Genetic relationships between ferralsols, podzols and white kaolin in Amazonia

摘要

White kaolin has frequently been observed to be associated with ferralsol-podzol soil systems in Amazonia. In order to evaluate whether such systems favour kaolin genesis and to identify the associated genetic processes, we studied soil organization, mineralogy and groundwater properties of a ferralsol-podzol soil system with white kaolin located in the High Rio Negro Basin, Brazil. We found that the kaolin was situated near the ferralsol-podzol transition and that its thickness was related to the depth of landscape incision by regressive erosion. The kaolin was characterized by silicon, iron and titanium (Ti) leaching and aluminium (Al) absolute accumulation. The groundwater that percolates from the podzol to the kaolin can enhance kaolinite precipitation, by supplying Al originating from kaolinite dissolution in the overlying Bh, and kaolin bleaching, by low pH and Eh of the percolating waters favouring iron reduction. The system dynamics imply that the quartz dissolution rate in the kaolin is of at least the same order of magnitude as the kaolinite dissolution rate in the overlying Bh. Within the whole system, Ti appeared to be very mobile