Characteristics of clay minerals in soil particles of two Alfisols in China

• There was the phenomenon of desilicification and allitization in the two Alfisols with decreasing particle size.
• With decreasing particle sizes, the clay mineral structures were transformed from well crystalline to poor crystalline.
• The major clay minerals in nanoparticles (25–100 nm) of the two soils were illite and kaolinite.
• The 1:1 clay minerals of nanoparticles were mainly inherited and transformed from the minerals in larger particles.

The composition, structure and transformation characteristics of clay minerals in various size particles (< 2000, 450–2000, 100–450 and 25–100 nm) of two Alfisols (Haplustalf and Hapludalf) in China were investigated by elemental analysis, X-ray diffraction (XRD), Fourier-Transform Infrared Spectroscopy (FT-IR) and 27Al and 29Si Magic-angle Spinning Nuclear Magnetic Resonance (MAS NMR). The clay minerals in < 2000, 450–2000 and 100–450 nm size particles of the two soils were illite, vermiculite, smectite, kaolinite and kaolinite interstratified minerals. In Haplustalf, vermiculite was dominant clay mineral in 450–2000 nm particles and illite was dominant clay mineral in 100–450 nm particles. In Hapludalf, kaolinite (and kaolinite interstratified minerals) predominated in all size particles. With decreasing particle sizes, the two clays showed a decrease in K2O, Na2O, MgO and SiO2 contents and an increase in Al2O3 content, with the minimum molar ratio of SiO2 to Al2O3 observed in nanoparticles (25–100 nm). Meanwhile, the smectite and vermiculite contents decreased or disappeared, leaving illite and kaolinite (and kaolinite interstratified minerals) as the main clay minerals in the two soil nanoparticles. Additionally, with decreasing particle sizes, the clay mineral structures were transformed from well crystalline to poor crystalline, with illite being the poor crystalline in nanoparticles of the soils. The hydroxyl groups between the tetrahedral and octahedral structures of the 2:1 clay minerals and the surface of kaolinite gradually broke down, and IVAl of the tetrahedral structure gradually reduced. In nanoparticles, Al mainly appeared in the octahedral structure as VIAl, and Si in the structure of Q3 (Si linked to three other Si atoms via oxygen). The composition and structure of clay minerals in different size particles showed that the clay minerals weathered from larger particles (> 100 nm) were an important mineral source of nanoparticles, and the 1:1 clay minerals were mainly inherited from the minerals in larger particles and transformed from 2:1 clay minerals into nanoparticles.