The assignment of the 29Si CP/MAS-NMR spectrum of naturally-occurring sepiolite clay was re-examined using 29Si COSY and 1H-29Si HETCOR pulse sequences. Each of the three main resonances at −92.1, −94.6 and −98.4 ppm has been attributed to one of the three pairs of equivalent Si nuclei in the basal plane, and the resonance at −85 ppm to Q2(Si-OH) Si nuclei. On the basis of the COSY experiment, the resonance at −92.1 ppm is unambiguously assigned to the intermediate, near-edge Si sites. The HETCOR experiment revealed that the resonance at −94.6 ppm cross-polarizes almost entirely from the Mg-OH protons, and therefore is assigned to the central Si position. The remaining resonance at −98.4 ppm correlates strongly to the protons of the structural water molecules and therefore is assigned to the edge Si sites. Nearly complete rehydration was achieved at room temperature by exposing sepiolite samples that had been partially dehydrated at 120°C to water vapor or to D2O vapor. The rehydration results support the 29Si NMR peak assignments that were made on the basis of the COSY and HETCOR experiments.

The 29Si CP/MAS-NMR spectrum corresponding to the folded sepiolite structure in which approximately one half of the structural water has been removed by heating to 350°C is reported for the first time. The chemical shift values and relative intensities are significantly different compared to the resonances that are observed in the corresponding spectrum of the true sepiolite anhydride. These observations support the earlier claim that sepiolite heated to ∼350°C exists as a distinct phase to be differentiated from that of the completely dehydrated state.

The aim of this study was to compare the redox properties of oxides and smectite structural Fe (FeStr). Structural Fe in smectite often accounts for about half of the Fe pool in soils and sediments, and is naturally reduced by indigenous bacteria. This reaction is, however, seldom taken into account in environmental studies. The equation for the oxidation-reduction of FeStr and its application to the calculation of the standard electrode potential of FeStr, E0, are discussed on the basis of literature review. The resulting redox properties were used in a new analysis of previously reported field monitoring data. This agreed well with experimental observations that remained unexplained, and showed that if equilibrium conditions prevail, the FeStr pool in soils will reduce prior to the Fe in oxides. The FeStr pool is, therefore, likely to play a major role in controlling the bio-geochemistry of temporarily water-logged soils, and this reaction should be taken into account in further studies.

Structural Fe2+ in montmorillonite is readily oxidized by contact with water, salt solutions or on mild heating. This is shown clearly by the Mössbauer spectra and is associated with a sharpening of the infrared absorption near 880 cm−1. It was inferred that this band comprises the Fe2+—OH—Al and Fe3+—OH—Al deformations. The rate at which oxidation occurs depends on the exchangeable cations. High acidity of the interlayers is conducive to oxidation, as is contact with Cu2+-containing solutions or concentrated H2O2 solutions.

The results show clearly that any chemical treatment of montmorillonite causes changes in the oxidation state of structural iron.

Orthopyroxene (En85) weathers initially by vacancy diffusion, and through this process hydration occurs and a sequence of biopyriboles develops, culminating in a talc-like layer silicate whose structure joins coherently to the orthopyroxene structure. Oxidation of Fe2+ to Fe3+ colors the altering pyroxene yellow. The ‘talc’ does not remain in structural coherence with the pyroxene after it has exceeded a few tens of nanometers in size; it is replaced by a mixture of talc and smectite. In some areas the mixture has an epitactic relation to the pyroxene, but commonly it fills faceted solution holes without crystallographic relation to the parent structure. Continued weathering extends the yellow zone at the periphery of the orthopyroxene, and the alteration product increases in smectite and decreases in 'talc’ During this stage of the reaction, MgO and SiO2 are released to form colorless true talc around the altering pyroxene. Eventually, the yellow alteration may become a smectite pseudomorph after orthopyroxene or it may be changed entirely to a mixture of vein talc and iron oxides. The complete conversion of orthopyroxene to talc plus oxides thus takes place through three sequential mineral reactions without the development of a noncrystalline phase.

