Development of preferred orientations of illite-smectite (I-S) has been studied using X-ray diffraction (XRD) texture goniometry to produce pole figures for clay minerals of a suite of 16 mudstone samples from a core from the Gulf Coast. Samples represent a compaction-loading environment in which the smectite-to-illite (S-I) transition occurs. In five shallow, pre-transition samples, there is no significant preferred orientation for smectite-rich I-S. Development of preferred orientation of I-S, although weak, was first detected at depths slightly less than that of the S-I transition. The degree of preferred orientation, which is always bedding-parallel, increases rather abruptly, but continuously, over a narrow interval corresponding to the onset of the S-I transition, then continues to strengthen only slightly with increasing depth. The degree of post-transition preferred orientation is also dependent on lithology, where the preferred orientation is less well-defined for quartz-rich samples.

Previously obtained transmission electron microscope (TEM) data define textures consistent with the change in orientation over many crystallites. The smectite in pre-transition rocks consists largely of anastomosing, “wavy” layers with variable orientation and whose mean orientation is parallel to bedding, but which deviate continuously from that orientation. This results in broad, poorly defined peaks in pole figures. Post-transition illite, by contrast, consists of thin, straight packets, with most individual crystallites being parallel or nearly parallel to bedding. This results in pole figures with sharply defined maxima. By analogy with development of slaty cleavage in response to tectonic stress during metamorphism, the S-I transition is marked by dissolution of smectite and neocrystallization of illite or I-S locally within the continuous “megacrystals” of smectite. The transition is inferred to have some component of mechanical rotation of coherent illite crystals within a pliant matrix of smectite. The data suggest that change in orientation and coalescence of clay packets plays an important role in the formation of the hydraulic seal required for overpressure generation.

The amount of Mn2+ adsorbed or removed from solution by birnessite is several times greater than its reported cation exchange capacity. Extractability of the sorbed Mn2+ decreases with aging. It is uncertain whether the sorbed Mn2+ is oxidized on the surface or incorporated into the structure of birnessite. Using X-ray powder diffractometry and transmission electron microscopy, a study was conducted to examine the mineralogical alteration of birnessite after treatment with various concentrations of MnSO4 and solution pH.

The sorbed Mn2+ was not directly oxidized and remained on the birnessite surface. The sorption of Mn2+ was followed by alteration of birnessite with the formation of new Mn minerals. The specific Mn minerals formed were governed by the pH of the reaction, and the rate of the transformation was determined by Mn2+ concentration and pH. Nsutite and ramsdellite were identified at pH 2.4, crypto-melane at pH 4, groutite at pH 6, and manganite at pH 8. Other Mn minerals formed at these and other pH levels could not be identified. As the concentration of Mn in the solution decreased, the time required to form new minerals from the birnessite increased. The newly formed phases were the result of structural conversion since dissolution of birnessite and reprecipitation of new phases were not observed.

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 structural damage produced by dry grinding and acid leaching of chrysotile was studied by transmission and scanning electron microscopy, infrared spectroscopy, X-ray powder diffraction, and thermogravimetric analysis. Severe dry grinding converted the chrysotile fibers into fragments having strong potential basic reaction sites. These sites were immediately neutralized by molecules present in the atmosphere (e.g., H2O, CO2). Acid leaching transformed the chrysotile fibers into very porous, non-crystalline silica, which was easily fractured into short fragments. The damage produced in the chrysotile structure by grinding or leaching was assessed by monitoring the intensity of various infrared absorption bands.

The transformation of sodium and potassium smectite into mixed-layer clay was followed in hydrothermal kinetic experiments. Glasses of beidellite composition and the Wyoming bentonite were used as starting materials. Temperatures ranged between 260 and 490°C at 2 kbar pressure, and run times ranged between 6 hr and 266 days.

The course of the reactions was found to be strongly affected by interlayer chemistry. When potassium was the interlayer cation, increasing reaction produced the series: randomly interstratified illite/smectite-ordered interstratified illite/smectite-illite. This sequence is equivalent to that formed in shales during burial diagenesis. With interlayer sodium and temperatures above 300°C, an aluminous beidellite (Black Jack analog)-rectorite-paragonite series was realized. The difference between these two diagenetic families is discussed. Below 300°C, sodium beidellite formed randomly interstratified mixed-layer clay much like potassium beidellite, except that a higher layer charge was required to produce sodium mica-like layers. The higher charge resulted from sodium's higher hydration energy. The difference in hydration energy between potassium and sodium may account for the fixation of potassium rather than sodium in illite during burial diagenesis.

