The increasing exploration and exploitation of hydrocarbon resources hosted by oil and gas shales demands the correct measurement of certain properties of sedimentary rocks rich in organic matter (OM). Two essential properties of OM-rich shales, the total specific surface area (TSSA) and cation exchange capacity (CEC), are primarily controlled by the rock’s clay mineral content (i.e. the type and quantity). This paper presents the limitations of two commonly used methods of measuring bulk-rock TSSA and CEC, ethylene glycol monoethyl ether (EGME) retention and visible light spectrometry of Co(III)-hexamine, in OM-rich rocks. The limitations were investigated using a suite of OM-rich shales and mudstones that vary in origin, age, clay mineral content, and thermal maturity.

Ethylene glycol monoethyl ether reacted strongly with and was retained by natural OM, producing excess TSSA if calculated using commonly applied adsorption coefficients. Although the intensity of the reaction seems to depend on thermal maturity, OM in all the samples analyzed reacted with EGME to an extent that made TSSA values unreliable; therefore, EGME is not recommended for TSSA measurements on samples containing >3% OM.

Some evidence indicated that drying at ⩾200°C may influence bulk-rock CEC values by altering OM in early mature rocks. In light of this evidence, drying at 110°C is recommended as a more suitable pretreatment for CEC measurements in OM-rich shales. When using visible light spectrometry for CEC determination, leachable sample components contributed to the absorbance of the measured wavelength (470 nm), decreasing the calculated bulk rock CEC value. A test of sample-derived excess absorbance with zero-absorbance solutions (i.e. NaCl) and the introduction of corrections to the CEC calculation are recommended.

The term ‘montmorillonite’ encompasses a wide range of chemical compositions and structures. Comprehensive and reliable characterization is essential for unambiguous classification. Twenty eight purified, Na-exchanged smectites (<0.2 µm) were characterized by layer-charge measurement using the alkylammonium method, by cation exchange capacity (CEC) measurement with Cu-triethylenetetramine, by determination of the chemical composition using X-ray fluorescence analysis, by calculation of the structural formula following determination of the octahedral structure (trans-vacant vs. cis-vacant) by simultaneous thermal analysis, and by X-ray diffraction analysis. Mössbauer spectroscopy was applied to determine the oxidation state and coordination of Fe and changes thereof during purification of the source materials.

The charge derived from chemical composition was considerably greater (by up to 30%) than the measured layer charge. The independently measured layer charge was used to calculate the structural formula. The measured CEC values, comprising the permanent charges and the pH-dependent edge charges, were consistent with measured layer charge but not with layer charge derived from the chemical composition. Therefore, the structural formula of smectites should be calculated using the measured layer charge.

The dehydroxylation temperature, which conveys information about the structure of the octahedral sheet, was correlated to the amount of Mg and Fe3+ and the location of charges. No relationship was found among the dehydroxylation temperature and the mean layer charge or the Mg content. In contrast, a clear relationship was observed between the Fe content and the dehydroxylation temperature. Montmorillonites with an Fe content <0.3/f.u. are cis-vacant and those containing Fe3+ > 0.3 mol/f.u. are trans-vacant, mostly with additional cis-vacancies. Tetrahedral substitution also appeared to be a function of the number of trans-vacancies.

The parameters analyzed provide the basis for a new descriptive classification system.

The total specific surface area (TSSA) and smectitic layer charge (Qs) calculated from the structural formulae and unit-cell dimensions of 12 pure smectite samples were used as a reference in the design and evaluation of TSSA and Qs measurement techniques based on cation exchange capacity (CEC), H2O retention at 47% RH, and ethylene glycol monoethyl ether (EGME) retention. A thermogravimetric analysis-mass spectrometry (TGA-MS) technique was used to study the release of H2O from smectite on heating, and to introduce a correction for H2O remaining in the smectite after heating to 110°C, because the sample weight at this temperature has been used routinely as a reference in CEC and EGME sorption measurements. A temperature of 200°C was found to be the optimum reference for such measurements.

