Two different illites in Luochuan Loess, northern Shaanxi province

Two different illites in samples from So to L-10 loess-paleosol sequence at Heimugou section in Luochuan are identified by decomposition of X-ray diffraction (XRD) curve, transmission electronic microscopy (TEM) and energy dispersive X-ray detector (EDX) analyses. Type I illite displays relatively poor crystallinity, wide and blunt XRD peaks, the highest content of which in < 2 mu m clay minerals is up to 55%. Type 11 illite shows good crystallinity, narrow and sharp XRD peaks. The contents of these two types of illites show mutually exclusive relationship in the studied section. The EDX analyses indicate that in contrast to the illite with good crystallinity, the illite with poor crystallinity is enriched in K+, Si4+ and depleted in Al3+.