Bentonite Beds Research Articles

High-precision U–Pb zircon ID–TIMS dating of two regionally extensive bentonites: Cenomanian Stage, Western Canada Foreland BasinThis article is one of a series of papers published in this Special Issue on the theme ofGeochronologyin honour of Tom Krogh.

The stratigraphy of Cretaceous rocks in the Western Canada Foreland Basin is well constrained by dense borehole data that allow three-dimensional mapping of transgressive–regressive events, paleogeography, and subsidence patterns. However, it is difficult to estimate rates of change or to place events in a precise temporal framework because very few of the many bentonite beds have been dated using modern techniques. In this study, two bentonites, of at least regional extent, were dated using isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb methods on chemically abraded zircon crystals. The older bentonite correlates with the “X” bentonite of the late middle Cenomanian and yields an age of 95.87 ± 0.10 Ma. The Bighorn River Bentonite, which lies just below the Cenomanian–Turonian boundary, yields an age of 94.29 ± 0.13 Ma and is interpreted to equate with the “B” bentonite and bed 80 at Pueblo, Colorado. Both bentonites dated here are older than the previously reported40Ar/39Ar ages from correlative United States samples, supporting the observation that40Ar/39Ar ages may systematically underestimate ages of Cretaceous bentonites by ∼1%, as suggested by other recent studies. The X bentonite immediately precedes a major late middle Cenomanian eustatic transgression that inundated the Dunvegan delta complex. The new ages for the X and Bighorn River bentonites indicate an average minimum subsidence rate of ∼0.27 mm/year in the most proximal part of the Cenomanian foredeep in northeast British Columbia during this time. The new age of the Bighorn River Bentonite, coupled with the orbitally tuned time scale of Sageman et al. (2006) , suggests a Cenomanian–Turonian boundary age of 94.12 ± 0.13 Ma.

Evolution of Boron and Nitrogen Content During Illitization of Bentonites

AbstractThe incorporation of boron (B) and nitrogen (N) into illite is the key demand-side process responsible for the diagenetic budget of these elements in sedimentary basins, with important implications for pore-water chemistry, natural-gas composition, and borehole geophysics. The purpose of the present study was to take advantage of recent advances in quantitative mineral analysis of sedimentary rocks which have opened new possibilities for investigating this particular process. In order to avoid complications with recycled (detrital) N and B, clays from pyroclastic horizons of sedimentary rocks (bentonites) were used. The B and N contents in illite-smectite were measured in samples from different sedimentary basins, representing a complete range of diagenetic alteration. The bulk-rock chemical measurements, performed on raw rock samples in order to avoid any loss of exchangeable B and N, were referred to the contents of illite-smectite clays and to the content of illite alone, both measured by a combination of XRD and chemistry-based techniques.Both B and N (as NH4) are present in illite, so their contents in illite-smectite clay increase in a more or less linear manner with progressing illitization. Thus, during diagenesis, the illite-smectite clay is a net consumer of B and N from the pore water. The amount of N in individual illite layers decreases during diagenesis and the amount of B either decreases or remains stable. Bentonitic illite must acquire both B and N from outside of the bentonite bed. In one diagenetic cycle, bentonitic illite fixes up to 800–1000 ppm B and up to >1% N expressed as (NH4)2O, corresponding to >20% of the fixed cation sites.

Planform and Facies Variability in Asymmetric Deltas: Facies Analysis and Depositional Architecture of the Turonian Ferron Sandstone in the Western Henry Mountains, South-Central Utah, U.S.A.

