In order to understand the influence of salt concentration and temperature on the behavior and properties of clays used in drilling muds, we studied montmorillonite supensions (4 g clay/100 g solution) in 0.1, 0.5 and 1 M NaCl and KCl solutions. A fraction of each sample was heated to 200 °C in a closed vessel for 7 d, then cooled at room temperature (RT, 25 °C). Small-angle X-ray scattering (SAXS) spectra were recorded, for all the samples, at RT. The structure of the clay particles was determined by comparing the experimental intensity with the theoretical intensity computed from a model that took into account the number of layers per particle, the hydration state of the layers (0, l, 2, 3 or 4 water layers) and the order in the succession of these states. With this set of parameters, we can compute the mean statistical parameters M¯ (mean number of layers per particle), d¯ (mean interlayer distance) and δ¯2/d¯2 (parameter describing the disorder of the distribution of interlayer distances). The evolution of these parameters shows that:

1. 1) At low concentration (0.1 M NaCl or KCl), the samples do not consist of particles but of isolated layers (M = 1). The suspensions form gel-like structures. The difference between Na and K suspensions, or between heated and nonheated samples, is unnoticeable at the studied scale (5–500 Å).

2. 2) An increase in salt concentration (from 0.1 to 0.5 M) brings the sample in a granular state: we notice the appearance of particles at 0.5 M (M¯ ≥ 25). Differences appear between NaCl and KCl suspensions, and the temperature effect becomes visible. Thus, we noticed that in NaCl suspensions, particles are composed of hydrated layers (1, 2, 3 or 4 water layers) and internal porosity (d > 30 Å), whereas suspensions in KCl are characterized by the presence of interlayer distances of 10 Å, that is, of collapsed layers. Particles in the KCl suspensions are much thicker than in the NaCl corresponding ones, and also less hydrated at the interlayer level as well as at the internal porosity level. Further increase in salt concentration (0.5 to 1 M) amplifies this effect. As far as temperature is concerned, its effect is to promote the clay dispersion by breaking up the particles, dehydrating and disordering them. This effect is more important for low salt concentration, that is, when the system is less stressed.

The microtextural changes in the kaolinization of primary phyllosilicates including biotite, sericite, clinochlore and muscovite were investigated by scanning electron microscopy (SEM) and microchemical analysis of thin sections of weathered anorthosite. Kaolinization began at grain edges and propagated toward the interior. Grains were highly fanned out from the edges and exfoliated into several flakes along the basal cleavages, producing lenticular voids. Finally, long vermicular kaolinite pseudomorphs were formed after primary phyllosilicates. Statistical analysis showed a ninefold increase in volume during the kaolinization of biotite, suggesting that most AI in the kaolinite was imported from ambient weathering solution. Weathering primary phyllosilicates supplied templates suitable for the thick epitactic overgrowth of kaolinite to form long vermicular pseudomorphs. AI was sufficiently available due to the intense weathering of soluble anorthosite. Although present in small amounts, primary phyllosilicates gave high volumetric and mineralogical contributions to the weathering profiles by facilitating kaolinite precipitation.

Hisingerite, first described in 1810, has been variously regarded as noncrystalline, as a septechlorite, as ferric allophane, as ferric halloysite and as poorly crystalline nontronite. Hisingerite from the original localities of Gillinge and Riddarhyttan in Sweden has a composition close to Fe2O3·2SiO22H2O. X-ray diffraction (XRD) analysis of Riddarhyttan hisingerite yields very broad maxima at 7.7, 4.44, 3.57, 2.56, 2.26, 1.69 and 1.54 Å, and that from Gillinge is similar. Cation exchange capacities are 2.2 meq/100 g (Riddarhyttan) and zero (Gillinge). Transmission electron microscopy (TEM) shows a fabric of concentric spheres and part spheres, with diameters of about 140 Å and walls up to six 7-Å layers thick. High-resolution images of the sphere walls reveal a 2-layer structure similar to that of kaolinite. A calculated diffraction pattern based on a model of 4 concentric shells of ferric kaolinite structure matches the observed pattern quite closely. Some other hisingerites, notably that from Bellevue King Mine, Idaho, show 10-Å layers as well as 7-Å layers, and this hisingerite has a CEC of 32 meq/100 g and a weak 15.5-Å X-ray reflection in addition to a pattern similar to Riddarhyttan hisingerite. It is concluded that hisingerite is a curved ferric 7-Å 1:1 layer silicate analogous to halloysite, and that many of the hisingerites reported in the literature contain admixed nontronite.

