Expandable Layer Silicates Research Articles

Nitrogen isotope homogenization of dissolved ammonium with depth and 15N enrichment of ammonium during incorporation into expandable layer silicates in organic-rich marine sediment from Guaymas Basin, Gulf of California

Sedimentary nitrogen isotopic ratios are used as a proxy for ancient biogeochemical cycles on Earth's surface. It is generally accepted that sediment hole tops record primary signatures because organic nitrogen (ON) is predominant in this part of the hole. In contrast to such early to middle diagenetic stages, it is well known that heavier nitrogen isotope 15N tends to enrich in sedimentary rocks during later diagenetic and metamorphic stages. However, there are some critical gaps in our understanding of nitrogen isotopic alteration associated with abiotic processes during early-middle diagenesis. In this study, we examined the isotope ratios of ammonium nitrogen in interstitial water (IW) and total nitrogen (TN), including exchangeable ammonium and mineral nitrogen, in the solid-phase of organic-rich-sediment recovered by International Ocean Discovery Program (IODP) Expedition 385 cores drilled in the Guaymas Basin, Gulf of California, that contained ammonium-rich IW. The isotopic ratios (δ15N value) of TN are the most variable with depth compared to any other type of nitrogen. This variation can be interpreted as reflecting changes in the water mass environment in the basin caused by glacial–interglacial climate changes, modifying the δ15N values of the marine primary producers. Thus, the δ15N value of TN is a proxy for environmental change in the basin, while each component of TN shows different trends. The δ15N values of IW and exchangeable ammonium did not exhibit significant changes with depth, but the latter values are about 3 ‰ enriched in 15N. This may be due to advective transport of solute into adjacent layers followed by the formation of an isotopic equilibrium between IW and exchangeable ammonium in the case of fast sediment accumulation rate. The δ15N value of exchangeable ammonium is the highest among the other types of nitrogen with one exception, where the δ15N value of TN is the highest. The calculated δ15N values of ON based on mass balance are almost the same as those of associated TN in the shallow sediment layers (< 150 m below seafloor), but the difference in the δ15N values of TN and ON are significant in the deeper layers, where proportions of ON contents are <50%. In particular, in the layer where the δ15N value of TN is the highest, that of ON shows an even higher value and the difference reaches 3.5 ‰. The δ15N values of mineral nitrogen are similar to that of IW ammonium except the surface layers. Under such conditions, when δ15N value of TN is intermediate between those of mineral nitrogen and exchangeable ammonium, calculated δ15N value of ON is close to that of TN. On the other hand, if δ15N value of TN is out of the range between mineral nitrogen and exchangeable ammonium, it causes further difference in δ15N value of ON. It means that the fluctuation of δ15N values of TN is reduced relative to those of ON through depth. It has been considered that δ15N value of TN in sediment is similar to that of ON, and changes in the δ15N value of TN due to diagenesis are limited, but in such environment ON fluctuations over depth may be slightly underestimated.

Smectite clay minerals reduce the acute toxicity of quaternary alkylammonium compounds towards potentially pathogenic bacterial taxa present in manure and soil

Quaternary alkylammonium compounds (QAACs) are a group of cationic surfactants which are disinfectants with numerous industrial and agricultural applications and frequently released into the environment. One recent hypothesis is that bacteria present in soil will be protected from acute toxic effects of QAACs in the presence of expandable layer silicates due to interlayer sorption. We therefore studied bacterial growth kinetics with high temporal resolution and determined minimal inhibitory concentrations (MICs) of two QAACs, benzyldimethyldodecylammonium chloride (BAC-C12) and didecyldimethylammonium chlorid (DADMAC-C10), for eight strains of different bacterial taxa (Escherichia coli, Acinetobacter, Enterococcus faecium, Enterococcus faecalis, and Pseudomonas fluorescens) in relation to QAAC sorption to smectite and kaolinite. The MICs of BAC-C12 and DADMAC-C10 were in the absence of smectite and kaolinite in the order of 10 to 30 µg mL−1 and 1.0 to 3.5 µg mL−1 for all strains except the more sensitive Acinetobacter strain. For all tested strains and both tested QAACs, the presence of smectite increased apparent MIC values while kaolinite had no effect on MICs. Sorption curves without bacteria showed that smectite sorbed larger amounts of QAACs than kaolinite. Correcting nominal QAAC concentrations employed in toxicity tests for QAAC sorption using the sorption curves explained well the observed shifts in apparent MICs. Transmission electron microscopy (TEM) demonstrated that the interlayer space of smectite expanded from 13.7 ± 1 Å to 19.9 ± 1.5 Å after addition of BAC-C12. This study provides first evidence that low charge 2:1 expandable layer silicates can play an important role for buffering QAAC toxicity in soils.

