Kinetics and mass transfer of pseudomorphic replacement; application to replacement of parent minerals and kaolinite by Al, Fe, and Mn oxides during weathering

Abstract

Parent rock minerals are universally replaced by kaolinite and oxides of Al and Fe (and, occasionally, Mn) during lateritic weathering. The kaolinite is itself also replaced by the oxides. The replacement is pseudomorphic and globally involves enormous volumes of parent rock. For each parent mineral in a weathering profile, the replacement horizon is systematically overlain by one in which the parent mineral exhibits dissolution voids. This spatial association of textures forms through two coupled local reactions, a congruent dissolution above and a pseudomorphic replacement below. The dissolution of a grain is driven by H[sup +]s supplied from above, and it releases aqueous Al and/or Fe. In turn, this Al (or Fe) travels a short distance downward and drives the replacement of another grain by gibbsite (or hematite). Any silicate grain that becomes pseudomorphically replaced by gibbsite or hematite requires import of Al or Fe, unless the porosity of the replacing aggregate is very high. Combining the condition for pseudomorphic recplacement (that the replacing mineral grows about as fast as the replaced mineral dissolves) with reaction rate expressions leads to predicting windows in local water chemistry within which pseudomorphic replacement is possible but outside of which it is not. The modelmore » predicts replacement rates, and correctly accounts for the systematic occurrence of a horizon of partially replaced grains followed upward in the weathering profile by a horizon where grains exhibit dissolution voids but no more replacement. 41 refs., 5 figs., 1 tab.« less