Submitted to: Meeting Abstract
Publication Type: Abstract only
Publication Acceptance Date: 3/3/2005
Publication Date: 5/1/2005
Citation: Lebron, I., Robinson, D.A., Goldberg, S.R. 2005. The effect of calcite crystallinity on the dielectric permittivity. EOS Trans. AGU, 86(18), Joint Assem. Suppl. Abstract No. H13B-01. Interpretive Summary:
Technical Abstract: Measurement of soil dielectric properties, epsilon are widely used to estimate water content in soils from remote sensing data and from in situ soil sensors such as time domain reflectometry. Investigations have also explored the use of dielectric measurement to estimate other soil properties such as cation exchange capacity, soil aggregation, and particle size analysis. Understanding the dielectric properties of soils is also required for the improved interpretation of data from methods such as ground penetrating radar and microwave remote sensing. 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. However, calcite (CaCO3), a major constituent of some arid and semi-arid zone soils, has a permittivity of 8-9, nearly twice the permittivity of quartz (epsilon=4.6). However, tabulated permittivity values for minerals generally refer to pure crystalline samples, crystals formed by crystal growth in the absence of impurities and without isomorphic substitutions; this is not the most likely scenario for pedogenic calcite. We studied 4 soils, with micaceous mineralogy, but with two soils having ' 40% pedogenic calcite. We also measured the permittivity of Iceland Spar calcite (epsilon=9.1) and a microcrystalline calcite (epsilon=8.3), and used atomistic modeling to account for differences in permittivity based on the crystal density. We found dielectric 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 7.4-7.9, 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.