**1. Introduction**

Rare earth elements (REEs) are 16 chemical elements grouped by their atomic number, and classified as light (LREEs), middle (MREEs), and heavy (HREEs). The weathered crust elution-deposited REE ores in southern China have drawn much attention because of their abundance of granitic residuum, their simple extraction processes, and their well-distributed composition [1–3].

The migration and enrichment of REEs are controlled by several factors, such as parent rock lithology, pH value, intensity of weathering, and topography [4–6]. Previous studies have shown that, for chemical index of alteration (CIA) values of 65%–85% in granite, clay minerals increase rapidly with an increasing degree of weathering. There is a positive correlation between the loss on ignition (LOI) of 2%–6% in the weathering crust and REE content [7].

Clay minerals have a controlling e ffect on the migration and release of REE ore. The completely weathered layer of a weathering crust mainly comprises quartz, feldspar, and clay minerals. The clay mineral content decreases gradually from the weathering crust surface to the lower layer, where clay minerals are converted from hydromica and montmorillonite to halloysite, kaolinite, and gibbsite [8].

The distributions of REEs in the weathering crust are controlled by both the composition of the parent rock and the clay mineral content of the weathering crust. Halloysite, a clay mineral, plays a significant role in the di fferentiation of cerium [9]. Halloysite has a stronger e ffect than kaolinite in the adsorption of REEs; however, this adsorption mechanism is not ye<sup>t</sup> fully understood. Previous studies found that the adsorption of REEs is controlled by the properties of the clay minerals rather than the electrolyte solution or dissolved carbon dioxide content [10]. The adsorption capacity of kaolinite increases linearly with increasing pH. A fractionation between HREEs and LREEs due to selective sorption is observed, with HREEs being more sorbed than LREEs at high ionic strengths [10]. For montmorillonite at pHs below 4.5, the REE adsorption capacity is constant, and is modeled by cation exchange [11]. Di fferent clay minerals have di fferent adsorption capacities for REEs. Chi et al. [12] showed that for three common clay minerals, the cation adsorption capacity follows the order: montmorillonite > halloysite > kaolinite. This result shows that di fferent parent rock lithologies will result in di fferent weathering crust structures and clay mineral compositions. Intimate grain-to-grain contacts promote a unique chemical environment at the microscale, bringing about the formation of transient clay mineral phases which quickly disappear in the overlying soil [13]. The bulk of illite in the weathering crust is due to the weathering of mica minerals. A study of unstable soil profiles found that illite is converted into vermiculites or interstratified illite-smectite [14].

Climatic and environmental change is one of the causes of compositional di fferentiation in clay minerals. Kaolinite and kaolinite interlayer minerals are dominant in strongly leached soil layers [15], while illite and montmorillonite represent a cold and humid climate with weak chemical weathering [16]. Clay minerals of di fferent crystal characteristics di ffer in physical structure and properties [13]. Clay minerals that host ion-adsorbed REE ores have large specific surface areas and a strong capacity to adsorb REE ions. The clay mineral content thus controls the migration and enrichment of REEs—processes of grea<sup>t</sup> significance for REE mineralization.

Although the clay mineralogy of weathered crust elution-deposited REE ores varies, several studies have demonstrated that the clay minerals in these ores commonly comprise halloysite, kaolinite, some illite, and rare montmorillonite [17]. It is widely believed that the horizon enriched in REE generally contains abundant halloysite and kaolinite [18,19], and that clay mineral migration is controlled by soil particle size and specific leaching conditions [20,21]. The metallogenetic mechanism of weathered crust elution REE deposits could involve the weathering of granodiorite and volcanic rocks in warm and humid climates, with the transformation of their parent mineralogy into kaolinite, halloysite, and montmorillonite [22]. In weathering crust elution-deposited REE ores, REEs adsorbed on the clay minerals by ion-exchangeable phases account for more than 80% of the total REE content [3]. However, leaching is controlled by the properties of the REE ore, by the nature and concentration of the leaching reagent, and by the hydrodynamics, kinetics, and mass transfer of the leaching process [22]. We postulate that the weathered crust elution-deposit REE ore is associated with REE ion enrichment, which is dissociated with hydrated or hydroxyl hydrated minerals and adsorbed by clay minerals, which are subsequently deposited, and mineralized in the weathered crust over a long period. In contrast, this is not to say that all REE mineralization can be explained by a single model. It is important to understand that the clay mineralogy in di fferent environments of REE ore formation varies with di fferent conditions of parent rock, pH values, degrees of weathering [4,5], and mining conditions [9]. This study investigates the types and changing characteristics of clay minerals in several ion-absorbed REE ores in southern Jiangxi Province, China, during weathering and in situ leaching, with an aim of improving the recovery rate. To achieve this, soils on the surface of the weathered crust in a typical rare earth mining area in southern Jiangxi Province were sampled. Then, methods such as in situ leaching profile monitoring and indoor leaching simulation experiments were used to study the characteristics of the clay mineral properties and soil particle size.
