Rare earth element and clay minerals of paddy soils from the central region of the Mekong River, Laos

The rare earth elements (REE) content, particle-size distribution, and clay mineralogical composition were analyzed for the paddy soils collected from the central region of the Mekong River, Laos, to study the origin and inherent potentiality of soils, REE as the chondrite-normalized curve or the plot of Ce/Eu against Eu/Sm were found to be useful for grouping soils according to their origins. The soils in the upper area (Luang Prabang Province, approximate elevation 300-400 m) were estimated to originate from the composite materials of the sediment derived from local granite and limestone and the sediment transported by the Mekong River. The predominant clay mineral was mica and its weathering product or kaolins, and the mineralogical composition varied from site to site, The soils were generally characterized by high clay and silt contents. Inherent potentiality was evaluated to be of intermediate to high level. The soils in the lower area (Vientiane Prefecture and Borikhamxay Province, approximate elevation 90-150 m) were estimated to originate from sediment transported by the Mekong River or from sediment derived from local sandstone, The soils originating from the Mekong River sediment were medium-textured and mica-dominated and were evaluated to be at the intermediate level of inherent potentiality. The soil originating from the sediment of local sandstone was coarse-textured and contained mica and kaolinite as the predominant clay minerals. Its inherent potentiality was evaluated to be poor.