Weathering of Micaceous Minerals in Some Salt-Affected Soils

Mineralogy of some sodic soil profiles developed on alluvium was investigated both by X-ray and optical methods. The sand fractions showed a dominance of quartz and feldspars in association with white mica, biotite and other accessory minerals. Semiquantitative estimates of minerals in the silt fractions indicated illite as the dominant mineral followed by mixed layers, chlorite and vermiculite. Subsequent weathering produced vermiculite, smectite and two-component (10–14 Å) as well as three-component (10.14–14 Å) interstratified minerals in the clay fractions. A plausible mechanism involving K depletion together with Fe²⁺ oxidation and subsequent movement of Fe³⁺ from the octahedral positions under the seasonally arid environment appeared to be responsible for the transformation of biotite to smectite through the intermediate stages of (10–14) Å interstratification. The principle of ‘nucleation and silication’ has been invoked to explain the three-component interstratification.