Mineralogy of Red Soils in Southern China in Relation to Their Development and Charge Characteristics

Abstract

The mineral composition of the different size fractions of red soils from southern China was investigated using X-ray diffraction (XRD), infrared spectroscopy analysis (IR), transmission electron microscopy (TEM) and chemical methods. Surface charge and phosphate adsorption characteristics of the soils were also determined. The results showed that kaolinite and/or halloysite are the most common clay minerals in the soils. However, the abundance and mineral composition of the soils varied with latitude, elevation, parent materials, and topographic position. Types of kaolin mineral in the soils changed with soil types, the relative proportion of kaolinite increasing with weathering degree. Under current subtropical climates, a transformation process of mica to vermiculite also occurs in these soils. Goethite and hematite are the only two crystalline Fe oxides occurring in significant quantity in these soils. The goethite/(goethite+hematite) ratios ranged from 0 to 1, being the highest in yellow soils, and the lowest in soils derived from purple sandstone where only hematite was present. Substitution of Al in goethite ranged from 6.6 to 29.9 mole % with a mean value of 16.7 mole %Al, while substitution of Al in hematite ranged from 0 to 12.1 mole %. The values of Al substitution of hematite were closely related to the corresponding values of the coexisting goethite. Phosphorus (P) adsorption of all the soils studied was well described by the Freundlich and Langmuir equations. The maximum buffering capacity (MBC) of P ranged from 66.5 to 9880 mg kg-1 in the increasing order of red-purple sandstone soil < purple sandstone soil < red sandstone soil < red soil < lateritic red soil < yellow soil < latosol. The zero point of charge (ZPC) values obtained by salt titration-potential titration varied from 3.03 to 5.49, the highest value being found in the latosol derived from basalt and the lowest in purple sandstone soil. Correlation analysis indicates that the main minerals responsible for phosphate adsorption in the soils were gibbsite, amorphous iron oxides and kaolin, whereas the PZC was mainly controlled by kaolin, gibbsite and oxides.