Argon physisorption at 87 K is the new standard for texture analysis of microporous media recommended by the International Union of Pure and Applied Chemistry (IUPAC). However, geoscientists routinely use nitrogen (77 K) and carbon dioxide (273 K), both molecules with permanent polarization and the preference to interact with specific surface sites. In this work, N2, CO2, and Ar physisorption isotherms were measured and classical physisorption theories applied to investigate the suitability of Ar physisorption for the porosity assessment of mudrocks, clays, and (non)-porous analogs.

N2 and Ar physisorption isotherms are qualitatively similar with the most significant discrepancies in the submonolayer range. Textural parameters reveal linear relations but parameter ratios vary randomly, independent of the sorbent class. While N2 and CO2 (mostly) underestimate micropore volumes, nitrogen BET areas are consistently larger than argon BET areas. Those differences are probably associated with differences in polarization. But its effect on molecular orientation, for example, is presumably masked by microporosity and a narrow spacing of specific surface sites.

Mesopore size distributions and Gurvich (total) pore volumes agree well for N2 and Ar indicating similar pore size and pore volume access. Combining both parameters proves effective in identifying saturation pressure offsets which pose the largest uncertainty factor in the present study. Ar-based micropore size distributions reveal three distinct classes of mudrocks differing in organic matter maturity, and its contribution to microporosity. Empirical αs plots corroborate this classification underlining the discrepancies in the micropore range of mudrocks. Comparative hysteresis loop analysis indicated cavitation as one dominant evaporation mechanism in mudrocks and clays effecting a sample-specific compartmentalization of their pore networks.

Greigites from a gley and a Tertiary sediment were investigated by X-ray diffraction and Mössbauer spectroscopy. The cell-edge length a of 9.8639 Å ± 0.0003 Å for the soil greigite was significantly smaller than that of the sedimentary greigite (9.8737 Å ± 0.0004 Å), but both cell-edge lengths were smaller than the value given on JCPDS card #16-713 (9.876 Å). Both greigites had 440 as strongest peak rather than 311 (as indicated on JCPDS card #16-713), but the other relative intensities did not deviate from the values given on this card within experimental error. Mean X-ray diffraction coherence lengths of 23 ± 2 nm for the soil greigite and of 60 ± 5 nm for the sedimentary greigite suggest superparamagnetic behavior. Mössbauer spectra nevertheless comprised two sextets with hyperfine fields of about 31.2 T (tetrahedral sites) and 30.7 T (octahedral sites), which resemble published values. It is postulated that aggregation may play an important role in determining the magnetic properties of the described samples.

Synthetic hematites with Al substitutions between 0 and 18 mol % were synthesized at different temperatures and water activities. The cell-edge lengths a for different synthesis conditions decreased linearly with increasing Al substitution. The regression lines, however, had different slopes and intercepts: the series with the highest synthesis temperature (1270 K) had the most negative slope. With increasing Al substitution, the hematites contained increasing amounts of non-surface water. Significant correlations were found between these chemically determined water contents and the deviations of the unit-cell parameters a, c, and V relative to the corresponding 1270 K regression lines. To explain the measured X-ray peak intensities, structural OH had to be included into the theoretical calculations. From intensity ratios normalized to I113, it is possible to determine the structural OH separately from the Al substitution, which can be assessed by the shift of the cell-edge lengths relative to the 1270 K regression lines. The incorporation of Al and OH into the hematite structure induces strain, which was quantified by X-ray diffraction.

