Influence of clay mineral composition on complex permittivity of clays and soils Heike Kaden Competence Center for Material Moisture (CMM), Karlsruhe Institute of Technology The title of my presentation is “Influence of clay mineral composition on complex permittivity of clays and soils”. My name is Heike Kaden and I work for the Competence Center for Material Moisture at Karlsruhe Institute of Technology.

Agenda Motivation and objectives Permittivity – theory Samples and excursus of clay mineralogy Summary and outlook At the beginning of my presentation, I would like to point out the motivation and objectives of my PhD studies. After that I will provide a short introduction to permittivity. Then I am going to introduce my own samples to you which will be accompanied by an excursus of clay mineralogy. At the end I will provide a summary and outlook.

Motivation and objectives w = f(εr) Permittivity (ε) in high MHz range (Geophysical Survey Systems Inc.,2001) material permittivity vacuum 1 air 1.00059 dry soil 3-6 dry clay 4 dry sand 4-6 wet soil 29 wet clay (non-swellable) 27 swellable clays 100 – 250* wet sands 15-25 water 81 bound water 3.6-3.8 Soil is a three-phase system consisting of solid, liquid, and gaseous components. the permittivities of the solid and gaseous phases differ strongly from the permittivity of water Hence, permittivity of porous materials mainly is a function of water content. But especially with highly swellable clays an anomaly in permittivity occurs. Even if it is a simple three-phase system (clay-water-air), the permittivity of the bulk material exceeds the permittivity of the individual components and can reach permittivity values of up to 250. TDR measures the propagation time of a signal (fast rising pulse) travelling along a sensor (wires). Because the relation of the propagation velocity in air to that in soil is directly related to the permittivity (c (in vacuum or air (approximately))/v (in medium) = sqrt(permittivity)). Via a calibration function the context to the volumetric water content is established. FDR uses single frequencies for the measurement. (Chen & Or 2006) * Low MHz range (Rayhatna & Sen, 1986)

Permittivity of clays and soils Frequency Temperature Density Water content Porosity Electrical conductivity Specific surface area Mineral composition X-ray diffractometry (XRD) X-ray fluorescence analysis (XRF) Simultaneous thermal analysis (STA) Cation exchange capacity (CEC) The permittivity of a porous material is influenced by some already known factors, which are frequency temperature density porosity specific surface area water content and electrical conductivity Furthermore I assume that permittivity is also influenced by mineralogical composition, especially when measuring permittivity of clay-water-mixtures. Therefore I additionaly determine the mineralogical properties of my materials by X-ray diffraction X-ray fluorescence analysis simultaneous thermal analysis cation exchange capacity and others.

Permittivity – theory Complex permittivity ε* Relative permittivity εr transmissibility of materials for electrical fields, complex number ε* = ε’ + jε’’ Relative permittivity εr polarizability of materials, dimensionless εr = ε* / ε0 Polarization mechanisms interfacial polarization orientation polarization ionic polarization electronic polarization (displacement polarization) Before I go further into detail on clay minerals, I would like to provide some background information about permittivity. Permittivity (also called dielectric conductivity and formerly called dielectric constant) characterizes the transmissibility of materials for electrical fields Permittivity is a complex number, hence ε* = ε’ + jε’’ (“Epsilon complex is the sum of epsilon prime and epsilon double prime.”) where ε’ = real part of permittivity (dispersion) ε’’ = imaginary part of permittivity (absorption), dielectric loss The permittivity of a material is often expressed as the relative permittivity εr The relative permittivity characterizes the polarizability of materials, this value is dimensionless There are different polarization mechanisms that contribute to permittivity. At low frequencies (kHz to low MHz, depending on material), interfacial polarization occurs and with increasing frequency it skips to orientation polarization, ionic polarization, and electronic polarization. Orientational polarization: orientation of dipols in the electrical field, Orientational polarization bulk water at 20 GHz and adsorbed water at 10 MHz Displacement polarization: displacement of positive and negative charge centers increasing frequency