A good agreement between Qs from the structural formula and from CEC was obtained when this correction was applied. The TSSA of smectite was measured with similar accuracy (mean error of ±5–7%) by three techniques: (1) using mean H2O coverage; (2) using mean EGME coverage; and (3) using a combination of H2O coverage and CEC. A reduction of the mean error from 5–7% to 4% can be obtained by averaging these measurements, and a further reduction to 3% by introducing corrections for the dependence of H2O and EGME coverage on layer charge. The study demonstrates that Ca2+-smectite samples at 47% RH have H2O contents corresponding to 88–107% of the theoretical mass of a monolayer and offers an explanation of this variation.

The Gourougou volcanic massif (northeastern Morocco) is actively prospected for bentonite deposits. Five bentonites originating from different environments were selected for the present study: hydrothermal alteration of obsidian perlite glass inside the volcanoes themselves (Providencia and Tribia deposits); alteration of pyroclastic flows in a marine shallow water to lagoonal lacustrine environment (Ibourhardayn deposit); ash falls in marine or lacustrine systems (Moulay Rachid and Melg el Ouidan (formerly Camp Berteau) deposits). All of these bentonites were probably formed from volcanic glass originating from a rhyolitic magma at different stages of differentiation as shown by slight variations of REE and incompatible element abundances. The crystal-chemical characteristics of the smectite vary according to alteration conditions: beidellite predominates in hydrothermal systems, whereas montmorillonite predominates in lagoonal and lacustrine environments, and mixed-layer beidellite-montmorillonite in the sea-water-altered pyroclastic flows. All these dioctahedral smectites exhibit a heterogeneous distribution of charge as shown by the presence of partially expandable (1 EG) or non- expandable (0 EG) layers in the K-saturation state. The proportion of the collapsed or partially expandable layers is not related to the average layer charge or to the cation exchange capacity. This militates for an overall heterogeneous charge distribution. Compared to other natural or experimental alteration systems of similar rhyolitic glass, the formation of beidellite or montmorillonite appears to be controlled by the amounts of Mg in the system.

The nature of component layers of mixed-layer illite-smectite and their possible evolution in the course of illitization have been debated since the 1960s. The present study is a new attempt to solve these problems, using samples collected from diverse geological formations around the world. Twenty three purified illite-smectites from bentonites and hydrothermal rocks, covering the complete range of expandability, were analyzed chemically, including ${\text{NH}}_4^{+}$\$\end{document} determination, and their structural formulae were calculated. The exchangeable cations (EXCH) were plotted vs. the fixed cations (FIX) yielding the following experimental regression:$\text{EXCH}=-0.43\times \text{FIX}+0.41\left({\text{R}}^2=0.98\right)$\$\end{document}

FIX and EXCH depend on the charge of the illite (Qi) and smectite (Qs) interlayers, and the fractions of these interlayers in the bulk clay leading to: $\text{EXCH}=-Q_{\text{s}}\text{/}{Q}_{\text{i}}\times \text{FIX}+Q_{\text{s}}$\$\end{document}

Analysis of these relations and independent measurements of the total specific surface area (TSSA) indicate that the layer charges of both types do not change in the course of illitization. The smectite layer charge is equal to 0.41 and the illite layer charge is equal to 0.95 per O10(OH)2. End-member illite has a well defined composition that is close to intermediate between muscovite and phengite, with fixed cations content greater than that specified in the AIPEA classification of layer silicates:${\text{FIX}}_{0.95}\left({\text{Si}}_{3.25}{\text{Al}}_{0.75}\right)\left({\text{Al}}_{1.81}{\text{Fe}}_{0.01}{\text{Mg}}_{0.19}\right){\text{O}}_{10}{\left(\text{OH}\right)}_2$\$\end{document}

The established relationship allows the calculation of the mean number (N) of 2:1 layers in all fundamental particles and also the fraction of smectitic layers (fs) from FIX:$N=Q_{\text{i}}\text{/}(Q_{\text{i}}-\text{FIX})f_{\text{s}}=(Q_{\text{i}}-\text{FIX})\text{/}{Q}_{\text{i}}$\$\end{document}

N and fs can be used to calculate TSSA, and all three parameters can also be calculated from cation exchange capacity and from X-ray diffraction peak positions, utilizing the regressions established here.