Fatty acids promote fulvic acid intercalation by montmorillonite

Organic matter (OM) adsorbed to the interlamellar surfaces of swelling clay minerals is particularly stable, with residence times as long as 2000 to 10,000years. Intercalation of OM by smectitic clay minerals may therefore increase the stable C reservoir in both soils and sediments. However, the precise mechanisms of adsorption and intercalation of the myriad organic constituents by swelling clay minerals in soils and sediments remain unresolved. Model adsorption experiments with montmorillonite (Mt), a ubiquitous expanding layer silicate, showed that environmentally relevant fatty acids possessing long-chain alkyl C facilitate the adsorption and subsequent intercalation of fulvic acid (FA). Palmityl palmitate, comprising two C16:0 chains, promotes greater FA adsorption than does stearic acid, the latter possessing a single C18:0 chain. Furthermore, maximum FA interlayer adsorption was observed where the fatty acid was adsorbed to Mt prior to introduction of the humic material; the alkyl C chains of the fatty acid evidently serving as attachment points to enhance uptake and optimize placement of FA at the margins of the interlayer space. These data reveal a previously unknown mechanism through which humic materials become intercalated by expanding layer silicates, thus contributing to the stable OM pool in soils and sediments.

Origin of Talc, Iron Phosphates, and Other Minerals in Biosolids

Biosolids (i.e., sewage sludge) contain as much inorganic as organic materials. The inorganic materials include local soils, sediments, and other materials washed down residential, industrial, and storm drains. In this study, the chemistry and mineralogy of the inorganic materials in New York, NY (NYC) biosolids applied to soil surfaces in western Texas from 1992 to 1999 were examined. Inorganic residues were 44 to 77% of whole biosolid weights after oxidation of the organics with H2O2 Inorganic residues consisted largely of silt‐ and clay‐size particles with smaller amounts of fine and very fine sand. Selective dissolution and chemical analyses of the residues indicated &gt;20% Fe phosphates, formed during anaerobic digestion, that contained Al, As, Cr, Cu, Hg, and Zn. X‐ray diffraction, selective dissolution, and surface area analyses showed that the Fe phosphates were poorly crystalline. The mostly quartz and feldspar sands also contained glass shards, textile fibers, and zircon grains. New York City area biosolids contained quartz, feldspars, kaolinite, mica, and expandable layer silicates. Electron microscopy and x‐ray spectra of the clay fractions revealed aggregates of layer silicates and Fe phosphates containing Al, Ca, and Ti. Pigment TiO2, commonly used in foods and cosmetics, contributed Ti to the layer silicate aggregates. Talc derived from cosmetic products was identified in silt and clay fractions by x‐ray diffraction and electron microscopy. Less talc occurred in the 1999 and 1997 biosolids relative to 1992, 1993, and 1994 biosolids suggesting reduced use of talc products. Nationally, cosmetic talc use decreased during that time, probably due to health concerns.

Permanganate-mediated dissolution of hydroxy-Cr interlayers from montmorillonite

Expandable layer silicates may be intercalated with a variety of hydrolyzed metal cations. Chromium is among those metals that have received the most attention, due to its presence in many polluted environments and toxicity when present as Cr(VI). Of all potential oxidants of Cr(III), high-valence Mn remains one of the most potent. Manganese(VII) species were, therefore, introduced to suspensions of hydroxy-Cr(III) interlayered montmorillonite to: (i) establish the role that Mn(VII) plays in the oxidation of intercalated Cr(III) and, (ii) attempt to intercalate Mn species that would impart catalytic activity to the montmorillonite. The oxidation of intercalated hydroxy-Cr(III) polymers by Mn(VII) was rapid initially then decreased sharply due, in part, to accumulation of residual Mn within the interlayer. As this intercalated Mn was present as Mn(II), the oxidative capacity of the clay was not enhanced. Furthermore, the hydroxy-Cr(III) pillars readily collapsed upon oxidation, eliminating much of the gallery space required for effective catalysis. However, the intercalation of Mn onto montmorillonite by means of reduction, described for the first time in this paper, may find application in the synthesis of related pillared clay catalysts.