The appearance of ordered interlayering in mixed-layer phases is also related to interlayer chemistry. Ordering formed in sodium clays at high expandabilities, whereas it never appeared in the potassium clays above approximately 35% expandable. The appearance of ordering may be partly related to the polarizing power of the mica-like layers.

Phase diagrams, constructed from the kinetic experiments and from the composition and occurrence of natural clays, are presented for the systems paragonite and muscovite-2 quartz-kaolinite-excess water. This study also reports the first synthesis of a Kalkberg-type mixed-layer clay.

The heterogeneity of the interlayer cation density and the corresponding mean layer charge density due to isomorphous substitution have been determined for six montmorillonite samples. This has been achieved by considering the transition of a monolayer to a double layer complex with alkylammonium ions. The transition is related to the interlayer cation density, together with the variation of the apparent spacings, which in turn is related to the composition of this two-component mixture.

All the samples showed a heterogeneous interlayer cation distribution and a differing mean charge density.

The clay mineralogy of soil samples from the Morrow Plot Experiment (University of Illinois, Urbana Campus) was investigated. Analysis of soil samples taken at various intervals between 1913 and 1996 indicates that there is a significant influence of cropping method on the clay minerals in the soils. Curve decomposition methods were used to identify and follow the evolution of the different clay minerals: mica, illite and two randomly mixed-layered illite-smectite phases. The most striking difference is seen for continuous corn and corn-oats-hay rotations. Little change in clay mineralogy is seen in the rotation plot while a significant loss of illitic material from different phases was noted for the continuous-corn cultivation plots. Use of NPK fertilizer since 1955 appears to restore the clay mineralogy in continuous-corn cropping compared to that of the 1913 samples. From these data it appears that the I-S minerals play the role of a K buffer, becoming K-poor when the soil cannot furnish enough K from mineral reserves of detrital phases and K-rich when the soil is able to release enough K to enter into the I-S minerals, where it is available during a growing season, for plant growth.

Kaolinites with varying degrees of defect structures have been studied by both mid-infrared (IR) and near-IR diffuse reflectance spectroscopy (DRIFT). Difference bands were observed in the 2650- to 2750-cm−1 region. This region coincides with the kaolinite-deuterated hydroxyl stretching region. Summation bands were observed in the near-IR spectra in the 4500- to 4650-cm−1 and in the 7050- to 7250- cm−1 region. Each of the spectral regions of the summation and difference bands is both kaolin polytype and sample dependent. It is proposed that each of these sets of bands arises from the combination of the hydroxyl stretching frequencies and the hydroxyl deformation frequencies and, to a lesser extent, the silicon-oxygen symmetric stretching vibration of the siloxane layer. Additional difference bands of very low intensity were also observed at 2930 and 2856 cm−1. Combination bands were observed in all kaolinites at 2137 and 2227 cm−1. Each of the 3 major combination spectral regions was composed of 5 component bands corresponding to the 4 IR active and the 1 Raman active kaolinite hydroxyl stretching frequencies. Combination bands were also observed at ~1932 and 1821 cm−1.

Sodium-saturated Wyoming bentonite was hydrothermally reacted at 150° and 250°C for 30 to 180 days to determine smectite alteration rates that might be applied to nuclear-waste repository design. Na-Ca solutions deficient in K were used to determine the role of interlayer cations in the creation of high layer-charge in the smectites. The results provide insight into the mechanism and timing of various steps in the diagenetic alteration of smectite to illite. X-ray powder diffraction (XRD) analyses of the reacted clay showed little effect on the character of the 17-Å reflection even after 180 days at 250°C. Potassium saturation of these reacted clays and re-examination by XRD indicated collapse of some smectite layers, leaving at most only 60% expandable layers. The development of layer charge sufficient to cause collapse on saturation with a low hydration energy exchange-cation does not require K in the reacting fluid. Rate constants for the illitization reactions as determined by K-saturated collapse are between 1.0 × 10−3 and 2.8 × 10−3/day with activation energies <3.5 kcal/mole. Ca in a Na-silicate-bicarbonate solution slightly reduced the illitization rate constants. These rate constants are higher than expected from extrapolation of studies of beidellite-composition glasses at higher temperatures, but lower than values obtained in studies of natural clays in artificial sea water. The release of Si, Al, and Mg in the 150°C experiments suggests congruent dissolution of the smectite. In contrast, at 250°C the release of Al was not stoichiometric with Si; as little as one half of the relative available Si was released. Rather than different mechanisms for dissolution at the two temperatures, the conclusion is that noncrystalline Al-rich phases formed at greater rates at higher temperatures. The cation-exchange capacities for several of these reacted smectites were significantly less than expected, suggesting a clogging of interlayer sites, perhaps by Al-complexes.

