Variation of clay mineral contents in completely decomposed granite

Abstract

There are three major components in completely decomposed granite (CDG): quartz, feldspar and clay mineral. How about the variation and the value range of their contents, especially the clay mineral? Taking samples from 18 blocks (each side 30 cm length) from 3 trial pits in Kowloon, Hong Kong, as an example, we try to solve this problem. The specimens for XRD test are cut from different zones in the blocks. The test results (involving the three components contents, percentages of kaolinite and of illite in clay size particles) are analyzed and compared. It is found that the mineral contents generally vary within a range of 10%-20%; the varied range of kaolinite content is bigger than that of illite; the change values of specimens with coarse-particle structures are larger than that of finer-particle structures; and the results from different analyzers vary to an extent. After analysis it is known that both the inhomogeneity of CDG and the experiences of analyzers mostly make the clay mineral contents change even in the same zone of one block. Furthermore, the XRD results are compared with those from statistic values via thin sections to check the variation of mineral contents. As a result, it is shown that for the mineral contents from XRD, the quartz is usually higher, the feldspar is smaller, and the clay mineral higher and smaller for different specimens. The mineral contents from XRD and thin section are different in a range of 0%-35%. Totally this research quantitatively describes the inhomogeneity of the CDG in block and trial pit sizes, which together with the identification of initial mineral or secondary clay mineral (e.g. feldspar or kaolinite, mica or vermiculite), causes the variations of the content results to a great extent.