Three Ultic Haplorthods with significant illuviation of clay and spodic materials in the subalpine forests located in the Alishan area of central Taiwan were selected to identify the clay mineral compositions by X-ray diffraction (XRD) and to explain the transformation of the clay minerals. The three pedons are dominated by vermiculite and vermiculite-illite interstratified minerals, and they have minor kaolinite, quartz and gibbsite. No hydroxy interlayered vermiculite (HIV) was found in the E horizon of the three pedons because the forest soils are very acidic and have very low Al contents. The presence of HIV in the spodic (Bhs) and argillic horizons (Bt) of the three pedons was associated with greater free Fe and Al contents (Fed and Ald), more favorable pH ranges, and coatings of organo-metallic complexes which prevented continuous weathering. The specific pedogenic process, clay illuviation and podzolization occurred sequentially in the Alishan area, and induced the unusual clay mineral distribution and transformation. The largest amounts of illite are in the C horizon and the amounts of vermiculite increased with decreasing soil depths. A reverse distribution between illite and vermiculite through the soil profile was observed. Illite was transformed to vermiculite due to the strong weathering environment associated with extremely low exchangeable K contents. The weathering sequence of clay minerals of Spodosols with fine textures in the study area is proposed as: illite → vermiculite (or interstratified vermiculite-illite minerals) → HIV and vermiculite.

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 footslope. 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.