Origin of clay minerals on section of Luochuan loess- palaeosol in Shaanxi Province, northwest China

摘要

Crystallinity, polytype, and morphology of clay minerals in the Luochuan loess-palaeosols in Shaanxi Province, northwest China were studied in order to have an insight into their origin using X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) methods. The SEM observations showed that the morphology of some illites seems to be lath-shaped crystals. An analysis of illite crystallinity (IC) on the Luochuan section indicated that the origin of illite was related to the rocks of an anchizone. Most illite in the Luochuan loess-palaeosol section were of 2M1 polytype, but some were of 1M polytype formed by weathering of feldspar in the process of pedogenesis. Illite in the Luochuan section has undergone both physical and chemical weathering. These results revealed that most illite were of detrital origin related to the source area of an anchizone, but parts of the illite were of an authigenic origin formed during pedogenesis after sedimentation. Chlorites in the samples of the Luochuan section were mainly composed of irregular flaky grains and their crystallinity was good. These showed that chlorite had the detrital origin formed by physical weathering. Kaolinite crystallinity was relatively good. The value of CIA on the Luochuan section ranges from 61.9 to 69, and therefore kaolinite could not be formed during weathering and pedogenesis. These results indicated that the kaolinite had a detrital origin. Morphology of smectite seems to be capillaceous. The XRD patterns of all samples contained diffraction peaks at 1.5218 A (nontronite) and 1.5052 A (montmorillonite), thus indicating an intermediate composition between trioctahedral and dioctahedral smectite. The smectite crystallinity was very poor. These results revealed that smectite in the study area was authigenic rather than detrital origin.