Kaolinite, halloysite, and iron oxide influence on physical behavior of formulated soils
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Soils of some tropical regions have low bulk density, high permeability, and resistance to erosion even under high rainfall. Soils with this combination of characteristics frequently contain halloysite and Fe oxides. We hypothesized that platy kaolinite particles form a more compact mass than tubular halloysite particles, and that Fe oxides promote porous aggregation. To test this hypothesis the settling densities were determined for soil analogs constructed of kaolinite and halloysite clays with ferrihydrite and nonionic, anionic, and cationic organic surfactants as organic matter proxies. The samples were mixed in water and compacted by centrifugation. The hydraulic conductivity of each kaolinite- and halloysite-based mixture was determined with a falling head permeameter. Halloysite samples had lower bulk densities than kaolinite samples with the same centrifugation. The addition of ferrihydrite decreased bulk density of sets of both mineral mixtures. Numerous small particles of ferrihydrite adhered to halloysite tubes in contrast to larger oxide masses formed among clean kaolinite plates, as shown in transmission electron micrographs. The greater frequency of ferrihydrite particles associated with halloysite than kaolinite crystals provided more sites for inter-particle bonding and clay aggregation. Cationic and neutral organic polymers decreased the bulk density of halloysite soil analogs. The kaolinite analog response to polymer treatment varied. Anionic polymers appeared to increase the bulk density of both mineral mixtures that is attributed to increased particle dispersion. These results support the hypothesis that clay morphology and bonding agents influence soil bulk density and hydraulic conductivity of fabricated clay soils.