Iron oxides in surface environments generally form at temperatures of 25 ± 10°C, but synthesis experiments are usually done at higher temperatures to increase the rate of crystallization. To more closely simulate natural environments, the transformation of 2-line ferrihydrite to hematite and goethite at 25°C in the presence of different Al concentrations and at pH values from 4 to 7 was studied in a long-term (16–20 y) experiment. Aluminum affects the hydrolysis and charging behavior of 2-line ferrihydrite and retards crystallization. Al also promotes the formation of hematite over goethite and leads to multidomainic discoidal and framboidal crystals instead of rhombohedral crystals. The strong hematite-promoting effect of Al appears to be the result of a lower solubility of the Al-containing ferrihydrite precursor relative to pure ferrihydrite. Hematite incorporates Al into its structure, as is shown by a decrease in the a and c-cell lengths and a decrease in magnetic hyperfine fields (Mössbauer spectroscopy). With hematite formed at low-temperature, these decreases were, however, smaller for the cell length and greater for the magnetic field than for hematite produced at higher temperatures. Both phenomena are removed by heating the hematite at 200°C. They are attributed to structural OH and/or structural defects. The relative content of Al in the structure is lower for hematite formed at 25°C than for hematites synthesized at higher temperatures (80 and 500°C). The maximum possible substitution of one sixth of the Fe positions was not achieved, similar to soil hematites. These results show that properties of widely distributed soil Al-containing hematites can reflect formation environment.

The influence of water vapor on bentonites or smectites is of interest in many different fields of applied mineralogy such as nuclear-waste sealing or casting in the foundry industry. The water vapor affects the smectite surface and perhaps its structure probably leading to mostly unfavorable changes in its properties. In this first part of the present study, the influence of hot water vapor (200°C) on the physicochemical and mineralogical properties of smectite-group minerals was studied. After the steam treatment, turbidity measurements, methylene-blue sorption, water adsorption, and cation exchange capacity (CEC) were measured on both untreated and treated samples. Mineralogical changes were monitored by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) was used to measure O, Al, and Si. Only a few parameters showed differences between the untreated and vapor-treated samples. Sedimentation volumes (SV) decreased following the treatment. As shown by XRD and XPS, the crystalline structure of smectite remained unaffected by the steam treatment. Equivalent sphere diameters (ESD) were not affected systematically by the steam treatment. Differences in CEC values between untreated and treated samples were observed, but only for smectites with monovalent interlayer cations. From the variety of different measurements the conclusion of the present study was that steam treatment changes the charge properties at or near the smectite particle surface.

Investigation of material from three core sections of the RWTH-1 drill-hole in the Wurm syncline of Aachen, Germany, shows mineralogical and structural evidence of intensive hydrothermal activity in the footwall of the Aachen thrust. Mineral and microstructural data indicate minimum temperatures of 200–250°C. CISillite 001 values of 0.45–0.61 (Δ°2θ) and insignificant amounts of smectite indicate a late diagenetic grade for illite pointing to temperatures <200°C. Chlorite, mainly formed in veins and cleavage planes, has CISchlorite 002 values between 0.35 and 0.26 (Δ°2θ) which only in part point to anchizonal grade. In contrast to these illite and chlorite data, maximum temperatures up to 370°C can be expected based on comparison with recently published fluid inclusion and mineral thermometric data. Illite is neither significantly affected by the hydrothermal event nor by deformation, and mirrors the burial history of the Wurm syncline.

Chlorite grew syntectonically as is shown by bent and predominantly stretched sheets which do not, however, have deformed structures. Syntectonic hydrothermal growth by incipient nucleation along crystal edges limited domainsize and thus also the CISchlorite 002 values. The hydrothermal event did not last long enough to allow further crystal growth. The retarded CISillite and CISchlorite grades can be best explained by limited duration(probably <5000 y) of the hydrothermal event which for a short time reached epithermal temperatures. The hydrothermal fluid flow was caused by dewatering of sedimentary rocks during thrusting and tectonic thickening within the Variscan orogen and it was focused along the Aachen thrust which represents the frontal Variscan thrust.

Bentonite-bonded molding sand is one of the most common mold materials used in metal casting. The high casting temperatures cause dehydration and alteration of the molding sand, thereby degrading its reusability. Neutron radiography and neutron diffraction were applied to study these processes by using pure bentonite-quartz-water mixtures in simulation casting experiments. The aim of the experiments was to compare the dehydration behavior of raw and recycled mold material in order to assess possible causes of the limited reusability of molding sands in industrial application. Neutron radiography provided quantitative data for the local water concentrations within the mold material as a function of time and temperature. Dehydration zones, condensation zones, and areas of pristine hydrated molding sand could be established clearly. The kinematics of the zones was quantified. Within four cycles of de- and rehydration, no significant differences in water kinematics were detected. The data, therefore, suggest that the industrial handling (molding-sand additives and the presence of metal melt) may have greater effects on molding-sand reusability than the intrinsic properties of the pure bentonite–quartz–water system.