A new method for the prediction of Gibbs free energies of formation for hydrated clay minerals is proposed based on the parameter ΔGO= Mz+(clay) characterizing the oxygen affinity of the cation Mz+. The Gibbs free energy of formation from constituent oxides is considered as the sum of the products of the molar fraction of an oxygen atom bound to any two cations multiplied by the electronegativity difference defined by the ΔGO= Mz+(clay) between any two consecutive cations. The ΔGO= Mz+(clay) value, using a weighting scheme involving the electronegativity of a cation in a specific site (interlayer, octahedral, or tetrahedral) is assumed to be constant and can be calculated by minimization of the difference between experimental Gibbs free energies (determined from solubility measurements) and calculated Gibbs free energies of formation from constituent oxides. Results indicate that this prediction method compared to other determinations, gives values within 0.5% of the experimentally estimated values. The relationships between ΔGO= Mz+(clay) corresponding to the electronegativity of a cation in either interlayer or octahedral sites and known ΔGO= Mz+(aq) were determined, thereby allowing the prediction of the electronegativity of transition metal ions and trivalent ions in hydrated interlayer sites and octahedral sites. Prediction of Gibbs free energies of formation of any clay mineral with various ions located in the interlayer and with different cations in octahedral sites is possible. Examples are given for Al-rich montmorillonite from Aberdeen, transition element-exchanged montmorillonite, and Ni-rich stevensite, and the results appear excellent when compared to experimental values.

The interface between the surfaces of clay minerals and other materials (aqueous solution, organic moieties, biomolecules, etc.) is of great importance to many geological, technological and biological processes. The examination of the structure and composition of mineral surfaces is advancing rapidly. However, the crystal structure of a mineral is only part of the information needed to understand the activity of the surface; we still do not fully understand the chemical and physical interactions at the surface or interface. One very powerful methodology for the study of surfaces and interfacial interactions is the determination of the thermodynamic properties of the surface. It became possible in the late 1980s to determine quantitatively the apolar and polar surface-tension components and parameters of liquids and solids (van Oss et al., 1988; van Oss, 1994), and the experimental techniques necessary for obtaining surface free-energy components of minerals, and particularly of colloid-sized minerals, are also a recent development (Costanzo eta!., 1990; van Oss et al., 1990, 1992; Giese et al., 1991).

A clay-rich Callovo-Oxfordian sedimentary formation was selected in the eastern Paris Basin (MHM site) to host an underground laboratory dedicated to the assessment of nuclear waste-disposal feasibility in deep geological formations. As described initially, this formation shows a mineralogical transition from an illite-smectite (I–S) mixed-layered mineral (MLM), which is essentially smectitic and randomly interstratified (R = 0) in the top part of the series to a more illitic, ordered (R ⩾ 1) I–S in its deeper part.

This description has been challenged by using the multi-specimen method developed by Drits et al. (1997a) and Sakharov et al. (1999). It is shown that all samples contain a physical mixture of an unusually (?) illitic (∼65% I) randomly interstratified I-Exp (illite-expandable MLM) and of a discrete smectite, in addition to discrete illite, kaolinite and chlorite. Structural parameters of the different clay phases vary little throughout the series. According to the proposed model, the mineralogical transition corresponds to the disappearance of smectite with increasing burial depth.

Comparison with clay minerals from formations of similar age (Oxfordian-Toarcian) throughout the Paris Basin shows that the clay mineralogy in the deeper part of the series originates from a smectite-to-illite transition resulting from a low-temperature burial diagenesis. The anomalous lack of evolution of clay minerals in the upper part of the series is thought to be related to specific interactions between organic matter and clay minerals.

In this study, the feasibility of using surfactant-modified palygorskite (PFl-1) and sepiolite (SepSp-1) for removal of anionic contaminants from water was evaluated from batch experiments. The results showed that both minerals had strong affinity for hexadecyltrimethylammonium (HDTMA), a cationic surfactant used for surface modification. The HDTMA sorption capacities were 520 and 260 mmol/kg for PFl-1 and SepSp-1, respectively. Accompanying HDTMA sorption, the sorption of counterion bromide reached 380, and 210 mmol/kg, for PFl-1 and SepSp-1, respectively, indicating that the sorbed surfactant molecules form admicelles on the minerals’ surfaces. After modification by HDTMA to sorption maxima, these clays showed strong affinity for anionic contaminants such as chromate and nitrate. The chromate sorption capacities were 42 and 34 mmol/kg for HDTMA-modified PFl-1 and SepSp-1, respectively. Desorption of counterion bromide due to sorption of chromate followed a straight-line relationship, suggesting that the sorption of chromate on surfactant-modified palygorskite and sepiolite was also due to anion exchange as with other surfactant-modified clay minerals and zeolites.

