Reaction Pathways of Clay Minerals in Tropical Soils: Insights from Kaolinite-smectite Synthesis Experiments

Pedogenic smectite from a young (Holocene) tropical soil was reacted in Al-rich solution at 150oC for a range of reaction times (3 to 120 days) in order to study mechanisms and rates associated with the transformation of smectite to kaolinite via interstratified kaolinite-smectite (K-S). As has been observed in tropical soils, the overall reaction rate is logarithmic, with rapid initial transformation of smectite to K-S with ~50% smectite layers, followed by progressively slower transformation of intermediate K-S to kaolinite-rich K-S and eventually Fe-kaolinite. Sub-micron hexagonal non-Fe-bearing kaolinite forms in the final stage (after 120 days) as a minor mineral in an assemblage dominated by Fekaolinite. The pedogenic smectite used as starting material consisted of two end-members, Fe-beidellite and Al-smectite, enabling comparison of reaction pathways. Fe-beidellite transforms to K-S or Fekaolinite within 3 days, whereas Al-smectite transforms much more slowly, appearing to reach a maximum rate in intermediate stages. This difference is probably due to hydrolysis of relatively weak Mg O and Fe O bonds (relative to Al O bonds) in Fe-beidellite octahedral sheets, which drives rapid reaction, whereas the driving force behind transformation of Al-smectite is more likely to be related to stripping of tetrahedral sheets which reaches its maximum rate at intermediate stages. Multiple analytical approaches have indicated that Al is rapidly fixed from solution into smectite interlayers within K-S, and that K-S and Fe-kaolinite inherit octahedral Fe and Mg from precursor smectite; as the reaction progresses, octahedral sheets become progressively more Al-rich and Fe and Mg are lost to solution. These results demonstrate that: (1) early-formed pedogenic smectite in tropical soils is expected to transform to kaolinite via interstratified K-S; (2) K-S has a strong potential to sequester plant-toxic Al in tropical soil; and (3) the presence in tropical soils of Fe-kaolinites with relatively large cation exchange capacities may be related to inheritance of octahedral sheets from precursor smectite and K-S. Key Words—Beidellite, Fe-kaolinite, Interstratified, Kaolinite-smectite, Mixed-layer, Pedogenic, Smectite, Soil, Synthesis, Transformation, Tropical.