The lacustrine Sucker Creek Formation (Miocene) of eastern Oregon includes unaltered vitric tuffs as well as tuffs altered to the following diagenetic fades: (1) bentonite, (2) interbedded bentonite and opal-CT, (3) K-clinoptilolite, and (4) Ca-clinoptilolite. Bentonite beds contain Fe-rich smectite (8–10 wt. % Fe2O3), quartz, plagioclase, and Ca-clinoptilolite. Opal-CT-rich layers contain inorganic silica (opal-CT), Fe-rich smectite, and minor diatoms. K-clinoptilolite beds typically contain clinoptilolite that can be extremely K-rich (≤7.6 wt. % K2O), opal-CT, smectite, plagioclase, and K-feldspar. This diagenetic facies also includes smectitic tuff and unaltered tuff. Ca-clinoptilolite beds contain Ca-clinoptilolite, quartz, K-feldspar, smectite, and illite.

Based on its chemistry and mineralogy, the bentonite appears to have been derived from dacitic volcanic ash. Chemical considerations and the close spatial relationship between beds of bentonite and opal-CT suggest that the diagenetic alteration of glass to smectite provided silica to the adjacent opal-CT beds. Based on the presence of late-stage Ca-clinoptilolite, alteration appears to have proceeded in a relatively closed chemical system.

Based on the composition of preserved vitric tuff, the zeolitic tuffs appear to be derived from rhyolitic ash, which diagenetically altered in an open hydrologic system and produced vertical zonations in mineralogy. In this model, bentonite horizons at the top of the K-clinoptilolite diagenetic fades formed by reaction of volcanic glass with dilute fluids that had a relatively low (Na+ + K+ + Ca2+)/H+ activity ratio and ${\mathrm{a}}_{{\mathrm{H}}_4{\mathrm{S}\mathrm{i}\mathrm{O}}_4}$, whereas the underlying K-clinoptilolite beds formed from reactions between glass and dilute fluids having a higher (Na+ + K+ + Ca2+)/H+ activity ratio and ${\mathrm{a}}_{{\mathrm{H}}_4{\mathrm{S}\mathrm{i}\mathrm{O}}_4}$. Unaltered vitric ash between these beds may represent zones of higher permeability that inhibited secondary mineral alteration. Ca-clinoptilo-lite-rich beds appear to have undergone alteration similar to K-clinoptilolite-rich beds as well as to have been subjected to later, low-temperature (perhaps 75°–150°C) hydrothermal alteration which enhanced cation exchange in the zeolite and formed quartz from opal-CT.

The wetting contact angle was measured for water drops settled on the surface of pressed discs of kaolinite, alumina, bentonite, marble, montmorillonite, and quartzite immersed in hexane, octane, dodecane, cis-decalin, and air. Minimum and maximum values of the contact angle were obtained for the given systems of solid-water drop-hydrocarbon, depending on the manner of disc preparation. Using both minimum (θmin) and maximum (θmax) values of the contact angle, values of the dispersion component (γsd) of surface free energy of these solids were calculated from the equation which was derived on the basis of an equilibrium state of the system solid-water drop-hycrocarbon for two different hydrocarbons. The values of γsd for kaolinite, alumina, bentonite, marble, montmorillonite, and quartzite obtained from θmin are 83.5, 98.1, 98.9, 80.2, 95.9, and 89.7 mJ/m2, and from θmax are 73.1, 85.0, 84.4, 75.8, 85.5, and 75.5 mJ/m2. These values for marble and quartzite are similar to those in the literature (marble = 67.7 mJ/m2; quartzite = 71.3 and 76.0 mJ/m2). The values of the dispersion components of surface free energy for marble and quartzite covered with a water film (γsfd) were found to be: 41.8, 36.9; 49.2, 42.5; 49.6, 42.2; 40.2, 38.1; 48.1, 42.8; and 44.9, 38.0 mJ/m2, respectively. Values of γsfd for kaolinite, bentonite, and montmorillonite agreed well with those obtained from hydrocarbon adsorption isotherms determined by differential thermal analysis (35.5, 36.5, and 37.4 mJ/m2).