Sorption of two polar herbicides in soils and soil clays suspensions

Adsorption of the polar herbicides thiazafluron (1,3-dimethyl-1-(5-trifluromethyl-1,3,4-thia-diazol-2-yl)urea) and metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one) in the aqueous suspension of five soils of southern Spain, their respective clay fractions (with diverse organic carbon content and clay mineralogy) and model pure clay minerals has been monitored as an integrated study to assess the role of the diverse colloidal soil components and their solid/solution ratio, as relevant to the transport of contaminants by particulate matter in water. Adsorption isotherms obtained were analysed and fitted to the logarithmic form of the Freundlich equation and adsorption coefficients K f calculated. Thiazafluron adsorbs on soils, soil clays and model mineral sorbents to a higher extent than the herbicide metamitron due to their different molecular structure. The sorption of both herbicides in clay fractions increases with decreasing solid/solution ratio. The highest K f value at high solid/solution ratio for both herbicides is found in a saline soil with its clay fraction predominantly composed of an altered illite mineral which behaves as a montmorillonite. Thiazafluron and metamitron also show the highest adsorption capacity (at low solid/solution ratio) on a predominantly montmorillonitic clay fraction of low cation exchange capacity (CEC), whereas low adsorption is found on a montmorillonitic clay fraction of high CEC. The negative influence of the clay CEC is confirmed in adsorption studies on pure clay minerals suspensions. The sorption of both herbicides by soil clays after removing organic matter (OM), shows that contribution of the colloidal OM is very low for thiazafluon, although rather important for metamitron. The influence of the different nature of the OM associated to the clay fractions of diverse soils is suggested. The mineral components of the soil clays, especially expandable layer silicates such as montmorillonite and a type of altered illite, are revealed to be responsible for the adsorption and hence the transport of these polar herbicides by waters in contact with soils or fine-size soil separates. Not only the relative amounts of the organic and inorganic components are important, but also the surface properties and the accessibility of the functional active groups of the herbicide molecule to those surfaces.

Oxidative dissolution of hydroxy-cr interlayers from montmorillonite

Expandable layer silicates can sorb considerable amounts of hydrolyzed Cr(III). This study was conducted to determine the effect of the interlayer environment on the oxidative dissolution of intercalated hydroxy-Cr polymers. Seven montmorillonite clays with pure hydroxy-Cr interlayers and three with coprecipitated hydroxy-Cr and -Al interlayers were suspended in dilute NaOCI. The supernatant Cr concentrations of each were monitored over time. As the amount of Cr held by the pure Cr clays increased, the time required for complete interlayer dissolution decreased. Chromium(III) oxidation was consistent with a proton-catalyzed, second order reaction. Aluminum in the AlCr interlayers decreased, but did not stop, the Cr(III) oxidation rate by forming a barrier around the AlCr polymers. The slow but sustained oxidation of Cr(III) from the AlCr clays was due to diffusion of Cr through the polymers. Despite modest protection offered by Al, all intercalated Cr(III) was rapidly oxidized in the presence of OC1 −.

Properties of Hydroxy-Al and -Cr Interlayers in Montmorillonite

AbstractIn environments contaminated with Cr, the interlayers of expandable layer silicates may serve as sinks for this potentially toxic element. As a means of determining the potential for smectites to serve as sinks for Cr, the precipitation products of Al and Cr in the interlayers of a montmorillonite were examined. Five montmorillonite (SWy-1) clay suspensions were treated with preweighed amounts of AlCl3 and CrCl3 to give five Al/(Al + Cr) molar ratios (1.0, 0.67, 0.5, 0.33, 0) with a total trivalent cation (M3+) concentration of 600 cmol(+)/kg clay. The clay-cation suspensions were titrated with 0.1 N NaOH to give a NaOH/M3+ molar ratio of 2.5. Analysis of the solid-phase reaction products showed that the cation exchange capacity and specific surface of all clays were reduced. Chromium reduced the exchangeability of the interlayers while Al increased the thermal stability. X-ray diffraction analysis revealed that all Al-containing interlayer materials formed similar gibbsitelike polymers. Data from infrared spectroscopy indicated that both Al and Cr were present within the same polymer. Differential thermal analysis and thermogravimetric tracings showed that the rapid collapse of the interlayer in the Cr end-member upon heating was due to a low-temperature loss of hydroxyls. It was not possible to identify all interlayer structures in the Cr end-member. Data from X-ray photoelectron spectroscopy showed all Cr to be Cr(III). Displacement of the interlayer material became more difficult as Cr content increased. The least exchangeable interlayers, therefore, may be found in environments containing the most Cr.

