FREQUENCY DISTRIBUTION OF CLAY MINERALS IN THE SOILS OF CHINA

The present paper deals with the frequency distribution of clay minerals in the soils of China. The soils in the arid and semiarid regions of northern China contain large amount of illite. South of the Hwai River, the content of illite is decreased markedly with the increase of rainfall. However, illite is still dominant in the soils derived from micaceous parent materials. Except the laterite and some terra-rossa, all soils including red and yellow earths contain more or less illite. Vermiculite and chlorite are always associated with mica and illite. In the soils of semiarid and subhumid regions, vermiculite is a common accessory mineral of illite. In the tropic and subtropic regions, vermiculite is usually associated with kaolin in the red and yellow earths, but not found in the laterites. Montmorillonite has been identified in the clays of chernozem and chestnut earth. Small amount of montmorillonite is also found in rendzinas, paddy soils and some young soils derived from recent effusive volcanic rocks. Kaolinite is an indicative mineral of tropic and subtropic soils. It is present obviously in the yellow-brown earths and is higher in the red earths. In the lateritic soils, the content of kaolinite is very high but not absolutely predominant. In the laterites, where both illite and vermiculite do not exist, kaolinite is then present dominantly as the only mineral of layer silicate. Gibbsite is an indicative mineral of allitic weathering. It is absent in the calcareous soils of northern China, but occurs popularly in the soils south of the Yangtze River or south of 30° north latitude. Both iron and titanium oxides also accumulated in the red earths and laterites. Illite is a predominant clay mineral in the desert soils. In chernozem and chestnut earth, montmorillonite has been identified although a large amount of illite is present. In the drab soils, vermiculite is usually associated with illite. In the soils of northern subtropic region, kaolinite increases markedly, while illite and vermiculite decrease. In the red earths of middle subtropic region, though kaolinite predominates, there is still appreciable amount of illite and vermiculite. In the soils of southern subtropic and tropic regions, liberal amounts of gibbsite and iron oxide are associated with kaolinite. According to the distribution of soil clay minerals in different climatic regions, it is supposed that the alteration of layer silicates obeys the following sequence: illite→illite, vermiculite→illite, vermiculite, kaolinite→kaolinite, vermiculite→kaolinite, gibbsite. The constitution of clay minerals in soils is also influenced by factors such as the parent materials, topography, time and biology. Although the effects of pedogenic factors are interdependent and very complex, the different constitution of soil clay minerals is characterized by the specific predominant factor occurring under different conditions.