Using values of γsfd and contact angles measured in the system solid-water drop-air, the nondispersion component of the surface free energy of solids with adsorbed water film (γsfn) was calculated from the modified Young equation. The values of γsfn for kaolinite and quartzite are as follows: 55.8, 69.0; 85.6, 94.0; 52.1, 75.0; 64.7, 68.9; 54.9,71.3; and 59.2,74.4 mJ/m2. The values of the nondispersion components determined for kaolinite, bentonite, and montmorillonite agreed well with those obtained by differential thermal analysis (67.6, 78.3, and 65.5 mJ/m2, respectively). Further, based on the assumption that the adsorbed water film decreased the surface free energy of these solids by the value of the work of spreading wetting, the nondispersion component (γsn) of the surface free energy of the solids was calculated to be: 86.9,129.6; 169.5, 187.7; 67.1, 144.8; 117.5, 129.3; 83.0, 135.7 and 100.2, 143.4 mJ/m2. These calculated values of the nondispersion component of marble and quartzite surface free energy agree with those obtained from adsorption isotherms determined by chromatographic and differential thermal analysis (marble = 103.8, 106.4; quartzite = 112, 115, 153.6 mJ/m2).

Junction probability diagrams show variation in both composition and layer arrangement in mixed-layer clay minerals. These diagrams can represent short-range and long-range ordered, random, and segregated interstratifications. Mineralogical analyses of illite/smectite from shale cuttings, bentonites, and hydrothermally altered tuffs define characteristic reaction pathways through these diagrams. Shale and bentonite analyses fall along pathways joining smectite and illite on diagrams showing nearest-neighbor (R1) layer arrangements. Transition from random to R1-ordered interstratifications occurs in shale samples containing 60–70% illite layers, and in bentonites containing 55–67% illite layers. Analyses of alteration products, however, fall near a line connecting rectorite and illite, which represents the maximum degree of R1 layer ordering. No mineralogical evidence is available to suggest that these alteration samples formed from a smectite precursor. All samples develop next-nearest (R2) and thrice-removed (R3) neighbor ordering along similar pathways. Transition to R2 ordering occurs gradually in samples composed of 65–80% illite layers, and samples containing more than 85% illite layers may show strong R3 ordering.

Dilute aqueous solutions of quinoline were contacted with Na-montmorillonite to elucidate the sorption process of the neutral and protonated species. Sorption occurs via a combination of ion exchange and molecular adsorption and yields S-type isotherms. Exchange between the quinolinium ion (QH+) and Na can be described by means of Vanselow selectivity coefficients and a thermodynamic exchange constant (Kex). Due to the apparent adsorption of the neutral species at high mole fractions (x) of the solid phase, the thermodynamic standard state was defined as 0.5 mole fraction. The selectivity at pH ~4.95 of the QH+ species over Na (at XQH+ = 0.5) was determined to be Kv = 340. At pH ≥ 5.5 surface mole fractions of 0.5 could not be obtained without adsorption of the neutral species. This study suggests that at dilute solution concentrations quinoline is sorbed preferentially as the cation even at pHs ≫ pKa. A critical surface-solution concentration is apparently necessary for adsorption of the neutral species.