Abstract Facies analysis of the Upper Cretaceous (Turonian) Ferron Sandstone in the western Henry Mountains of south-central Utah, U.S.A., indicates sediment accumulation in a series of flood-dominated, marine current- and wave-influenced, deltas. Twelve lithofacies are recognized: 1. erosionally based, thick cross-bedded sandstone bodies (Distributary Channels), 2. similar though thinner bodies containing common bioturbation (Marine-Influenced Distributary Channels), 3. root-penetrated, plant fossil bearing siltstone with minor sandstone (Coastal Floodplain and Floodbasin), 4. coal and carbonaceous shale (Coastal Mire), 5. thin-bedded, carbonaceous and bioturbated sandstone–siltstone (Coastal Lagoon), 6. erosionally based sandstone with large- and small-scale cross-bedding and bioturbation (Mouth-Bar Complex), 7. sharply based sandstone bodies internally dominated by hummocky cross-stratification, soft-sediment deformation, or lacking structure (Proximal Delta Front), 8. sharply bounded, calcareous, fossiliferous and bioturbated sandstone sheets (Abandoned Delta Lobe), 9. thickly interbedded sandstone, coarse- and fine-grained siltstone (Medial Delta Front), 10. thinly interbedded sandstone, coarse- and fine-grained siltstone (Distal Delta Front), 11. mainly siltstone with minor thin-bedded sandstone (Prodelta) and 12. fine-grained siltstone with bentonite beds (Offshore). Deltas prograded into shallow water, forming sharp-based mouth-bar sand bodies. The upper delta front was evidently fluidal and prone to failure, leading to the development of rotational slope failures, debris–flow filled gullies, and, in places, growth faults. Paleocurrent data indicate that the regional sediment dispersal direction was eastward. Data from delta-front facies, however, suggest that outflow plumes and associated bottom currents were deflected towards the southeast, giving rise to an asymmetric delta planform. The Holocene and modern Burdekin River Delta of NE Australia is considered a close planform, process, and facies analog for the Ferron Notom deltas. The Burdekin Delta facies assemblage is vertically and laterally heterogeneous, despite being the product of a consistent array of environmental controls. Adopting a model that incorporates such a degree of heterogeneity negates the need for multiple depositional models for complex stratigraphic intervals such as the Ferron Sandstone. The facies model also suggests that asymmetric deltas may be produced by directional growth of delta lobes, rather than by deflection of beach ridges. Stratigraphic stacking patterns strongly suggest that sediment accumulation in the Ferron Sandstone of the western Henry Mountains was forced by a regime of progressively more limited accommodation through time.

New insights into smectite illitization: A zoned K-bentonite revisited

The illitization reaction in a thick K-bentonite bed located in upper Cretaceous marine shale in the Montana disturbed belt was studied by X-ray diffraction, chemical analysis, and thermal gravimetric analysis. Modeling of the experimental XRD patterns from oriented clay specimens in air-dried and glycolated states shows that at each sample location in the bentonite bed a mixture of R0 illite-smectite (I-S) and R1 I-S coexist. Each of these phases in all samples consists of the same or similar content of illite and expandable layers independent on location in the bed. In particular, the illite content in the R0 I-S and the R1 I-S from the <0.5 μm fractions is equal to 30 and 62%, respectively. The main difference between the samples at different locations is the different weight concentrations of the coexisting I-S phases. The R1 I-S content decreases progressively from the lower and upper contacts of the bed to its center. The reverse trend was observed for the R0 I-S. The layer unit-cell parameter b increases from samples located near the middle of the bed toward samples near the bed margins. The DTG patterns of the samples contain two endothermic maxima at about 640 and 470 °C, corresponding to cis -vacant ( cv ) illite and trans -vacant ( tv ) smectite layers coexisting in the R1 I-S and R0 I-S. Analysis of the crystal-chemical features of the R1 I-S and R0 I-S shows that, in the middle of the bed, both phases are characterized by the lowest octahedral Mg and the highest tetrahedral Al contents. In the structural formula of the R1 I-S, the tetrahedral Al content is significantly higher than the (K+Na) content independent of sample location. In contrast, tetrahedral Al in the R0 I-S located near the bed boundaries is lower compared with (K+Na) content. To account for the crystal-chemical features of the coexisting I-S, a first assumption is that the initial volcanic ash was altered into tv smectite having a homogeneous Al-rich composition throughout the bed. Second, along with K, the active role in illitization was controlled by Mg. Mineralogical zonation of the K-bentonite is explained by the progressive migration of K from the margins toward the bed center with the associated decrease of K cations in the pore fluids. However, the decrease in K concentration was accompanied by a successive increase in R0 I-S content, but not a progressive decrease in illite layer content in a single I-S phase. The main role of Mg was to redistribute octahedral and tetrahedral Al in the 2:1 layers of the R0 I-S and R1 I-S in such a way that the amount of Al in the tetrahedral sheets increased at the expense of the substitution of Mg for Al in the octahedral sheet of the 2:1 layers in the initial smectite. These results demonstrate a new insight into mineralogical sequences of intermediate members of smectite illitization. Instead of a statistically homogeneous and continuous reaction associated with the increase of illite layers in I-S and the simultaneous increase of order of the layer stacking sequence, the illitization reaction in the thick K-bentonite consists of the formation of a physical mixture of two I-S having a contrasting content of layer types and their distribution. Factors responsible for the formation of the coexisting R0 I-S and R1 I-S are discussed.

