The less than 2-micron fractions of modern deltaic sediments of the Godavari River were analyzed for their clay-mineral assemblage by X-ray-diffraction technique. The results obtained are as follows: with progressive increase in the salinity of the water downstream, there is a corresponding increase in the concentration of Na⁺ montmorillonite, illite, and chlorite, and a decrease in kaolinite. Na⁺ montmorillonite is the predominant clay mineral in the paludal deposits of the mangrove swamps, illite in the lagoonal facies, and chlorite in the deep marine facies (prodelta?) of the Godavari delta. Ca⁺⁺ Na⁺ montmorillonite is restricted specifically to the fresh-water Godavari channel. Within the saline environment of the delta, ill te concentration is lowest in the swamps.

It is obvious that the observed lateral variations in the clay minerals are a response to the changes in the physicochemical conditions of the depositional environments. The increase in Na⁺ montmorillonite toward more saline environments is due apparently to the cation exchange of Na⁺ for Ca⁺⁺ in the Ca⁺⁺ Na⁺ montmorillonite, whereas the increase in illite and chlorite is probably an outcome of regeneration--by K⁺ and Mg⁺⁺ adsorption--from their respective degraded forms. The abrupt fall in the illite content within the mangrove swamps is attributed to a return of the regenerated illites to their original degraded state, which in turn, is a consequence of the preferential leaching of K⁺ by the dens mangrove vegetation.

The geochemistry of potassium, sodium, calcium, and magnesium pertaining to the clay fractions of the deltaic sediments generally tends to support the above contention. Attempts to define the polymorphic forms of the illite and chlorite have proved futile because it is nearly impossible to fractionate these minerals from the other associated minerals.

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