Characteristics and properties of complexes of a smectite (hectorite) with 1,10-phenanthroline (phen) chelates with iron or copper were determined by a variety of physical and chemical measurements. The complex ions showed high selectivity for the hectorite surface. Basal spacings of 17.4 Å were produced by Fe(II) or Cu(II) analogues of M(phen)32+ hectorite. Adsorption of gases and vapors by the M(phen)32+ hectorite complex revealed large surface areas and reflected intrinsic characteristics of the complex ions. Lower surface areas were found for copper phen hectorite than iron phen hectorite probably because of the loss of a ligand from the Cu(II) ion. ESR spectra confirmed that appreciable Cu(II) existed as the bis-phen complex under certain conditions. An increase in the oxidation potential of the Fe(phen)32+-Fe(phen)33+ couple above that in pure solvent was noted when these complexes were supported by the mineral surface.

Muscovite and K-feldspar (adularia) were dry-ground to — 200/inch particles, which were suspended in water. These suspensions were “titrated” with KCl, and the pH was recorded as a function of KCl concentration and temperature. The results indicate that muscovite and adularia react with water to produce a surface film in which H+ has displaced K+. The “titration” curves show some characteristics attributable to exchange reactions, others apparently related to equilibria among solids of fixed composition. Maximum release of K+ from adularia by reaction with water is much greater than that from mica. The interpretation is made that the first result of reaction of mica and adularia with water is a surface layer that grades from an outer portion that is structurally disrupted to an inner portion that retains the original silicate structure but with H+ substituted for K+. Addition of K+ as KCl to the suspending solution displaces H+ from the disrupted zone, but all H+ originally taken up by the solids was not returned to the solution by concentrations of KCl up to 1.0 M. Experiments were of a few hours duration; work by others has shown that the disrupted zone releases appreciable concentrations of silica and alumina to solution over longer time intervals.

These hydrolysis experiments indicate that at 25°C an H-feldspar or H-mica structure is favored over a K-feldspar or K-mica structure except in solutions in which the ratio of aK+ aH+ exceeds 109–10 or 107–8 respectively. These ratios decrease in the temperature range 25-65°C by a factor of about 100.7. These results, where considered in relation to the observed behavior of feldspar and mica under weathering conditions, indicate that the major energy change for the reactions, $2\,mica + 5{H_2}O = \,3\,kaolin + \,2{K^ + }\, + \,2O{H^ - }$$3K - feldspar + 2{H_2}O = \,K - mica + 6quartz + \,2O{H^ - }$ can be considered to result from H+-K+ exchange, and that the energy contribution from other changes is small.

A new method for calculation of layer charge and charge distribution of smectites is proposed. The method is based on comparisons between X-ray diffraction (XRD) patterns of K-saturated, ethylene glycol-solvated, oriented samples and calculated XRD patterns for three-component, mixed-layer systems. For the calculated patterns it is assumed that the measured patterns can be modeled as random interstratifications of fully expanding 17.1 Å layers, partially expanding 13.5 Å layers and non-expanding 9.98 Å layers. The technique was tested using 29 well characterized smectites. According to their XRD patterns, smectites were classified as group 1 (low-charge smectites) and group 2 (high-charge smectites). The boundary between the two groups is at a layer charge of −0.46 equivalents per half unit-cell. Low-charge smectites are dominated by 17.1 Å layers, whereas high-charge smectites contain only 20% fully expandable layers on average. Smectite properties and industrial applications may be dictated by the proportion of 17.1 Å layers present. Non-expanding layers may control the behavior of smectites during weathering, facilitating the formation of illite layers after subsequent cycles of wetting and drying. The precision of the method is better than 3.5% at a layer charge of −0.50; therefore the method should be useful for basic research and for industrial purposes.