Bentonites are candidate materials for encapsulation of radioactive waste. The cation exchange capacity (CEC) has proved to be one of the most sensitive parameters for detecting changes of mineral properties such as swelling capacity and illitization in alteration experiments. Whether measured differences in CEC values of bentonite buffer samples before and after an experiment are (1) actual differences caused by clay structural changes such as illitization or (2) simply data scatter due to the different methods used by international research teams is an open question. The aim of this study was to measure the CEC of clay samples in five different laboratories using the same method and to evaluate the precision of the values measured. The Cu-trien method and four reference materials of the Alternative Buffer Material (ABM) test project in Äspö, Sweden, were chosen for this interlaboratory study. The precision of the Cu-trien method, which uses visible spectroscopy, was very good with a standard deviation of ±0.7–2.1 meq/100 g for CECs that ranged from 11 to 87 meq/100 g. For the same CEC range, analysis of Cu-trien index cations using inductively coupled plasma (mass spectrometry) and atomic absorption spectroscopy were less precise with a standard deviation of ±2.8–3.9 meq/100 g. Based on the measured precision, greater measured differences in Cu-trien CEC and exchangeable cation values of bentonite buffer samples, before and after an experiment, might be actual differences. Great care must be taken when interpreting measured CEC differences, and analytical characterization of any structural changes may be needed. Compared with results from the ‘International Soil-Analytical Exchange’ (iSE) program for soils, most absolute concentrations were much larger for the clays studied; however, for the two parameters exchangeable Ca2+ and CEC the range was similar to the iSE ring test and, most importantly, the precision was comparable. Future studies should discuss the accuracy of CEC and exchangeable cation values and compare them to alternative CEC methods in which care is taken to prevent dissolution of soluble minerals, such as calcite and gypsum.

Smectites are considered to be an important component in backfill barriers due to their marked swelling and high cation exchange capacity. Both properties are affected considerably when these clays transform under natural conditions. However, we have recently described a chemical interaction between high-activity radionuclide simulators and smectites which could prove to be effective at immobilizing radionuclides definitively. Investigating the efficiency of this mechanism, independent of bentonite ageing, is a challenge. For this purpose, the reactivity shown by a non-expandable layered aluminosilicate, muscovite, has been compared to that shown by an expandable one, beidellite. Both samples were treated hydrothermally with a solution of lutetium nitrate, and the transformations were studied by X-ray diffraction, nuclear magnetic resonance and scanning electron microscopy/energy dispersive X-ray analysis. Lutetium cations react with the silicon framework of both 2:1 layered aluminosilicates under hydrothermal conditions, and new phases, lutetium disilicate, kaolinite, boehmite and natrosilite are generated. The results demonstrate that the efficiency of the chemical mechanism is not determined by the swelling and the cation exchange capacity of 2:1 layered aluminosilicates. Thus, the rare earth disilicate formation might account for the success of the clay barrier, once bentonite has lost its swelling and cation exchange capacity.

Illite from Muloorina, just south of Lake Eyre in South Australia, is commonly used as a standard ferric iron-bearing illite which has a cation exchange capacity (CEC) of ~22 meq/100 g, too high a value for this clay to conform to the most recent conclusions about illite composition. The objective of the present study was to reassess the Muloorina illite to determine the reason for this high CEC value. The composition determined by X-ray fluorescence analysis calculated to a cation charge of +22 is K0.68(Mg0.39Al1.02Fe0.543+Fe0.042+)[Si3.59Al0.38Fe0.033+]O10(OH)2. X-ray diffraction of glycolated and heated K-saturated Muloorina illite revealed no evidence of expandable layers. The cell dimensions are a = 5.314(1)Å, b = 9.040(1)Å, c = 10.135(3)Å, and b = 100.97(3)° (3σ in parentheses). Transmission electron microscopy revealed that Muloorina illite has a remarkably fine and uniform particle size in the form of irregularly stepped hexagonal crystals, averaging 60 nm in diameter at their widest, and 35 nm thick, each step being ~7 nm high. At most steps and between many steps defects occur where a 2:1 layer terminates, with a wide interlayer spacing beyond, commonly reaching to the crystal edge. Even though uninterrupted stacking of >10 or so 2:1 layers across one crystal is uncommon, the irregular distribution of the dislocations leaves appreciable structural continuity so that the mean diffracting thickness is of the order of 16 nm. Muloorina illite is not an interstratified illite-smectite, but a mica mineral with low-charge regions associated with terminating 2:1 layer defects. Such regions are wedge shaped at the nm scale and stabilized by the bonding of the illitic remainder of that interlayer. Approximately 17% of the total volume of each Muloorina illite crystallite is occupied by such low-charge 2:1 layers.

