Eocene-aged bentonite deposits in Gonzalez (Texas, USA) carrfy scattered manganese oxide-rich pods where rare earth element (REE) concentrations as high as 7800 ppm have been detected — 10 times greater than in the already enriched bentonite. This study investigated the nature of the association between REEs and Mn oxides as well as REE modes of occurrence in these pods, in order to understand the processes that mobilize and accumulate REEs in the low-temperature depositional environment of the Texas coastal plain. Embedded, polished blocks of Mn pods were analyzed by Scanning Electron Microscopy (SEM); sections were then extracted by the FIB liftout technique from regions bearing REEs for further analysis by Transmission Electron Microscopy (TEM). The SEM and TEM results revealed the presence of abundant yttrium phosphate nanoparticles identified as xenotime, forming large globular aggregates that reach a few microns in diameter; these xenotime aggregates also host trace concentrations of Zr, Dy, Er, Yb, and Ce. Further, rhabdophane nanoparticles were detected that host predominantly light REEs and Y. The relative proportions of REEs in rhabdophane vary between particles and show decoupling of Ce from other light REEs, suggesting oxidation from Ce3+ to Ce4+. The REE enrichment of these Mn pods is due to extensive weathering along fractures cutting through the bentonite deposit that drove their remobilization and transport downward. At the base of the bentonite, where fractures terminate against the boundary with a cemented sandstone, changes in flow regime combined with gradual downward changes in fluid composition, prompted co-precipitation of Mn oxides and REE phosphates (xenotime and rhabdophane).

The total concentrations of rare-earth elements (REE) in the mined kaolin (0.02–0.06 wt.%), kaolin mine tailings (0.03–1.9 wt.%), and the kaolin-associated Marion Member sand lithology (0.03–4.6 wt.%) opened questions regarding the modes of occurrence of the REE and the role(s) of chemical weathering and secondary processes to explain the presence of REE in these materials. The REE were hosted primarily by phosphate minerals (monazite, xenotime) based on mineralogic analyses (scanning electron microscopy, X-ray diffraction). Enrichments in the light rare-earth elements (LREE: La–Gd) and the high correlation coefficient values were noted between P and the total REE concentrations (r2 = 0.99) for the sands and the mine tailings. Lower correlation coefficient values were noted between total REE concentrations and Zr (r2 = 0.31). The coarse fractions of the mined kaolins were enriched in the heavy rare-earth elements (HREE: Y, Tb–Lu) relative to the kaolin-associated sand lithologies. The REE inventory cannot be explained solely by mineral inheritance within the mined kaolins. Lower correlation coefficient values between P and total REE, positive Eu/Eu* anomalies, and the presence of xenotime overgrowths on zircon showed the importance of the role of chemical weathering of the detrital minerals during post-depositional processes (such as diagenesis) leading to redistributed and fractionated REE within the mined kaolin. The possibility of adsorption of the REE to kaolin mineral surfaces in the fine fraction of the mined kaolins remains open and permits further study to characterize fully the multi-modal fractionation of REE possible in the Georgia kaolin deposits.

In the search for rare earth and other critical elements in coal measures, the coals are emphasized with lesser consideration for the accompanying rocks. In this investigation, the focus is on a lanthanide-rich, 315–317 Ma (after Machlus et al., Chemical Geology, 539, art. no. 119485, 2020) volcanic ash-fall trachyandesite to trachyte tonstein which occurs in association with the Middle Pennsylvanian Duckmantian-age Fire Clay coal in eastern Kentucky. The tonstein was deposited largely during peat accumulation, although it is known to occur at the base of the coal or within the underclay. The mineralogy is dominated by kaolinite with illite and quartz as minor to major minerals. A number of accessory minerals, as detected by X-ray diffraction + Siroquant XRD software and scanning and transmission electron microscopy (S/TEM), include REE-bearing phosphates (apatite, crandallite, florencite, monazite), and Y-bearing zircon. The highest rare earth element + Y concentrations occur in the weathered tonsteins, probably due to the concentration of these minerals after weathering of kaolinite from the rock.

Ion adsorption-type rare earth deposits (IADs) are developed via prolonged weathering of REE-rich volcanic and metamorphic rocks. Intense magmatic activity which occurred during the Yanshanian (199.6–65.5 Ma) and Caledonian periods (542–359.2 Ma) provided an abundant material basis for the formation of IADs in South China. High concentrations of REE and the high proportion of ion-exchangeable REE were found in the Maofeng Mountain regolith, Guangzhou city. However, the geochemical patterns and mechanisms of REE enrichment in the regolith were still poorly understood. The present study investigated the regolith profile (0–8 m) developed in Maofeng Mountain based on metallogenic and geochemical characteristics, sequential extraction, and physical and chemical parameters of the regolith profile. The bedrock contained abundant REE resources (245–287 mg kg–1) and the chondrite-normalized REE patterns showed the enrichment of light REE (LREE) and negative cerium (Ce) and europium (Eu) anomalies. The distribution patterns of REE in the bedrock were inherited by the regolith. REE enrichment of the regolith occurred mainly in the completely weathered layer (B1, B2, and B3 horizons), particularly in the depth range 2.5–4.5 m (849–2391 mg kg–1). The position of REE enrichment was controlled by the soil pH (5.52–6.02), by the amount of kaolinite and halloysite, and by the permeability of the metamorphic rock. In the REE-enriched horizon (2–8 m), the REE were hosted mainly in ion-exchangeable fractions (75–2158 mg kg–1), representing 79% of the total REE. Given the pH of 4.73–6.02, REE fractionation driven by the adsorption of kaolinite was limited. Fe–Mn (oxyhydr)oxides played an important role in REE enrichment and the reducible fraction holds up to 21% (139 mg kg–1) of the total REE. The enrichment of LREE was observed in the reducible fraction potentially because of the preferential release of LREE from the LREE-bearing minerals (monazite) and then scavenged by Fe–Mn (oxyhydr)oxides. Positive Ce anomalies (Ce/Ce*: 10) were found in the reducible fraction because trivalent Ce was oxidized by Fe–Mn (oxyhydr)oxides to cerianite (CeO2). The present study helps to understand the enrichment and fractionation of REE in the IADs of South China.

The wide application of rare earth elements (REEs) in the development of a carbon–neutral society has urged resource exploration worldwide in recent years. Regolith-hosted REE deposits are a major source of global REE supply and are hosted mostly in clay minerals. Nonetheless, the ways in which changes in the physicochemical properties of clay minerals during weathering affect the concentrations of REEs in the regolith are not well known. In the current study, a world-class regolith-hosted REE deposit (Bankeng, South China) has been studied to illustrate further the effect of clay minerals on sorption and fractionation of REEs during weathering to form economic deposits. In the weathering profile, halloysite and illite are abundant in the saprolite due to weathering of feldspars and biotite from the bedrock. During weathering, halloysite and illite transform gradually to kaolinite and vermiculite. The large specific surface area, pore volume, and cation exchange capacity of the clay mineral assemblages are favorable to the sorption of REEs, probably because of the formation of vermiculite. The abundance of vermiculite could explain the enrichment of REEs in the upper part of the lower pedolith. For the saprolite-pedolith interface, halloysite is probably the main sorbent for the REEs, as indicated by the distinctive appearance of pore sizes of 2.4–2.8 nm characteristic of halloysite. The progressive transformation of halloysite to kaolinite reduces the pores and desorbs the REEs, causing REE depletion in the shallower soils. As a result, REEs were mobilized downward and re-sorbed in the lower pedolith-upper saprolite causing gradual enrichment and formation of these regolith-hosted deposits.