Permittivity – theory εr’ = real part of relative permittivity (dispersion) εr’’ = imaginary part of relative permittivity (absorption), dielectric loss This illustration shows the permittivity of water at different frequencies with the corresponding polarization mechanisms. There is one curve for the real part of relative permittivity and one for the imaginary part. Especially at lower frequencies the dielectric loss is not only due to relaxation, but also caused by electrical conductivity. I measure in the frequency range of 100 MHz to 1100 MHz where no interference due to high electrical conductivity occurs and εr’ (epsilon prime) of pure water is relatively constant. That makes it possible to recognize the mineralogical contribution to permittivity. Relaxation: Due to molecular dipole rotation associated with reorientation (relaxation) under an electrical field working range Relative permittivity of water (Agilent Technologies ,2009)

Permittivity of multi-phase-mixtures mixing rules bulk water soil air ε : permittivity v: volumetric weight α = geometry factor Roth et al. 1990: α = 0.46 We have already seen that there is a large difference between the permittivities of air and soil particles on one hand and water on the other hand. The measured permittivity of the bulk material lies in between and can be evaluated to extract the volumetric water content of this multi-phase mixture. A lot of empirical mixing rules have been elaborated to calculate the water content from the permittivity of bulk material. This formula is only one example: it takes the permittivity of the individual components and the volumetric weight of each component into account. The exponent alpha is a geometry factor, which can range from -1 to +1. Roth et al. 1990 calculated the geometry factor α on the basis of 13 different soils (sandy, clayey, and organic soils) α is an empirical constant to take porosity into account α varies in literature between 0.3-1.0 The problem of the existing calibration curves for calculating the water content is the underestimation and overestimation, respectively of the volumetric water content in presence of highly swellable clays. εb = bulk permittivity α = geometry factor depends on the spatial arrangement of the 3-phase-mixture; assumption: α = +1  external field parallel to layers; α = -1  external field is perpendicular to layers

Samples and excursus clay mineralogy clay minerals 1:1 layer silicates 2:1 layer silicates swellable non-swellable smectites vermiculites talc-pyrophyllite group illites micas chlorites kaolinite serpentine Since I mainly focus on clays, I now would like to provide a short introduction to clay mineralogy. Clay minerals can be divided into 1:1 and 2:1 layer silicates. 1:1 layer silicates consist of SiO4-Tetrahedra + M(O, OH)6-octahedra, TO 2:1 layer silicates have a further tetrahedral sheet, TOT 1:1 layer silicates are mainly non-swellable, whereas 2:1 layer silicates can be divided into swellable and non-swellable clays. The ability of swelling depends on the layer charge. Those clays with no layer charge (talc-pyrophyllite) or high layer charge (illite and micas) are non-swellable. Clays with a medium layer charge are swellable. Smectites are structurally complex, which we will see on the next slide. swellable: medium negative layer charge (0.3-0.4 charges per FU) Non-swellable: no layer charge (0 charges/FU) or high layer charge (0,6-0,9 charges/FU (illite); 1 charges/FU (micas)) Jasmund & Lagaly (1993)

M+x+y(H2O)n(Al3+, Fe3+2-y Mg2+, Fe2+y) (Si 4-x Alx)O10(OH)2 Smectites oxygen hydroxyl groups water SiIV M+x+y(H2O)n(Al3+, Fe3+2-y Mg2+, Fe2+y) (Si 4-x Alx)O10(OH)2 montmorillonite, beidellite, nontronite AlIII for SiIV MgII for AlIII FeII/FeIII for MgII; AlIII AlIII Clays of the smectite group are negatively charged due to isomorphous substitutions. Substitution can take place in the tetrahedral or octahedral sheet. Depending on where the substitution took place and which ion was substituted, different minerals are formed, which are then named montmorillonite, beidellite, or nontronite, for instance. Interlayer cations like sodium, calcium, magnesium, potassium and ammonia neutralize the negative layer charge. AND those cations can be hydrated. substitutions in the octahedral sheets: Mg2+ instead of Al3+ ( most common substitutions in montmorillonites) consequence is negative layer charge which results in the swelling ability of smectites, Cations in interlayer: sodium, calcium, magnesium, potassium, ammonia, … Emmerich et al. (2009) and Wolters et al. (2009)