The cation exchange capacity (CEC) is one of the most important properties of clays in terms of their performance in both natural and technical processes. For decades, common methods for determining exchangeable cations have failed when calcareous clays or soils were examined, because calcite is at least partly dissolved throughout the exchange experiment which in turn increases measureable Ca2+ concentrations. As a result, exchangeable Ca2+ and the sum of exchangeable cations by far exceed the amount of negative charges. In the past, the silver-thiourea method (AgTU) has been modified to overcome this problem (AgTUcalcite), but is unsatisfactory as the method is laborious. In the present study three new methods based on two alternative metal complexes, cobalt(III) hexamine (CoHex) and copper(II) triethylenetetramine (Cu-trien), are proposed. The optimum solid/liquid ratios of these methods and the optimum complex concentration of Cu-trien are reported, depending on the mineralogical composition of the samples. The key development is that the exchange solutions are saturated with respect to calcite prior to the experiment. Approximately 70–90% of the dissolution of calcite present as an admixture in a clay sample is suppressed in the subsequent cation exchange experiment, but not all. The Ca2+ exchange is not suppressed and there is no evidence for any precipitation of this Ca2+. Three possibilities for how to handle this problem are discussed, one of which is to perform no further correction. The resulting error arises from the remaining calcite solubility of the different solutions after pre-treatment with calcite. This corresponds to errors of 0.2–1.3 (CoHexcalcite)and 0.7–8.4 (Cu-triencalcite) meq/100 gCa2+ for samples with small and large CEC values, respectively. As a consequence of the poor performance of Cu-triencalcite for samples with large CEC, a more concentrated Cu-trien5 × calcite solution was developed which performed much better: 0.1–0.8 meq/100 g(Cu-trien5 × calcite). For Cu-trien5 × calcite and CoHexcalcite at least, the errors are in the range of the non-systematic scattering for exchangeable Ca2+ determination. Therefore, the methods suggested provide ‘operationally correct’ Ca2+ values without additional effort. Moreover, owing to the high selectivity of the index cations applied in the present study, only one exchange step is required, providing a significant advantage over the AgTUcalcite method.

This study uses the data from Miocene rocks of the Carpathian Foredeep to test the performance of the computer programs QUANTA and BESTMIN in aiding the interpretation of geophysical log data. These programs were designed to help trace trends in the mineral composition of rocks, the chemical composition of minerals, and the effects of these data on petrophysical and geophysical logging parameters. Chemical and X-ray diffraction data for 65 samples of shales, sandstones, and carbonates taken from cored wells in the molasse basin of the Carpathian Foredeep were processed. Compositional differences were detected between rocks sourced from the platform and rocks sourced from the Carpathians. Quartz, K-feldspar, and zircon were more abundant in the coarse-grained rocks (sandstones), while calcite, ankerite, siderite, pyrite, barite, halite, celestite, apatite, anatase, chlorite, 2:1 minerals, and organic matter were more abundant in the fine-grained rocks (shales). Plagioclase reached its maximum in coarse shales. Ankerite, chlorite, and dioctahedral 2:1 minerals had more Fe in the coarse-grained rocks. The dioctahedral 2:1 minerals in fine-grained rocks had a greater concentration of smectitic layers. This information permitted the precise calculation of grain density, porosity, adsorbed water, and some geophysical logging parameters. It also permitted the calibration of well-log response, in particular, the macroscopic neutron absorption cross-section (Σa) combined with the photoelectric absorption factor (P{e}) or with Pe + Ca (calcium content, measurable in wells by spectroscopic techniques) with porosity and cation exchange capacity (CEC). The NaCl concentration in the pore waters was found to range from the values typical for seawater in shales to the freshwater level in clean sandstones.

