ON KAOLINITIC AND MIXED MINERALOGY CLASSES OF SHRINK-SWELL SOILS

Spatially associated red (Typic Hapludalf) and black (Vertic Eutropept ) soils developed on the Deccan plateau in the Western Ghats of India were analysed for clay mineralogy and also physical properties relatin g to shrink-swell. This was done in order to examine a possible correl ation between shrink-swell phenomena and the content of expansible cla y minerals, and to reconcile the apparent incompatibility between such a correlation and the classification of some Vertisols into kaoliniti c, illitic, and mixed mineralogy classes. The fine clay mineralogy of the red soil was dominated by interstratified smectite/kaolinite with a little amount of smectite, but it had low cation exchange capacities and other associated non-vertic physical properties. Some of the smec tite was interlayered with chlorite. This red soil is grouped into the kaolinitic mineralogy class. The fine clay mineralogy of the black so il was dominated by a highly smectitic interstratified smectite/kaolin ite and also some smectite, which also shows some interlayering with c hlorite. This soil has vertic physical properties but has a mixed mine ralogy classification. The results suggest that there is an incompatib ility between marked shrink-swell characteristics and mineralogical cl assification of soils in Soil Taxonomy, in view of the fact that it is smectite content which governs the vertic character of soils.