We determined the complex permittivity and clay mineralogy of grain-size fractions in a wet silt soil. We used one clay-size fraction and three silt-size fractions, measured permittivity with low error from 25MHzto24GHz with time-domain spectroscopy, and estimated mineral weight percentages using X-ray diffraction (XRD). The volumetric water contents were near 30%, and the temperature was 25°C. For the whole soil, standard fractionation procedures yielded 2.4% clay-size particles by weight, but XRD showed that the phyllosilicate clay minerals kaolinite, illite, and smectite made up 17% and were significantly present in all fractions. Above approximately 500MHz, all real parts were similar. Below approximately 500MHz, the real and imaginary permittivities increased with decreasing grain size as frequency decreased, and the imaginary parts became dominated by direct-current conduction. Similarly, below approximately 500MHz, the measured permittivity of montmorillonite, a common smectite, dominated that of the other clay minerals. Total clay mineral and smectite mass fractions consistently increased with decreasing grain size. Below 2GHz, a model with progressively increasing amounts of water and parameters characteristic of montmorillonite matches the data well for all fractions, predicts permittivities characteristic of free water in smectite structural galleries, and shows that the similar real parts above 500MHz are caused by a small suppression of the high-frequency static value of water permittivity by the smectite. We conclude that the clay mineral content, particularly smectite, appears to be responsible for permittivity variations between grain-size fractions. Small model mismatches in real permittivity near the low-frequency end and the greater fractions of kaolinite and illite suggest that the total clay mineral content might have been important for the coarser fractions.

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