Origin of illite in the loess from the Luochuan area, Loess Plateau, Central China

Illite, the predominant component of the clay fraction of the Chinese loess , has been studied by X-ray diffraction (XRD) in 33 samples taken from the Holocene palaeosol (SO), the last glacial loess (L1) and the last interglac ial palaeosol (S1) in the Louchuan loess section of the Loess Plateau in Ch ina. The XRD investigations indicate that it is mainly a 2M(1) dioctahedral mineral with crystallinity values ranging from 0.23 to 0.36 degrees Delta 2 theta (CIS Index). The 2M(1) polytype illites with low IC values preclude a pedogenic formation of illites in the surface horizons of aridic and sem i-aridic soils or deserts, and thus the clay mineral composition was largel y determined by provenance. The illites are considered to be of detrital or igin derived from pre-existing sediments and very low- to low-grade metamor phic rocks, eroded from the northern part of the Tibetan Plateau. The clay content of the loess and palaeosols was largely controlled by the strength of the winter monsoon, rather than by effects of in situ pedogenetic proces ses.