Bentonite chemical features as proxy of late Cretaceous provenance changes: A case study from the Western Interior Basin of Canada

Bentonite beds are fairly common in both marine and terrestrial Upper Cretaceous (Campanian–Maastrichtian) deposits of the Western Interior Basin of western Canada and northwestern United States. A detailed stratigraphic, sedimentologic, geochemical (X-ray fluorescence), and mineralogical (X-ray diffraction) study of twenty-one bentonites from the Puskwaskau and Wapiti formations in the Grande Prairie area (west-central Alberta, Canada) is here presented. Major and trace-element concentrations from altered volcanic ashes document the presence in the study area of predominantly trachyandesitic and rhyolitic volcanogenic products, resulted from intense volcanic arc to within-plate pyroclastic activity. Concentration values of high field strength elements (HFSE) and selected large ion lithophile elements (LILE) (e.g. Nb, Zr, Th, and Y) obtained by X-ray fluorescence spectroscopy strongly support the presence of multiple volcanic sources. Integrated paleoenvironmental and geochemical criteria for provenance determination indicate a bimodal occurrence of basic and acid volcanic products interpreted as reflection of source areas characterized by different tectonic setting and magmatic composition. A comparative analysis of geochemical compositions between Grande Prairie bentonites and 30 known volcanic beds from central and southern Alberta, Manitoba and Montana 1. documents a trend toward more acidic and alkali-depleted volcanic products during the late Campanian–early Maastrichtian interval, and 2. suggests a well constrained stratigraphic and geographic subdivision of the non-marine successions of the foreland basin on the basis of geochemical characteristic of volcanic ash beds. Furthermore, geochemical “fingerprints” of several decimeter to meter thick bentonite beds have been coupled with volcanic ash subsurface signature in order to investigate their role as marker beds. This multiple-approach provides a reliable tool for basin-scale identification and correlation of non-marine sedimentary successions.

Numerical age calibration of the Albian/Cenomanian boundary

New biostratigraphic and sequence stratigraphic correlations of the U.S. Western Interior Albian/Cenomanian sections with North Texas and European sections calibrate the Albian/Cenomanian boundary at 97.2 Ma, which supports previous correlations in the Western Interior. This age, however, is 2.4 myr younger than the recent calibration at 99.6 Ma derived from radiometric ages of volcanic beds with diagnostic marine fossils in northern Japan. This discrepancy suggests that additional analyses are required to test the age calibration of the geologic time scale. New data of cosmopolitan dinoflagellates enable direct correlation with the European reference sections of the Albian and Cenomanian. Previous correlations in the Western Interior were based on endemic or Boreal ammonites. The first and last occurrences of dinoflagellate species bracket the Albian/Cenomanian boundary in Europe and many of these species also bracket the Clay Spur Bentonite bed previously dated 97.17±0.69 Ma. The high number of radiometrically dated bentonites in the Western Interior provides a numeric age control for the ranges of many dinoflagellates. Sequence stratigraphic correlation of the North Texas section with the U.S. Western Interior sections also demonstrates that the Clay Spur Bentonite bed correlates with the Ablian/Cenomanian boundary in Texas defined by the ammonite succession. The flooding contact SB WA6 in Texas at the Albian/Cenomanian zonal boundary of Stoliczkaia dispar and Mantelliceras mantelli correlates into the Western Interior with an erosional sequence boundary SB4 at the base of the Romeroville Sandstone in New Mexico and the Dakota Sandstone in Colorado. This contact correlates with the base of the Belle Fourche Shale that overlies the Clay Spur and the Mowry Shale in Montana. The revised correlation of the Western Interior poses two questions. 1) What is the accurate age of the boundary? Both cannot be correct. 2) What were the durations of the Albian and Cenomanian stages? Cyclostratigraphic analyses estimate that the Albian was 11.9 to 11.6 myr in duration but the Western Interior data project the duration at about 15 myr.