Various workers have studied the mechanism of swelling of Na-montmorillonite and have demonstrated that its ability to swell is due to water penetrating between the individual silicate sheets. In the present study intraerystalline swelling was followed by diffraction techniques which enabled the movement of the silicate sheets with respect to one another to be measured as a function of electrolyte concentration, with or without an externally applied load. The observations have been made on oriented flakes of Na-montmorillonite and single crystals of Li-vermiculite.

Qualitatively the minerals behave similarly when swollen in salt solutions. They both show an initial stage of crystalline swelling, after which there is an explosive increase to the gel state, and then the distance apart of the silicate sheets increases linearly with C-½, where C is electrolyte concentration. Quantitatively, however, their swelling is very different, particularly after the “explosion”, where vermiculite generally gives higher spacings than montmorillonite. There are two other important differences in the swelling of montmorillonite and vermiculite. Firstly, while the swelling of vermiculite appears to be reversible both with respect to electrolyte concentration and applied pressure, the swelling of montmorillonite shows a marked hysteresis. Secondly, in montmorillonite, swelling depends very strongly on pH or chemical treatment.

There is little doubt that the development of diffuse “double layers” gives rise to repulsive forces, which cause the silicate sheets to move apart. Van der Waals’ forces have been regarded as providing attraction, but it is found that, at the observed interlayer separations, the magnitude of these attractive forces is inadequate to balance repulsion, both in montmorillonite and vermiculite.

The present results demonstrate that in vermiculite swelling proceeds until the interlayer separation causes the repulsion between sheets to drop to a value of ~ 2.5 × 104 dyn/cm2. This attractive force, which limits swelling, appears to be independent of sheet separation and electrolyte concentration. In montmorillonite swelling is opposed by edge-to-face bonds between sheets, whose number and strength can be controlled by chemical treatment. These bonds act to resist any displacement of the sheets, either swelling or contraction. This mechanism is compatible with the observed swelling behavior of montmorillonite, and explains the differences between the swelling of this mineral and that of vermiculite where, because the sheets are considerably larger, the force arising from edge-to-face bonds is small or absent.

Varieties of kaolin, a rock, may be classified geologically, mineralogically, crystallographically, genetically, texturally, morphologically, by industrial use, and in other ways which are desired. In this paper, the first-order of classification used is geological, i.e., transported and residual, after which other categories are used as subdivisions.

Scan electron micrographs, SEM, of the textures of kaolin show that distinctive textures characterize the several categories of classification. Varieties in texture of kaolin include similarities to those typical of sedimentary, igneous and metamorphic rocks. Because word descriptions of the textures are inadequate in comparison to pictures of them, the reader is referred to the micrographs.

SEM's illustrate differences between kaolins which were transported, formed, or deposited from solution, a colloidal phase, or as orthodox clastic particles. The parent source of kaolin deposited, or “grown,” from solution may be difficult to ascertain. It is suggested that the total role of solution work in kaolin petrology can be more important than has ordinarily been credited.

Specimen samples of Crook County montmorillonite and Silver Hill illite, purified and prepared in the Na-form, were imaged under 80% relative humidity using an atomic force microscope. The direct images showed clearly the hexagonal array of hexagonal rings of oxygen ions expected for the basal planes of 2:1 phyllosilicates. Fourier transformation of the digital information obtained by the microscope scanning tip led to an estimate of 5.1 ± 0.3 Å for the nearest-neighbor separation, in agreement with the ideal nearest-neighbor spacing of 5.4 Å for hexagonal rings as derived from X-ray powder diffraction data. The atomic force microscope should prove to be a useful tool for the molecular-scale resolution of clay mineral surfaces that contain adsorbed macromolecules.

