Rare Earth Elements

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This entry is adapted from the peer-reviewed paper 10.3390/min13081031

The use of analytical techniques is important and critical in all areas related to REE (Rare Earth Elements), such as basic fundamental research, exploration, mining, extraction, and metallurgical activities at different stages by different industries. At every stage of these activities, rock, ore, minerals, and other related materials have to be analyzed for their REE contents in terms of elemental, isotopic, and mineralogical concentrations using different analytical techniques.

rare earth elements ICP-MS SHRIMP dating LIBS analysis of REE

1. Introduction

At present there is intense exploration activity going on worldwide for critical elements, like rare earth elements (REEs), lithium, gold, platinum, palladium, copper, and cobalt, because of their wide applications in several green technologies required to transition to a low-carbon economy ^[1]^[2]. In fact, currently, these elements are fulfilling thousands of different industrial needs in our technology-powered society. The REE group of elements consists of the 15 lanthanide elements (La to Lu) plus Y and Sc. Based on atomic numbers, they are divided into two groups. The lower atomic weight elements from La to Sm, and the most abundant ones, with atomic numbers 57–62, are referred to as light REEs (LREEs); while Eu to Lu, and the least common and the most valuable, with atomic numbers 63–71, are known as heavy REEs (HREEs). Despite their low atomic weights, Y and Sc are included in the HREE subgroup because of their co-occurrence, similar ionic radii, and closer behavioral properties to HREEs than LREEs (Figure 1). Because of their unique physical, chemical, electronic, optical, mechanical, catalytic, and magnetic properties, they are being extensively used to make different high-technology devices, such as computers, televisions, smartphones, catalysts for fuel cells, light emitting diodes, hard drives for computers, corrosion inhibitors, and magnets for wind turbines and other power generating systems. The concentrations of Ce and Eu are extremely sensitive to changes in atmospheric conditions with two different oxidation states each (Ce³⁺/Ce⁴⁺ and Eu²⁺/Eu³⁺); hence, these two redox pairs are used to understand oxygen fugacity (fO₂) in different geological environments ^[3]^[4]. Because of these similarities, all these elements are usually studied as a group in several research and development studies. Though REEs exhibit similar properties chemically and frequently occur together in several geologic formations in many ores or minerals as major or minor constituents, they differ in some physical respects and possess different electronic and magnetic properties.

Figure 1. List of 17 and classification of REEs.

The demand for REEs, Y, and Sc is increasing day by day, especially because of their utility in green technology applications as mentioned above. As a result, there has been a significant surge in the exploration, mining, and extraction activities for these elements worldwide. The European Commission and the US government declared REE groups as economically critical elements ^[5]^[6] and, currently, there is intense exploration activity going on the world over for discovering new economically viable deposits. Even the mining, efficient extraction of REEs from the ore materials, and their metallurgy studies assume a lot of importance at present. Hence, there is a great need to have rapid, ecological, and cost-efficient analytical techniques during exploration, geochemical mapping, mining, and ore processing operations, and even during metallurgical works ^[7]^[8]. In all these activities, it is essential to analyze several types of geological and industrial materials for REE, Y, and Sc. Usually, it is very complicated to determine them by classical methods, such as gravimetry, titrimetric methods, and spectrophotometry, because of close similarities in their physical and chemical properties, particularly when a selected REE element among them has to be determined in the mixture of the other REEs, because of numerous interferences and coincidences. Moreover, it is extremely difficult to determine them at crustal levels of concentrations (ranging from 0.3 to 33 µg/g in crustal rocks) in various geological materials ^[6]. But astonishing advances have taken place during the last three decades in the analytical techniques for the detection and determination of different elements including REE not only in terrestrial materials but also in faraway bodies, such as the Moon and Mars. Recently, eight elements, including one of the REEs, Tb, were detected in an exoplanet’s (KELT-9 b’s) atmosphere using high-resolution spectrographs ^[9]. Exoplanets are planets that are in other solar systems than our own. Usually, sophisticated instrumental analytical techniques, like instrumental neutron activation analysis, (INAA) and different forms of inductively coupled plasma–mass spectrometry (ICP-MS) including tandem-ICP-MS, ICP-time-of-flight-MS (ICP-TOF-MS), high-resolution-ICP-MS (HR-ICP-MS), multi-collector-ICP-MS (MC-ICP-MS), and Mattauch–Herzog geometry-ICP-MS (MH-ICP-MS) are commonly used for REE determination in different kinds of materials because of their multi-element capability, high sensitivity, wide linear dynamic range, fewer interferences, and ease of operation. Dai et al. ^[10] n a study related to the understanding of modes of occurrences of different elements including REEs, used techniques ranging from a simple AAS to the complex SHRIMP instrument.

