Clay mineralogical and geochemical responses to weathering of intrusive vs. extrusive rocks under a subtropical climate

Abstract

Clay mineralogy and geochemical characteristics of two soil profiles developed on intrusive (quartz monzonite) and extrusive (rhyolite) rocks in subtropical China were investigated to reveal clay mineralogical responses to weathering of different parent rocks under a subtropical climate. Overall chemical weathering intensity and biotic weathering intensity do not show significant differences between the two profiles, suggesting that the difference in parent lithology and rock's cooling history did not exert a significant control on weathering intensity. The differences between two profiles are mainly reflected in clay mineralogy and evolution of elemental loss/gain. The soil developed on intrusive rock is characterized by having morphologically diverse halloysite, little chlorite, less illite, and poorly crystallized illite. Elemental addition/depletion shows evident differences between the two profiles. Differences in clay mineralogy and geochemical characteristics can be linked to parent lithology and magma cooling history, which influence texture, parent composition and condition of water-rock interaction. For example, beyond expectation, soil developed on extrusive rock is conducive to the formation of halloysite because prolonged water stagnation favors preferential transformation of feldspar into halloysite. In addition, for each profile, biotic weathering and abiotic weathering jointly determined the formation and transformation of clay minerals. The widespread presence of porous volcanic glass in the rhyolite-derived soil may provide favorable nutritional conditions for microbial growth, producing large amounts of organic acids, which are an important factor influencing the alteration of illite to secondary minerals.