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A silver-palladium alloy with structural formula close to AgPd has been found in laterite from the Bahia lateritic gold deposit. The alloy occurs in a void of an iron oxide nodule, associated with goethite and hematite. The angular shape and protuberances of the alloy grains suggest crystal growth in a lateritic environment, indicating that the alloy is a secondary mineral precipitated during lateritisation. The oxidation of sulphides of the parent rocks probably favoured the migration of palladium and silver as transient thiosulphate and sulphite complexes. Destruction of the thiosulphate and sulphite ligands could result in precipitation of both palladium and silver as an alloy. Eh-pH phase diagrams for PdHzO-C1 and Ag-H20-C1 systems show that both palladium and silver are stable in lateritic environments under lower redox potentials. Such an environment may exist at the top of the ferruginous zone due to the abundant organic matter near the surface.

Recent observations on the geochemistry, biogeochemistry and stable isotope systematics of waters, soils and sediments of the Amazon region indicate the profound influence of biological processes on the nature of the surface and near surface materials. These same materials provide evidence for major change in vegetation types and climate during the recent past. The chemistry of local soils and waters provides key indicators of the agricultural and mineral resource potential of various regions. The extremely deep bio-weathering in the Amazon system has produced many important and new types of mineral deposits. In regions of intense and deep leaching, the input of many chemical species may be dominated by rain and aerosols.

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