From hydroxylbastnäsite to bastnäsite: A crystal chemistry perspective of the incomplete solid solution

In recent years, recovery of rare earth elements (REEs) from coal fly ashes (CFAs) has been considered as a promising resource recovery option. Yet, quantitative information on REE speciation in CFAs and its correlation with REE extractability are not well established. This study systematically investigated the REE speciation-extractability relationship in four representative CFA samples by employing multiple analytical and spectroscopic techniques across the micro to bulk scale and in combination with thermodynamic calculations. A range of REE-bearing phases are identified, such as REE oxides, REE phosphates, apatite, zircon, and REE-bearing glass phase. REEs can occur as discrete particles, as particles encapsulated in the glass phase, or distribute throughout the glass phase. Although certain discrepancies exist on the REE speciation quantified by X-ray adsorption spectroscopy and acid leaching due to intrinsic limitations of each method, both approaches show significant fractions of REE oxides, REE phosphates, apatite, and REE-bearing Fe oxides. This study contributes to an in-depth understanding of the REE speciation-distribution-extractability relationship in CFAs and can help identify uncertainties associated with the quantification of REE speciation. It also provides a general methodology for future studies on REE speciation in complex environmental samples and a knowledge basis for the development of effective REE recovery techniques.

Distribution of rare earth elements (REEs) in supergene environment around a typical ion adsorption–type REE deposit

Skyrocketing worldwide demand for heavy rare earth element (HREE) resources has driven the exploration of ion adsorption−type rare earth element (REE) deposits beyond China. The contribution of REE deposits in Southeast Asia to global REE output recently exceeded 20%. However, a comprehensive understanding of the characteristics and genesis of these tropical region deposits remains insufficient, in contrast to the extensively investigated REE deposits in subtropical regions of South China. The newly discovered Menghai REE deposits of Southwest China are situated in the same tropical region. This study investigated the influence of hydrodynamic zonation, climate, and topography on the geochemical and mineralogical characteristics of the regolith, and analyzed the primary variables governing the activation, migration, enrichment, and differentiation of REEs. Compared to the REE deposits in the Jiangxi area, the Menghai REE deposits are characterized by thicker and more intensely weathered regolith, and more of the REE resources are derived from the weathering-resistant REE minerals. The development of thick regolith results from substantial precipitation and diminished physical erosion due to abundant vegetation cover. Topographic conditions influence the combination of regolith horizons by regulating physical erosion. This, along with active microbial and plant processes in tropical regions, facilitate the chemical weathering of primary and secondary minerals, particularly those resistant to weathering in the humic soil (A horizon). The hydrodynamic zonation in the Menghai regolith significantly influenced REE mineralization. The REE ions released migrate vertically in the high-velocity vadose zone and are deposited in the low-velocity capillary fringe. The distinctive redox environment led to positive Ce anomalies above the capillary fringe and negative Ce anomalies within it. The vertical differentiation of REEs is primarily controlled by the diffusion process of REE complexions after they enter the capillary fringe. This process forms an upper light REE−enriched ore body overlying a lower HREE-enriched ore body within the regolith.

Interpretation of vertical migration and enrichment processes of rare earth elements (REEs) in ion-adsorption-type mineralization in Japan based on REE speciation analyses

Regolith-hosted rare earth element (REE) deposits have been the focus of recent studies. Most studies concern deposits formed over granites and felsic volcanic rocks, but little is known about those deposits developed over silica-undersaturated alkaline igneous rocks. The recently discovered Puxiong REE deposit in Southwest China formed through the weathering of nepheline syenite that has REE concentrations ranging from 177 to 9,336 ppm. Hydrothermal processes partially enriched the parent nepheline syenite in REEs. About 60% of the REEs in the bedrock are hosted in britholite-(Ce), tritomite-(Ce), and cerite-(Ce) and ~21% in REE minerals that occur as inclusions in K-feldspar, with the rest in titanite, hiortdahlite, apatite, fluorite, and calcite. These minerals all can be easily decomposed to release REEs into soil solutions during weathering. The released REEs are adsorbed on clay minerals or precipitate as supergene rhabdophane and an Fe-Mn-REE oxyhydroxide phase. Nepheline syenite-derived regolith-hosted REE deposits are enriched in illite and halloysite, which have a higher ion exchange capacity than the parent granites. Illite formed through the weathering of primary alkali minerals in the nepheline syenite. In the strongly eroded midslope and valley, the regolith has the lowest total REE concentration (997 and 1,001 ppm on average, respectively) across the ore-bearing catchment, whereas the regolith in the hilltop and footslope has REE concentrations of up to 1,564 and 1,677 ppm, respectively. Moreover, regolith at the footslope has the highest heavy REE (HREE) concentration of 110 ppm on average. The light REEs (LREEs) tend to be concentrated in the B horizon and laterally across the hilltops, whereas the HREEs are mobilized by groundwater and soil solutions and accumulated in the upper C horizon vertically and the footslope profiles laterally. In conclusion, nepheline syenite was hydrothermally enriched in the REEs, and these elements were released to the weathering solution and then adsorbed onto clay minerals in sufficient concentrations to form economic regolith-hosted REE deposits. This process, which was controlled at Puxiong by the nature of clay minerals, pH, the redox conditions, the mobility of the REEs, and topography, led to maximum enrichment of the LREEs in the lower B horizon at the hilltop, and HREE enrichment in the upper C horizons vertically and in the footslope laterally.

