There has been limited research on clay mineral transformation in serpentinitic soils
under humid tropical conditions. In this study, four soil pedons were selected along
a toposequence from the summit (Entisol), shoulder (Vertisol), backslope (Alfisol)
to footslope (Ultisol) positions to explore the contributions and the significance of
landscape and weathering status of serpentinitic rock with regard to clay mineral
transformations in eastern Taiwan. Experimental results indicated that the large
amount of dithionite-citrate-bicarbonate-extractable Fe (Fed) and clay in the
subsurface horizon were mainly caused by the strong leaching potential from
intensive rainfall and weathering of the fine-grained parent rocks. The clay
mineralogy reflected the clear weathering trend of the soils along the toposequence:
(1) the soils on the summit and shoulder contained smectite and serpentine, which
are predominant in the young soils derived from serpentinitic rocks; and (2)
vermiculite gradually increased in the relatively old soils on backslope and footstope.
The mineralogical transformations observed along the toposequence indicated that
chlorite and serpentine, initially present in the Entisol on the summit, weather into
smectite and interstratified chlorite-vermiculite in the intermediate soil on the
shoulder under strong leaching and oxidizing conditions. Furthermore, vermiculite
formed as the major weathering product of chlorite and smectite in the soil
developed on the backslope. In addition to vermiculite, kaolinite and quartz formed
in the soils on the footslope with the greatest concentration of Fed along the
toposequence. Copyright © 2007, The Clay Minerals Society.