2. Isotopic Studies

The study of isotopic abundances (radiogenic as well as stable) and their distribution provides clues for understanding various geological processes that these elements/isotopes underwent during the crustal evolution. Particularly, radiogenic isotope signatures are used to understand the long-term evolution and the origin of sources of volcanic rocks and the evolution of the Earth. Stable isotopes have a lot of applications in exploration, water and soil management, and in tracer studies. On the other hand, radiogenic isotope pairs, like ¹⁴⁷Sm-¹⁴³Nd, and ¹⁷⁶Lu-¹⁷⁶Hf, are used for dating applications to determine the ages of rocks and minerals, and also to evaluate the nature and evolution of their source regions. REE isotopes in geological materials are applied very extensively in geological sciences to obtain new insights into several natural processes, ranging from crustal formation to chemical weathering and ocean circulation. Different mass spectrometric techniques, such as TIMS, MC-ICP-MS, SIMS, and SHRIMP, are used for the precise determination of isotopic abundances and isotopic ratios both in liquid and solid materials.

2.1. Multi-Collector ICP-MS (MC-ICP-MS)

A basic MC-ICP-MS instrument contains an ICP source, an energy filter, a double-focusing mass spectrometer of Neir–Johnson geometry consisting of two analyzers, namely a traditional electromagnet and an electrostatic analyzer (ESA), and an array of detectors, typically Faraday cups for the simultaneous detection of a number of isotopes of interest When the sample, in the form of a solution or direct solid sample using a laser ablation sampling, is introduced into the high-temperature ICP, the sample aerosol is evaporated, dissociated, atomized, and ionized to generate positively charged ions of different elements. The positively charged ions are then extracted from the argon plasma into the high vacuum of the mass spectrometer. The ions are accelerated and focused into an electrostatic analyzer (ESA). The ions emerging from the ESA enter a magnetic field region, which acts to disperse the ions according to their mass-to-charge ratios. Careful alignment of the detectors to the individual isotope beam needs to be carried out when analyzing different elements ^[11]^[12]. Among REEs, Nd, for example, is highly incompatible and refractory and has seven isotopes. The variations in Nd isotopes can be caused by both mass-dependent and mass-independent effects which are used to understand a number of geological processes ^[13]. However, their precise determination is very challenging when utilizing some instruments, like TIMS, because of severe interference effects. But the advent of MC-ICP-MS, together with advanced sample preparation methods and separation methods, such as the use of a two-column ion-exchange separation procedure, has allowed the high precision determination of isotope ratios of various REEs ^[14]. In addition to the precise isotope ratios, MC-ICP-MS is also used for the determination of elemental concentrations.

Lee and Ko ^[15] determined REE concentrations in a natural river water CRM, SLRS-6, very accurately and precisely after group separation by 2-hydroxyisobutyric acid (HIBA). As isotope dilution methods are highly accurate and robust, Kent et al. ^[16] used isotope dilution-MC-ICP-MS (ID-MC-ICP-MS) to accurately determine eleven REE abundances in several international geological CRMs (USGS, NIST, GSJ) using rock powders and glass discs made of these rock powders. The authors also made a comparative study of the determination of REE concentrations by both ID-MC-ICP-MS and ID-TIMS. Unfortunately, ID methods cannot be used for monoisotopic elements, like Tb, as the method requires a minimum of two isotopes. Replicate analysis of both rock powder and glass materials agreed very well with analytical uncertainty, which is typically about 1%. Pourmand et al. ^[17] devised a novel extraction chromatography method after reducing the REE impurities in the fusion flux and used the MC-ICP-MS technique for rapid analysis of REE, Sc, and Y in primitive chondrites using a desolvating nebulizer and standard-sample bracketing technique. Baker et al. ^[18] developed a method for a high-precision analysis of REE by ID-MC-ICP-MS which is superior in terms of the analytical reproducibility or rapidity of analysis, on smaller sample amounts compared with ID-ICP-MS or with ID-TIMS. Bastnäsite is one of the important REE minerals and is the end member of a large group of carbonate–fluoride minerals which represents the major economic LREE deposits related to carbonatite and alkaline intrusions. Yang et al. ^[19] used LA-MC-ICP-MS for the determination of age by the U-Th-Pb geochronology method and Sr-Nd isotopic composition. Monazite and bastnasite are two principal REE ore minerals in the most famous Bayan Obo REE deposit in China. An Sm-Nd chronometer and LA-MC-ICP-MS were used to understand the primary crystallization timing of the REE deposit at 1293 ± 48 Ma ^[20].