Eolian influence on oxygen isotope abundance and clay minerals in soils of Hokkaido, northern Japan

Oxygen isotope geochemistry and clay mineralogy of five pedons developed on Pleistocene terraces in Hokkaido were studied to evaluate the eolian dust contribution from inland China. Soil quartz derived from the dust has fine and uniform particle-size distribution. The particle-size distribution is distinguishable from that of the underlying local terrace deposits which have a variable texture. Oxygen isotope compositions (δ 18O) of fine quartz (1–10 μm) from surface and subsurface horizons fell within a narrow range of + 16.3 to + 17.0ℵ. The values are consistent with those of quartz present in long-distance eolian dusts. By contrast, higher δ 18O values were obtained for quartz of the same size from the underlying Pleistocene terrace deposits ( δ 18O = + 17.5 to + 20.0ℵ ), indicating a larger contribution of low-temperature authigenic quartz derived from local sedimentary rocks. Clay minerals of the surface horizons are characterized by well-defined X-ray diffractograms consisting mainly of micaceous minerals with minor amounts of kaolinite. The layer silicates are inherited from eolian micas. With depth, halloysite, with or without random interstratification with 2:1 expandable layer silicates, becomes common. High-charge smectite and halloysite are major constituents in some clays from the underlying local alluvium which represent detrital materials from weathering products of local parent rocks. The eolian addition was not obvious in a residual pedon on low hills of green tuff, probably due to frequent rejuvenation of the landscape surface by erosion.

Intercalation of copper metal clusters in montmorillonite

EXPANDABLE layer silicates such as montmorillonite can be converted to efficient heterogeneous catalysts by introducing catalytically active sites or guest species between the layers or on the external surfaces. Attempts to produce intercalated zero-valent transition-metal particles in layer silicates, by hydrogen reduction for example, have, however, failed: the layers tend to collapse1, sometimes followed by deposition of metal particles on the external surfaces2–4. Here we describe the successful intercalation of copper metal clusters of 4–5 A in montmorillonite by in situ reduction of Cu2+ ions using ethylene glycol. These metal-cluster intercalates were stable up to at least 500 °C. The clusters prop the silicate layers apart, much as metal oxides do in pillared clays5, and may thus be able to introduce unique catalytic product selectivity through a molecular sieving effect similar to that in cluster-loaded zeolites. As metal clusters of these dimensions behave very differently from the bulk metal6, intercalates of this sort may prove to be versatile catalysts.

Epitaxial Growth of Nickel(II) Hydroxide on layer Silicate and Derived Nickel‐(Layer Silicate) Nanocomposite

Nickel(II) hydroxide crystals were deposited on a substrate of layer silicates from the liquid phase by decomposing nickel(II) ammine complex solutions dispersed with expandable layer silicates; the (001) planes of the nickel(II) hydroxide aligned parallel to the basal (001) plane of the layer silicates. From these layer silicate complexes with nickel(II) hydroxide, nickel‐coated layer silicate composite particulates were derived. Moreover, metal–ceramics alloys, composed of nickel containing uniformly dispersed layer silicates several tens of angstroms in thickness, were produced by sintering the metal‐coated ceramic powders.

Longevity of Bentonite as Buffer Material in a Nuclear-Waste Repository

Longevity of bentonite as a nuclear waste barrier depends on mineralogical stability of the smectite component of the clay as well as on its ability to perform as a buffer in the repository for long periods of time. Processes affecting the stability of smectites under the geochemical and thermal conditions in a high-level nuclear waste repository can be construed by examining the geological analogues and the results of the hydrothermal experiments. The physical and chemical properties of bentonite seem to be related to the nature of smectites in them. Smectites represent a range of expandable layer silicates of Al, Mg, Fe and other transition metals. Their thermal, hydrothermal, or solution stabilities vary from one smectite species to another. This is also true to a certain extent for the physical properties of bentonites, i.e. swelling, permeability, plasticity and adsorption. Swelling ability and permeability of naturally altered (metabentonites) and steam-treated bentonites are badly damaged. Temperature distribution in a clay barrier that was simulated in situ under the repository conditions showed a sharp drop from 175°C at the clay/canister contact to 81°C at a distance 65 mm away. Microstructures of bentonites were noticeably altered after hydrothermal treatment at 200°C.

