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 orderto 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 Fe-kaolinite. 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 Fe-kaolinite 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 WordsBeidellite Fe-kaolinite Interstratified Kaolinite-smectite Mixed-layer Pedogenic Smectite Soil Synthesis Transformation Tropical
Unable to display preview. Download preview PDF.
- Barnhisel, R.I. and Bertsch, P.M. (1989) Chlorites and hydroxy-interlayered vermiculite and smectite. Pp. 729–788 in: Minerals in Soil Environments (2nd edition) (J.B. Dixon and S.B. Weed, editors). Soil Science Society of America, Madison, Wisconsin, USA.Google Scholar
- Eswaran, H. and de Coninck, F. (1971) Clay mineral formations and transformations in basaltic soils in tropical environments. Pedologie, 21, 181–210.Google Scholar
- Hong, H., Churchman, G.J., Gu, Y., Yin, K., and Wang, C. (2012) Kaolinite-smectite mixed-layer clays in the Jiujiang red soils and their climate significance. Geoderma, 173-174, 75–83.Google Scholar
- Hughes, R.E., Moore, D.M., and Reynolds, R.C., Jr. (1993) The nature, detection and occurrence, and origin of kaolinite/smectite. Pp. 291–323 in: Kaolin Genesis and Utilization (H.H. Murray, W.M. Bundy and C.C. Harvey, editors). Special Publication No. 1, The Clay Minerals Society, Boulder, Colorado.Google Scholar
- Mackenzie, R.C. (1970) Simple phyllosilicates based on gibbsite- and brucite-like sheets. Pp. 498–537 in: Differential Thermal Analysis (R.C. Mackenzie, editor). Academic Press, London.Google Scholar
- Moore, D.M. and Reynolds, R.C. Jr. (1997) Identification of mixed-layer minerals. Pp. 261–297 in: X-ray Diffraction and the Identification and Analysis of Clay Minerals (D.M. Moore and R.C. Reynolds, editors). Oxford University Press, New York.Google Scholar
- NAP (National Academy Press) (1982) Ecological Aspects of Development in the Humid Tropics. National Academy Press, Washington D.C.Google Scholar
- Russell, J.D. and Fraser, A.R. (1994) Infrared methods. Pp. 11 —67 in: Clay Mineralogy: Spectroscopic and Chemical Determinative Methods (M.J. Wilson, editor). Chapman & Hall, London.Google Scholar
- Wilson, M.J. (1987) Soil smectites and related interstratified minerals: recent developments. Pp. 167–173 in: Proceedings of the International Clay Conference, Denver (L.G. Schultz., H. van Olphen and F. Mumpton, editors). The Clay Minerals Society, Colorado, USA.Google Scholar