Iddingsite rimming olivine in a basanite from the Limberg, Germany, is composed of saponite and goethite. Transmission electron microscopy of ion-thinned, oriented crystals suggests a two-stage alteration process. At first, the olivine breaks into a mosaic of 50-Å diameter {110} bounded needle-shaped domains which change to a metastable hexagonal phase having a = 3.1 Å and c = 4.6 Å, probably of close-packed, metal-oxygen octahedra. This reaction opens solution channels in the olivine which are detectable from about 20-Å diameter and are parallel to the olivine y-axis. Laths of smectite, one or two layers thick, 20 Å wide, and as much as 100 Å long parallel to their y-axis nucleate from the metastable phase and begin to fill in the solution channels. The laths orient with smectite (001) parallel to olivine (100). As the channels widen, prismatic {110} goethite crystals form directly from the metastable hexagonal phase. This first stage thus provides heterogeneous nuclei of smectite and goethite, formed epitactically and perhaps topotactically from a metastable intermediary.

In a second stage,these nuclei enlarge by deposition from solution as water migrates readily through the solution channels. A reduction in total volume allows smectite veins to form, misoriented with respect to the olivine. The reaction conserves iron, requires the addition of aluminum and water, and releases magnesium and silicon. Electron microprobe analyses of the iddingsite indicate that the smectite is saponite.

Chemical analysis by X-ray fluorescence (XRF) and calculated structural formulae of clay-size fractions of smectites from Cretaceous bentonites and illitic clays from Cretaceous, Devonian, and Ordovician bentonites and Jurassic and Permian sandstones indicate the nature and extent of various types of ionic substitution. The determination of tetrahedral (Al, Si) and octahedral (Al, Mg, Fe) composition shows the variable chemistry of these materials. Structural formulae of the illitic clays show that they have tetrahedral charges between 0.4 and 0.8 per half unit cell, and can be divided into phengitic types having octahedral charges of 0.2-0.4 and muscovitic types having octahedral charges <0.2. Evaluation of the formulae in the light of X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) data shows that the occupancy of non-exchangeable interlayer sites (predominantly K) varies from 47% to 90% of that of ideal muscovite. In some minerals as much as 20% of these sites is occupied by ammonium ions (determined independently). The amount of surface silicate charge balanced by non-exchangeable cations versus that balanced by exchangeable cations has been examined in conjunction with TEM data and suggests that in most samples the charges are about equal. The octahedral composition of smectites in Cretaceous bentonites precludes their having served as transformation precursors for most of the Cretaceous illitic bentonites. The results suggest that these illitic clays originated by neoformation.

FTIR studies of six partially-deuterated montmorillonites (MS) reveal the presence of two O-D stretching bands, one between 2702–2728 cm-1 and another near 2680 cm-1. For homoionic (Li, Na, Mg, Ca, or La) Wyoming-type MS, the position of the higher frequency band, designated as (O-D)h, is between 2714–2728 cm-1, whereas for homoionic Cheto-type MS it is between 2702–2706 cm-1. The lower frequency band, designated as (O-D)1, is in the narrow range of 2674–2684 cm-1. Resolution of two corresponding O-H bands, appearing near 3670 and 3635 cm-1, was observed only after partial dehydroxylation of the smectites. The changes in the relative intensities of the two O-D stretching bands as a function of the smectite type and of the Lewis acidity (charge density) of the exchangeable ion were determined. For Wyoming-type MS, the intensity of the (O-D)h band is much lower than that of the (O-D)l band, whereas for Cheto-type MS, the intensity of the (O-D)h band is about equal or slightly higher than that of the (O-D)l band. The observed resolution can be ascribed tentatively to the presence of (at least) two types of octahedral OH groups in the smectites, the (O-D)h band being assigned to AlMgOH and the (O-D)1 band to AlAlOH groups. Pillaring of Cheto-type MS with hydroxy-Al13 oligocations resulted in products showing much higher thermal stability between 400–600°C compared to that of identically pillared Wyoming-type MS. Compositional and other factors, e.g., CEC values and mode of pillaring, may cause this difference in stability.