Magnetite (Fe3O4) is a key economically valuable component in iron ore and is extracted by dissolution processes, but among the Fe (oxyhydr)oxides its solubility behavior is one of the least understood. The objective of this study was to improve understanding of magnetite dissolution mechanisms leading to thermodynamic equilibrium by comparing the dissolution of two solid samples, one synthetic and one industrial, using oxalic, sulfuric, and nitric acids at varying concentrations and temperatures. Of the three solid-liquid systems investigated, only the system consisting of magnetite and oxalic acid reached an equilibrium state within the duration of an individual experiment (6 h). In this system, increasing the acid concentration resulted in a significant increase in the equilibrium concentration of dissolved Fe. When dissolving synthetic and industrial magnetite, increasing the temperature not only increased the rate of reaction but also affected the concentration of dissolved Fe. Significant effects were observed when increasing the temperature from 15 to 35°C, but only slight differences were seen on further increases in temperature. Observations regarding the equilibrium state of the sulfuric and nitric acid systems could not be made because equilibrium was not reached. The most important individual observation regarding the equilibrium state of the nitric- and sulfuric-acid systems seems to be that in future studies a much longer reaction time is necessary, due to slow kinetics of the dissolution mechanism. A proton-based mechanism has been hypothesized as the one governing the dissolution of magnetite by these two acids, but only the dissolution of the industrial sample yielded results that were similar for these two acids and consistent with that hypothesis.

Formation of kaolinite-smectite (K-S) mixed layers in a soil toposequence developed from basaltic parent material was examined. The soil formed in a temperate climate with alternating dry and wet seasons in Sardinia (Italy). Chemical composition and charge characteristics of the smectite component in the K-S mixed layers were analyzed to help determine a mechanism for formation. Soils were sampled at the top, intermediate, and base of a steep (35%) slope. As indicated by X-ray diffraction data, the fine clay fraction (>0.1 μm) in the soils is dominated by K-S with a decreasing proportion of kaolinite from the top (70%) to the base (30%). Rapid internal drainage induced by the slope is probably the major factor responsible for the formation of K-S. Chemical composition and charge characteristics of the smectite component in the K-S were analyzed by X-ray diffraction (intercalation with alkylammonium ions), cation exchanged capacity (CEC) and surface area measurements, and infrared spectroscopy. Results indicate that the smectite component is nearly identical over the soil toposequence. The smectite component is the same with respect to charge magnitude and chemical composition, independent of the proportion of kaolinite and smectite components. This suggests the pedogenic formation of K-S by transformation of smectite through dissolution of some smectite layers and subsequent crystallization of kaolinite between the layers of the remaining smectite crystallites.

Illite diagenesis in Tertiary and Mesozoic shales in the Bergen High area, northern North Sea, was studied using mineralogic, isotopic, and computerized thermal modeling techniques. The Tertiary shales are dominated by smectite, with lesser amounts of illite, kaolinite, and chlorite. At present burial temperatures of >70°C smectite is absent, and the shales contain abundant lath-shaped illite which yields a mixed-layer illite/smectite (I/S) X-ray powder diffraction (XRD) pattern. Transmission electron microscopy (TEM) indicates that the illite laths increase in abundance and thickness with increasing depth; XRD patterns indicate a progressive increase in the illite component of the I/S. The deepest samples were found to contain long-range ordered (R=3) I/S, which showed platy particle morphology with the TEM. K-Ar ages of most of the <0.1-μm-size illite separates imply that illitization was a relatively brief event affecting a thick sequence of sediments during late Cretaceous to early Paleocene time (65–87 Ma); however, measured ages were affected by trace levels of detrital Ar contamination and do not represent the true age of diagenesis.

Several methods of quantifying Ar contamination were used to correct measured ages to obtain a reasonable estimate of the true age of diagenesis. The corrected ages are imprecise due to uncertainties in quantifying the levels of sample contamination, but generally suggest a Paleogene (38–66 Ma) period of illitization. In contrast, simple kinetic models of smectite-illitization suggest much younger ages of diagenesis (0–40 Ma at the Veslefrikk field; 0–60 Ma at the Huldra field). The timing of the diagenesis and the morphologic aspects of the authigenic illite suggest that illite precipitated before late Tertiary compaction and resulted in a decrease in fluid permeability. Low trapping efficiency of early Tertiary sediments, vertical escape of warm fluid from the Brent sandstone, and high heat flow may have promoted illite diagenesis in the shales prior to deep late-Tertiary burial.

