CHEMICAL AND MINERALOGICAL COMPOSITION OF AMAZON RIVER FLOODPLAIN SEDIMENTS, BRAZIL

The total and exchangeable concentration of major elements and the mineralogical composition of sediments collected along the Amazon River floodplain (varzea) were determined. Average elemental composition (wt%) decreased in the order: Si > Al > Fe > Na > Ca > K > Mg > Ti. There was some spatial variability: the concentration of Al, Na and Mg decreased downstream; the concentration of Fe, K and Ti remained constant; the Si concentration increased toward the Amazon River mouth. Mineralogically, there was a downriver increase in the concentration of quartz and kaolinite, and a concurrent decrease in the concentration of Na- and Ca-plagioclases. On the other hand smectite-vermiculite clay minerals increased and the mica (illite) concentration remained constant. A mass balance indicated that, except for illite, downriver changes in the other minerals were not caused solely by mixing of Amazon River water with water from the major tributaries, which have a distinctly different composition. Therefore, the mass balance indicated that plagioclases and smectite-vermiculite had been lost in the varzea, while kaolinite was formed there.