A unique interstratified expandable mica was obtained by cation exchange treatments using an expandable mica synthesized from talc. The 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectrum of the expandable mica used as a starting material showed that it had 2 kinds of Na+: one was exchangeable and the other was not exchangeable. Half of the Na+ per unit cell of the expandable mica was replaced with Mg2+ by cation exchange treatments. The X-ray powder diffraction (XRD) analysis of the Mg2+-exchanged expandable mica, after heating at 73°C, indicated that Na+ in the interlayer sheets was exchanged with Mg2+ in every second layer and that it had an interstratified structure with a 12.5-Å layer thickness and a 9.6-Å layer thickness. The structure of the Mg2+-exchanged expandable mica was changed into a unique interstratified structure by the calcination at 600°C; one component had a stacking sequence of talc and a small amount of OH−, but the other had a different stacking sequence from talc and no structural OH—.

The transformation of smectite into kaolinite and kaolinite-smectite interstratification were studied in samples belonging to the Argiles Plastiques formation of the Paris basin, by high-resolution transmission electron microscopy (HRTEM). Two original smectite phases, 1) beidellite with 1-nm-thick layers, and 2) beidellite-montmorillonite with 1.25-nm-thick layers, are progressively transformed into kaolinite-smectite mixed-layer minerals, and into kaolinite. As the percentage of kaolinite layers increases in the interlayered minerals, the kaolinite-smectite layer sequences, initially disordered, become locally more ordered, with the presence of KS and KKS units repeated 2 to 4 times (K = kaolinite layer, S = smectite layer). Two solid-state mechanisms seem to be responsible for the formation of kaolinite: 1) the transformation of 1 smectite layer into 1 kaolinite layer, denoted S → K, by stripping of a tetrahedral sheet and the adjacent interlayer region; 2) the intercalation of 1 kaolinite layer into smectite, denoted 0 (zero)→K. Structural and chemical incidences of these mechanisms are discussed.

A modified version of the Bertaut-Warren-Averbach (BWA) technique (Bertaut 1949, 1950; Warren and Averbach 1950) has been developed to measure coherent scattering domain (CSD) sizes and strains in minerals by analysis of X-ray diffraction (XRD) data. This method is used to measure CSD thickness distributions for calculated and experimental XRD patterns of illites and illite-smectites (I-S). The method almost exactly recovers CSD thickness distributions for calculated illite XRD patterns. Natural I-S samples contain swelling layers that lead to nonperiodic structures in the c* direction and to XRD peaks that are broadened and made asymmetric by mixed layering. Therefore, these peaks cannot be analyzed by the BWA method. These difficulties are overcome by K-saturation and heating prior to X-ray analysis in order to form 10-Å periodic structures. BWA analysis yields the thickness distribution of mixed-layer crystals (coherently diffracting stacks of fundamental illite particles). For most I-S samples, CSD thickness distributions can be approximated by lognormal functions. Mixed-layer crystal mean thickness and expandability then can be used to calculate fundamental illite particle mean thickness. Analyses of the dehydrated, K-saturated samples indicate that basal XRD reflections are broadened by symmetrical strain that may be related to local variations in smectite interlayers caused by dehydration, and that the standard deviation of the strain increases regularly with expandability. The 001 and 002 reflections are affected only slightly by this strain and therefore are suited for CSD thickness analysis. Mean mixed-layer crystal thicknesses for dehydrated I-S measured by the BWA method are very close to those measured by an integral peak width method.