Comparison of speciation and bioavailability of rare earth elements between wet rhizosphere soil and air-dried bulk soil

Factors controlling the generation and diversity of giant carbonatite-related rare earth element deposits: Insights from the Mianning–Dechang belt

Carbonatite-related rare earth element (REE) deposits, the most significant source of REEs globally, are normally generated in extensional settings, such as intracontinental rifts, mantle plume-related environments, or postcollisional orogens. Syncollisional orogens represent overall compressional regimes, so carbonatites and related REE deposits are rarely identified in such a setting. However, this study reports an anomalous syncollisional carbonatite-related REE deposit, Dong Pao, in the India-Asia collision zone in northwestern Vietnam. The Dong Pao deposit is dated at ca. 52 to 51 Ma through zircon and bastnäsite Th-U-Pb chronometers. The ore-hosting carbonatites were emplaced as stocks with associated syenite. The carbonatite-syenite complex is significantly enriched in light REEs, Ba, and Sr and depleted in high-field strength elements, and has high (87Sr/86Sr)i ratios (>0.707) and low εNd(t) values (–6.5 to –5.6). These geochemical signatures imply that the carbonatite-syenite complex was derived from partial melting of subcontinental lithospheric mantle previously metasomatized and fertilized by REE- and CO2-bearing fluids. Timing of the REE-rich carbonatite-syenite complex indicates that it was related to a far-field stress within the early Eocene main-collision stage at 52 to 51 Ma rather than the late-collision stage at 42 to 35 Ma as previously thought. Collisional tectonism involving block rotation and fault activation are interpreted to have induced disturbance of the lithosphere mantle and created localized, transtensional/extensional environments oblique to the trend of the orogen that facilitated emplacement of the REE-rich carbonatitic magmas. Dong Pao appears to be the first identified, high-tonnage REE deposit that formed in the syncollisional geodynamic setting. Such a finding highlights that tectonic disturbance of an REE-rich lithosphere mantle distal to collision sutures has the potential to generate REE deposits, even during prominent convergence and collision of continents. As such, it defines additional search spaces for exploration of other REE orebodies of this style in complex collisional orogens.

Optimized thermodynamic properties of REE aqueous species (REE3+ and REEOH2+) and experimental database for modeling the solubility of REE phosphate minerals (monazite, xenotime, and rhabdophane) from 25 to 300 °C

Complex REE systematics of carbonatites and weathering products from uniquely rich Mount Weld REE deposit, Western Australia

Exploration and research progress on ion-adsorption type REE deposit in South China

The ion-adsorption rare earth element (REE) deposit, a valuable type of REE deposit, has been thought to be derived from the release and enrichment of REE during granite weathering. Understanding the REE occurrence in regolith-hosted deposits is crucial for more efficient extraction. We investigated a weathering granite profile of a regolith-hosted REE deposit located in South China. X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) analysis of the clay fractions reveal that the highest Ce(IV) content located in intensely weathered layers and cerianite nanoparticles (CeNPs) can be observed, besides invisible adsorbed REEs. Interestingly, most of the CeNPs scatter on halloysite basal surface and exhibit preferred orientation. Detailed analysis demonstrates that the diagonal plane of cerianite matches with the exposed basal surfaces (Si-O tetrahedron) of halloysite. Such a lattice match may contribute to the nucleation and growth of CeNPs after oxidation of the adsorbed Ce(III), which results in great REE enrichment by clays. The findings provide new insight into the mechanism of Ce precipitation and REE mineralization during granite weathering.