2.2. Thermal Ionization Mass Spectrometry (TIMS)

In TIMS, ions are produced by the interaction of analyte species with a heated surface, which is usually a metal, like tungsten. Mostly singly charged ions are extracted, accelerated, and focused into a mass spectrometer where these ions can be separated based on their mass/charge ratios and detected using a Faraday cup detector. A multi-collector-TIMS uses a number of detectors for the simultaneous detection of multiple isotopes. For isotope dilution analysis by TIMS, chemical separations of REEs are usually made into two or more factions to separate LREEs and HREEs in order to reduce the inter-elemental interferences during the analysis. It requires a minimum of two isotopes for isotope dilution and it is not possible to analyze monoisotopic elements, like Pr, Ho, and Tm. TIMS is used mostly for dating applications in addition to elemental and isotope determinations. The new ages of 296.9 ± 1.65 Ma and 296 ± 4.2 Ma were obtained by U–Pb zircon dating by TIMS from tonstein layers interbedded with coal seams from the Candiota coalfield in the southern Paraná Basin, Brazil ^[21]. The Pope’s Hill REE deposit in Labrador, Canada, was studied by LA-ICP-MS and ID-TIMS U-Pb geochronology and in situ Sm-Nd isotopes using LA-MC-ICP-MS in monazite from the ore and host rock to understand the timing of the deposit formation and to determine the source of the REEs ^[22]. Ramesh et al. ^[23] determined REEs and heavy elements in surficial sediments of the Himalayan River system by TIMS to understand the behavior of REEs during weathering and transport in a secondary sedimentary environment. In addition, the decay systems of the radioactive REE isotopes ¹³⁸La, ¹⁴⁷Sm, and ¹⁷⁶Lu were used to establish the ages of a range of geological events, starting from the first steps of planetary formation to some of the younger events, like Deccan volcanism, and the TIMS technique was proved to be very valuable in such studies. Uranium isotope (²³⁴U/²³⁸U, ²³⁵U/²³⁸U) ratio data generated by TIMS was used to understand the occurrence and provenance of Kanyakumari Beach placer deposits of REEs in India ^[24].

2.3. Sensitive High-Resolution Ion Micro Probe (SHRIMP)

SHRIMP is a high-resolution and highly sensitive instrument developed by William Compston at the Australian National University, primarily used for geological applications, especially in situ U–Pb dating of the mineral apatite, as well as zircon ^[25]. SHRIMP is basically a mass spectrometer that works on the principle of secondary ion mass spectrometry (SIMS). The ion beam made up of negatively-charged O₂ at mass 32 is found to be the most effective primary species for bombarding the sample surface, producing a larger second ionization yield. The collisions between the primary ions and the surface physically erode, or ‘sputter’, the sample, ejecting particles from the surface. The secondary ions, released in all directions, are then focused and steered towards a magnetic analyzer. Different ions (isotopes) of interest are simultaneously detected by a multi-collector (detector) assembly ^[26]. SHRIMP dating methods have been extensively applied for the dating of a number of REE deposits the world over. REE deposits, such as carbonatite deposits, can be dated very precisely using the decay systems of the radioactive REE isotopes ¹³⁸La, ¹⁴⁷Sm, and ¹⁷⁶Lu not only for understanding the age of REE deposits but also a range of events, starting from the first steps of planetary formation to younger steps of geodynamic development ^[27]. Thus, the abundance of all REEs occurring in a large range of concentrations as well as precise isotope ratios must be analyzed in different geomaterials for such studies. Campbell et al. ^[28] dated zircon from the Bayan Obo Fe–Nb–REE deposit to understand carbonatite-related magmatism and REE mineralization events. These SHRIMP dating studies indicated that Bayan Obo zircon cores represent part of a magmatic, intrusive, carbonatitic protolith suite that crystallized at 1325 ± 60 Ma. Bhunia et al. ^[29] used SHRIMP to date the carbonatite-hosted REE deposit of Kamthai, northwest India. This is one of the well-studied carbonatite REE deposits in India which gave an age of 68.4 ± 1.8 Ma which is linked to the carbonatite emplacement of the Deccan Large Igneous Province that triggered the mass extinctions near the K–Pg boundary. SHRIMP is capable of producing high transmission at high-resolution data of REE abundances in some minerals, like zircon and apatite ^[30]. SHRIMP-RG was used to determine REEs to understand their distribution in coal, as the knowledge of the REE distribution is essential to design cost-effective REE extraction procedures from coal and its by-products (Kolker et al. ^[31]. Carbonatite–alkaline complexes in northwest Pakistan were dated using the SHRIMP U-Pb dating technique of zircons, namely Koga syenites, which are spatially associated with the carbonatite deposit and which gave mean ages of 283.6 ± 1.7(1σ) providing a useful timing for alkaline magmatism in that area ^[32]. Such in-depth studies on well-proven REE deposits in general shed light on the full origin of these world-class mineral deposits.

