CHEMICAL AND MINERALOGICAL COMPOSITION OF AMAZON RIVER FLOODPLAIN SEDIMENTS, BRAZIL

The total and exchangeable concentration of major elements and the min eralogical composition of sediments collected along the Amazon River f loodplain (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 decre ased downstream; the concentration of Fe, K and Ti remained constant; the Si concentration increased toward the Amazon River mouth. Mineralo gically, 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 miner als 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 wa ter with water from the major tributaries, which have a distinctly dif ferent composition. Therefore, the mass balance indicated that plagioc lases and smectite-vermiculite had been lost in the varzea, while kaol inite was formed there.