Hydroxy-aluminosilicate interlayers in montmorillonite: implications for acidic environments

Hydroxy-aluminosilicate (HAS) ions are one of the major forms of soluble aluminium and the precursor of non-crystalline aluminosilicates in acidic (pH<5.5) environments1–6. Acidification in acid-vulnerable areas7,8 would thus promote HAS formation. Over four decades the study of interactions of soluble aluminium species with clay minerals has been focused on the deposition of hydroxy-aluminium in interlayer spaces of expandable layer silicates9–12. Although the synthesis of 'hydroxy-silicoaluminium pillared smectites' as cracking catalysts has been recently attempted13, the nature of the dominant species adsorbed by smectites from the mixture of HAS, hydroxy-aluminium ions and silicic acid remains obscure. Here we provide evidence that HAS ions can be adsorbed in the interlayer spaces of montmorillonite, resulting in an expanded structure, and so the natural occurrence of HAS-interlayered layer silicates merits attention. Furthermore, the transport of HAS ions, their transformation to non-crystalline aluminosilicates and the levels of toxic solution aluminium species in acidic terrestrial and aquatic environments can thus be affected by naturally occurring expandable layer silicates such as montmorillonite.

Humic macromolecule Interlayering in nontronite through interaction with phenol monomers

Humic substances, which are the principal organic components of soils and waters1, form complexes with clay minerals2–4. These complexes profoundly affect the physical, chemical and biological properties of soils and sediments4,5. Various theories on the reaction sites of clay minerals with humic substances have been proposed1,5–15; however, evidence for the retention of humic substances between the layers of expanding layer silicates in field conditions is very limited, and comes only from soils in acidic environments10,11,14,16 (pH<5). Studies on the laboratory synthesis of humic substance interlayers in expanding layer silicates have shown that the formation of interlayers was strongly favoured at pH 2.5 and diminished significantly between pH 4 and 5 (refs 6–9, 12). Aniline polymerizes in interlayers of Fe(III)-saturated montmorillonite but not in those of Ca-saturated montmorillonite17. Furthermore, aniline is not very representative of biologically generated, natural aromatics. We report here that one of the well identified precursors18,19 for the formation of humic substances, hydroquinone, in aqueous solution at near-neutral pH (6.5) can be transformed to humic macromolecules deposited in the interlayers of nontronite (Fe(III)-bearing smectite) saturated with calcium, which is the most common and abundant exchangeable cation of soils and sediments.

Expandable layer silicates associated with hydrothermal talc deposits of Murphy, North Carolina

Corrensite and other mixed-layer chlorites occur in the insoluble residues from the Murphy Marble. They are found in an ordered arrangement around one of the eight zones in the Murphy Marble. The envelope, 60 m wide, is centered on the mottled zone in the upper part of the marble formation. The progression of the layer silicates is: vermiculite --> vermiculite-smectite --> smectite --> smectite-chlorite --> corrensite + chlorite --> chlorite. In addition, quartz, plagioclase, and muscovite may occur. The mottled zone appears to have been a conduit of the hydrothermal fluids which have altered the silicates in the marbles; isotopic data supports this hypothesis. The altered carbonate rock is depleted in heavy isotopes. A genetic connection, between the talc bodies and the hydrothermal plumbing system, is indicated by an increase in the amount of expanding and contracting layers in the mixed-layer minerals of the mottled zone near the orebody.--Modified journal abstract.

Clay Mineralogy in Relation to Landscape Instability in the Coast Range of Oregon

AbstractField and laboratory data reported in this study indicate that the kind of mass movement and the mineralogy of the materials involved vary with the parent material. The clay fraction of debris avalanches is dominated primarily by nonexpanding layer silicates that have large particle sizes and small water‐holding capacities. Dehydrated halloysite, chloritic intergrade, and mica were the common minerals in those areas underlain by sandstones and siltstones of the Tyee Formation as well as the massive basalt flowrock of the Siletz River Volcanic Series. The clay fraction of soils derived from Tertiary sandstones of the Galice and Lookingglass Formations is dominated by chloritic intergrade, chlorite, mica, and kaolinite. Serpentine, chlorite, and mica were the soil clays associated with debris avalances on serpentinite of the Otter Point Formation. Expandable layer silicates, or those with high charge or water‐holding capacity, were not major constituents of debris avalanches although smectite and vermiculite commonly occurred in a thin layer of soil above the underlying bedrock.The clay fraction of samples from sites undergoing failure by creep and slump consisted primarily of smectite. Smectite, chloritic intergrade, dehydrated and hydrated halloysite, and mica were the minerals commonly associated with soil creep and slump on slopes underlain by siltstones of the Tyee Formation. Montmorillonite was the major constituent of a large rotational slump at the contact between the Nye Mudstone and the sandstones of the Astoria Formation. Smectite, chlorite, and serpentine were identified in sites underlain by serpentinite of the Otter Point Formation.The mineralogy of soils involved in earthflow consisted predominantly of hydrated and dehydrated halloysite, amorphous material, and chloritic intergrade. No difference in mineralogy could be detected between sites underlain by siltstones of the Tyee and Nestucca Formations and tuffaceous siltstones and tuff of the Siletz River Volcanic Series. Surface samples were more poorly crystallized than samples taken at lower horizons. Electron micrographs reveal an abundance of amorphous gels and “coatings” on the surface of mineral grains. The abundance of “pores” may account for the fluid behavior of these materials during failure.

