Summary

Selected samples of well and spring waters from the weathered profiles of a typical Deccan Basalt outcrop reveal strong controls of soil mineralogy. Calcium plagioclase, augite and volcanic glass in the bed rock weather to montmorilloniterich black soils in the foothills. Soils at higher slopes are lateritic. Lime concretions are common and quartz is ubiquitous.

The HCO3/SiO2 mole ratio in the waters is in the range predicted for alteration of plagioclase to montmorillonite and augite to kaolinite. The silica level is above quartz solubility and generally below solubility of amorphous silica. Montmorillonite and calcite are thermodynamically stable in the ground water environment. Geochemical mobility of elements during weathering decreases in the order Na, Mg, Ca, K, Si and Fe.

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