In the nearly 2000 occurrences of zeolites in sedimentary rocks of volcanic origin about 15 zeolite minerals have been identified. The mode of occurrence of six of these, clinoptilolite, erionite, chabazite, phillipsite, analcime, and mordenite, is described, and their morphology is illustrated with scanning electron micrographs.

X-ray diffraction and electron microscopy were employed in conjunction with core flooding experiments to investigate clay migration phenomena.

Severe water sensitivity or loss of permeability was observed in a suite of sandstones in spite of the almost total absence of montmorillonite or swelling mixed layer clays. Clay migration was found to cause total or partial plugging even in sandstones of 500 millidarcy permeability. Bacterial plugging was ruled out by prefiltering and bactericide treatments of waters.

X-ray diffraction and electron microscopy analyses were performed on the sandstones and produced effluents. The direct cause of damage was displacement of submicroscopic natural clay crystals of needle-shaped mica and hexagonal-shaped kaolinite (Rex, 1965). The mobile clays were identified as authigenic crystals that are present on the pore walls and are dislodged by changes in water chemistry combined with water movement.

Flooding sandstones with alkali metal brines “sensitized” the cores, i.e. triggered clay dispersion upon subsequent flooding with fresh water. Flooding with divalent calcium brine prevented water sensitivity and suppressed the undesirable effect of alkali metal brines. A double layer expansion effect is suggested as the dispersion mechanism.

The vibrational modes of clay minerals in aqueous suspension are uniquely accessible to Raman spectroscopy, but this potentially powerful approach has not been applied heretofore to study clays in aqueous samples. In this paper, Raman spectra in the 100- to 4000-cm−1 region were obtained for kaolinite in aqueous suspension and in air-dry samples. Contact with water perturbed the low-wavenumber Raman spectrum (100 to 1000 cm−1) significantly with respect to relative band intensities and resulted in a pH dependence of the integrated intensity in the OH-stretching region. Comparison of the Raman and infrared (IR) spectra of air-dry kaolinite samples confirmed five Raman-active OH-stretching modes at 3621, 3652, 3668, 3688, and 3696 cm−1, in contrast to four IR-active modes at 3621, 3652, 3668, and 3695 cm−1. The Raman spectra of two kaolinites of different origin showed differences in band positions and intensities. These results suggest that Raman spectroscopy may provide a useful method to study clay mineral-water interactions, colloidal behavior in clay suspensions, and variations in clay mineral structure.

A kaolinite-polymer intercalation complex was apparently formed for the first time by the polymerization of acrylonitrile between the kaolinite layers. A kaolinite-ammonium acetate intercalation complex was dispersed in acrylonitrile monomer. The monomer was apparently incorporated between the layers by displacing intercalated ammonium acetate. After the removal of excess monomer, the intercalation complex was heated to cause polymerization. The resulting kaolinite-polyacrylonitrile (PAN) intercalate showed a basal spacing of ∼ 13–14 Å. On heating the complex at 220°C for 1 hr in air, the spacing decreased slightly. The hydrogen bond between the hydroxyls of kaolinite and probably the C≡N group of PAN was not affected after heating at 220°C. Even after heating at 400°C, the layers expanded. Because the starting kaolinite-ammonium acetate intercalation complex decomposed at a much lower temperature, these observations strongly suggest the presence of PAN between the layers.

The ammonia electrode serves as the basis of a simple, accurate method for determination of cation exchange capacity of small (ca. 50 mg) samples of clays. The technique is also capable of accurate measurement of CEC values on the order of 0·01 m-equiv/100 g if larger (ca. 500 mg) samples are used. The procedure, which requires saturation of the exchange sites with ammonium as in the usual methods, utilizes the electrode in the determination of ammonia released by treatment of the ammonium clay by strong base. For a Wyoming bentonite, the technique gave a CEC of 86 m-equiv/100 g with an S.D. (four determinations) of 0·83 m-equiv/100 g. Duplicate runs on the same sample by the conventional Kjeldahl method gave results of 86·0 and 85·5 m-equiv/100 g.