Different changes in the structural and thermal properties of various types of talc have been reported in the literature which have made comparison of analytical results difficult. The objective of the present study was to obtain some fundamental insights into the effects of the thermal behavior of talc and to carry out kinetic analyses of the decomposition of talc under high temperature. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetry-differential scanning calorimetry (TG-DSC) were used to study the thermal decomposition mechanism. The Coats-Redfern decomposition model was used to determine the decomposition mechanism of talc samples. The results showed that the decomposition of talc commenced at ~800°C, peaking at ~895°C, with the formation of enstatite and amorphous silica. An isothermal treatment at 1000°C caused the complete dehydroxylation of talc. The XRD and FTIR results indicated that the enstatite and amorphous silica phases were transformed into clinoenstatite and paracrystalline opal phases, respectively, after the decomposition stage at 1200°C. Good linearity in the Coats-Redfern model was observed from room temperature to 1300°C and the activation energy was calculated to be 69 kcal/mol.

The mechanism of decolorization of crude maize and sunflower oils was studied by means of adsorption of β-carotene by a low-grade bentonite, containing mixed-layered illite-smectite. Decolorization depends on temperature and the time required for equilibrium decreases with increasing temperature. The study of the kinetics of adsorption showed that decolorization of maize oil is a first-order process which occurs in two steps: a first fast step with higher activation energy (25.6 kJ mol−1), indicating the influence of a chemical interaction between the pigment and the clay surface, followed by a second slow step with low activation energy (12.3 kJ mol−1), characteristic of physical adsorption on the previously adsorbed molecules. Decolorization of sunflower oil is also a first-order process, described by a single mechanism with intermediate activation energy (19.0 kJ mol−1). Adsorption isotherms of decolorization of maize oil follow the Freundlich equation, indicating the existence of heterogeneous adsorption sites on the solid's surface. Heterogeneity is attributed both to different active centers on the smectite surface (Brönsted and Lewis centers) and to the different phases present in bentonite, such as illitic layers and clinoptilolite, which also have active centers on their surfaces.

Hydrotalcites of high aluminum content have been synthesized from aluminate liquors of varying composition and activated magnesia obtained by calcination of hydroxide or hydroxycarbonate precursors. Lattice parameter measurements and chemical analyses of 21 synthetic hydrotalcites show that the aluminum substitution $$\left( {\rm{X = \frac{{Al}}{{Al + Mg}}}} \right)$$ for most of the products is about 0.35, which is at the maximum experimentally-observed limit of solid solubility. Pillared hydrotalcites were also prepared by molybdate, Chromate, and silicate anion replacement. A maximum distance of 10.4 Å between the brucite-like layers was observed for the Mo7O246− intercalated material.

Clay minerals from soils of a red-black soil complex developed from basaltic parent material in Sardinia are formed along a short toposequence (200 m). At the foot of the sequence, a clay-rich, black Vertisol forms, whereas at the summit, the soil is a dark reddish-brown Inceptisol. X-ray diffraction, infrared spectroscopy (FTIR), cation exchange capacity (CEC) and permanent and variable charges analyses were used, and the data show that clay minerals varied according to soil horizon and topographic position of the soil. Clay minerals in the Inceptisol are dominated by kaolinite and mixed-layer kaolinitesmectite (K-S, K:S >0.5), whereas the Vertisol contains smectites and K-S with K:S proportions <0.5. In the Vertisol, the proportion of kaolinitic layers in the K-S increases from the C horizon (K:S ∼0.35–0.40) to the Ap horizon (K:S ∼0.40–0.45). This soil clay-mineral distribution, in relation to topography, is similar to that reported for other (kaolinitic) red-black (smectitic) soil associations in subtropical and tropical areas. The sequence forms by downward drainage on summits and slopes, and buildup of ions in ‘lows’ produces smectites. Fourier transform infrared spectra indicate that two types of smectite are formed in the C horizon of the Vertisol; one is more ferric (Fe-beidellite, nontronite), the other more aluminous. Mineralogical evolution in the soil profile (from C to Ap horizon) shows a decreasing proportion of ferric smectite layers (compared to the more aluminous smectite layers). This would indicate that ferric smectite layers are preferentially transformed (or dissolved) to give kaolinite layers, with Fe precipitating as oxides and/or oxy-hydroxides or retained partly in kaolinite layers. Because the surface properties of clay minerals are related to mineralogy, the CEC (33–41 cmol kg−1) in the brown Inceptisol is ∼50% pH-dependent charge while in the Vertisol up to ∼75% of the CEC (48–61 cmol kg−1) comes from accessible permanent charges.

