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Clay minerals are usually considered as important indicators to identify sources in both terrigenous and marine sediments. In particular, clay minerals in metalliferous sediments (MS) have long been studied in global oceans except in South Mid-Atlantic Ridge (SMAR) due to limited explorations. Thus, 32 MS and 34 non-MS (NMS) samples were analyzed to clarify the distribution characteristics and mineral compositions of clay minerals. All the sediments were collected along the SMAR between 12°S and 27°S. After removal of organic matter and carbonate, clay fractions (<2 µm) were investigated by X-ray diffraction (XRD) analysis. Results show that clay mineral assemblages of surface MS consist dominantly of smectite, less abundant illite, chlorite, and kaolinite in average weight percentage of 30%, 21%, 18%, and 16%, respectively. On the other hand, clay mineral assemblages in the NMS consist mainly of illite, less abundant kaolinite, chlorite, and very scarce smectite in average weight percentage of 47%, 29%, 24%, and 0.2%, respectively. The clay fractions in MS are mainly composed of amorphous or poorly crystallized Fe/Mn oxyhydroxides, clay mineral, quartz, and plagioclase. However, the counterparts in the NMS are mainly composed of well-crystallized clay minerals, quartz, and plagioclase without the presence of Fe/Mn oxyhydroxides. It is suggested that most of the illite, kaolinite, and chlorite in both MS and NMS are likely aeolian dust in origin from South Africa continent. In addition, the abundance of kaolinite dominates the clay mineral assemblage at low latitudes, where the intensive chemical weathering of continental source rocks facilitating the formation of kaolinite. In terms of smectite, it is indicated of authigenic origin in consideration of only smectite is available in several MS and the absence in NMS. Moreover, the MS samples with only smectite available are always accompanied by goethite. Therefore, it is assumed that most of smectite occurred in studied area is the results of interaction between hydrothermal Fe-oxyhydroxide, silica, and seawater. [ABSTRACT FROM AUTHOR]

Ion-adsorption deposits (IADs) are important types of REE deposits. Previous studies have found that, in IAD, REE3+ adsorb on the surface of clay minerals in the form of multi-coordinated hydrated complexes, [REE(H 2 O) 8/9 ]3+, forming exchangeable outer-sphere adsorption complex. Actually, there are two types of basal surfaces in clay minerals, the (001) surface and the (00−1) surface, and it is not clear whether there are differences in the adsorptions on these two types. Additionally, the electron transfer mechanism in the adsorption structures cannot be revealed. To address these issues, we employ first-principles calculations based on density functional study to simulate the outer-sphere adsorption structures of representative Y3+ ion on the two types of basal surface of kaolinite. The study reveals that both the (001) surface and the (00–1) surface can serve as adsorption surfaces, but the (00–1) surface forms more stable adsorption structures with adsorbate-[Y(H 2 O) 8 ]3+ due to the absence of hydroxyl group repulsion effects. The adsorption process involves electron transfer from the kaolinite basal surface to the adsorbate, forming hydrogen bonds. The primary orbital interaction in this process is between the O-2p orbitals of the adsorbate and the surface O-2p orbitals. This work contributes to a quantum-level understanding of the nature of ion adsorption in ion-adsorption deposit. • DFT studied Y3+ adsorption on kaolinite's (001) and (00-1) surfaces. • Y3+ mainly forms [Y(H 2 O) 8 ]3+ complexes in pure water. • (00-1) kaolinite surface is more stable for adsorption than (001). • Outer-sphere adsorption involves hydrogen bonding between adsorbate hydroxyl groups and surface O-2p orbitals. [ABSTRACT FROM AUTHOR]