Clay mineral evolution and formation of intermediate phases during pedogenesis on picrite basalt bedrock under temperate conditions (Yunnan, southwestern China)

Hanlie Hong (1) , Kaipeng Ji (1) , Hetang Hei , Chaowen Wang (1) , Chen Liu (1) , Lulu Zhao (1) , Bruno Lanson (2) , Chenlei Zhao (1) , Qian Fang (1) , Thomas Algeo (1, 3)

1 CUG - China University of Geosciences [Wuhan]
2 ISTerre - Institut des Sciences de la Terre
3 UC - University of Cincinnati

In order to better understanding clay mineral evolution coupled with geochemical changes during weathering of igneous rocks under temperate conditions, the picrite basalt-derived soil in Dali (South China) was investigated using X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscope (SEM), and high-resolution transmission electron microscopy (HRTEM) methods. Our results show that smectite and mixed-layer clays occur throughout the weathered soil profile, while discrete kaolinite and illite phases are absent in the soil. From saprolite to topsoil, smectite decreases sharply, while mixed-layer kaolinite/smectite (K/S) and illite/smectite (I/S) clays increase markedly. Clay minerals in the saprolite consist of both dioctahedral and trioctahedral species, while those of the middle to upper profile display a uniform dioctahedral structure. I/S phases are characterized by interstratification of 12-Å smectite and 10-Å illite layers, and K/S phases by interstratification of 12-Å smectite and 7-Å kaolinite layers, with I/S/K clays containing all three layer types. Kaolinization of smectite occurs at the initial stage of weathering, earlier than that of smectite illitization. Desilication and K-fixation of smectite take place simultaneously during advanced weathering, resulting in formation of illitic and kaolinitic phases continuously throughout the developmental history of the soil. Notably high K concentrations in the weathering profile may be related to K-fixing in the interlayer of illite due to smectite illitization in response to more advanced weathering and pedogenic processes, while the increasing K content in the topsoil may be ascribed to fertilizer in land use and uptake of K by plants from deeper soil horizons. Fe2O3 and TiO2 accumulation in the topsoil is probably mediated by microorganisms.

Keywords: kaolinite/smectite (K/S); illite/smectite (I/S); desilication; weathering; soil formation; saprolite

Domains: Geochemistry

Hanlie Hong, Kaipeng Ji, Hetang Hei, Chaowen Wang, Chen Liu, et al.. Clay mineral evolution and formation of intermediate phases during pedogenesis on picrite basalt bedrock under temperate conditions (Yunnan, southwestern China). CATENA, 2023, 220, pp.106677. ⟨10.1016/j.catena.2022.106677⟩. ⟨insu-03833532⟩