China has been the world’s leading rare earth element (REE) and yttrium producer for more than 20 years and hosts a variety of deposit types. Carbonatite-related REE deposits are the most significant REE deposit type, with REY (REE and yttrium)-bearing clay deposits, or ion adsorption-type deposits, being the primary source of the world’s heavy REEs. Other REY resources in China include those hosted in placers, alkaline granites, pegmatites, and hydrothermal veins, as well as in additional deposit types in which REEs may be recovered as by-product commodities. Carbonatite-related REE deposits in China provide nearly all the light REE production in the world. Two giant deposits are currently being mined in China: Bayan Obo and Maoniuping. The carbonatite-related REE deposits in China occur along the margins of Archean-Paleoproterozoic blocks, including the northern, southern, and eastern margins of the North China craton, and the western margin of the Yangtze craton. The carbonatites were emplaced in continental rifts (e.g., Bayan Obo) or translithospheric strike-slip faults (e.g., Maoniuping) along reactivated craton margins. The craton margins provide the first-order control for carbonatite-related REE resources. Four REE metallogenic belts, including the Proterozoic Langshan-Bayan Obo, late Paleozoic-early Mesozoic eastern Qinling-Dabie, late Mesozoic Chishan-Laiwu-Zibo, and Cenozoic Mianning-Dechang belts, occur along cratonic margins. Geologic and geochemical data demonstrate that the carbonatites in these belts originated from mantle sources that had been previously enriched, most likely by recycled marine sediments through subduction zones during the assembly of continental blocks. Although the generation of carbonatite magma is debated, a plausible mechanism is by liquid immiscibility between silicate and carbonate melts. This process would further enrich REEs in the carbonatite end member during the evolution of mantle-derived magma. The emplacement of carbonatite magma in the upper crust, channeled by translithospheric faults in extensional environments, leads to a rapid decompression of the magma and consequently exsolution of a hydrothermal fluid phase. The fluid is characterized by high temperature (600°–850°C), high pressure (up to 350 MPa), and enrichment in sulfate, CO2, K, Na, Ca, Sr, Ba, and REEs. Immiscibility of sulfate melts from the aqueous fluid, and phase separation between CO2 and water may take place upon fluid cooling. Although both sulfate and chloride have been called upon as important ligands in hydrothermal REE transport, results of our studies suggest that sulfate is more important. The exsolution of a sulfate melt from the primary carbonatite fluid would lead to a significant decrease of the sulfate activity in the fluid and trigger REE precipitation. The subsequent unmixing between CO2 and water may also play an important role in REE precipitation. Because of the substantial ability of the primary carbonatite fluid to contain REEs, a large-volume magma chamber or huge fluid flux are not necessary for the formation of a giant REE deposit. A dense carbonatite fluid and rapid evolution hinder long distance fluid transportation and distal mineralization. Thus, carbonatite-related alteration and mineralization occur in or proximal to carbonatite dikes and sills, and this is observed in all carbonatite-related REE deposits in China. Ion adsorption-type REE deposits are primarily located in the South China block and are genetically linked to the weathering of granite and, less commonly, volcanic rocks and lamprophyres. Indosinian (early Mesozoic) and Yanshanian (late Mesozoic) granites are the most important parent rocks for these REE deposits. Hydrothermal alteration by fluids exsolved from late Mesozoic granites or related alkaline rocks (e.g., syenite) may have enriched the parent rocks in REEs, particularly the heavy REEs. Furthermore, this alteration process led to the transformation of some primary REE minerals to secondary REE minerals that are more readily broken down during subsequent weathering. During the weathering process, the REEs are released from parent rocks and adsorbed onto kaolinite and halloysite in the weathering profile, and further enriched by the loss of other material to form the ion adsorption-type REE deposits. A warm and humid climate and a low-relief landscape are important characteristics for development of ion adsorption REE deposits.

The Bayan Obo rare earth element (REE) deposit in Inner Mongolia, northern China, is the largest REE deposit in the world, whose mineralization process remains controversial. There are dozens of carbonatite dykes that are tightly related to the deposit. Here we report the petrological and mineralogical characteristics of a typical dolomite carbonatite dyke near the deposit. The dolomite within the dyke experienced intense post-emplacement fluids metasomatism as evidenced by the widespread hydrothermal REE-bearing minerals occurring along the carbonate mineral grains. REE contents of bulk rocks and constituent dolomite minerals (>90 vol.%) are 1407–4184 ppm and 63–152 ppm, respectively, indicating that dolomite is not the dominant mineral controlling the REE budgets of the dyke. There are three types of apatite in the dyke: Type 1 apatite is the primary apatite and contains REE2O3 at 2.35–4.20 wt.% and SrO at 1.75–2.19 wt.%; Type 2 and Type 3 apatites are the products of replacement of primary apatite. The REE2O3 (6.10–8.21 wt.%) and SrO (2.83–3.63 wt.%) contents of Type 2 apatite are significantly elevated for overprinting of REE and Sr-rich fluids derived from the carbonatite. Conversely, Type 3 apatite has decreased REE2O3 (1.17–2.35 wt.%) and SrO (1.51–1.99 wt.%) contents, resulting from infiltration of fluids with low REE and Na concentrations. Our results on the dyke suggest that post-magmatic fluids expelled from the carbonatitic melts dominated the REE mineralization of the Bayan Obo deposit, and a significant fluid disturbance occurred but probably provided no extra REEs to the deposit.

Iron and sulfur isotopic compositions of carbonatite-related REE deposits in the Mianning–Dechang REE belt, China: Implications for fluid evolution