dielectric permittivity of calcite and arid zone soils with carbonate minerals

Measurement of soil dielectric properties, epsilon are widely used to estimate water content in soils from remote sensing and from in situ soil sensors such as time domain reflectometry (TDR). The mineral permittivity also plays an important role in geochemical dissolution and precipitation. Models used to estimate water content from soils often assume a value of 5 for the mineral permittivity epsilon(s). However, calcite (CaCO3), a major constituent of some arid and semi-arid soils, has a permittivity of 8 to 9, nearly twice the permittivity of quartz (epsilon(s) = 4.6). We studied four soils, with micaceous mineralogy, but with two soils having approximately 40% calcite. We also measured the permittivity of Iceland Spar calcite (epsilon(s) = 9.1) and a microcrystalline calcite (epsilon(s) = 8.3), and use atomistic modeling to account for differences in permittivity based on the crystal density. We found permittivities for our soils to be in the range of 5.8 to 6.6, higher calcite contents resulting in increased permittivity. The estimated permittivity of the calcite in the soils was 7.4 to 7.9, lower than the highly crystalline samples. We estimate, for a soil with a porosity of 0.5 that assuming a permittivity of 5 instead of 6.6 will result in an overestimation of water content of about 1% at saturation. This demonstrates that a large quantity of pedogenic calcite (40%) in soil is unlikely to cause substantial error in the determination of water content using standard calibration equations. However, the lower dielectric permittivity predicted for pedogenic calcite may have consequences for the interpretation and understanding of geochemical processes.