Halloysite and kaolinite occur in gneissic weathering profiles developed under a temperate climate in the Leucogia area of NE Greece. X-ray diffraction analyses of samples from three profiles of partially weathered gneisses have revealed that kaolinite becomes more abundant than halloysite with increasing intensity of the decomposition processes. To resolve the genetic relationship between halloysite and kaolinite, microtextures were examined by scanning electron microscopy using polished thin-sections and freshly exposed fracture surfaces, and their chemical compositions were determined using an analytical scanning electron microscope fitted with energy dispersive and wavelength dispersive spectrometers. Different morphological forms of halloysite and kaolinite were detected within the same rock mass. In the earliest stage of weathering, spheroidal aggregates consisting of microcrystalline halloysite are formed on the plagioclase surface. With progressive weathering, spheroidal halloysite converts to tubular halloysite. As weathering advances, tubular halloysite converts to platy halloysite, which in turn converts to kaolinite. Halloysite and kaolinite may coexist in the upper parts of the profiles. Electron microprobe analyses of spheroidal, tubular and platy halloysite and kaolinite show that the chemical composition of the various forms indicate a progressive Fe enrichment and Al depletion with advancing kaolinization, from VIFe0.08VIAl3.70 a.p.f.u. in spheroidal halloysite through VIFe0.29VIAl3.50 in platy halloysite to VIFe0.53VIAl3.32 in newly formed kaolinite. Final-stage, book-type kaolinite approaches its ideal chemical composition (VIFe0.04VIAl3.95). The mineralogy and composition of the kaolin minerals and the development of the various morphological forms of halloysite and kaolinite in different parts of the profiles are attributed to the chemistry of the ambient solutions. However, other physical and microenvironmental conditions (e.g. time and space available) also seem to have strong influence on the precipitation processes producing the morphological variations.

Many important properties of clay minerals are defined by the species of charge-balancing cation. Phenomena such as clay swelling and cation exchange depend on the cation species present, and understanding how the cations bind with the mineral surface at a fundamental level is important. In the present study the binding affinities of several different charge-balancing cations with the basal surface of the smectite mineral, montmorillonite, have been calculated using molecular dynamics in conjunction with the well-tempered metadynamics algorithm. The results follow a Hofmeister series of preferred ion adsorption to the smectite basal surfaces of the form:

K+ > Na+ > Ca2+ > Cs+ > Ba2+

The results also revealed the energetically favorable position of the ions above the clay basal surfaces. Key features of the free-energy profiles are illustrated by Boltzmann population inversions and analyses of the water structures surrounding the ion and clay surface. The results show that weakly hydrated cations (K+ and Cs+) preferentially form inner-sphere surface complexes (ISSC) above the ditrigonal siloxane cavities of the clay, while the more strongly hydrated cations (Na+) are able to form ISSCs above the basal O atoms of the clay surface. The strongly hydrated cations (Na+, Ca2+, and Ba2+), however, preferentially form outer-sphere surface complexes. The results provide insight into the adsorption mechanisms of several ionic species on montmorillonite and are relevant to many phenomena thought to be affected by cation exchange, such as nuclear waste disposal, herbicide/pesticide-soil interactions, and enhanced oil recovery.