Wenlock bentonites in Lithuania and correlation with bentonites from sections in Estonia, Sweden and Norway

Seven Wenlock bentonite beds from four drill cores in Lithuania with sufficient thickness for sampling were analysed by XRD and XRF. The sanidine composition of bentonites combined with graptolite biozonation of the host rock were used for correlations. Two of the studied bentonite samples at the antennulariuslflexilis biozone boundary can be confidently correlated between two Lithuanian sections. Four of the studied bentonites can be correlated with lower or higher probability with earlier studied bentonites in Estonia and Sweden. Identification of the Grötlingbo Bentonite in the Viduklė-61 drill core, and descriptions of other east Baltic sections enables composition of a thickness map of the Grötlingbo Bentonite. The thickness pattern with two axes, and the layered composition of the Grötlingbo Bentonite, indicate a change of the wind direction during a long lasting eruption. Hypothetical location of the volcanic source in the Silurian was within the zone of the Baltica-Laurentia collision.

Multiphase Silurian bentonites in the Baltic Palaeobasin

The mineral composition of altered volcanic ash beds, bentonites, deposited in marine environments is typically characterized by smectite or illite–smectite type clay minerals. Silurian bentonites from an unmetamorphosed normal marine sedimentary sequence in the eastern Baltic Palaeobasin, however, show a complex and varied mineral composition with illite–smectite, K-feldspar and kaolinite as the dominant authigenic phases. The whole-rock XRD study of 52 bentonite beds from two drill-cores — Ruhnu and Ohesaare, southwestern Estonia, reveals a range of compositions varying between different beds as well as laterally within a single bed. The observed variations are found to reflect the bentonite development and diversification during initial stages of volcanic ash transformation and diagenesis that was probably overprinted by a later K-metasomatic event.

Geochemical characterization of bentonite beds in the Two Medicine Formation (Campanian, Montana), including a new 40Ar/ 39Ar age

Terrestrial deposits of the Upper Cretaceous (Campanian) Two Medicine Formation of northwestern Montana preserve multiple bentonite beds (n ≥ 19) that reflect recurrent pyroclastic events in the Western Interior Basin. Major and trace element concentrations were determined on 27 samples derived from four bentonites using X-ray fluorescence spectroscopy. This study evaluates the potential for geochemically distinguishing three of these bentonite beds using a stepwise discriminant analysis of trace element concentrations. Seven elements were found sufficient to establish 100% classification in the group matrix. The elements (in order of decreasing contribution to the canonical discriminant functions) are Zr, Sc, V, Cr, U, Ga, and Th. The validity of these results is strongly supported by cross-validation methods that correctly assigned 100% of randomly-selected bentonite samples left out of the stepwise analysis to their correct bed. These findings indicate geochemical discrimination is a viable tool for correlation within the formation and suggests its application to more distant coeval strata. We also report here a new 40Ar/ 39Ar age of 77.52 ± 0.19 Ma for one of the analyzed bentonite beds. This new radioisotopic age affords insights into the timing of regional eruptive events, and further constrains the age of the Two Medicine Formation and its renowned fossil resources. Finally, the inferred magmatic composition of the original ash (based on trace element compositions) from the two older bentonites beds suggest a source in the Elkhorn Mountain Volcanics whereas the younger bentonites may have been sourced from the Adel Mountain Volcanics.

