Vladimír Šucha

K-Ar dating of illite fundamental particles separated from illite-smectite

Abstract Fundamental particles of illite-smectite from bentonites were separated into classes by high-speed centrifugation after infinite osmotic swelling of mixed-layer crystals, achieved by Naexchange and dispersion in distilled water. In samples free of detrital contamination, the thinnest fundamental particles yield older K-Ar ages than the thicker fundamental particles. This implies that they do not preferentially lose radiogenic 40Ar due to size, and that the illitization process is a crystal growth mechanism (not nucleation plus growth). As a result, any K-Ar age of fundamental illite particles from bentonites is an integral over longer or shorter periods of time, depending on the thermal history of the rocks. In thick bentonite beds, the measured age difference between the beginning of the illitization process at the contact with the host rocks and the end in the centre of the bed records extremely slow K diffusion in these well compacted rocks. These data explain why measured K-Ar ages of illite-smectite from bentonites are younger than the corresponding age of shale illitization, inferred from the burial history of the basin. The finest technically separable size-fractions of associated shales (<0.02 μm) yield K-Ar dates* greater than the stratigraphic age. This observation points to incomplete recrystallization of detrital illite during burial diagenesis.

Effect of non-swelling layers on the dissolution of reduced-charge montmorillonite in hydrochloric acid

Abstract A series of reduced-charge montmorillonites (RCM), prepared from the same parent Li-montmorillonite (Jelšový Potok, Slovakia) by heating at various temperatures (105-210°C) for 24 h, was treated with 6 M HCl at 95°C for periods up to 30 h. Reaction solutions obtained were analysed for Al, Fe, Mg and Li and the solid reaction products were investigated by FTIR spectroscopy. Both analyses provided evidence that the extent of dissolution decreased with increased amounts of Li fixed within the montmorillonite structure, i.e. with increased heating temperature. Differences in the acid dissolution process were reflected in the structural changes which occurred within the RCM samples, due presumably to different positions of fixed Li. The ethylene glycol monoethyl ether (EGME) surface areas, and XRD and HRTEM analyses of the RCM series revealed an increased amount of non-swelling layers in the samples prepared at higher temperatures, which caused a substantially slower decomposition of M7 and M8 in HCl. The [calculated XRD patterns of M6 and M7 confirmed the presence of 20% and 45% pyrophyllite-like layers, respectively, in these samples. Mixed-layer pyrophyllite-like-smectite and pyrophyllite-like crystals, containing only non-swelling layers, were found in sample MB. The results confirmed that the amount of swelling layers in RCM significantly affects their dissolution rate in HCl.

MEASUREMENT OF FUNDAMENTAL ILLITE PARTICLE THICKNESSES BY X-RAY DIFFRACTION USING PVP-10 INTERCALATION

The thicknesses of fundamental illite particles that compose mixed-layer illite-smectite (I-S) crystals can be measured by X-ray diffraction (XRD) peak broadening techniques (Bertaut-Warren-Averbach [BWA] method and integral peak-width method) if the effects of swelling and XRD background noise are eliminated from XRD patterns of the clays. Swelling is eliminated by intercalating Na-saturated I-S with polyvinylpyrrolidone having a molecular weight of 10,000 (PVP-10). Background is minimized by using polished metallic silicon wafers cut perpendicular to (100) as a substrate for XRD specimens, and by using a single-crystal monochromator. XRD measurements of PVP-intercalated diagenetic, hydrothermal and low-grade metamorphic I-S indicate that there are at least 2 types of crystallite thickness distribution shapes for illite fundamental particles, lognormal and asymptotic; that measurements of mean fundamental illite particle thicknesses made by various techniques (Bertant-Warren-Averbach, integral peak width, fixed cation content, and transmission electron microscopy [TEM]) give comparable results; and that strain (small differences in layer thicknesses) generally has a Gaussian distribution in the log-normal-type illites, but is often absent in the asymptotic-type illites.

STRUCTURAL TRANSFORMATIONS OF INTERSTRATIFIED ILLITE-SMECTITES FROM DOLNA VES HYDROTHERMAL DEPOSITS: DYNAMICS AND MECHANISMS

Structural transformations of illite-smectite samples of hydrothermal genesis with increasing contents of illite layers were studied by X-ray powder diffraction. The samples were K-saturated and subjected to wetting and drying cycles to increase three-dimensional structural ordering. Diffraction profiles were analyzed with the help of a specially devised computer program based on the approximation of individual diffraction reflections by “bell-shaped” functions, with minimization of the differences between experimental and simulated profiles. The data indicate that the transformations of these illite-smectite samples were accompanied not only by variations in the proportion of illite and smectite layers and in the pattern of their alternation, but also by a change in structure within 2:1 layers.

Weathering of smectite and illite-smectite under temperate climatic conditions

Abstract Weathering profiles developed on the top surface of a bentonite (containing Al-Mg montmorillonite) and a K-bentonite (containing mixed-layer illite-smectite (I-S)) under Central European temperate conditions were studied by XRD, HRTEM, FTIR, K-Ar and chemical analyses. Weathering of montmorillonite results in the decrease of cation exchange capacity (CEC), total surface area and Mg content. The process is interpreted as montmorillonite dissolution and precipitation of amorphous SiO2. Weathering of I-S produces an increase in CEC and total surface area The XRD data suggest dissolution of I-S and appearance of smectite as a separate phase at intermediate depths. The fixation of ammonium is documented in the topmost sample. In both profiles, abundant aeolian contaminants, including mica, were identified and their migration was traced using K-Ar dating.