Batch sorption experiments at fixed initial Np(V) concentration (∼1 × 10−6 M 237Np), M/V ratio (4 g L−1), and ionic strength (0.1 molal NaNO3) were conducted to determine the effects of varying pH and $P_{C{O}_2}$ on Np(V) sorption on SAz-1 montmorillonite. The results show that Np(V) sorption on montmorillonite is strongly influenced by pH and $P_{C{O}_2}$. In the absence of CO2, Np(V) sorption increases over the entire pH range examined (∼3 to ∼10), with measured sorption coefficients (KD) of about 10 mL g−1 at pH < 6 to KD ∼ 1000 mL g−1 at a pH of 10.5. However, for experiments open to atmospheric CO2 ($P_{C{O}_2}={10}^{-3.5}atm$), Np(V) sorption peaks at KD ∼ 100 mL g−1 at pH of 8 to 8.5 and decreases at higher or lower pH. A comparison of the pH-dependence of Np(V) sorption with that of Np(V) aqueous speciation indicates a close correlation between Np(V) sorption and the stability field of the Np(V)-hydroxy complex NpO2OH0 (aq). In the presence of CO2 and aqueous carbonate, sorption is inhibited at pH > 8 due to formation of aqueous Np(V)-carbonate complexes. A relatively simple 2-site Diffuse-Layer Model (DLM) with a single Np(V) surface complexation reaction per site effectively simulates the complex sorption behavior observed in the Np(V)-H2O-CO2-montmorillonite system. The good agreement between measured and DLM-predicted sorption values suggests that surface complexation models based on parameters derived from a limited set of data could be useful in extrapolating radionuclide sorption over a range of geochemical conditions. Such an approach could be used to support transport modeling and could provide a better alternative to the current use of constant KD values in performance assessment transport calculations.

An ancient Egyptian limestone sculpture was found to be undergoing major structural decay when stored in a museum environment. Mineralogical and petrographic analysis of the limestone showed a high proportion of clay (≥ 10% by weight) that was concentrated along bedding planes. The clay fraction consisted mostly of sepiolite (>90%) and palygorskite (<10%). Minor quantities (≤l%) of soluble salts (NaCl and NaNO3) were also found. Wetting/drying with distilled water and relative humidity cycling resulted in the same delamination cracking damage as that observed in the museum environment. Thermomechanical analyses (TMA) confirmed that the damage was due to expansion (>4.5%) parallel to bedding planes when the limestone was immersed in water. The expansion due to swelling of the clays was directly observed at high magnification in an environmental scanning electron microscope (ESEM) when wetting/drying cycles were performed. X-ray diffraction (XRD) analysis showed that crystalline swelling of sepiolite occurred. This was determined by a shift of (110) reflection (from 12.07 to 12.20 Å) and a decrease of (060) reflection (4.47 Å, to 4.44 and 4.41 Å), when in contact with ethylene glycol (EG) and dimethyl sulfoxide (DMSO), respectively. Swelling also occurred due to hydration of the clay surfaces and to electrostatic forces between clay particles, which, it was assumed, was promoted by the presence of Na counterions in water solution. Possible treatments for the conservation of these artistic objects are proposed and discussed.

Nanomorphological structure of well-crystallized Georgia and poorly crystallized North Queensland kaolinite particles have been compared using field emission scanning electron microscopy (SEM) and atomic force microscopy (AFM). In general, there is good agreement in information from the 2 very different imaging techniques. AFM gives more detailed information on step and ledge dimensions, microvalleys and crystallographic orientation of irregularities on basal planes and edges of the crystallites. There are major differences in nanomorphology and surface structure between the 2 kaolin samples with the Georgia kaolin showing 200–500-nm, relatively flat basal planes with some cascade-like step growth 50–100 nm wide. The edges, apparently flat and right-angled in SEM images, appear beveled in AFM images due to artifacts from the aspect ratio of the AFM tip. The North Queensland kaolinite has much more complex surface structure with anhedral crystallites attached to larger particles, high density of steps and nm-scale irregularities (often crystallographically directed). The additional step edge site contribution from the attached crystallites is estimated as a minimum of 6%, giving a total edge contribution above 30% of the kaolinite total surface area. These structures will generate a substantial pH-dependent charge across the surfaces of the North Queensland kaolinite platelets. An idealized, uniform, pH-independent, negatively charged basal plane cannot be assumed from these structures. There is also some evidence, from both SEM and AFM images, of curvature in the thinner, poorly ordered structures of the North Queensland kaolinite particles.

