Abstract

Leaching tests with solutions at several different pH's in the natural pH range (4 to 9) were performed on crushed latite, nepheline soda-andesite, and peridotite at 35 degrees C. and 0 degrees C. Both by immersing the rock in the solutions and by allowing the solutions to percolate through the rock, enough more silica was removed than Al and Fe to cause a considerable residual enrichment of the 2 metals. The amount of silica dissolved was much greater in slightly acid solutions than in neutral and slightly basic solutions. A comparison of the results of recent solubility studies shows that the solubility differences between amorphous silica and the hydroxides of Al and ferric iron are sufficient to explain the observed results of the leaching experiments and to support the concept of laterization as a chemical process whereby silica is extensively removed from silicate rocks by ground water as a result of solubility differences, with the resulting residual concentration of Al and/or ferric iron. The greater amount of silica dissolved at acid pH's than at neutral and basic pH's probably represents a dependence on pH of the rate of decomposition of the rock, since the solubility of amorphous silica should be constant in the natural pH range. These results suggest that acid ground water is more effective in leaching silica from rocks during weathering than basic solutions, a conclusion that contradicts the widely accepted hypothesis that laterite is the result of leaching by a basic soil solution. This conclusion is supported by recent papers describing areas of contemporary laterization with acid soil solutions. The occurrence of gibbsite in temperate soils suggests that the chief effect high temperatures have laterization is to accelerate the chemical reactions involved.

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