Smectite-to-illite alteration in salt-bearing bentonites (the East Slovak Basin)

The effect of a saline environment on illitization in volcanoclastic rocks is examined in deep boreholes in the East Slovak Basin. Based on X-ray diffraction analysis, it is concluded that illite-smectite (I–S) expandability is always less in the salt-bearing bentonites (SBB) than in the salt-free bentonites (SFB) for a given depth interval. These two lithologies can be distinguished easily by water-leachate chemistry. Within the depth interval 2100–2500 m, the expandability in SBB varies within the range 25–10% expandable with R1 and R3 ordering in SBB and 68–35% expandable with R0 ordering in SFB. In two shallow SBB samples the expandability is close to that of SFB, suggesting that salinity alone does not enhance the illitization; but salinity may enhance it when combined with higher burial temperature. Vitrinite reflectance and Tmax of RockEval pyrolysis measured in adjacent shales confirm that the increased illitization in SBB is not due to heating and/or erosion. The model of burial and thermal history calibrated by organic maturity suggests that the same thermal history produces two different expandabilities in the two lithologies (SBB and SFB). Particle thickness measurements and K-Ar data were used to deduce the crystal growth mechanism of illitization in SBB. Whereas surface-controlled growth is typical for SFB, simultaneous nucleation and growth played a more important role in the case of SBB. The effect of a salty environment on the illitization is not yet fully understood and may have severe consequences for the utilization of bentonites as engineering barriers in radioactive waste disposal sites if salt formations used as host rocks are taken into account.

Evolution of pyrophyllite particle sizes during dry grinding

Abstract The Bertaut-Warren-Averbach (BWA) technique and high-resolution transmission electron microscopy (HRTEM) were used to characterize the products of dry-ground pyrophyllite. Mean crystallite thickness and crystallite thickness distributions were measured for each sample using the BWA technique. Mean crystallite thickness decreases during the treatment with respect to grinding time and energy applied per unit mass. The BWA data were checked by HRTEM measurements and good fits were obtained for samples having small mean particle thicknesses. Samples with thicker particles could not be measured properly by HRTEM because the number of particles counted from images is statistically insufficient. The shape of the crystallite and the particle-size distribution were used to determine the mechanism of pyrophyllite particle degradation. Particles initially having a lognormal size distribution are first delaminated randomly, then some are delaminated preferentially, thereby producing polymodal thickness distributions. Finally all particles undergo delamination yielding a lognormal thickness distribution.

PARTICLE SHAPE VERSUS COHERENT SCATTERING DOMAIN OF ILLITE/SMECTITE: EVIDENCE FROM HRTEM OF DOLNA VES CLAYS

Fundamental particle thickness measurements of Dolná Ves hydrothermal illite/smectite (I/S) samples confirmed earlier findings regarding the content of fixed cations in illite interlayers (ca. 0.9/O10(OH)2). The distributions of fundamental particles and mixed-layer crystals of a given sample are internally consistent. In samples dominated by bilayer fundamental particles, mixed-layer crystals most often contain even numbers of layers. The expandabilities measured by XRD are much higher than so-called minimum expandabilities obtained from HRTEM measurements. This discrepancy is explained by assuming that the coherent scattering domains of Dolná Ves clays do not correspond to natural mixedlayer crystals but are thicker, probably due to parallel association of crystals on the oriented XRD slide. This tendency to produce intercrystal contacts is probably related to the unusually large ab dimensions of crystals of Dolná Ves clays.

Effect of illite particle shape on cesium sorption

Samples containing illite and illite-smectite, having different crystal shapes (plates, “barrels”, and filaments), were selected for sorption experiments with cesium. There is a positive correlation between total surface area and Cs-sorption capacity, but no correlation between total surface area and the distribution coefficient, Kd. Generally Kd increases with the edge surface area, although “hairy” (filamentous) illite does not fit this pattern, possibly because elongation of crystals along one axis reduces the number of specific sorption sites.

High-resolution transmission electron microscopy of mixed-layer clays dispersed in PVP-10: a new technique to distinguish detrital and authigenic illitic material

Abstract The results of a new technique for the measurement of the thickness distribution of fundamental particles are reported. The technique is based on high-resolution transmission electron microscopy (HRTEM) of Na-saturated mixed-layer illite-smectite dispersed in polyvinylpyrrolidone (PVP-10). Intercalation of PVP-10 increases the spacing of expandable interlayers and changes the arrangement of particles so that the number of layers per fundamental particle can be counted easily on HRTEM images. The data obtained by HRTEM on PVP-10-intercalated illite-smectite of hydrothermal origin are compared with data from the Pt-shadowing technique. A good agreement between the two methods for the measured thickness distributions, mean thickness and expandability confirms the reliability of the new technique. The same technique is applied to a set of four sedimentary samples with different expandabilities (83-18%). The thickness of illite particles from shales and claystones has a lognormal distribution. Detrital and discrete illite particles can be distinguished from the thickness distribution of authigenic illite.