The purpose of this report is to describe the appropriate use of indices relating to crystallinity, such as the ‘crystallinity index’, the ‘Hinckley index’, the ‘Kübler index’, and the ‘Árkai index’. A ‘crystalline’ solid is defined as a solid consisting of atoms, ions or molecules packed together in a periodic arrangement. A ‘crystallinity index’ is purported to be a measure of crystallinity, although there is uncertainty about what this means (see below). This report discusses briefly the nature of order, disorder and crystallinity in phyllosilicates and discusses why the use of a ‘crystallinity index’ should be avoided. If possible, it is suggested that indices be referred to using the name of the author who originally described the parameter, e.g. ‘Hinckley index’ or ‘Kübler index’, or in honor of a researcher who investigated the importance of the parameter extensively, e.g. ‘Árkai index’.

The electrical state of the interface between a kaolinite-dominated clay sample and aqueous electrolyte solutions was characterized using low-frequency conductance measurements. From these measurements, the ζ-potential and surface conductivity contributions from the diffuse and non-diffuse parts of the electrical double layer were obtained. The suspensions were studied as a function of volume fraction, electrolyte concentration, and electrolyte type (LiCl, NaCl, KCl, CsCl, CaCl2, SrCl2, and BaCl2). Interpretation in terms of the surface conductance revealed that a substantial part of the surface conductivity originates in the inner part of the double layer. Electrokinetic potentials and related diffuse double layer properties are highly dependent on the nature of monovalent counterions, whereas divalent counterions do not show such clear dependencies. Further presented was a simple way to estimate the order of magnitude of counterion mobilities in the inner part of the electrical double layer. All counterions were shown to have a substantial mobility in the inner part of the double layer. Finally, we suggest that the apparent ion-specific effects observed in the diffuse part of the double layer are at least in part related to the finite size of the counterions. Our findings are relevant to scenarios where fluid flow in porous media is accompanied by charged species transport, e.g., in electro-osmotic remediation, spectral-induced polarization, or permeability measurements.

The strongly coloured Cu-triethylenetetramine complex (Cu-Trien) provides a fast and easy way to measure the cation exchange capacity of clay samples, but the reliability of this method has been questioned in recent publications. This work identifies several reasons for the poor reliability, thus improving both the precision and accuracy of the CEC method. Pure Trien is shown here to intercalate as does the Cu-Trien complex. Hence, the purity of Trien and also the Cu/Trien ratio require special attention. Using cold Cu-Trien solutions produces systematic deviations in CEC determinations caused by errors in the pipetted volumes. The present study demonstrates, for the first time, that the Cu-Trien complex protonates and changes its effective charge beyond 2+. Based on titration experiments at three different temperatures, a correction function is derived for calculating the total charge of the Cu-Trien complex as a function of pH. Accounting for these pH-corrected charges indicates that Cu-Trien may well be insensitive to edge charges on smectite. Finally, it is shown here that the ratio of sample mass to the initial amount of Cu-Trien is critical because excess adsorption of Cu-Trien in addition to CEC sensu strictu was observed.

The low-frequency conductivity of aqueous kaolinite suspensions has been measured as a function of volume fraction and concentration of KCl, K2SO4 and BaCl2, respectively. These measurements were interpreted with a theoretical model accounting for surface conductivity and particle shape. For the first time, an internally consistent data set was established by measuring all parameters necessary to solve the relevant equations. The simultaneous availability of surface conductivity, surface charge density and diffuse layer charge density permitted the estimation of counterion mobilities in the stagnant layer and a consistency check for the evaluation procedure of the conductivity experiments. In agreement with current literature results, monovalent counterions were found to have a Stern layer mobility similar to their bulk mobility, whereas the mobility of divalent counterions in this layer is reduced by a factor of ∼2.