Transmission electron microscopy/analytical electron microscopy (TEM/AEM) were utilized to study pyrite and sphalerite inclusions in chlorite or mixed-layer chlorite-corrensite from an analcimized ash bed in the Etalian stage (Middle Triassic), South Otago, New Zealand. These sulfide inclusions occur as elongated crystals up to 1 × 15 µm in size, within lens-shaped voids between separated, deformed (001) layers of (primarily) chlorite and mixed-layer chlorite-corrensite grains of typical detrital shape or chlorite packets in chlorite-mica stacks (intergrowths of chlorite and phengite packets) up to 40 × 150 µm in size. Relict biotite layers within chlorite, mixed-layer chlorite-corrensite and berthierine have textures implying replacement of the former by the latter, whereas in other unaltered samples only fresh biotite was observed. Anatase occurs in otherwise Ti-free chlorite, whereas relict biotite contains significant Ti (0.3 moles per 22 oxygen atoms). No sulfide minerals have been found in fresh biotite and phengite.

Mass balance considerations indicate that S and Zn were introduced via pore fluids and that the Fe was provided by the decomposition of biotite to secondary phyllosilicates. The alteration of biotite and the reaction of biotite to form chlorite and pyrite is controlled by aH+/aK+ as well as oxidation of reduced S species or reduction of oxidized S species from solution. Simple calculations with the observed compositions of chlorite and biotite suggest that some of the Fe in biotite was actually removed in solution rather than precipitated in pyrite and chlorite. Similar textures are abundant in ferroan phyllosilicates elsewhere, implying that the mechanism may apply widely to precipitation of sulfides in phyllosilicates during early diagenesis of sediments.

Cores of pelitic sediments (Eocene-Miocene) of the drillings Puchkirchen 1 and Geretsberg 1 (Molasse Basin, Upper Austria) have been studied to determine the mineralogical and chemical changes taking place during burial diagenesis. Mineralogical and chemical investigations of the bulk samples show that the deepest samples of the profiles are derived from a different source area. In particular, there is an increase in kaolinite and chlorite with depth and a decrease in quartz related to the initial sedimentology and provenance.

Investigations of the <2 µm and <0.2 µm fractions of the profiles Puchkirchen 1 and Geretsberg 1 reveal the diagenetic overprint of the mineral constituents: The gradual illitization of mixed-layer illite-smectite, also reflected in an increase of K2O and Al2O3, is displayed most prominently in the <0.2 µm fraction. The source for the Al and K is the dissolution of K-feldspar (<2 µm fraction), as indicated in many previous studies.

The I-S mixed-layer phases are randomly interlayered to a depth of 1600 m; from there on a regular interstratified I-S phase appears in coexistence with the randomly interlayered I-S mixed layer. The randomly oriented phase is still present in major amounts to depths of 2500 m, presumably as a result of the low geothermal gradient (2.9 °C/100 m) in the Molasse Basin.

The calculation of the structural formula of the end members illite and smectite from this series of I-S mixed-layer phases gave the following results: Smectite: $${K_{0.14}}{X^ + }_{0.44}\left( {A{l_{1.10}}M{g_{0.46}}F{e_{0.36}}T{i_{0.01}}} \right)S{i_{4.03}}{O_{10}}{\left( {OH} \right)_2}$$

Illite: $${K_{0.44}}{X^ + }_{0.19}\left( {A{l_{1.26}}M{g_{0.42}}F{e_{0.38}}T{i_{0.01}}} \right)\left( {S{i_{3.52}}A{l_{0.48}}} \right){O_{10}}{\left( {OH} \right)_2}$$

The end-member interlayer charge for the smectite component (+0.58) is higher than reported for typical smectites (+0.32 to +0.47). It is suggested that the I-S phases of the Molasse Basin are probably intergrowths of 3 layer-silicate members: illite, low-charged smectite and high-charged smectite. The determined smectite end-member composition represents, therefore, an average for a variable 2-component smectite system. The charge-differences of the 2 smectites would likely reflect the differences in source material, which in turn would have led to the formation of different early, highly smectitic I-S phases in the sedimentary basin.

