MINERALOGY OF SALT EFFLORESCENCES IN PADDY FIELD SOILS OF KOLLO, SOUTHERN NIGER

This study examines the mineralogy of salt crusts developed on the irr igated paddy soils in the Soudano-Sahelian region of Niger. It involve d field observations supplemented by laboratory determinations: powder XRD, SEM-EDAX, IR spectral analyses. The salt crusts were found in th e concave zones near to the dike along the Niger River. They consisted of white mamillated konyaite crystals supported by a continuous hexah ydrite crust. Upslope, konyaite disappeared progressively and the disc ontinuous crust was constituted by hexahydrite only. At a few centimet ers below the soil surface, epsomite crystallized in the cracks and vo ids. In the subsoil, gypsum and calcite were deposited. The salt miner als were distributed vertically and laterally in the soil horizons acc ording to the chemical composition of shallow groundwater, in particul ar the NaMg ion ratio, the water table depth and the extent of surface runoff. Human activity, particularly superfluous irrigation, constitu tes the most pertinent explanation of the salinization processes at Ko llo.