Study on Kaolinitization of Soil Clays

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Samples from 40 profiles of upland soils and river sediments from Thailand were investigated for chemical and physical characteristics and mineralogy. The soils were generally strongly acid. The contents of exchangeable bases and organic matter were generally low. Soil texture varied widely, from sandy to clayey, with profiles depending on the type of parent materials. The clay content increased downward in each profile probably as a result of illuviation as well as loss of clays from the surface by run-off. The content of silt was generally very low as compared with the temperate soils, indicating its transient nature in the tropical soils.The potential contents of micro-nutrients, such as Zu, Cu, Mo, and Mn, were low in general as extracted by aqua regia.The sands and silts were entirely quartziferous, except for those from relatively young soils and from basaltic lave-derived soils. The former contained weatherable minerals such as feldspars in measurable quantities and the latter, hematite in addition to quartz which was considered as a inheritance from xenocrysts contaminated in the parent lava.The clays were almost kaolinitic, with kaolinite alone, kaolinite plus kaolinite/smectite mixed layers, or kaolinite plus halloysite and/or gibbsite. The kaolinite/smectite mineral was considered as a product of far-advanced stage of kaolinitization from smectite. The halloysite was considered as a product of transformation from kaolinite in a dry soil moisture regime. Most abundant of the accessory components were free iron oxides which consisted mostly of hematite with some goethite. Illite, vermiculite, and chlorite were recognized in certain amounts in most soils.The river sediments contained large amounts of vermiculite and illite, in addition to kaolinite. High contents of 2 : 1 type minerals in the lowland soils may be accounted for by incorporation of such materials by flooding.