Bentonites are candidate materials for encapsulation of radioactive waste. The cation exchange capacity (CEC) has proven to be one of the most sensitive parameters for detecting changes in mineral properties in bentonite-alteration experiments. An interlaboratory study of CECs and exchangeable cations for three reference bentonite buffer materials that were used in the Alternative Buffer Material test (ABM) project in Äspö, Sweden, was conducted to create a suitable database. The present study focused on CEC accuracy and compared CEC methods where care was taken to prevent dissolution of soluble minerals such as calcite and gypsum. The overall quality of the CEC and exchangeable cation values measured using non-Cu-trien CEC methods were good, with CECs of 74–91±0.5–3.3 meq/100 g and exchangeable cation values of 22–61±1.2–3.9 meq/100 g Na+, 7–23±0.8–1.5 meq/100 g Mg2+, and 19–39±0.8–1.6 meq/ 100 g Ca2+. The precision was comparable to the standard Cu-trien method even for exchangeable Ca2+, although the laboratories used different solution/solid ratios and reaction-time parameters for Cu-trien which affect carbonate dissolution. The MX80 and Dep.CAN bentonite exchangeable Ca2+ values were more accurate than standard-Cu-trien values. Using the optimized methods of this study, MX80 and Dep.CAN exchangeable Ca2+ values averaged 20.2±1.6 and 38.8±1.4 meq/100 g which amounts to ~70% of the inflated Cu-trien values. A more complex analysis of the CEC data using different methods, anion analyses, and mineralogical analysis is necessary to obtain plausible and accurate CEC values. Even with a more complicated analytical procedure, the CEC and exchangeable cation values were still not accurate enough because of excess anions. Chloride, sulfate, and dolomite might have increased the exchangeable Na+, Mg2+, and Ca2+ values.

The aim of the present study was to find methodological tools to obtain reasonable results for exchangable Ca2+ of gypsiferous bentonites. Cation exchange capacity (CEC) is an important property of clays. Numerous methods for calculating CEC and exchangeable cations exist; determination of exchangeable Ca2+ fails, however, when gypsiferous clays are examined because gypsum is dissolved throughout the exchange experiment, which in turn increases measureable Ca2+ concentrations. Several new methods (AgTUcalcite, CoHexcalcite, and Cu-trien5×calcite) have been developed to overcome a similar problem occurring with calcite by using exchange solutions saturated with respect to calcite prior to the experiment. In the present study these three solutions were also pre-treated with gypsum and labeled AgTUCcGp, CoHexCcGp, and Cu-trien5×CcGp. The special solutions were applied first to a gypsum- and calcite-free bentonite with known reference values for exchangeable Ca2+. The resulting exchangeable Ca2+ values obtained did not match with reference values. The solutions were then applied to natural calcareous and gypsiferous bentonites but only the proposed AgTUCcGp test method was successful. The performance of AgTUCcGp was relatively poor when applied to calcareous non-gypsiferous bentonites, the third group of test materials. Reasonable values for exchangeable Ca2+ of gypsiferous clays were obtained using a combination of two separate results: (1) calcite saturation of exchange solution (e.g. Cu-trien5×calcite) and (2) quantification of gypsum with suitable mineralogical methods. Result 1 eliminates errors caused by calcite dissolution though it is still incorrect because it contains significant amounts of Ca2+ from gypsum dissolution. After proving that gypsum was completely dissolved during the exchange experiment, result 2 was used to subtract the theoretical Ca2+ portion of gypsum from result 1. The initial concentration of gypsum of the samples studied was <1 wt.%, typical of many commercial bentonites. Using this combined procedure the sum of exchangeable cations is very close to the CEC, though it still exceeds the CEC by, on average, 3%, which is a satisfactory improvement. The resulting exchangeable Ca2+ values can be considered as operationally correct using this approach. Ca2+ saturation (Ca/CEC in %) of seven gypsiferous bentonites ranges from 1 to 69%.

