Abstract
The argillic subsoil B-horizon in the calcareous alluvial plain of Pacabit-Madulao area contains soil clay materials dominated by smectite with occasional association of kaolinite. X-ray diffraction and chemical data demonstrated that smectite is Fe-montmorillonite which is characterized by mainly Ca 2+ of the interlayer cation reflecting the parent material of basic rocks. This clay contains more octahedral Fe 3+ than ordinary montmorillonite, exhibits no substitution of Al to Si in the tetrahedral layer and dehydroxylates below 600°C. The morphology showed both detrital and euhedral habits suggesting a neo-formed origin. Clay illuviation and redeposition of translocated clay materials are considered to be significant processes ultimately responsible for the accumulation of this nearly monomineralic clay assemblage. Large amounts of smectite produced initially by the weathering of basic rocks were transported in suspension in soil water and redeposited at deeper zones in the soil profile. The genesis of smectite speculated here is supported by the absence of mixed layer minerals or any transitory weathering products of the original detrital clay in the samples. The possibility of neo-formed smectite in this soil environment is also considered as several hints of authigenic clay formation were observed in some clay mineralogical examinations.
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