We measured the visible to near-infrared (IR) spectra of 176 synthetic and natural samples of Fe oxides, oxyhydroxides and an oxyhydroxysulfate (here collectively called “Fe oxides”), and of 56 soil samples ranging widely in goethite/hematite and goethite/lepidocrocite ratios. The positions of the second-derivative minima, corresponding to crystal-field bands, varied substantially within each group of the Fe oxide minerals. Because of overlapping band positions, goethite, maghemite and schwertmannite could not be discriminated. Using the positions of the 4T1←6A1, 4T2←6A1, (4E;4A1)←6A1 and the electron pair transition (4T1+4T1)←(6A1+6A1), at least 80% of the pure akaganeite, feroxyhite, ferrihydrite, hematite and lepidocrocite samples could be correctly classified by discriminant functions. In soils containing mixtures of Fe oxides, however, only hematite and magnetite could be unequivocally discriminated from other Fe oxides. The characteristic features of hematite are the lower wavelengths of the 4T1 transition (848–906 nm) and the higher wavelengths of the electron pair transition (521–565 nm) as compared to the other Fe oxides (909–1022 nm and 479–499 nm, resp.). Magnetite could be identified by a unique band at 1500 nm due to Fe(II) to Fe(III) intervalence charge transfer. As the bands of goethite and hematite are well separated, the goethite/hematite ratio of soils not containing other Fe oxides could be reasonably predicted from the amplitude of the second-derivative bands. The detection limit of these 2 minerals in soils was below 5 g kg−1, which is about 1 order of magnitude lower than the detection limit for routine X-ray diffraction (XRD) analysis. This low detection limit, and the little time and effort involved in the measurements, make second-derivative diffuse reflectance spectroscopy a practical means of routinely determining goethite and hematite contents in soils. The identification of other accessory Fe oxide minerals in soils is, however, very restricted.

Adsorption-desorption of Cd to Ca montmorillonite (SAz-1) was studied at concentrations ranging from 44.5 to 266.8 μM. An adsorption model was employed in the analysis of the data. The procedure consists of solving the electrostatic Gouy-Chapman equations and calculating adsorbed amounts of the cations as the sum of the cations residing in the double-layer region, and the cations chemically bound to the surface, in a closed system. The model also accounts explicitly for cation complexation in solution. The model yields good predictions for the adsorbed amounts of Cd, Ca and Mg, by employing binding coefficients from previous studies for the divalent cations and for Na, K and CdCl+. The model calculations also yield good predictions for the apparent hysteresis observed in the adsorbed amounts of Cd after each of 3 cycles of desorption. The apparent hysteresis is explained by the reduction in the total concentrations of Ca and Mg in desorption cycles, and the corresponding increase in the magnitude of the surface potential. Our estimates indicate that adsorption of Cd is mostly to planar, rather than edge sites of the clay mineral.

Halloysite and kaolinite coexist in anorthosite weathering profiles in the Sancheong district, Korea. X-ray diffraction (XRD) analysis on a transect of partially weathered anorthosites reveals an increasing amount of kaolinite development with weathering age. Microtextures were examined by scanning electron microscopy (SEM) of thin sections and raw samples in an attempt to resolve the genetic relation between halloysite and kaolinite. In the earliest stage of weathering, halloysite ellipsoids and short tubes form on the plagioclase surfaces. The successive formation of new grains from the early grains results in globular aggregates where halloysite grains are partially interconnected. With the further progress of weathering, the halloysite grains coalesce from the inner part of the aggregate outward, and convert into stacked kaolinite plates. Continuous addition of halloysite grains on existing plates and their subsequent conversion into plates form long vermicular kaolinite. Solid-state transformation is suggested as a major conversion mechanism. Concurrently with its successive transformation into kaolinite, halloysite also continuously precipitates, giving rise to overgrowths on vermicular kaolinite. Halloysite forms as a kinetically favored metastable precursor and is subsequently transformed into thermodynamically stable kaolinite. Halloysite and kaolinite coexist temporarily in the weathering profile far from equilibrium.

Ludox SiO2 sols with a reasonably uniform size of 12 nm and in the high concentration of 10 to 40 wt% were transformed to gels by lowering the pH. The process from sol to gel, followed by light scattering, did not show a sign of fractal growth within the length scale of observation (>0 nm). Rather, a density fluctuation due to a dynamic (non-equilibrium) disorder was apparent. Together with the results of viscosity measurements, it was concluded that the process consists of 3 stages: 1) A dynamic (non-equilibrium) density fluctuation grows rapidly. 2) Such density fluctuation acquires some order. 3) The entire bulk loses fluidity and becomes a macroscopic gel.