Steeply Dipping Heaving Bedrock, Colorado: Part 3 Environmental Controls and Heaving Processes

This paper examines the environmental processes and mechanisms that govern differential heaving in steeply dipping claystone bedrock near Denver, Colorado. Three potential heave mechanisms and causal processes were evaluated: (1) rebound expansion, from reduced overburden stress; (2) expansive gypsum-crystal precipitation, from oxidation of pyrite; and (3) swelling of clay minerals, from increased ground moisture. First, we documented the effect of short-term changes in overburden stress, atmospheric exposure, and ground moisture on bedrock at various field sites and in laboratory samples. Second, we documented differential heaving episodes in outcrops and at construction and developed sites. We found that unloading and exposure of the bedrock in construction-cut areas are essentially one-time processes that result in drying and desiccation of the near-surface bedrock, with no visible heaving response. In contrast, wetting produces a distinct swelling response in the claystone strata, and it may occur repeatedly as natural precipitation or from lawn irrigation. We documented 2.5 to 7.5 cm (1 to 3 in.) of differential heaving in 24 hours triggered by sudden infiltration of water at the exposed ground surface in outcrops and at construction sites. From these results, we interpret that rebound and pyrite weathering, both of which figure strongly into the long-term geologic evolution of the geologic framework, do not appear to be major heave mechanisms at these excavation depths. Heaving of the claystone takes two forms: (1) hydration swelling of dipping bentonitic beds or zones, and (2) hydration swelling within bedrock blocks accommodated by lateral, thrust-shear movements along pre-existing bedding and fracture planes.

Sorption and diffusion behaviours of strontium in sodium-type bentonite bed

Sorption and diffusion of Sr were examined using a typical Japanese bentonite. The experimental results showed that Sr sorption on the bentonite had linear relationship between the equilibrium Sr concentration and Sr sorption amount, i.e., Henry’s type of sorption, in wide initial Sr concentration from 1.1 × 10−9 to 1.1 × 10−4 mol L−1 at pH 10. The Sr sorption also indicated pH dependence in pH range between 2 and 12. Sorption modelling calculation indicated that cation-exchange reactions contributed to Sr sorption in the pH range studied and a surface complexation reaction was predominant above pH 8. Diffusion of Sr in loosely compacted bentonite bed was described by pore and surface diffusion and surface complexation of Sr. Chemical-transport calculations reproduce the diffusion data at pH 5 using the cation-exchange parameters obtained in the analysis of the batch sorption experiment.

Chemical signature of two Permian volcanic ash deposits within a bentonite bed from Melo, Uruguay

A Permian bentonite deposit at Melo, Uruguay is composed of a calcite-cemented sandstone containing clay pseudomorphs of glass shards (0-0.50 m) overlying a pink massive clay deposit (0.50-2.10 m). The massive bed is composed of two layers containing quartz and smectite or pure smectite respectively. The smectite is remarkably homogeneous throughout the profile: it is a complex mixed layer composed of three layer types whose expandability with ethylene glycol (2EG 1EG or 0EG sheets in the interlayer zone which correspond to low-, medium- and high-charge layers respectively) varies with the cation saturating the interlayer zone. The smectite homogeneity through the profile is the signature of an early alteration process in a lagoonal water which was over saturated with respect to calcite. Compaction during burial has made the bentonite bed a K-depleted closed system in which diagenetic illitization was inhibited. Variations in major, REE and minor element abundances throughout the massive clay deposit suggest that it originated from two successive ash falls. The incompatible element abundances are consistent with that of a volcanic glass fractionated from a rhyolite magma formed in a subduction/collision geological context.

Identification of the O-bentonite in the deep shelf sections with implication on stratigraphy and lithofacies, East Baltic Silurian

Pyroclastic sanidine composition is used for correlation of the Osmundsberg bentonite (O-bentonite) bed from Estonia to Latvia. Chemical changes during conversion of volcanic ash to authigenic silicates in shallow and deep shelf sediments differ, leading to the formation of kaolinite-rich bentonite in deep shelf and feldspar-rich bentonite in shallow shelf environments. The Rumba Formation in Estonia is proved to be of Lower Telychian age. Firmly based correlation between shallow and deep shelf sediments allows tracing of facies variations that occurred during environmental changes in the Late Llandovery times.

The mobility of rare-earth and other elements in the process of alteration of rhyolitic rocks to bentonite (Lebombo Volcanic Mountainous Chain, Mozambique)

Ca-type bentonite deposits of economic interest occur associated with rhyolitic rocks in the Lebombo volcanics. From previous geological studies on the deposits, besides the economic aspects, little is known about the main formation mechanism of beds of bentonite that can be more than 15 m thick, in some places. Chemical analyses of bentonite samples indicate that elements such as Ca, Mg, Sr, Zn, Cu and Sc are markedly enriched in bentonite in comparison to the parent rock. Other elements such as K, Na, Rb and Ba are depleted in bentonite. HREE (Yb, Lu) and Y are depleted in the bentonite samples, as well as Cs, Rb, Ta, U, Ba, Co, Cr and Pb. Concentrations of the LREE (La, Ce, Nd, Sm, Eu), Sc and Ga are slightly higher in the bentonites than in the parent rocks. The chondrite-normalized REE patterns show identical trends both in the rhyolites and bentonites samples. Alteration of the parent rocks to bentonite is associated with leaching and subsequent removal of, principally, K and Na in open-system conditions.

Correlation between the basal part of the Lower Cretaceous Speeton Clay Formation, Yorkshire, and the Purbeck Limestone Group, Dorset: a bentonite tie line

SUMMARY Four beds in the middle and upper parts of the Upper Jurassic–Lower Cretaceous Purbeck Limestone Group at Durlston Bay, Dorset, contain acid insoluble residues that suggest a penecontemporaneous volcanogenic origin, probably as sub-aerial ashfalls. Their particle size distribution patterns, mineralogy, geochemistry and K–Ar signatures indicate post-depositional alteration of fine-grained volcanic glass by pore waters. Major and trace element chemistry indicates that: (1) two beds in the Cherty Freshwater Beds (Lulworth Formation) were derived from ashes of trachyandesite-andesite-rhyodacite/dacite composition that were argillized in saline pore waters enriched in Mg 2+ ; (2) a bed in the Unio Beds (Durlston Formation) developed from an ash of trachytic composition; and (3) a bed in the Viviparus Shales and Clays (Durlston Formation) originated from an ash of trachyandesite-andesite-rhyodacite/dacite composition within the suboxic zone of brackish pore waters. Comparison of the four Purbeck volcanogenic beds with bentonite beds in the lower part of the Speeton Clay on the Yorkshire coast and offshore in the North Sea suggests that those from the Unio Beds and Viviparus Shales and Clays are equivalent respectively to Beds D7D and D6J at Speeton.

Composition variation of illite-vermiculitesmectite mixed-layer minerals in a bentonite bed from Charente (France)

AbstractMineralogical and chemical variations were studied in the upper half of a 1 m thick discontinuous bentonite bed interlaminated in the Lower Cenomanian sedimentary formations of the northern Aquitaine basin (France). X-ray diffraction patterns obtained from the &lt;2 mm fraction in the Ca and K-saturated states were decomposed and compared to those calculated from decomposition parameters. They revealed the presence of two highly expandable illite-expandable (I-Exp) mixedlayer minerals (MLMs). The relative proportions of the two MLMs evolve steadily with depth leading to the decrease of the cation exchange capacity and of the (Na + Ca) content towards the centre of the bentonite bed. However, the system is essentially isochemical and Mg, Al, Si, K and Fe are roughly constant in the bulk samples. It is thought that the mineralogical zonation results from the initial stages of the smectite formation in an ash layer.In the Ca-saturated state, the expandable component of these MLMs was for the most part homogeneous with the presence of 2 sheets of ethylene glycol molecules in the interlayer. However, the heterogeneous hydration behaviour of these expandable layers was enhanced by the K-saturation test. From this test, the presence of three layer types with contrasting layer charge was evidenced from their contrasting swelling abilities. The C12-alkylammonium saturation test applied to samples in which the octahedral charge had previously been neutralized (Hofmann-Klemen treatment) showed that the tetrahedral charge is located on specific layers. These layers are responsible for the heterogeneous hydration behaviour. Low-charge smectite layers are mostly octahedrally substituted, whereas for intermediate- and high-charge layers this montmorillonitic charge is complemented by additional tetrahedral substitutions (0.30 and 0.35–0.40 charge per O10(OH)2, respectively).

Mercury in Western Canadian subbituminous coal—a weighted average study to evaluate potential mercury reduction by selective mining

Western Canadian subbituminous coal is mined using strip-mining methods. The coal is used to feed coal-fired power plants for electricity generation. Parameters that influence the mercury content of these coals include the occurrence and frequency of partings in the seam, weathering, and epigenetic mineralization. Most partings have higher mercury content than the coal layers in the same seam, with the single bentonite bed having the highest Hg content. However, some high ash components, such as sandy siltstone, do not follow this trend. The incorporation of the high mercury partings into the as-mined coal may increase the Hg content in the feed coal. Epigenetic mineralization may also increase the mercury content of coal, depending on the nature of the mineralization. Calculation of the weighted mean of mercury content for various combination of lithological components of a seam illustrates how these components influence the total mercury content of the seam. This approach also suggests how selective mining might be used to reduce mercury in “as-mined” coal. Results of weighted average calculations of mercury indicate that the thin lithological units with moderate mercury may not necessarily influence that overall weighted average of Hg for a seam. However, there can be exception, such as presence of thin (3.5 cm) bentonite parting with high mercury in a 94.5 cm seam that increased the mercury content of seam by 83.4%.

A bentonite bed in the Aceguá (RS, Brazil) and Melo (Uruguay) areas: a highly crystallized montmorillonite

Two occurrences of bentonite beds have been studied in Aceguá (RS, Brazil) and Melo (Uruguay). Despite their different thicknesses, the two profiles seem to belong to the same bentonite bed because of their similar petrographical and mineralogical properties (color, density, composition). Both are composed of a nearly pure smectite mineral that is dominantly a montmorillonite. High amounts of Mg ions in the octahedral sheet elevate the layer charge approximately 0.5 per Si 4O 10. The layer charge is mostly compensated for by Ca 2+ ions in the interlayer. The rare earth element (REE) patterns are different from the post-Archean Australian average shale (PAAS) standard. In particular, when the data are PAAS normalized, the pattern obtained shows that the bentonite is LREE depleted. Thus, it cannot have originated from detrital sources. Despite diagenetic conditions that favor the crystallization of random illite/smectite mixed layers from detrital clays in the surrounding formations, the mineral reactions in the bentonite bed are limited to a few Al for Si substitutions in the tetrahedral sheets. The clays remain dominantly montmorillonite with abnormally high crystallinity.

The smectitic minerals in a bentonite deposit from Melo (Uruguay)

AbstractA nearly monomineralic 1.5 m thick bentonite bed sampled in Melo (Uruguay) appears to be a pure high-charge montmorillonite: [Si3.94Al0.06](Al1.40Fe3+0.11Ti0.02Mg0.49Mn0.01)O10 (OH)2Na0.01K0.08Ca0.18. However, contrasting swelling behaviours have been demonstrated by fitting the experimental X-ray diffraction patterns which were recorded on oriented preparations of the same sample in different saturation states. According to the expandability of the layers in the Ca-, K- and K-Ca-saturated (i.e. saturated first with K+ and subsequently with Ca2+) states, three ‘layer types’ were defined. Low-, intermediate-, and high-charge layers are fully, partly, and not expandable, respectively, after K-saturation. Collapse of high-charge layers is not reversible after subsequent Ca-saturation, probably because of tetrahedral substitutions. These three different layer types are segregated in two distinct randomly interstratified mixed-layer phases. Total surface area and cation exchange capacity are shown to depend on the interlayer cation composition.

Diffusion measurement of caesium in compacted bentonite

Caesium diffusion in compacted bentonite beds was studied to determine the transport mechanism. Apparent diffusion coefficients determined at various temperatures indicated a surface diffusion mechanism when compared with values estimated under pore diffusion control assumption. However, temperature dependence of the diffusion coefficients showed a tendency similar to that of pore diffusion. Chemical-transport calculations showed that the cation-exchange reactions possibly described the above disagreement in the mechanism.