Observations on tropical weathering: a study of the movement of iron, aluminum and silicon in weathering rocks at Singapore

Abstract

Summary The precipitation of iron, aluminium and silicon released during the weathering of rocks in a humid equatorial climate is described. While the composition of the weathering rock determines the composition of the solution from which the precipitates are deposited the type of precipitate is determined largely by the texture of the rock in which it is formed. Pure iron-oxide concretions are found only in weathered igneous rocks and shales, seams of secondary quartz crystals are found in weathering quartzites, and composite seams of crystalline quartz enclosed in iron-oxide layers are found in beds ranging in composition from argillaceous quartzites to sandy shales. It is suggested that this separation of iron and silicon from a composite solution is due to dialysis, the colloidal iron when first deposited on the walls of an intermittently drying crack forming a semi-permeable membrane which allows the crystalloid silica to pass through into the crack. The precipitation of aluminium as gibbsitic concretions is associated with shales, the form of the concretions being controlled by the structure of the weathering rock. Weathering processes are controlled by the density of the surface vegetation, the decomposition of which may account for the low p H values found in certain rocks. In other rocks the sulphide content is sufficient to account for the very low p H values found in the weathering zone.