A method using 1,10-phenanthroline (phen) to quantify Fe(II) and total Fe in nonrefractory minerals was modified to improve the accuracy and precision and to eliminate the inconvenience of performing much of the analysis under darkroom conditions. Reagents were combined to minimize solution-handling errors, volumes of the reagent additions were determined gravimetrically and the acid-matrix solution was preheated to near-boiling before sample contact. The darkness requirement, which stems from the photoreduction of Fe(III) to Fe(II) in the presence of phen, was eliminated by the use of opaque amber-colored high-density-polyethylene bottles during the digestion step and for storage of the digestate and subsequent dilutions before Fe(II) analysis. Reduction of Fe(III) for total-Fe analysis was accomplished either by exposure to light from a Hg-vapor lamp or by reaction with hydroxylamine, NH2OH. Although the minimum periods required for adequate reduction ranged from 1.5 to 4 h, the optimum reduction periods were between 6 and 10 h. When standard samples containing Fe(II) and MnCl2 were digested and analyzed for total-Fe using the light treatment (with incidental heating to 35–45 °C), significant decreases and in some instances, oscillations, in absorptivity were obtained. Similar experiments with NH2OH, or with CrCl3 showed no effect. The absorptivity of most digestates stored in opaque bottles was stable for at least 2 weeks, although digestates with Mn concentrations above 3 µg mL−1 showed proportional decreases in absorptivity. Analysis of 8 geochemical reference materials by the modified method (using NH2OH) yielded excellent agreement with published values and a mean relative standard deviation of 0.6%. Total-Fe results obtained using the light treatment, however, were generally lower (∼2% relative) than the NH2OH values, although this difference decreased with longer irradiation periods. Use of NH2OH was deemed preferable because it was simpler, faster, minimized interferences from Mn and eliminated the need for specialized apparatus. Lastly, MICA Fe was shown to be unreliable as a primary reference material for Fe(II) determinations.

Eight kaolinite and 2 halloysite samples were analyzed using 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, chemical analysis and magnetic susceptibility to understand the effect of isomorphously substituted Fe3+ and secondary Fe phases on the NMR signal. Known additions of goethite and hematite were made to determine the response of kaolinite 27Al MAS NMR spectra and sample magnetic susceptibilities.

Results from high field (11.7 T) NMR studies show positive correlations between 1) Fe content, 2) magnetic susceptibility and 3) relative intensity of the spinning side band (SSB) to central band (CB) ratio. No correlation is observed between the mass-corrected NMR spectral intensity and Fe content. Comparative high/low field (11.7 T/8.46 T) NMR studies show a decrease in the relative ratio of line broadening with increasing Fe content. Projected trends of known additions of hematite and goethite versus magnetic susceptibility extrapolate back to zero y intercepts that have Fe concentrations higher than actually measured.

Absolute intensity observations have negative implications for the use of 27Al MAS NMR spectroscopy in assessing Fe-ordering in kaolinites. First, high-energy, short (1/6 of π/2 solutions) pulse sequences do not produce reliable quantitative data needed to assess paramagnetic line-broadening affects caused by different Fe-ordering clustering scenarios. The lack of perfect correlation between SSB/CB, Fe content and magnetic susceptibility indicates that differences exist with respect to 1) the amount of isomorphously substituted Fe, 2) the ordering of the Fe within kaolinite, 3) the concentration of secondary Fe phases and 4) magnetic susceptibility of the secondary Fe assemblage. Variability of line-width ratios at different field strengths indicates an increasing second-order quadrupole effect (SOQE) with increasing Fe. Finally, the difference between the observed Fe content and that predicted from magnetic susceptibility measurements suggest that magnetic domain properties of secondary Fe phases behave differently from Fe domains bound in kaolinite.

