Precipitation and dissolution of gibbsite during weathering of andesitic boulders in humid tropical West Java, Indonesia

Abstract

Several weathered andesite boulders, sampled in Typic Humitropepts from the Cigudeg area, West Java (Indonesia), were studied using micromorphological, mineralogical (XRD, TG, DTA, SEM, EDXRA) and chemical techniques. Because all data of the boulders are similar, only the results of the largest boulder are presented in this paper. Three zones are distinguished: the rim of the boulder core, a transition layer, and outer layers. In the rim of the core weathering attacks the glass matrix and the anorthite rich parts of the plagioclases. Even before sufficient iron is liberated from pyroxenes goethite is deposited in their cracks, as a first step to the formation of a boxwork. In the transition layer mono- and divalent cations have been lost, but Al shows an absolute accumulation as gibbsite in plagioclase pseudomorphs and pores. It is presumed that the Al originates from the more internal parts of the core. Towards the outer layer gibbsite gets completely removed, supposedly as the result of a lowering of pH, and the plagioclase pseudomorphs become filled with illuvial kaolinitic clay. Illuviation of clay, and the import of iron, from the surrounding soil material, put limits to the use of isovolumetric calculations. The goethite boxworks after pyroxenes reflect the original pattern of fractures, rather than cleavage patterns.