ABSTRACT:The clay mineral composition is one of the factors that affect the physical properties of soils, and a knowledge of it is required to promote a fuller understanding of the origin of these properties. The relationships between the clay content and the plastic and liquid limits of natural montmorillonitic and kaolinitic soils and of artificial mixtures have been examined and compared. Factors affecting the relationships are discussed, and illustrated by the effect of particle aggregation on the measurement of the liquid limit of tropical red clays and on the sedimentation analysis of the Keuper Marl. The effect of muscovite and of silt-sized material on the position of soils on the Casagrande classification chart has also been examined.
Summary
The mineralogy of the processes leading to the formation of soils by the tropical weathering and leaching of rocks is reviewed, and illustrated by the results of an investigation into the formation, mineralogy and plasticity properties of some soils formed on basalt, granite, shale and sandstone in west Malaysia.
It is shown that hydrolysis followed by differential solution can account for the formation of the samples studied. Kaolinite was always present and generally the dominant clay mineral, although it gave place to gibbsite under severe weathering, and illite was also present in some of the less weathered soils on granite and shale. Quartz was prominent in soils on granite and sedimentary rocks, which are rich in this mineral. Iron oxide generally occurred, usually as goethite in the soils but as hematite in the nodules which were generally present in the soils selected for study, although gibbsite (hydrated aluminium oxide) was the main constituent of the nodules in one case. The observed mineralogical composition and clay content of the soils are consistent with the range of plasticity properties observed. The activity of some soils was reduced by the presence of gibbsite or goethite in the clay-sized fraction.
Clay mineral analyses of Caribbean soils illustrate the differences in composition that can arise from differences in topography and parent material, and emphasize the differences between the groups of proposed new engineering classifications of the soils based on these factors. Within one set of environmental conditions the clay mineral assemblage was reasonably constant. Examples are given of local differences in clay type which were correlated with variations in climate and mode of formation.
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