The formation of representative lateritic weathering covers in south-central Guangxi (southern China)

Abstract

The mineralogy and geochemistry have been studied in three representative lateritic weathering covers close to the Tropic of Cancer (Guangxi, South China) with the purpose of studying formation and evolution processes of lateritic covers in this region. The X-ray diffraction and scanning electron microscope analysis results indicate that the contents, particle sizes, shapes and structures of secondary minerals, such as iron and clay minerals, are distinctly different in ferruginous nodular horizon and mottled clay layer of lateritic profiles. The ferruginous nodules are characterized by high concentrations of iron and aluminum and low contents of silicon. The energy analysis suggests that there are element exchanges between hematite crystal and clay minerals. The mass balance calculation results show that there is exogenetic input of iron into lateritic covers, especially within the ferruginous nodular horizon, compared to the bottom layer of the profile. The scanning electron microscope images also confirm that voids and fractures in ferruginous nodules were filled with ferruginous material derived from soil solution. This could be a significant interpretation for the negative correlations between Fe2O3 and Al2O3. The iron and manganese oxides in ferruginous nodular horizon have an apparent effect on the adsorption and precipitation of cerium. Strontium, compared to other alkali metal and alkali-earth metal elements, concentrates in mottled clay layer. Variation in the He/Gt ratio reflected by the O/Fe ratio in a small ferruginous concretion indicates that climate changes altered aqueous activity of the soil environment. Finally, a model for the formation and evolution of typical lateritic weathering covers in south-central Guangxi, supported by all the data, was proposed.