An Evaluation of Zonal Centrifugation as a Research Tool in Soil Science: I. Methodology

AbstractThe methodology involving zonal centrifugation of clay sized mienrals through density gradients is presented. Various aspects of the procedure such as gradient solvents, formation of the gradient, dispersing agents, and methods of band removal are discussed. The parameters of centrifugal force, saturating cation, and particle size were evaluated as to their effect on time and isopycnic banding location within the gradient. It was concluded that an ultracentrifuga is required (a force field greater than 623 g) to band isopycnically clay particles of &lt; 0.2 microns in diameter. The saturating cation influenced banding of 2:1 expandable layer silicates to a greater degree than 1:1 layer silicates, and the spatial relationshp of the expanding layer silicate associated with specific cation saturation played a larger role in banding than differences in atomic weight among the saturating catons.

Replacement of OH by OD in layer silicates, and identification of the vibrations of these groups in infra-red spectra

SummaryConditions necessary for exchange between lattice OH groups and D2O in expanding layer silicates are reported. From spectral changes in the 200–1200 cm−1 region resulting from this exchange, in-plane librations of OH co-ordinated to (A1A1), (A1Mg), (A1Fe3+), and (MgMgMg) have been identified in montmorillonite, beidellite, saponite, hectorite, and vermiculite. Both in-plane and out-of-plane librations have been identified for pyrophyllite and celadonite, from comparison of synthetic specimens containing OD with natural or synthetic OH forms. Comparison of hydrogen and deuterium forms of Mg- and Ni-talcs leads to the identification of translational and librational OH vibrations.

Crystalline clay minerals of the soils derived from recent volcanic ashes in Hokkaido, Japan

Abstract In the previous study on clay minerals in the soils derived from recent volcanic ashes, it has been shown that expanding layer silicates with more or less interlayer-Al are found in large amounts among crystalline clay minerals, and that chlorite, illite and possibly kaolin minerals are found in small amounts (1). Montmori11onite shows a tendency to be dominant in the soils of the younger group and to be less Alinterlayered, However, vermiculite tends to increase, and both montmorillonite and vermiculite become more Al-interlayered in the soils of the older group. The purpose of the present report is to investigate 2: 1 type clay minerals, chiefly expanding layer silicates.

Infrared Absorption Spectrometry in Clay Studies

AbstractThe relationships between spectrum and structure in layer-silicates are reviewed, and applied in the study of structural changes occurring during the heating of montmorillonites up to dehydroxylation, and their subsequent rehydroxylation. Information given by infrared spectroscopy on the binding of water in expanding layer silicates is presented, and the physical and chemical processes associated with entry of basic, neutral and acidic molecules into the interlayer space of these minerals are illustrated for ammonia, ethylamine, pyridine, nitrobenzene, and benzoic acid. Problems associated with the study of soil clays, which are often complex mixtures including poorly ordered and amorphous constituents, frequently firmly combined with organic matter, are discussed.New evidence is presented concerning the environment of the two types of hydroxyl group in beidellite. The thermal stabilities of NH4+ and lattice OH in montmorillonite and beidellite, and the properties of their dehydroxylates, are contrasted. The nature of the collapsed phase formed in Li-, Mg-, and NH4-montmorillonite at 300–500°C is discussed. The presence of weak hydrogen bonds between lattice oxygens and interlayer water is established, although it is shown that the strength of hydrogen bonds formed between NH4+ and lattice oxygens is dependent on the sites of substitution in the layer lattice.