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 coll ected from the central region of the Mekong River, Laos, to study the origin and inherent potentiality of soils, REE as the chondrite-normal ized 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 es timated to originate from the composite materials of the sediment deri ved from local granite and limestone and the sediment transported by t he Mekong River. The predominant clay mineral was mica and its weather ing 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 intermedia te to high level. The soils in the lower area (Vientiane Prefecture an d Borikhamxay Province, approximate elevation 90-150 m) were estimated to originate from sediment transported by the Mekong River or from se diment derived from local sandstone, The soils originating from the Me kong River sediment were medium-textured and mica-dominated and were e valuated to be at the intermediate level of inherent potentiality. The soil originating from the sediment of local sandstone was coarse-text ured and contained mica and kaolinite as the predominant clay minerals . Its inherent potentiality was evaluated to be poor.