A complete conversion series for mica/smectites was found in a hydrothermal alteration envelope around Kuroko-type ore deposits at the Shinzan area, Akita Prefecture, Northeast Japan. The minerals are an alteration product of volcanic glass of Miocene age and are commonly associated with zeolites and silica minerals. Degrees of ordering of interstratification of the minerals change discontinuously from Reichweite g = 0 (100–55% expandable layers) to g = 1 (45–20% expandable layers), and from g = 1 to g = 2 (<20% expandable layers). This pattern of conversion differs from the behavior of mica/smectites during burial diagenesis which undergo a continuous change in ordering type, and from the behavior of rectorite which displays a constant expandability and ordering (45–55%) over a wide range of conditions. Differences between these minerals were also found in the relationships between expandability and total layer charge, and between expandability and number of non-exchangeable interlayer cations. In mica/smectites from the Shinzan area, chemical changes in the interlayers and tetrahedral and octahedral sites are consistent with a reaction in which K-enrichment and K-fixation in the interlayers are controlled by an increase in negative layer charge. This conversion occurred in response to a steep geothermal gradient and migrating hydrothermal solutions.

The term ‘montmorillonite’ encompasses a wide range of chemical compositions and structures. Comprehensive and reliable characterization is essential for unambiguous classification. Twenty eight purified, Na-exchanged smectites (<0.2 µm) were characterized by layer-charge measurement using the alkylammonium method, by cation exchange capacity (CEC) measurement with Cu-triethylenetetramine, by determination of the chemical composition using X-ray fluorescence analysis, by calculation of the structural formula following determination of the octahedral structure (trans-vacant vs. cis-vacant) by simultaneous thermal analysis, and by X-ray diffraction analysis. Mössbauer spectroscopy was applied to determine the oxidation state and coordination of Fe and changes thereof during purification of the source materials.

The charge derived from chemical composition was considerably greater (by up to 30%) than the measured layer charge. The independently measured layer charge was used to calculate the structural formula. The measured CEC values, comprising the permanent charges and the pH-dependent edge charges, were consistent with measured layer charge but not with layer charge derived from the chemical composition. Therefore, the structural formula of smectites should be calculated using the measured layer charge.

The dehydroxylation temperature, which conveys information about the structure of the octahedral sheet, was correlated to the amount of Mg and Fe3+ and the location of charges. No relationship was found among the dehydroxylation temperature and the mean layer charge or the Mg content. In contrast, a clear relationship was observed between the Fe content and the dehydroxylation temperature. Montmorillonites with an Fe content <0.3/f.u. are cis-vacant and those containing Fe3+ > 0.3 mol/f.u. are trans-vacant, mostly with additional cis-vacancies. Tetrahedral substitution also appeared to be a function of the number of trans-vacancies.

The parameters analyzed provide the basis for a new descriptive classification system.

Deposits of sepiolite, trioctahedral smectite (mixed-layer kerolite/stevensite), calcite, and dolomite, found in the Amargosa Flat and Ash Meadows areas of the Amargosa Desert were formed by precipitation from nonsaline solutions. This mode of origin is indicated by crystal growth patterns, by the low Al content for the deposits, and by the absence of volcanoclastic textures. Evidence for low salinity is found in the isotopic compositions for the minerals, in the lack of abundant soluble salts in the deposits, and in the crystal habits of the dolomite. In addition, calculations show that modern spring water in the area can precipitate sepiolite, dolomite, and calcite following only minor evaporative concentration and equilibration with atmospheric CO2. However, precipitation of mixed-layer kerolite/stevensite may require a more saline environment. Mineral precipitation probably occurred during a pluvial period in shallow lakes or swamps fed by spring water from Paleozoic carbonate aquifers.

The use of neutron diffraction to determine some of the structural properties of montmorillonite-water systems at low water concentrations is described. The samples were prepared by compression or suction to give clay samples with between one and three molecular layers of water between the plates.

About 10% of the platelets in the clay are randomly oriented. The remainder are partially oriented in the plane of the sample, with an angular spread of 40° about the mean orientation. It is suggested that these oriented domains are formed from the larger platelets present in the system. The Bragg diffraction pattern is better explained by a disordered lattice model rather than by a mixture model with small particles having a well-defined lattice spacing. We have fitted both the intensities of (00l) reflections and the shape of the (001) reflection quantitatively to a model which allows for a Gaussian spread of platelet spacing about a mean value. The half width of the spread is about 10% of the lattice spacing.

No significant structural differences are found between Li, Na, K, and Cs montmorillonites. The method of preparation has no effect on the structural properties of the large platelet particles but does affect the randomly oriented fraction. The lattice spacing of the latter appears to be better defined for samples prepared by compression.

Experiments on the variation of lattice spacing with humidity indicate that the structural model we have used is adequate except at humidities where the system is changing over from one to two, or two to three water layers.