Interaction of smectites and water increasing relative humidity high cation exchange capacity 100-120 meq/100 g high specific surface area up to 750 m²/g water adsorption on inner surface and outer surface Hydration of cations in the interlayer leads to swelling of the clays. The swelling process is reversible and the amount of swelling depends on rel. humidity and the kind of cation in the interlayer. Thus, smectites are industrially important as they have some useful properties such as: High cation exchange capacity of about 100-120 meq/100 g and High specific surface area which can reach up to 750 m²/g These properties lead to a high water binding capacity as smectites can adsorb water on their inner and outer surface. Thus, they have great influence of water content of soils. Difficulties that arise with relative permittivity measurements in clay-water/fluid mixtures are due to different hydration states of the clays. Free water has a rel. permittivity of 80, but adsorbed water has a re. permittivity similar to that of ice (3.6-3.8) and there are several states in between that still have to be classified. high water binding capacity Jasmund & Lagaly (1993)

Cation Exchange Capacity (CEC) sample CEC [meq/100g] bentonite (EXM1912) 120 bentonite (Volclay) 84 bentonite (Calcigel) 63 illite (Arginotech GI) 25 ceramic kaoline (FW830) 4 limestone 3 finesand (N45) 2 silty quartz-feldspar-mixture (FS700) This table shows the Cation Exchange Capacity of my samples. The CEC describes the ability of a material to reversibly exchange cations. Bentonites are smectite containig clays and they have the highest CEC. Illite – the non-swellable 2:1 layer silicate has a CEC of 25 and kaoline – mainly consisting of 1:1 layer silicates has a CEC of 4. I expect a correlation of permittivity and CEC as the CEC influences the water binding capacity. The table also shows CEC of possible soil components that should have no CEC with respect to their composition, but show small CEC values that can be compared to kaoline.

Grain size of clays and soils silt clay This illustration shows the grain size distribution of my samples. The samples comprise one sandy soil, two silty soils, and five clayey soils. This shows that soils not only have different mineral compositions that have to be taken into consideration with permittivity measurements. In addition, one also always has to keep in mind the grain size effect when measuring permittivity. Another aim of my studies is to set up a new mixing rule, which pays attention to the grain size effect and some clay specific properties. This would make it possible to calculate the “grain size gap” within the diagram.

Summary and outlook Dielectric properties of soils can be used for volumetric water content measurements. Permittivity of bound water (3.6-3.8) is lower than permittivity of free water (≈ 81), which can lead to underestimation of water content in highly swellable clays as they adsorb large amounts of water. There are still phenomena to be explained: permittivity values of up to 250 in bentonite-water-mixtures Determination of moisture in soils with high amount of swellable clays requires special calibration of moisture-permittivity-relationship. . I would like to finish my presentation with a summary and outlook.

Thank you for your attention!

Literature Chen, Y. and Or, D., 2006: Geometrical factors and interfacial processes affecting complex dielectric permittivity of partially saturated porous media. Water Resources Research, 42: 1-9. Emmerich, K., Wolters, F., Kahr, G., and Lagaly, G., 2009: Clay profiling: The classification of montmorillonites. Clays and Clay Minerals, 57: 104-114. Jasmund, K. & Lagaly, G., 1993: Tonminerale und Tone. Steinkopff Verlag Darmstadt. 490 pp. Klein, K. and Wang, Y.-H., 2005: Towards a Better Understanding of the Electro-Magnetic Properties of Soils, IUTAM Symposium on Physicochemical and Electromechanical Interactions in Porous Media, 241-250. Raythatha, R. and Sen, P. N., 1986: Dielectric properties of clay suspensions in MHz to GHz range. Journal of Colloid and Interface Science, 109: 301-309. Roth, K., Schulin, R., Flühler, H., and Attinger, W., 1990: Calibration of Time Domain Reflectometry for Water Content Measurement Using a Composite Dielectric Approach. Water Resources Research, 26: 2267-2273. Wolters, F., Lagaly, G., Kahr, G., Nueesch, R., and Emmerich, K., 2009: A comprehensive characterization of dioctahedral smectites. Clays and Clay Minerals, 57: 115-133.