Bentonites are candidate materials for encapsulating radioactive waste within barrier systems in crystalline rocks. In the ‘Alternative Buffer Material’ (ABM) test in the hard rock laboratory in Äspö, Sweden, six packages of eleven different buffer materials (mainly bentonites) with various exchangeable cation populations were packed vertically with an iron tube used as a heater in the center. After installation, the second ‘ABM package’ (ABM-II) was first allowed to saturate with water for approximately 2.5 years. The blocks were then exposed to a temperature of up to 141°C for approximately 3–4 years. The hypotheses for the present study were: (1) no horizontal gradient of the cation exchange population was present in the individual blocks of ABM-II because ABM-II had a longer reaction time in comparison to the ABM-I package, which did not have horizontal gradients; (2) the exchangeable cation Ca2+:Na+:Mg2+ ratio was equal in all blocks of ABM-II and was independent of block position in the package. As expected from ABM-I, all blocks in the ABM-II experiment showed large differences between the measured values of the reference materials and the reacted samples. The exchangeable Na+ and Mg2+ values in ABM-II were reduced by up to 55% to 59% in comparison to the reference material. Contrary to the first hypothesis, horizontal gradients were observed in ABM-II; and, contrary to the second hypothesis, the exchangeable cation ratios differed markedly in the different reacted buffer materials. The largest total Na+ loss was observed in the middle part (-67%), whereas Mg2+ values decreased by 79% in the upper part. The exchangeable Ca2+ values increased strongly in ABM-II, particularly in the upper part. The most useful parameter to distinguish between ion exchange equilibria of ABM-I and ABM-II was the Na+/Mg2+ ratio. This ratio was constant in ABM-I (3.0) and had a similar ratio (3.5) in the lower part of ABM-II; however, the ratio strongly increased (5–10) in the upper part of the ABM-II package. The large Na+/Mg2+ ratios in the upper part of ABM-II could possibly be explained by water loss into the rock (caused by a pressure drop and boiling) and subsequent water uptake.

Bentonites are candidate materials for high-level radioactive waste (HLRW) repositories and, therefore, are investigated with respect to long-term stability. In order to identify possible bentonite alteration processes, long-term in situ tests are conducted in rock laboratories. The prototype repository in situ experiment (PR) is one of the best examples of this kind of test due to the size of the installation as well as the duration. In the present study, chemical and mineralogical alteration processes of the bentonite MX 80 after an 8 y heating period were investigated. The water content of all samples increased following inflowing Na-Ca-Cl-type granitic groundwater causing cation exchange in the bentonite buffer materials. Exchangeable magnesium was desorbed in the buffer and MgO concentration increased at the bentonite-Cu canister interface; the Mg sink could not be detected, however. CaO also accumulated at this interface mainly as Ca carbonate and Ca sulfate. Cu corrosion products were identified at the bentonite-canister interface by chemical analysis, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX), and differential thermal analysis. Up to 0.5 mm into the bentonites Cu could be detected by SEM-EDX. No cristobalite dissolution was observed in contrast to other in situ tests in which iron heaters were used. The corrosion products and the lubricant which was added during manufacturing of the bentonite blocks were mixed with the bentonite at the bentonite-canister interface. A quantitative measure of that mixture was the decrease in the cation exchange capacity (CEC). The CEC also reduced in all other samples, however, compared to the CECs of the reference samples, particularly in the warmer deposition hole 5 compared to the colder deposition hole 6. Overall, the PR in situ experiment proved that cation exchange reactions occurred in full-scale bentonite buffer experiments in all bentonite blocks but structural degradation of smectite could not be identified.

Knowledge of mineral composition is helpful in the interpretation of well-logging data, where the hydrocarbon saturation is expected as the final result.It is calculated from the electric resistivity, cation exchange capacity (CEC) and porosity.Porosity is calculated from the bulk density, which is measured directly in the well, and the matrix (mineralogical, grain) density, which can be calculated precisely only when the mineral composition is known.

In the shale-sand formations, which are the subject of this study, the rock matrix can conduct electric current.The Waxman–Smits formula is applied to calculate the water saturation of such rocks.This formula combines rock porosity, water saturation, water resistivity and the CEC with the total rock resistivity measured in the well.CEC is introduced to this formula as the parameter determining the shale ability to conduct electricity.Typically, CEC is controlled by the smectite content of the rock.

