Using ΔpH as a geochemical index of illite neoformation in saprolite
Résumé
Soil pH is routinely measured for agronomic purposes. When the difference between KCl pH and H2O pH, or ΔpH, yields positive values, it is used by soil scientists as a classification criterion for identifying anionic subgroups according to the Soil Taxonomy or geric properties according to the WRB. Negative values have not been granted much attention. Here we focus on the occurrence of highly negative ΔpH values in the weathering zone of profiles developed on gneiss in semiarid Northeast Brazil and semiarid South India and interpret them as proxies of a geochemical weathering process involving the neoformation of illite. Detailed optical, chemical and mineralogical characterizations involving scanning electron microscopy coupled with X-ray element mapping demonstrate the neoformation of illite inside plagioclase feldspar crystals after their partial dissolution. This study thus reveals that meteoric weathering is capable of producing illite not only from mica, i.e., by a transformation process, but also within non-alkali feldspar by a neoformation process. The ΔpH is shown to be a good proxy for detecting such weathering signatures because the recently neoformed illite flakes, which present a significant compositional deficit in K, reveal their presence by a detectable uptake of K from the KCl solution. This finding changes the perspective over the origin of illite in continental environments, which has most commonly been attributed to hydrothermal processes.