HALLOYSITE /SMECTITE (H/Sm) MIXED-LAYER CLAYS

clays from volcanic ash soils at different stages of weathering differ in their relative halloysite content with respect to kaolinite and several surface properties, namely CEC, and exchange s lectivity for K +. These three parameters are related to each other in that hey all decrease with increasing soil weathering stage. XRD data show that he hydrated 1:1 layer-silicates in these clays combine with smectite oform interstratified H/Sm clay minerals. In these mixed-layers, the content and layer charge of smectitic units decrease as the relative halloysite content in the clay decreases. These clays thus depict aweathering sequence that is parallel to the weathering sequence of the soils from which they originate. It is also shown that he smectites in the H/Sm minerals have the distinctive composition a d ESR spectrum of Fe-rich 2:1 clay minerals belonging to the beidellite-nontronite series. The information btained explains why these clays have high CEC and distinct affinities for K +.It is hoped that his study will help to clarify the controversy concerning the CEC and related surface properties attributed tohydrated halloysite. As noted by Newman & Brown (1987), the cation exchange capacity (CEC) of halloysite is still open to debate. The occurrence in soils, tephra and saprolites of halloysites exhibiting a high charge contrasts with that of halloysites exhibiting CEC values more usual for 1: