Clay Mineral Fraction Research Articles

Elucidating the crystal-chemistry of Jbel Rhassoul stevensite (Morocco) by advanced analytical techniques

AbstractThe composition of Rhassoul clay is controversial regarding the nature of the pure-mineral clay fraction which is claimed to be stevensite rather than saponite. In this study, the raw and mineral fractions were characterized using various techniques including Fourier transform infrared spectroscopy and magic angle spinning nuclear magnetic resonance (MAS NMR). The isolated fine clay mineral fraction contained a larger amount of Al (>1 wt.%) than that reported for other stevensite occurrences. The27Al MAS NMR technique confirmed that the mineral is stevensite in which the Al is equally split between the tetrahedral and octahedral coordination sites. The29Si NMR spectrum showed a single unresolved resonance indicating little or no short-range ordering of silicon. The chemical composition of the stevensite from Jbel Rhassoul was determined to be ((Na0.25K0.20)(Mg5.04Al0.37Fe0.20☐0.21)5.61(Si7.76Al0.24)8O20(OH)4). This formula differs from previous compositions described from this locality and shows it to be an Al-bearing lacustrine clay mineral.

Complex permittivity and clay mineralogy of grain-size fractions in a wet silt soil

We determined the complex permittivity and clay mineralogy of grain-size fractions in a wet silt soil. We used one clay-size fraction and three silt-size fractions, measured permittivity with low error from [Formula: see text] with time-domain spectroscopy, and estimated mineral weight percentages using X-ray diffraction (XRD). The volumetric water contents were near 30%, and the temperature was [Formula: see text]. For the whole soil, standard fractionation procedures yielded 2.4% clay-size particles by weight, but XRD showed that the phyllosilicate clay minerals kaolinite, illite, and smectite made up 17% and were significantly present in all fractions. Above approximately [Formula: see text], all real parts were similar. Below approximately [Formula: see text], the real and imaginary permittivities increased with decreasing grain size as frequency decreased, and the imaginary parts became dominated by direct-current conduction. Similarly, below approximately [Formula: see text], the measured permittivity of montmorillonite, a common smectite, dominated that of the other clay minerals. Total clay mineral and smectite mass fractions consistently increased with decreasing grain size. Below [Formula: see text], a model with progressively increasing amounts of water and parameters characteristic of montmorillonite matches the data well for all fractions, predicts permittivities characteristic of free water in smectite structural galleries, and shows that the similar real parts above [Formula: see text] are caused by a small suppression of the high-frequency static value of water permittivity by the smectite. We conclude that the clay mineral content, particularly smectite, appears to be responsible for permittivity variations between grain-size fractions. Small model mismatches in real permittivity near the low-frequency end and the greater fractions of kaolinite and illite suggest that the total clay mineral content might have been important for the coarser fractions.

Iron phases identified in bentonite from the Lieskovec deposit (Slovakia) by variable-temperature Mössbauer spectroscopy

AbstractThe clay-mineral fraction of samples from a bentonite deposit in the Zvolenská kotlina Basin (Slovakia), formed by the alteration of andesitic pyroclastics, was dominated by Fe-bearing montmorillonite of relatively low charge. The total Fe content of the samples ranged from 3.58 to 5.81 wt.% Fe. Variable-temperature Mössbauer spectroscopy revealed that structural Fe(III) in smectite accounts for 70–90% of the total Fe in the unfractionated samples. No structural Fe(II) was observed by Mössbauer spectroscopy, but a small amount was detected by wet chemical analysis. The remainder of the Fe is present in oxide and/or oxyhydroxide phases dominated by poorly ordered goethite and hematite. The hyperfine parameters of the Mössbauer spectra for the raw bentonite are consistent with structural Fe(III) in an aluminosilicate phase mixed with varying quantities of hematite, goethite and possibly maghemite. Except in the fine fraction of sample L28, the hematite failed to undergo the Morin transition, as shown by the negative values for quadrupole splitting at 298 K and 6 K.

Contribuição dos constituintes da fração argila de solos subtropicais à área superficial específica e à capacidade de troca catiônica

A área superficial específica (ASE) está relacionada com diversas propriedades que definem o comportamento físico-químico dos solos. Neste trabalho, foi estudada a fração argila de 23 amostras de solos desenvolvidos a partir de diferentes materiais de origem, encontrados na região Sul do Brasil, a fim de estabelecer a contribuição dos principais constituintes para a ASE e a capacidade de troca de cátions efetiva (CTCe). Foram identificados os minerais por difratometria de raios X (DRX); quantificados por análise termogravimétrica (ATG); determinados os valores de CTCe e de ASEt (total), pelo método do etilenoglicol mono-etil éter (EGME); de ASEe (externa), pelo método BET-N2; e de ASEi, pela diferença dos valores obtidos entre os dois métodos. Essas determinações foram feitas antes e após os procedimentos seqüenciais de dissolução seletiva, que visavam remover: a matéria orgânica (argila-NaOCl); a matéria orgânica e os óxidos de Fe livres (argila NaOCl + DCB); e a matéria orgânica, os óxidos de Fe livres, a caulinita, a gibbsita e os minerais aluminossilicatados de baixa cristalinidade (argila-NaOCl+DCB+NaOH 5 mol L-1). Os solos estudados apresentaram grande variação na mineralogia da fração argila; para a maioria deles, a caulinita foi o mineral predominante, seguido dos argilominerais do tipo 2:1 expansíveis ou dos óxidos e hidróxidos de Fe e Al. Entre a ASEe e a CTCe da fração argila-natural não se constatou relação de dependência, devido ao recobrimento da superfície da amostra pela matéria orgânica. O efeito agregante dos óxidos de Fe promoveu diminuição de 21 % da ASEe. Os argilominerais do tipo 2:1 encontram-se em concentrações variando de 3 a 65 % da fração silicatada, contribuindo com valores médios de 1.054 mmol c kg-1 e de 202 m² g-1 para CTCe e ASEt, respectivamente. A ASEi contribui com 58 % da ASEt na fração argila com remanescentes de argilominerais do tipo 2:1.

Impact of contaminant mass reduction in residuum sediments on dissolved concentrations in underlying aquifer

Contamination of aquifers in many cases occurs because of the downward migration of contaminants from the land surface through the vadose zone, including through residuum sediments in case of fractured or karst aquifers. It is therefore critically important to characterize and model fate and transport of contaminants in both the vadose zone/residuum and the underlying saturated zone as part of remedial design for aquifer restoration. Residuum sediments usually contain a significant fraction of clay minerals and fluid flow is often predominantly in vertical direction. Groundwater modeling is useful when evaluating alternatives of contaminant mass removal in the vadose zone/residuum and their impact on the downgradient dissolved concentrations in the underlying aquifer. Two case studies illustrate applicability of such modeling. The first study includes modeling of the expected natural longevity of a residual DNAPL mass accumulation in the saturated residuum, and its comparison to the effects of remedial DNAPL mass removal. The second study includes modeling contaminant transport from a residual DNAPL source within the vadose zone to the saturated zone in the residuum and the underlying bedrock aquifer. The natural longevity of the unsaturated DNAPL source is modeled, focusing on resulting down-gradient concentrations in the residuum and bedrock aquifer. For comparison, two remedial actions are simulated; 90% mass removal and soil flushing.

Palaeoenvironmental significance of the clay mineral composition of Olduvai basin deposits, northern Tanzania

Quaternary deposits in the southeastern part of the Olduvai basin, northern Tanzania, consist of lake margin deposits, followed by a series of fluvial sediments. The clay mineral fraction of the lake margin deposits (Bed I and lower part of Bed II) is composed of smectite and subordinate illite. All smectite is largely dioctahedral and shows indications for a limited degree of irregular interstratification by illite. In the overlying fluvial deposits (Beds II–IV), illite is the most abundant clay mineral. Smectite only occurs in lower parts of the fluvial deposits (up to the middle of Bed III), where it generally shows a high degree of irregular interstratification. Differences in clay mineral composition between the lake margin deposits and the fluvial deposits record differences in sediment source area and degree of alteration. Dioctahedral smectite in the lake margin deposits and the oldest fluvial deposits is derived from a region with volcanic material extending to the east and south of the basin, which also supplies a certain amount of illite. Illite in the fluvial deposits of Bed IV originates from an area with a metamorphic bedrock to the west and north. Alteration of detrital clay minerals resulted in Mg-enrichment of dioctahedral smectite in part of the lake margin deposits and partial illitization of smectite in the older fluvial deposits. Formation of clay minerals during diagenesis or soil development is not documented by analysis of the total clay fraction.

Osmotic interpretation of overpressures from monovalent based triple layer model, in the Callovo-Oxfordian at the Bure site

Overpressures in the range 0.2–0.6 MPa are measured in the Callovo-Oxfordian at the Bure site. An understanding of the origin of these pressures is needed in order to justify the hydrogeologic and transport model within this formation. A prior modeling work on this topic [Gonçalvès, J., Violette, S., Wendling, J., 2004. Analytical and numerical solutions for alternative overpressuring processes: Application to the Callovo-Oxfordian sedimentary sequence in the Paris basin, France. J. Geophy. Research 109, B02110, doi:10.1029/2002JB002278.] concluded that an osmotic process could explain at least partly these overpressures. Indeed, in these previous calculations, the presence of a maximum salinity within the clay-rock layer together with a membrane property results in a “build up” in pressure. In this paper, the pore space concentration of water activity determining solutes is analyzed taking into consideration the contribution of ions compensating the structural charge of clay minerals and the fraction of clay minerals in the total lithology according to their vertical distribution. The calculations presented here show that taking into account the fraction of the CEC compensating cations present in the diffuse layer calculated using a recent model coupling transport and a triple layer model for sorption in clayey porous media allows a reasonable representation of observed vertical over-pressure profile.

Exchangeable Cation Hydration Properties Strongly Influence Soil Sorption of Nitroaromatic Compounds

Nitroaromatic compounds (NACs) are commonly found as soil contaminants in military training sites and manufacturing facilities, and may adversely affect human and ecosystem health. Exchangeable cation effects on p‐nitrocyanobenzene (p‐NCB) and 1,4‐dinitrobenzene (1,4‐DNB) sorption by the Webster (fine loamy, mixed, mesic Typic Haplaquoll) soil series A (WA) and B (WB) horizons were determined from batch sorption experiments. Smectite is the most abundant mineral in the horizons of this soil. Soil organic matter (SOM) removal increased p‐NCB sorption to the WA and WB horizons by ∼1.5 times, and increased 1,4‐DNB sorption to the A and B horizons by ∼1.2 to 2.2 times, respectively. Greater sorption of NACs by whole soils after SOM removal suggests that SOM suppresses p‐NCB and 1,4‐DNB sorption by soil mineral components. Clay (<2‐μm fraction) removal decreased p‐NCB sorption to the WA and WB horizons by ∼8 and 11 times, respectively; clay removal decreased 1,4‐DNB sorption to the WA and WB horizons by ∼4.8 and 6.7 times, respectively. Nitroaromatic compound sorption to different soil fractions was measured to identify the independent effects of soil components and exchangeable cations on sorption. For this purpose, 1,2,4‐trichlorobenzene (1,2,4‐TCB) and 1,4‐DNB sorption to two organic soils and a soil devoid of smectites, and p‐NCB sorption to whole soil and the soil clay‐sized fraction were determined. Exchangeable cation type was found to minimally affect sorption of 1,2,4‐TCB, p‐NCB, and 1,4‐DNB by SOM. Sorption of p‐NCB to homoionic soil clay‐sized fractions was generally greater for clays saturated with monovalent cations than clays saturated with divalent cations. Greater p‐NCB sorption followed decreases in cation hydration energy in the order Na+ < Li+ < < K+ < Cs+ and Ca2+ < Ba2+ < Mg2+ Similar trends were observed for whole‐soil samples exchanged with these cations. This indicates that differences in nitroaromatic sorption are due to exchangeable cation effects on the clay mineral fraction.

Sorption study of 25 volatile organic compounds in several Mediterranean soils using headspace–gas chromatography–mass spectrometry

A sorption study of 25 volatile organic compounds (VOCs) in different agricultural soils was carried out by using headspace–gas chromatography–mass spectrometry. The extraction of the VOCs from soil samples was carried out following the EPA method with some differences such as addition of potassium chloride and different instrumental conditions which provide higher sample throughput. In addition, a complementary study on several procedures for soil fortification with VOCs was also assayed, fortification with minimal sample handling was selected in order to minimise evaporation losses of the VOCs. The effect of clay minerals (7.0–69.7%) and organic carbon (0.2–3.5%) contents on acid and alkaline (pH 5.3–8.8) soils were evaluated. Based on the results, all compounds assayed were more sorbed in alkaline soils than acid ones; chlorobenzenes interact more strongly with agricultural soils than do alkylbenzenes. The organic carbon content affects the sorption of 25 VOCs in alkaline soils (the highest sorption was found for the most organic soil), while in acid soils VOC sorption increases as the organic carbon content decreases. The clay mineral fraction plays an important role in the sorption of VOCs in acid soil owing to π-/ n-electron interactions, this effect being more marked for chlorobenzenes.

Defloculação de massas cerâmicas triaxiais obtidas a partir do delineamento de misturas

A avaliação das propriedades reológicas de suspensões é uma parte importante no desenvolvimento de massas cerâmicas. Em processamento via úmida, o fator reológico pode ser limitante ao emprego tanto de determinada matéria-prima quanto do agente defloculante. Nesse sentido, a abordagem integrada da formulação de massas cerâmicas triaxiais juntamente com o auxílio do delineamento de misturas visa a diminuição do número de experimentos necessários para determinação de propriedades ótimas do sistema em estudo, sejam essas propriedades físicas ou reológicas. O presente trabalho tem como objetivo estudar a defloculação de massas cerâmicas triaxiais (argila/feldspato/quartzo) obtidas a partir do delineamento de misturas, através da obtenção de um modelo matemático que represente a concentração ideal de defloculante apresentada por cada composição cerâmica do diagrama triaxial. Para tanto, foram preparadas suspensões aquosas (55% de sólidos), utilizando silicato de sódio anidro (razão molar SiO2:Na2O igual a 1.3) como agente defloculante. Curvas de defloculação foram obtidas com o objetivo de determinar a concentração ideal de defloculante. Os resultados mostraram que o aumento na fração argilosa das composições estudadas conduziu a um aumento no consumo de defloculante.

Arsenic-bearing smectite from the geothermal environment

AbstractArsenic-rich scales are widely associated with geothermal fields and constitute a potential hazard to human health. Such arsenic has hitherto been reported to be almost exclusively hosted by sulphide or oxide phases or occurring as surface species. We report here, however, the occurrence of an arsenic-rich (1500 to 4000 mg kg—1 As) smectite from geothermal precipitates from a geothermal field in northwestern Japan and present evidence that the arsenic is predominantly hosted within this silicate mineral.Consistently ∼80% of the total arsenic determined in these geothermal precipitates was found by selective chemical extractions to be associated with an operationally defined clay mineral fraction, with lesser proportions being associated with operationally defined amorphous silica, Fe oxide and sulphide fractions. Analysis by XRD, ATR IR and XRF showed the clay fraction to be dominated by Mg-rich trioctahedral smectite.Arsenic K-edge XAS spectra of the smectite suggested the dominance of As(III)-O coordinated species with significant contributions from As(V)-O coordinated species. Both XPS and a magnesium chloride chemical extraction indicated minimal adsorption of arsenic on smectite surfaces suggesting that the arsenic was predominantly either dissolved within the smectite or occurred within mineral occlusions. No such occlusions greater than 1 μm in size were observed in the As-rich smectites.The potential occurrence of arsenic-bearing clays should be considered when determining remediation strategies for geothermal environments or evaluating risks associated with the industrial usage of geothermal precipitates.

Behavior of lithium and its isotopes during weathering of Hawaiian basalt

We examined the pedogenic behavior of lithium (Li) and its isotopes in Hawaii by sampling same‐age lava flows under mean annual rainfall ranging from 18 to 300 cm. Lithium concentrations in these soils vary from 1 to 29 ppm. Whereas Na, K, and Ca are completely leached from the soil at the most humid and severely weathered site, Li, Mg, Si, and Al show significant retention due to their association with secondary clay minerals. In these soils, allochthonous Li delivered in marine and mineral aerosol mixes with basalt‐derived Li, modifying the isotopic composition of the Li pool. The ability of soil to retain Li is related to its effective cation exchange capacity, which in turn is governed by rainfall and leaching intensity and their resulting effect on mineralogy. Lithium isotope ratios have a large range (δ7Li of −0.4 to 14‰), well beyond the intershield or temporal variation of the lavas of Hawaiian volcanoes (2.5 to 5.7‰). The most unweathered samples have δ7Li (∼5–6‰) comparable to that of Mauna Kea and Mauna Loa lavas. The Li concentrations and isotope ratios together suggest that in arid to subhumid sites there is a net addition of isotopically heavier lithium from marine aerosol but that under greater rainfall there is a net loss of Li, and isotopically light Li is preferentially retained. Thus, during incongruent weathering of primary minerals, Li, especially 6Li, is adsorbed and sequestered by the clay mineral fraction of high cation exchange capacity soils in drier regions but lost from highly weathered, low cation exchange capacity acidic soils in the wetter regions.

Neogene palaeoceanography of Chatham Rise (Southwest Pacific) based on sediment geochemistry

Approximately one thousand sediment samples from ODP Site 1123 on the Chatham Rise, east of New Zealand, have been examined for inorganic elemental concentrations. ODP 1123 provides a record of sediment drift deposition under the Deep Western Boundary Current, the main inflow of deep water to the Pacific Ocean since the Early Oligocene, though a major hiatus spans the late Early Oligocene to the Early Miocene. Normalisation of the elemental concentrations by aluminium was used to allow for the effects of variable carbonate dilution. The elemental ratios were used as proxies for sediment composition and as palaeoceanographic indices. The samples were collected at a resolution designed to sample adequately any variation in elemental ratios at the scale of the Milankovitch orbital cycles. The sampled intervals span the Early Oligocene, Early Miocene, mid-Miocene and Late Pleistocene to Recent. Anomalous Si/Al, K/Al, Ti/Al values in the upper Pleistocene section, often associated with horizons of low carbonate, are attributed to tephras derived from North Island. Not all of the tephras detected geochemically had been detected visually in the cores. A total of 37 tephra events between 1.17 Ma BP and the present are recognised based on this and the shipboard investigations. The tephra events cluster at intervals of approximately 326 000 years (326 ka) perhaps due to variations in eruption frequency on North Island and/or to variations in the regional palaeowind intensity and direction. In the Late Pleistocene to Recent P/Al (inferred nutrient availability), percent calcium carbonate (%CaCO 3) and Ba/Al (inferred productivity) varied regularly at a period of 40 000 years with these factors lagging minimum global ice volumes (interglacials). During the mid-Miocene %CaCO 3, Ba/Al, P/Al and Si/Al all gradually increased with %CaCO 3 and P/Al showing regular 138 000-yr cyclicity and Ba/Al showing 44-ka cyclicity. Inferred productivity (Ba/Al) may have been rising in association with increasing nutrient availability (P/Al) at the same time as increased vigour of the Deep Western Boundary Current that was connected to a period of rapid ice-sheet growth in Antarctica. In the Early Miocene P/Al and Si/Al were much higher than subsequently and both %CaCO 3 and P/Al exhibited 131 000-yr cycles. By far the highest nutrient levels and inferred productivity at this site apparently occurred during the Early Oligocene as revealed by long-term changes in P/Al and Si/Al. A progressive rise in K/Al, but stable Ti/Al from the Early Oligocene to the Recent probably indicates increased proportions of illite in the clay mineral fraction of the drift sediments caused by increased flux of debris from the Southern Alps.

Control of 36Cl production in carbonaceous shales by phosphate minerals

When using 36Cl to date very old groundwater in regional aquifer systems, knowledge of the subsurface 36Cl input into the aquifer system is essential. Although 36Cl can be produced through nuclear reactions in the subsurface, in many situations, the input of 36Cl into sedimentary aquifer systems by this avenue of production can be neglected. This is a valid assumption when investigating long-flowpath groundwater systems composed of sandstones, limestones, and shales of typical composition. These rock types are not sufficiently enriched in radioactive elements to produce significant 36Cl in the deep subsurface. Carbonaceous shales, on the other hand, can concentrate the radioactive elements necessary to produce significant 36Cl in the deep subsurface. Chlorine-36 ratios ( 36Cl/Cl) for a suite of Late Devonian and Pennsylvanian carbonaceous shales were calculated from bulk-rock chemistry as well as measured using accelerator mass spectrometry. The poor agreement between calculated and measured ratios is the result of the assumption of chemical homogeneity used by the calculation algorithm, an assumption that was not satisfied by the carbonaceous shales. In these shales, organic matter, clay minerals, and accessory minerals are heterogeneously distributed and are physically distinct on a micron-order scale. Although organic matter and clay minerals constitute the overwhelming bulk of the shales, it is the phosphate minerals that are most important in enhancing, and suppressing, 36Cl production. Minerals such as apatite and carbonate-apatite (francolite)—by including uranium, rare earth elements (REEs), and halogens—have an important impact on both neutron production and thermal neutron absorption. By incorporating both uranium and fluorine, phosphate minerals act as neutron production centers in the shale, increasing the probability of 36Cl production. By incorporating REEs and chlorine, phosphate minerals also act to shield 35Cl from the thermal neutron flux, effectively suppressing the production of 36Cl. To reconcile the measured 36Cl ratios with the ratios calculated assuming chemical homogeneity, the shales were artificially split into three fractions: organic, clay mineral, and phosphate mineral. Neutron production was calculated separately for each fraction, and the calculation results demonstrated that the phosphate fraction exerted much more control on the 36Cl ratio than the organic or clay mineral fractions. By varying the uranium and chlorine contents in the phosphate fraction, a new, heterogeneous 36Cl ratio was calculated that agreed with the measured ratio for the overwhelming majority of the carbonaceous shales. When using rock chemistry to calculate the 36Cl ratio, rock types that show mineralogical heterogeneity on a micron scale can be divided into bulk fractions and accessory fractions for separate calculations of neutron production and neutron absorption. In this manner, a more accurate, heterogeneous 36Cl ratio can be calculated for the rock as a whole.

Caracterização e gênese de perfis plínticos desenvolvidos de arenito do Grupo Bauru: II - mineralogia

Nas paisagens do norte e oeste do estado de São Paulo, plintita e petroplintita constituem feições que se repetem com freqüência sobre os arenitos cretácicos da Formação Adamantina (Grupo Bauru). Com o objetivo de avaliar as características mineralógicas desses materiais e estudar sua gênese, selecionaram-se dois perfis de solos representativos da paisagem local e constituídos por feições plínticas, petroplínticas e mosqueados. O estudo foi realizado na baixa meia encosta de uma vertente situada na Estação Experimental de Agronomia de Pindorama, do Instituto Agronômico (IAC), região norte do estado de São Paulo. Com base nas observações em microscópio de varredura e microanálise pontual realizadas em glébulas selecionadas, bem como nas análises mineralógicas da fração argila desferrificada e dos óxidos de ferro de todos os horizontes dos perfis estudados, constatou-se que caulinita, hematita e goethita são os principais constituintes da fração argila dos nódulos e horizontes estudados. Os minerais mica, gibbsita e anatásio complementam a mineralogia da fração argila das glébulas, assemelhando-se em constituição ao material interglebular e aos demais horizontes dos perfis. Quartzo, feldspatos potássicos, traços de feldspatos sódicos e ilmenita foram identificados como componentes da fração silte e areia dos nódulos. A presença constante de minerais alteráveis nas glébulas petroplínticas é evidência de que a gênese desses materiais está relacionada com a ferruginização do saprolito. Este fato, associado aos baixos teores de Al na estrutura dos óxidos de ferro das glébulas, evidencia sua formação em condições hidromórficas, supostamente relacionadas com a solubilização e mobilização do ferro ferroso, lixiviado da paisagem a montante e reprecipitado na zona de vadosa, onde os maiores potenciais de oxidação favoreceram a segregação e a precipitação do ferro.

Sorption and Desorption of Pesticides by Clay Minerals and Humic Acid‐Clay Complexes

In soils, organic matter and minerals are often associated such that it is unclear how the presence of the former component influences the sorptive properties of the latter one. In this study, sorption and desorption of the herbicides 4,6‐dinitro‐o‐cresol (C7H6N2O5) and dichlobenil (C7H3Cl2N) by Ca2+–, K+–smectites, and humic acid–smectite complexes, was investigated using batch‐equilibrations and x‐ray diffraction (XRD). Greater sorption of 4,6‐dinitro‐o‐cresol compared with dichlobenil was observed for both smectites and humic acid–smectite complexes. For both pesticides, K+ smectites were more effective sorbents than Ca2+ smectites, with the lower charge‐density clay (SWy‐2) displaying a greater sorption capacity than the higher charge‐density clay (SAz‐1). The presence of humic acid did not impact pesticide sorption by K+ clays, but could enhance or suppress pesticide sorption by Ca2+–clays. A composite model for estimating pesticide sorption, which assumes mineral and organic matter function individually as sorbent phases, predicted sorption within a factor of 0.8 to 1.5 times the measured values. Humic acid did not contribute to pesticide desorption hysteresis in K+–humic acid–clay complexes, but was a source of hysteresis in the corresponding Ca2+ complexes. The basal spacings of K‐SWy‐2 and humic acid‐modified K‐SWy‐2 increased gradually from approximately 10.4 to 12.2 Å with increasing 4,6‐dinitro‐o‐cresol loadings. Also, XRD patterns of humic acid‐modified and unmodified K‐SWy‐2 smectite clays were found identical. These results demonstrate the intercalation of 4,6‐dinitro‐o‐cresol and suggest that humic acids are restricted to the external surfaces of clay tactoids. Together, these results indicate that clay mineral fractions in soils, including those with organic coatings, may play an important role in the retention of certain pesticides.

Problems with argon: K–Ar ages in Gulf Coast shales

K–Ar ages of Gulf Coast shales do not correspond to diagenetic ‘events’, but are a combination of inherited detrital ages and authigenic overprints. The key to understanding K–Ar ages of shales is the fate of the 40Ar when the host minerals dissolve or react during the illitisation reaction: is 40Ar incorporated within the illite, or is it lost to the porewaters? Ages are modelled as due to a single illitisation reaction, assuming a uniform precursor sediment and open-system import of K. Both whole-rock and clay mineral fraction data can be reproduced by modelling. The 40Ar incorporated into illite is a surprisingly high proportion (75–100%) of that produced within detrital phases (smectite, K-feldspar, mica), perhaps implying that the illitisation reaction is a solid-state transformation, or that 40Ar is imported into the shales along with incoming K. The model does not provide a good fit to the data below 4000 m, when the illitisation reaction is apparently complete, but shales lose 40Ar by solid-state diffusion or recrystallisation.

Silicate diagenesis in deep-sea sediments from the Tonga forearc (SW Pacific): a strontium and Rare Earth Elements signature

87Sr/ 86Sr isotopic ratios, strontium and Rare Earth Element concentrations obtained on volcano-sedimentary rocks and separated clay mineral and zeolite fractions reveal a formation by pore water–volcanic rock interaction for most of the hydrous silicate minerals of the Site 841 ODP collected from the Tonga forearc. Unusual strontium concentrations and isotopic ratios recorded in the Miocene tuffs associated with specific REE patterns indicate that the formation of these hydrous silicates does not follow a simple burial diagenesis model, but was related to the cooling of intruding basaltic sills in the Miocene volcano-sedimentary series. Migration of strontium into the pore water in response to the heat flow induced the formation of Sr-bearing zeolites such as clinoptilolite, heulandite and chabazite. No evidence of any influence of a further thermal pulse in the Eocene rhyolitic tuffs could be found. As recorded by the chemistry of their clay mineral fraction, the rhyolitic tuffs developed a polyphasic diagenetic process, which might have been influenced by a possible circulation of a fluid into structurally weak areas. © 2000 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS Tonga / silicates / strontium / Rare Earth Elements / diagenesis / heat flow

'Clay hutches': a novel interaction between bacteria and clay minerals.

Biofilm formation on a low-energy substratum floating on the surface of a water column overlying a polychlorinated biphenyl (PCB)-contaminated sandy clay soil was followed by light and electron microscopy. The biofilms that developed consisted of a dense lawn of clay aggregates, each one of which contained one or more bacteria, phyllosilicates and grains of iron oxide material, all held together by bacterial extracellular polysaccharides (EPS). The clay leaflets were arranged in the form of 'houses of cards' and gave the aggregates the appearance of 'hutches' housing the bacteria. Interestingly, although the soil is poor in carbon, and the weakly bioavailable PCBs constitute the principal source of carbon in this system, the bacteria contained electron-transparent structures presumed to be carbon storage granules. These, and the EPS material present in the hutches, indicate that carbon is not limiting in this system and, as PCBs have been found associated with the clay mineral fraction of the floating substratum, the clay particles may serve as carbon shuttles. The interesting possibilities that the 'clay hutches' may represent a 'soil microhabitat', a 'minimal nutritional sphere' and an 'effective survival unit' for autochthonous bacteria are noted. The formation of clay hutches by bacteria would seem to merit further investigation, particularly regarding their roles in bacterial processes in soil and in geological processes.

Illitization in a Paleozoic, peat-forming environment as evidence for biogenic potassium accumulation

Potassium enrichment is common in surface horizons of many paleosols and has been attributed to the formation of authigenic illite from smectites during burial diagenesis. In such cases, the presence of primary smectite is key to the illitization process, which then results in potassium enrichment as a byproduct. Potassium enrichment in paleosols where no primary smectite was present, however, requires a different process of accumulation. A Lower Pennsylvanian, tropical Ultisol from the Southern Anthracite Field in Pennsylvania is enriched in potassium in the surface horizon and formed in an environment unfavorable for smectite accumulation. Kaolinite dominates clay mineral fractions of soils that form in such highly weathering environments. In the absence of primary smectite to scavenge potassium during illitization, there should be no potassium enrichment during burial diagenesis. However, in Lykens Valley paleosol, the pattern of potassium concentration, organic matter distribution and authigenic illite growth suggests that organic matter may have provided an important source of potassium for illite formation. Illitization of kaolinite in the organic-rich surface horizon is complete but, illite coexists with kaolinite in the B horizon. Within the paleosol matrix, illitization is more complete in areas of high organic concentration where organic fragments often have illite haloes with illite whorls growing from their edges. We propose that the potassium enrichment both in the soil profile and in the overlying coal is the result of the luxury uptake of potassium by peat-forming plants (nutrient intake at rates exceeding metabolic requirements). Further, potassium immobilized by adsorption on the peat provided interlayer cations for illite authigenesis to begin. If, as we suggest, luxury nutrient uptake during peat formation sequestered sufficient potassium to promote later illite formation, the pattern and extent of illitization within the paleosol profile reflects early biogenic nutrient accumulation. Thus, illite, in some paleosols, may represent fossil evidence for a biological process: the luxury uptake of potassium.