earth elements: Light rare earth elements (LREEs) are a group of lanthanide elements that include the first half of the lanthanide series, from lanthanum (La) to europium (Eu). These elements are known for their unique chemical properties and play a crucial role in various high-tech applications, including electronics, magnets, and catalysts.
Market demand: Market demand refers to the total quantity of a good or service that all consumers in a market are willing and able to purchase at various prices during a specific time period. This concept is crucial in understanding how prices are determined and how resources are allocated in an economy, especially when it comes to commodities like rare earth elements, which have significant implications for technology and industry.
Mass spectrometry: Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions, allowing for the identification and quantification of chemical compounds. This method is crucial for understanding the composition and behavior of elements and compounds in various contexts, including natural processes and industrial applications.
Monazite: Monazite is a phosphate mineral that is primarily composed of rare earth elements, particularly cerium, lanthanum, and neodymium, along with thorium and uranium. This mineral is an essential source of these rare earth elements, which have significant industrial applications in electronics, renewable energy technologies, and various high-tech devices.
Neodymium: Neodymium is a chemical element with the symbol Nd and atomic number 60, belonging to the group of rare earth elements. This silvery-white metal is notable for its high magnetic strength and is commonly used in powerful permanent magnets, lasers, and various electronic devices. Neodymium is significant in industries such as renewable energy, defense, and electronics, making it a crucial component in modern technology.
Permanent magnets: Permanent magnets are materials that produce a persistent magnetic field without the need for an external power source. They are typically made from alloys of rare earth elements or transition metals, which give them their strong magnetic properties. These magnets are used in various applications, from household items to advanced technologies, due to their ability to maintain a stable magnetic field over time.
Radioactive waste: Radioactive waste refers to materials that are contaminated with radioactive substances and are no longer useful, requiring careful handling and disposal. This type of waste is a byproduct of various activities, including nuclear power generation, medical treatments, and scientific research, and can pose significant environmental and health risks if not managed properly. The connection between radioactive waste and rare earth elements arises from the extraction and processing of these elements, which often involves radioactive materials that need to be handled with caution.
Resource Evaluation: Resource evaluation is the systematic assessment of a natural resource's quantity, quality, and accessibility to determine its economic viability for extraction and use. This process is crucial for understanding how rare earth elements, which are essential for modern technologies, can be sustainably sourced while considering environmental impacts and market demand.
Soil contamination: Soil contamination refers to the presence of hazardous substances in the soil, resulting from human activities such as industrial processes, agricultural practices, and waste disposal. This contamination can lead to adverse effects on the environment and human health, as pollutants may enter the food chain or groundwater systems, making it a critical issue in land management and environmental protection.
Supply chain: A supply chain is the entire system involved in producing and delivering a product, from sourcing raw materials to delivering the final product to consumers. It includes all the processes, resources, and activities that are necessary for getting a product into the hands of the end-user, making it a crucial part of business operations and economic systems.
X-ray fluorescence: X-ray fluorescence (XRF) is a non-destructive analytical technique used to determine the elemental composition of materials by measuring the characteristic X-rays emitted from a sample when it is irradiated with X-rays. This technique is crucial in various fields such as geology, archaeology, and materials science, allowing for the analysis of bulk compositions, crustal elements, and magmatic processes.