Ferrihydrite was transformed to goethite and/or hematite at various temperatures, [OH], and [Al]. Increasing temperature and [Al] favored hematite, increasing [OH] favored goethite. A given [Al] induces hematite more effectively at lower [OH]. Al substitution in goethites increased linearly with log[Al], but was independent of temperature. At a given [Al], substitution increased with decreasing [OH]. In a plot of Al/Fe in the goethite against [Al]/[Fe(OH)4−] in solution a straight line was obtained for all preparations independent of [OH].

Ceramic clays are among the most complicated of ceramic systems because of the very intricate relationship between the behavior of minerals during ceramic processing and their modifications during heating. A major challenge is to predict the phase changes in clay ceramics. The aims of this study were to establish reference data of ceramic products that can be formed based on the mineralogical compositions of the local raw materials. These data, in turn, can be compared with archeological ceramics in order to study their origins.

The mineralogical compositions and modifications during firing (550–1100°C under oxidizing conditions) of seven clayey materials sampled from the main clay deposits of northern Morocco were evaluated by X-ray powder diffraction. Two groups of clays were distinguished according to the type of neoformed high-temperature minerals: non-calcareous clays and calcareous clays. For the non-calcareous raw materials, spinel was produced at 950°C. Cristobalite and mullite were formed at temperatures in excess of 1000°C from clays that contain illite, kaolinite, and chlorite. In clays containing vermiculite and large amounts of chlorite, hematite was formed at temperatures in excess of 950°C. Firing of calcareous clays at temperatures >950°C yielded Ca-silicates (diopside, gehlenite and wollastonite), spinel, cristobalite, hematite, and feldspars. Mullite may also form in the calcareous clay products when the carbonate content exceeds 10%.

The mechanism of boron uptake by clays, especially illitic clays, and the factors which control such uptake have long been debated issues. In an attempt to answer some of the questions of the controversy, three illites were treated in solutions containing boron. In the study, boron concentration, salinity, temperature, and time were varied independently over rather wide ranges.

For the illites studied, irreversible uptake of boron was increased by increasing boron concentration, salt concentration, temperature, and time of treatment. The amount of boron which was fixed also varied with the type of illite treated. The amount of fixation was controlled primarily by the specific surface area of the clay and also by the crystallinity, K content, and/or amount of mixed-layer material (“frayed-edge” development?), and apparently was independent of the original boron content of the clay.

A two-step mechanism is proposed for boron fixation by illite, consisting of rapid chemical adsorption of the tetrahedral B(OH)4− anion at the “frayed-edge” of the illite flake followed by much slower diffusion of boron into the tetrahedral part of the structure.

This paper compares the results of 27Al nuclear magnetic resonance spectroscopy (NMR) and Al-K-edge X-ray Absorption Near Edge Structure (XANES) of natural imogolite and allophanes and some crystalline reference minerals. All soil allophanes studied contain 4-coordinated Al (AlIV). The highest relative proportion of AlIV, 21% of the total Al, was found in Si-rich allophane. This value is close to that found in spring allophanes, which were previously considered to be different from soil allophanes. For a quantitative determination of the AlIV/Altotal ratio, NMR is more reliable than XANES, because of the sensitivity of the chemical shift to low AlIV concentrations, but XANES may be used even if paramagnetic impurities (mostly Fe) are present. Al-K XANES also yields more information than NMR on the local environment of AlVI and especially site multiplicity. AlVI XANES of imogolite and allophanes are similar regardless of the Al/Si ratio. They yield two well-resolved resonances with maxima near 1568 and 1570 eV, which indicates the presence of a unique AlVI site by comparison with crystalline references. The presence of only one AlVI site indicates that imogolite and allophanes have an octahedral sheet with a structure similar to 2/1 dioctahedral phyllosilicates but different from gibbsite or kaolinite, previously considered as structural analogues. The 27AlIV MAS NMR peak maxima of allophanes are between 58.6 and 59.8 ppm, in the range observed for crystalline and amorphous framework alumino-silicates, and less positive than those of sheet silicates, which are typically in the range 65–75 ppm. 27Al-H1 CPMAS NMR spectra suggest that both AlIV and AlVI have Al-O-H linkages.

