Characterization of clay rock-derived soils containing multi-mineral sand particles in upland areas of Sichuan Basin, China

Multi-mineral sand particles (MSPs), commonly found in upland soils, can change the erosion properties, structure, moisture level, porosity, and permeability of soil; however, their effects are generally neglected in previous studies. In this study, we explored the morphology using scanning electron microscopy (SEM), particle size distribution (PSD), aggregation state, and mineralogy of soils derived from two types of clay rocks containing MSPs: shale and mudstone. The results showed that: (1) small lamellate bodies formed granules and blocks in a face-to-face or side-to-side manner and that the aggregation particles ranged from several to tens of microns in diameter, which increased gradually in a downslope direction; (2) the sand content in shale- and mudstone-derived soils decreased from 41.0% and 31.3%, respectively, on the summit to 19.6% and 18.4%, respectively, on the toeslope, whereas their clay content increased from 23.3% and 20.2% to 45.0% and 38.9%, respectively; (3) regarding the aggregation states of soil particles, the content of 0.25–2 mm MSPs in the shale- and mudstone-derived soils decreased from 62.9% and 45.2%, respectively, on the summit to 7.1% and 1.4%, respectively, on the toeslope, with averages of 42.3% and 20.8%, respectively. The content of 0.25–2 mm aggregates in the shale- and mudstone-derived soils increased from 9.1% and 6.2%, respectively, on the summit to 44.4% and 21.1%, respectively, on the toeslope, with averages of 21.5% and 14.2%, respectively; and (4) the mineralogy of the clay rock-derived soils and MSPs was generally dominated by illite, montmorillonite, vermiculite and calcite, which was similar to the mineralogy of the parent rocks. Therefore, a large amount of MSPs was found in clay rock-derived soils whose particle characteristics were determined by the environment of diagenesis and the physical weathering of pedogenesis.