Distribution of clay minerals along a weathered pyroclastic profile, Hong Kong

Abstract

The distribution of clay minerals along a weathered profile developed over crystal-vitric tuffs are examined by X-ray diffractometry (XRD) and scanning electron microscopy (SEM) techniques. Chlorite and illite are the most abundant clay minerals in fresh and moderately decomposed rocks, respectively. Chlorite is limited to fresh samples, whereas illite and trace amounts of interstratified illite–smectite are ubiquitous along the profile. Halloysite and kaolinite are the dominant clay minerals in highly to completely decomposed rocks. Halloysite clusters in moderately decomposed samples provide clear evidence of crystallization from solution, whereas vermiform kaolinite books reveal evidence of transformation from illite in most samples. In general, as the intensity of weathering increases: (a) kaolin-to-illite and kaolinite-to-halloysite ratios increase; and (b) the morphology of halloysite varies from spheriodal clusters to tubular aggregates. It is concluded that the nature, type and abundance of clay minerals along the profile result from complex combinations of several different processes controlled mainly by three factors: the pre-weathering alteration history of parent rock, the degree of weathering, and microenvironmental conditions. The relative role of each factor can vary significantly within the same profile. As evidenced by mineralogical and petrographical characteristics, the nature and type of clay minerals in fresh and moderately decomposed rocks can be attributed to pre-weathering hydrothermal and/or deuteric alterations. On the other hand, the degree of weathering regulates the nature, type and abundance of clay minerals, particularly kaolin group minerals, in highly to completely decomposed rocks. Sesquioxides are sensitive to microenvironmetal conditions and can modify the distribution of clay minerals regardless of the degree of weathering of the rock.