3. Mineralogical Studies and In Situ Analytical Techniques

The mineralogical characteristics of different REE ores and resources are extremely important, as the leaching process depends on their mineralogical composition and the morphological distribution of different elements. Such studies help in the determination of the best leaching/extracting process during the extraction of REE from different ores. Mineral characterization techniques, such as XRD, scanning electron microscopy–energy dispersive spectrometry (SEM-EDS), and laser ablation-ICP-MS (LA-ICP-MS) are popular for the qualitative and quantitative identification of different REE minerals. During exploration studies, pXRD and Raman spectrometry are used to understand the mineralogy of a geological formation in the field itself for planning future courses of action.

3.1. X-ray Diffractometry (XRD)

Identification of different indicator minerals and their concentrations in rocks, ores, and soils is extremely important in mineral exploration studies. XRD is one of the most powerful methods used to identify and quantify minerals in different earth materials ^[33]. The interaction of the incident monochromatic X-ray beam with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg’s law (nλ = 2d sin θ). This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed, and counted. By scanning the sample through a range of 2θ angles, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. Conversion of the diffraction peaks to d-spacings allows the identification of a mineral because each mineral has a set of unique d-spacings. Typically, this is achieved by comparison of d-spacings with standard reference patterns. XRD was effectively used in the identification of different minerals in the clay fractions of REE-enriched weathered anorthosite complex in Hadong district, South Korea ^[34]. In order to understand the differences between the mineralogy and mineral chemistry of REEs in phosphate and carbonates in the Bahoruco karsr bauxites in the Dominican Republic, Villanova-de-Benavent et al. ^[35] utilized XRD, SEM-EDS, and EPMA. This study revealed that phosphates are mostly enriched in Y and HREE (Gd, Dy); on the other hand, carbonates display a wide range of compositions in terms of LREE (La, Ce, Nd, SM) and HREE (Gd). The lignite deposits of Barmer Basin, Rajasthan, India, were characterized using a host of analytical techniques including XRF, XRD, and ICP-MS to understand mineralogical and elemental distribution. XRD analysis revealed the presence of minerals, such as hematite (Fe₂O₃), nepheline, anhydrite, magnesite, andalusite, spinel, and anatase. Other minor minerals included albite, siderite, periclase, calcite, mayenite, hauyne, pyrite, cristobalite, quartz, nosean, and kaolinite. Major and minor elements were determined by XRF, and REEs and several other trace elements were determined by ICP-MS. These investigations revealed the predominant mineral concentration, elemental makeup, trace elements, and REEs associated with lignite reserves in the Barmer Basin ^[36].

3.2. Electron Probe Micro Analyzer (EPMA)

EPMA operates under the principle that when a solid material is bombarded by an accelerated and focused electron beam, electron–sample interactions mainly liberate heat, electrons, and X-rays due to inelastic collisions of the incident electrons with electrons in the inner shells of atoms in the sample. Different kinds of detectors are arranged around the sample chamber that is used to collect X-rays and electrons emitted from the sample. A light microscope allows for direct optical observation of the sample. X-rays are used to determine the chemical composition of the sample using an ED-XRF unit. EPMA can be used for the detection and determination of high concentrations of REEs in rocks and also to understand the REE distribution patterns in different minerals ^[37]. It is a non-destructive technique and uses EDS to accurately determine the chemical composition of small amounts of solid materials by comparing it with the standards of similar and known compositions. EPMA and other analytical techniques, such as ICP-MS and scanning electron microscopy (SEM), were used to study the occurrence and distribution of REEs in a fly ash sample from the Qianxi coal-fired power plant in Guizhou province in China. Discrete REE mineral particles distributed throughout were seen directly by SEM and EPMA, and these studies revealed that the ∑REE concentration in coal fly ash was 630.51 µg/g and wet-grinding followed by acid-leaching was found to liberate more REEs during the extraction process ^[38]. Many times, such multi-disciplinary approaches would help in devising economically viable and eco-friendly extraction procedures for REEs from coal fly ash which has been found to be a promising alternative source for REEs ^[39]. Sano et al. ^[40] determined all REEs in oceanic basalt glasses by EPMA. The concentrations measured by EPMA compared favorably with those obtained by ICP-MS. The same authors also developed a method to determine REEs in glass samples by SHRIMP with a detection limit of ~6 ng/g.