In order to evaluate the input data needed in the log interpretation the extended mineralogical (quantitative mineral composition, CEC) and chemical (major, trace and REE) analyses were made for over 200 core samples obtained from four gas fields in the Miocene formation of the Carpathian Foredeep.The most important clay component: the sum of mixed-layer illite-smectite and discrete illite has been quantified.The detailed recognition of clay minerals allowed for constraining the density-neutron cross-plot, which is a well log data interpretation technique for the determination of both porosity and shale content in shale-sand formations.

The statistical multivariate analysis of all data helped to set up a comprehensive petrophysical model.A reliable correlation (r2 = 0.96) of the thermal neutron absorption cross-section (Σama) and the total natural radioactivity (GR) with CEC was established.Such good correlation allows for a continuous on-line CEC determination and therefore a reliable application of the Waxman-Smits water saturation model.Experimental equations with similar level of correlation were established for quartz, clay and carbonate contents, allowing the calculation of matrix density, required in the porosity calculation.

The high correlation of Σama with CEC reflects the presence of boron in the illite-smectite clays.

The Cu-triethylenetetramine-complex (Cutrien) is one of the commonly used index cations for CEC determination in clay science. Cutrien-exchanged smectites show basal spacings between 13.0 and 13.5 Å after correction for the Lorentz and polarization factors. The full width at half maximum (FWHM) of the d001 reflection is today related to the percentage of tetrahedral charge (beidellitic character) and/or to the Fe content of the smectites. The structural Fe content and the tetrahedral charge correlate, so their individual influence on d001 cannot be resolved. Nevertheless, the FWHM of Cutrien smectites should depend on the charge distribution rather than the Fe content.

X-ray diffraction (XRD) and water uptake capacity measurements showed that the interlayer of Cutrien-exchanged smectites does not swell any more, but can take up a few water molecules. Accordingly, the water uptake capacity of the external surface area can be determined independently from the interlayer water uptake capacity. Adjusting the pH of Cutrien-bentonite dispersion to different values allows for the determination of the variable charge.

In conclusion, Cutrien exchange of smectites appears to be suitable for the study of external surfaces area related phenomena (e.g. edge adsorption processes) without any influence of the interlayer region.

Basalts and related magmatic rocks such as diabase are frequently used as raw materials in the building and construction industry as dimension stones or as aggregates, where they may form the sub-bases of roads or railway embankments. A major characteristic controlling the quality of the raw materials is the rock strength, which can be affected by the presence of swellable clay minerals (smectites). Increasing smectite content is generally considered to increasingly affect rock integrity. Industrial practice, in which the smectite content is monitored via cation exchange capacity (CEC) methods, on the other hand, showed that both mechanically stable smectite-rich and unstable smectite-poor materials exist. We therefore studied the CEC–strength relation based on twelve samples from a German diabase quarry.

As could be expected, the sample with the highest CEC was the least stable. However, the comparison of the rock strength with the CEC of the other samples was less clear. Comparing the CEC of powders and of the 1–2 mm fraction of the same sample provided additional information. In the case of samples which are relatively stable (in spite of high CEC values), the CEC values of the powders were significantly higher than those of the 1–2 mm fraction. Samples with less rock strength, on the other hand, showed equal CEC values of the powder and the 1–2 mm fraction. This difference may be explained by different accessibilities of the smectites for CEC exchange solutions during typical experiments.

In conclusion, the application of the CEC method for quality control considering both powdered and 1–2 mm samples allows a more precise estimation of the rock strength to be made, particularly for the comparison of different materials. Although measurement of two CEC values (after both sieving and grinding) would complicate quality control, the present study indicates that valuable additional information, i.e. semi-quantitative information on the accessibility of smectites, is gained.

Three clay minerals from southern and central Tunisia were characterized. Both XRD quantitative analysis and a chemical method were used to determine the mineralogical and physico-chemical characteristics of the <2 μm clay fractions. The XRD, CEC and specific surface measurement analyses showed that the dominant phase in all samples is illite-smectite with kaolinite and traces of quartz also present. From quantitative XRD, the abundances of the minerals identified are consistent with the measured CEC, specific surface area and TG/DTA properties. Analysis by XRD also showed that the illite-smectite phases are composed of thin stacks (two to a maximum of ten layers per stack) of random illite-smectite and ordered (R1) illite-smectite. There is also some discrete illite.