Solid phase transformation and metal solubility were monitored after coprecipitation of Cd2+, Cu2+, Pb2+ and Zn2+ with Fe3+ to form ferrihydrite by titration to pH 6. The (co)precipitates were aged at room temperature for up to 200 d and subsequently heated for 60 d at 70 °C. The mode of (co)precipitate formation, rapid and slow titration, was also investigated. Metal solubility was measured by anodic stripping voltammetry. Surface area, Fourier transform infrared (FTIR) and X-ray diffraction (XRD) analysis were used to follow the transformation of ferrihydrite after initial (co)precipitation. Electron microprobe analysis (EMPA) was used to show the distribution of metals within ferrihydrite aggregates. Thermal treatment produced a reduction in soluble Cd2+ and Zn2+, whereas Pb2+ appeared to be expelled from the solid phase. The more stable coprecipitate (formed by slow titration) maintained a constant Cu2+ solubility after thermal treatment. Characterization of the solid phase by XRD indicated that the presence of low levels of metals did not affect the initial or final transformation products, although metals present during the slow titration seemed to stabilize a higher surface area material. The rapid titration resulted in a less ordered (1-line) ferrihydrite than the slow titration (9-line). Furthermore, FTIR analysis indicated that the presence of metals promoted the formation of mixed (microcrystalline) end-products. The initial coprecipitation products seem to determine the final thermal transformation products. These transformation products include ferrihydrite, hematite (Hm), and goethite (Gt)- and lepidocrocite-like microcrystalline structures. Although experimental conditions were favorable for the homogeneous distribution of metals throughout the coprecipitate, EMPA suggests that Cu and Zn segregation within aggregates of Fe oxides occurs.

The fine clay (<0.1 μm) fraction of a clayey soil (Vertisol) from Sardinia (Italy) was studied by means of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, cation exchange capacity (CEC) and surface area measurements. Smectites were the dominant clay minerals both in the parent material and the soil horizons. The magnitude and location of the smectites’ layer-charges were analyzed using the Hofmann & Kiemen effect (suppression of the octahedral charge following lithium-saturation and heating). The amount of montmorillonite layers was evaluated by measuring the reduction of total surface area and CEC after suppression of octahedral charges and subsequent collapse of montmorillonite interlayers. Reduction of total surface area and CEC after fixation of K and irreversible collapse of interlayers were used to quantify high-charge layers before and after the suppression of octahedral charges. This allowed us to evaluate the amount of tetrahedral high-charge layers. The smectites in the parent material were montmorillonitic-beidellitic mixed layers and exhibited a small proportion of high-charge layers; in particular, layers with a high tetrahedral charge were a very minor component. In the upper soil horizons, the amount of montmorillonitic layers decreased whereas the amount of smectite layers with a high tetrahedral charge increased. FTIR spectroscopy indicated more Fe for Al substitution in the smectites of the soil horizons than in the smectites of the parent material. The results suggested that octahedral charged smectite layers (montmorillonitic) were altered, whereas high-charge beidellitic layers were formed in this soil environment characterized by rather high pH (>8.0).

The purpose of this study was to characterize more fully the surface charge characteristics of the end-member smectite in illite-smectite (I-S) mixed-layer phases found previously in pelitic sediments of the Molasse Basin in Austria. The smectite end member was shown to have an unusually high interlayer charge (0.58). Based on earlier work on pure smectites, it was hypothesized that this high charge represents the mean of a mixture of a higher- and lower-charged smectite component intermixed with illite. To test this hypothesis, the magnitude of the interlayer charge of the smectites was evaluated using 2 different methods: alkylammonium ion orientation and K-fixation by wetting and drying.

Using 2 I-S samples of different I-S ratios, saturated with alkyammonium ions of chain lengths nc = 5–18, X-ray diffraction patterns (XRD) could be interpreted as representing a 3-component system, consisting of randomly interlayered high- and low-charged smectite and illite.

K-fixation, carried out by K-treatment and followed by 100 wetting and drying (WD) cycles, confirmed the presence of a high-charged smectite component admixed with low-charged smectite, both interlayered with illite. The wetting and drying of the K-treated samples led to interlayer collapse of the high-charged smectite component and to the production of illite layers stable against exchange with 0.1 N SrCl2. The 2 smectites occur in the ratio of about 1:1 and consist of 1 phase with an interlayer charge of about 0.76 and another phase with a normal charge of about 0.40. During diagenesis, the 2 kinds of smectite are altering simultaneously to the same end-member illite along 2 different reaction paths.