Near-infrared (NIR) reflectance spectra for mixtures of ordered kaolinite and ordered dickite have been found to simulate the spectral response of disordered kaolinite. The amount of octahedral vacancy disorder in nine disordered kaolinite samples was estimated by comparing the sample spectra to the spectra of reference mixtures. The resulting estimates are consistent with previously published estimates of vacancy disorder for similar kaolin minerals that were modeled from calculated X-ray diffraction patterns. The ordered kaolinite and dickite samples used in the reference mixtures were carefully selected to avoid undesirable particle size effects that could bias the spectral results.

NIR spectra were also recorded for laboratory mixtures of ordered kaolinite and halloysite to assess whether the spectra could be potentially useful for determining mineral proportions in natural physical mixtures of these two clays. Although the kaolinite-halloysite proportions could only be roughly estimated from the mixture spectra, the halloysite component was evident even when halloysite was present in only minor amounts. A similar approach using NIR spectra for laboratory mixtures may have applications in other studies of natural clay mixtures.

The scanning electron microscope (SEM) is useful in the identification of biogenic opal. Opaline spheres, cups, and scrolled or convoluted sheets were identified in both soil and vegetative isolates. X-ray diffraction analysis indicates that both alpha quartz and cristobalite were co-associated with the amorphous opaline phase synthesized during life metabolism of deciduous tree leaves. Such crystalline phases were most abundant in the 2–5 μm fraction and many consist of anitsotropic rods with parallel extinction or equidimensional bodies with aggregate extinction. Between 2/3 and 3/4 of the total opal isolate from deciduous tree leaves was solubilized when digested for 2–5-min in boiling 0.5 N NaOH. Rate of dissolution was a function of particle-size and tree species. Biogenic opal of forest origin was about 10–15 times more soluble than grass opal, which probably reflects the higher specific surface of the former.

Infrared and Mössbauer spectroscopy show that the extent of the reduction of nontronite is dependent on the chemical composition of the nontronite and on the nature of the reducing agent. Hydrazine reversibly reduces about 10% of the iron in all of the nontronites studied irrespective of composition and it is suggested that the resulting ferrous iron occurs only in distorted octahedral sites. Similar conclusions are reached for the dithionite reduction of the nontronites containing little tetrahedral iron, but for those with more than one in eight silicons replaced by iron, changes brought about by dithionite treatment are irreversible due to dissolution of appreciable quantities of iron. Results from both spectroscopic techniques suggest that iron in tetrahedral sites is preferentially dissolved and that up to 80% of the structural iron can be reduced.

Evidence is presented for the formation in these extensively reduced nontronites of a small amount of a mica-like phase resembling celadonite or glauconite, and, as dithionite is used for the pretreatment of soils, the implication of this observation is briefly discussed.

The use of deuterated hydrazine as a reducing agent has enabled the nontronite absorption band near 850 cm-1 to be assigned to a Si-O (apical) stretching vibration, which is inactive in the infrared for perfect hexagonal symmetry, but which is activated by distortions in the tetrahedral layer.

The pH of aqueous bentonite suspensions is known to be influenced by carbonates present even in minor amounts. On the other hand, at high solid:liquid ratios (at standard pH measurement conditions: 2% w/w suspension), the type of exchangeable cation in the smectite is also known to determine pH (particularly Na+ or Ca2+). By cation-exchange tests we proved that exchanging the Ca2+ for Na+ results in an increase in the pH. However, this increase in pH was only found if excess salts were removed from the system (by washing or dialysis, respectively). The effect of the type of exchangeable cation can, at least partially, be explained by hydrolysis of Ca2+. On the other hand, a pronounced alkalinity of Na bentonites is observed which can, at least partially, be attributed to the hydrolysis of montmorillonite (Na+ is exchanged for H+ of water). The increase in the volume of the Stern layer, caused by increasing the degree of delamination, is also suggested to play a role. H+ and Na+ are concentrated in the Stern layer. Hence, increasing the Stern layer volume decreases the amount of H+ and Na+ in solution and thus increases pH. Unfortunately, both processes, montmorillonite hydrolysis and delamination, depend on the ionic strength. Distinguishing the processes quantitatively, therefore, is an analytical challenge, and impossible based on the data presented here.

