Electrical conductivity spectra of smectites as influenced by saturating cation and Thumidity

Electrical conductivityis an important soil propertyrelated to salinity, and is often used for delineating other soil properties. The purpose of this studywas to examine the influence of smectite properties on the complex electrical conductivity spectra of hydrated smectitic clays. Four smectites were saturated with Ca, Mg, Na or K and equilibrated at four relative humidities ranging from 56 to 99%. X-ray diffraction was used to determine fractions of the various smectite layer hydrates (0 to 4 layers of interlayer water molecules) in each sample. A vector network analyzer was used to determine the real component of the complex electrical conductivityspectra for frequencies (f) ranging from 300 kHz to 3 GHz. Values of the dc electrical conductivity (0), the frequencywhere the slope changes in the spectra (fr), and the slope at the high-frequencyend of the spectra (n) were determined by fitting ʹ to ʹ (f) = 0(1 + f\fr)n. Both 0 and fr increased with the total amount of water, the amount of interlayer water, and, for saturating cations in the order K < Mg < Ca < Na. The opposite trends were observed for n. The values of these parameters were influenced by the type of smectite, but the trends were not consistent for the effect of layer charge. The results indicate that interlayer water in smectites contributes to the electrical conductivity of hydrated smectites, and that polarization of water by local electrical fields has a substantial influence on the complex electrical conductivityspectra of smectites. The accuracyof salinityestimates for soils and sediments that are based on conductivitymeasurements maybe adverselyaffected unless the effects of hydrated clays on electrical conductivity are considered.