Comparison of clay mineral content variations in clay-size portions and bulky completely decomposed granite (in Chinese)

How about the relationship between clay mineral contents in bulky soils and in clay-size portions, respectively? Taking 51 nos. samples of completely decomposed granite (CDG) from 3 trial pits in 2 slopes of Kowloon, Hong Kong, as an example, the study is carried out with the same instrument, approaches and analyzers. In this procedure, the sampling pipette is adopted to measure the weight proportion of clay-size particles, the physicochemical method is used to test the specific surface area, and the X-ray diffraction (XRD) is applied to measure mineral contents in bulky rocks and clay minerals in clay-size portions. Furthermore, the correlation of statistic results from thin section with those from XRD is analyzed.For these CDG with clay minerals contents in a range of 20%～45%,it is found that:(a)In bulky rocks, clay minerals, e.g. kaolinite, vermiculite, are with a higher absolute content than the calculated content from clay-size portions percent timing the relative content of clay minerals in the portions; (b)The variation of relative contents of kaolinite and vermiculite is more prominent than that of the absolute content in bulky rocks, and these two with similar varying features; (c)The absolute content of clay minerals in bulky rocks is far larger than the clay-size portions (two limits of particle size: 2μm and 5μm). The portions within the two limits are with an concordant variation; (d)The specific surface area value is closely connected with the absolute content of clay minerals in bulky rocks; (e)Compared with results from thin section, the clay mineral content from XRD is usually bigger due to its uncertainty in identification of feldspars under weathering as clay minerals or not.