To model the pore-water chemistry of clays and clay stones, all of the above-mentioned processes have to be considered. It is possible that other reactions, not identified in the present work, contribute toward the pH values of aqueous bentonite suspensions.

The negative surface charge of synthetic allophanes with a range of Si/Al ratios decreased and positive charge increased with increasing alumina content at a given pH. The phosphate adsorption capacity also increased with increasing Al content. That this relationship between composition and chemical reactivity was not found for the soil allophanes is attributed to the presence of specifically adsorbed organic or inorganic anions on the natural material. Both synthetic and natural imogolites had a much lower capacity to adsorb phosphate than the allophanes and adsorbed anomalously high amounts of Cl− and ClO4− at high pH. It is proposed that intercalation of salt occurs in imogolite, although electron spin resonance studies using spin probes failed to reveal the trapping of small organic molecules in imogolite tubes. These spin probes in the carboxylated form did, however, suggest an electrostatic retention of carboxylate by imogolite and a more specific adsorption by allophane involving ligand exchange of surface hydroxyl. The results illustrate the inherent differences in charge and surface properties of allophane and imogolite despite the common structural unit which the two minerals incorporate.

Reinterpretation of published data for shale cuttings from the Gulf of Mexico sedimentary basin identifies three reaction zones for illite formation with increasing depth for well CWRU6. In a shallow zone (1.85 to 3 km), non-expanding illite-like layers formed primarily by the coalescence of smectite 2:1 layers around interlayer K+. In a middle zone (3 to 4 km), illite crystals neoformed from solution as coarser K-bearing phases and smectite were dissolved by organic acids. In the deepest zone (>4 km), illite recrystallized as less stable illite crystals dissolved, and more stable illite crystals grew during mineral ripening. The progressive loss of radiogenic argon in the deepest zone yielded a constant apparent age for the clays with depth, an effect previously attributed to “punctuated diagenesis.” The above hypothesis for illite formation emphasizes the need to establish the zone (i.e., the reaction mechanism) from which shales were derived before making detailed geologic interpretations based on illite mineralogy.

The importance of various sediment components in the oxidation of As(III) (arsenite) to As(V) (arsenate) by freshwater lake sediments in southern Saskatchewan was examined. Treating the sediments with hydroxylamine hydrochloride or sodium acetate to remove Mn greatly decreased the oxidation of As(III). Furthermore, synthetic Mn(IV) oxide was a very effective oxidant with respect to As(III): 216 µg As(V)/ml was formed in solution when 1000 µg As(III)/ml was added to suspensions of 0.1 g of the oxide. These results indicate that Mn in the sediment was probably the primary electron acceptor in the oxidation of As(III). The conversion of As(III) to As(V) by naturally occurring carbonate and silicate minerals common in sediments was not evident in the system studied. Sediment particles >20 µm in size are the least effective in oxidizing As(III); the oxidizing ability of the 5–20-, 2–5-, and <2-µm particle size fractions varies depending on the sediment. The concentration of As(V) in equilibrated solutions after adding increasing amounts of As(III) (as much as 100 µg/ml) to 1 g of the three sediments ranged from approximately 3.5 to 19 µg/ml. Because As(III) is more toxic and soluble than As(V), Mn-bearing components of both the colloidal and non-colloidal fractions of the sediments may potentially detoxify any As(III) that may enter aquatic environments by converting it to As(V). This is very important in reducing the As contamination and in maintaining the ecological balance in aquatic environments.

A fundamental property of electrostatic potentials is their additivity. This study demonstrates that the electrostatic potential of a negatively charged, cation-substituted phyllosilicate layer can be represented as the sum of two potentials. Viewing cation substitution as a defect, one potential is derived from the atoms in a charge-neutral, unsubstituted layer such as pyrophyllite or talc. The “neutral-layer” potential rapidly decays to zero with distance from the layer and is determined primarily by the atoms in the first two atomic planes parallel to the (001) surface, i.e., the basal oxygens and tetrahedral cations. The second component, characterized as a “defect” potential, is a long-range potential derived from cation-substitution. The model used to compute the electrostatic potentials, a two-dimensional Ewald lattice sum, represents the atoms of a single phyllosilicate layer as point charges.