3.3. Ion Microprobe (SIMS)

Secondary ion mass spectrometry (SIMS) or ion microprobe is one of the best techniques for the in situ analysis of REEs in geological materials ^[41]. The principle of SIMS analysis involves the use of a primary ion beam (consisting of O⁻ or Cs⁺) to strike the sample surface of a polished rock or a thin section, and the sputtering process produces secondary ions which are extracted into a mass spectrometer and analyzed. This technique provides a unique combination of extremely high sensitivity for all elements from hydrogen to uranium, including all REEs, with a detection limit down to both the ng/g level with extremely high spatial resolution. High-resolution SIMS (HR-SIMS) with large-diameter and double-focusing SIMS offer >5000 R and can measure the isotopic and elemental abundances in minerals at a 10 to 30 μm-diameter scale, and with a depth resolution of 1–5 μm. This instrument can also be used as an ion microscope for providing elemental distribution imaging maps. Zinner and Crozaz ^[42] used an ion microprobe for the quantitative measurement of REEs in phosphates. REE working curves were found to be linear over a wide range. The detection limits were found to be >50 ng/g for LREEs and ~200 ng/g for HREEs. It is possible to determine REE concentrations in individual mineral grains, such as zircon and monazite, using the SIMS technique.

3.4. Scanning Electron Microprobe (SEM-EDS)

A SEM is essentially a high magnification microscope, which uses a focused scanned electron beam to produce images of the sample, usually on thin sections of the samples. The X-rays emitted are characteristic of the elements in the top few μm of the sample and are measured by the EDX detector. Thus, SEM-EDS combines the capabilities of a scanning electron microscope and an ED-XRF for material characterization and has proved to be very useful for mineral characterization ^[43]. In a study related to the leaching recovery of REEs from coal fly ash, Pan et al. ^[44] used SEM-EDS and XRD techniques to prove that monazite, apatite, scheelite, and aluminosilicate were the major phases that were REE carriers. Such studies help in devising cost-effective procedures for the leaching and recovery of REEs from different ores. Palozzi, et al. ^[45] used SEM-EDS and integrated mineral analysis (TIMA) to study fine <5 μm monazite grains in the coal fly ash which were typically bound to Al/Si-rich phases. These studies helped to devise more efficient extraction procedures using aqua regia. In a study to find out the potential of the carbonatite tailings and to identify the major mineral phases for the REE recovery, by using XRD and SEM-EDS, Sarker et al. ^[46] found that monazite and florencite were the main REE-bearing minerals. The SEM-EDS-based liberation analysis indicated that REE-minerals were found to be primarily associated with goethite and were locked within the larger particle sizes over 100 µm, but, when the particle size was reduced to 50 µm, most REE mineral grains were liberated. These findings suggested that grinding the ore material to 63 µm would potentially liberate the most REE minerals for the subsequent separation processes, such as gravity, magnetic, and floatation methods ^[46]. Li et al. ^[47] detected anomalously high concentrations of REEs in acid mine drainage of a coal mine in northern Guizhou, China. SEM-EDS analysis revealed two types of REE-bearing minerals. Bastnaesite and monazite were detected in the claystone samples, and they were found to be adsorbed by a large amount of clay minerals, mainly kaolinite. Careful documentation of mineralogical and geochemical variations in REE deposits is extremely important for a clear understanding of difficulties in mineral processing and to arrive at an optimal process for mineralogical beneficiation. SEM-EDS and XRD were effectively utilized to understand the mineralogy of the Bear Lodge REE deposit (Wyoming, US) and to classify it into four types: (i) fluor-carbonates (bastnaesite, parisite, synchysite), (ii) phosphates (monazite, xenotime, florencite, rhabdophane, churchite), (iii) cerianite, and (iv) ancylite, and the distribution of REE was found to be very heterogeneous throughout the deposit ^[48].

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