Abstract: The clay mineral composition of surface sediments from the middle reach of Huaihe river drainage area was analyzed with the X-ray diffraction method. Results show that illite, averaging 52.3%, is the dominant clay mineral, followed by kaolinite (17.0%), smectite (16.8%), and chlorite (13.9%). The sediments from the Huaihe River are characterized by high contents of smectite and kaolinite compared with those from the Yellow and Yangtze Rivers. The ratio of illite/smectite is lower than 6, which is close to the ratio from the Yellow River-derived sediments. The ratio of kaolinite/chlorite is higher than 1, which is close to the number from the Yangtze River-derived sediments. The sediments from the middle reach of the Huaihe River can be easily distinguished from the Yellow river and Yangtze River sediments in the triangular diagram with illite, smectite and Kaolinite+chlorite as end members.
In general, weathering intensity, which is mainly controlled by lithology, climate, and morphology, determines the formation of clay minerals. That parent rocks with different lithology and weathering intensity in the upper reach and tributaries of the river is one of the main factors to the formation and distributions of clay minerals in the lower drainage basin. Illite is the most abundant clay mineral in the Huaihe River drainage basins, which could have been derived from physical erosion of metamorphic and granitic parent rocks, mainly located at the Qinling-Dabie region. The high contents of smectite and kaolinite in the Huaihe River sediments are interpreted in this paper as the indicator of the source area and climate of the drainage area. The Huaihe River drainage area is located in the climatic transition zone between the Northern and Southern China. The difference of climate and parent rock in the two regions may be the main cause for the formation of clay minerals assemblage of Huaihe River-derived sediments and their difference with those from the Yellow and Yangtze Rivers. In the southern part of the Huaihe River drainage area, the sediments are influenced by the parent rock from the Qinling-Dabie region, including the granitic and metamorphic rocks, and by the wet and warm climate favorable to the chemical weathering, presumably leading to the high content of Kaolinite. In the northern part, however, the sediments are influenced by loess and the deposits from the Yellow River, and by the dry and cold climate favorable to the physical weathering, which result in the high content of smectite. These weathering processes produced the high content of illite (primary mineral)and reasonable amount of kaolinite and smectite(secondary mineral)as found in samples analyzed.