Ordered illite/smectite (I/S) and illite in a pelitic rock from a prograde metamorphic sequence in North Wales were observed by transmission electron microscopy. The dominant phyllosilicate noted was diagenetic-metamorphic illite, occurring as subparallel packets of layers, each about a few hundred Ångstroms thick. It exhibited two-layer polytypism (presumably 2M1) and numerous strain features and had a composition of (K1.21Na0.12)(Al3.36Fe0.31Mg0.33)(Si6.28Al1.72)O20(OH)4.

I/S occurred as thick packets of wavy layers having 10-Å subperiodicity and sharp differences in contrast in successive lattice fringes. All stages in a replacement series were noted, from one or two layers of smectite within illite, through thin packets of I/S, to thick packets that contained inherited deformation textures of diagenetic-metamorphic illite. Deformed illite was replaced by I/S more commonly than was undeformed illite. The I/S replacing undeformed original illite had significantly greater order, primarily of R1 type (ISISIS…), than that replacing deformed illite. R> 1 I/S occurred as small crystallites and contained relatively less smectite than the ordered I/S, Single smectite layers were spaced within several illite layers, forming curved packets of layers. IISIIS… (R2) and IIISIIIS… (R3) ordering were present locally, as was discrete smectite. Analytical electron microscopic analyses indicated that the I/S, (K0.46Na0.43)(Al3.75Fe0.06Mg0.19)(Si6.26Al1.74)O20(OH)4, was rectorite-like in composition and had smaller (Mg + Fe) contents and greater Al/Si ratios than the coexisting illite, which was also anomalous in terms of general crystal-chemical relationships between coexisting illite and I/S in burial diagenesis environments. The I/S appears to have formed by replacement of diagenetic-metamorphic illite, presumably at very low temperatures under hydrous conditions via dissolution and crystallization.

The conversion of akaganéite to goethite and/or hematite in alkaline media has been followed by X-ray powder diffraction and transmission electron microscopy (TEM). The rate of transformation fell and the amount of hematite in the product increased as the [OH−] decreased to < 1 M. Kinetic studies and TEM indicated that the transformation involved dissolution of akaganéite followed by reprecipitation of goethite and/or hematite. The rate-determining step was the dissolution of akaganéite.

Silicate species retarded the formation of goethite + hematite principally by inhibiting dissolution of akaganéite; to a lesser extent, they interfered with the nucleation of goethite. Silicate modified the morphology of goethite, but not hematite.

Comparison of the transformation behavior of akaganéite with that previously observed for ferrihydrite indicated that the composition of the reaction product depended strongly on the transformation conditions, i.e., pH and the presence of foreign species. The nature of the solid precursor was important insofar as its degree of crystallinity governed the dissolution kinetics and its surface properties influenced interaction with any foreign species in the system.

Platy (Te Puke, New Zealand), cylindrical (Spruce Pine, North Carolina), and spherical (North Gardiner mine, Huron, Lawrence County, Indiana) halloysite samples were analyzed by high-pressure differential thermal analysis (HP-DTA) to determine the effect of morphological and chemical differences on their respective thermal stability. In halloysite, these morphological differences imply structural features. The metastable phase relations of each are analogous to those of kaolinite. At 1 bar, the platy, cylindrical, and spherical samples showed peak temperatures (maximum deflection in the dehydroxylation endotherm) of 560°, 578°, and 575°C, respectively, whereas at about 600 bars the peak temperatures were 622°, 655°, and 647°C. At low pressures the observed reaction is related to dehydroxylation: halloysite (H) metahalloysite (MH) + vapor (V), whereas higher pressures produce melting reactions, either H + V = metaliquid (ML) for conditions of P(H2O) = P(total), or H + MH = ML for P(H2O) < P(total). The PT conditions of the invariant point, H + MH + ML + V, for each system are: Te Puke, 612° ± 4°C, 25 ± 7 bars; Spruce Pine, 657 ± 2°C, 30 ± 7 bars; North Gardiner, 652° ± 2°C, 34 ± 7 bars. The lower thermal stability of the Te Puke sample may be related to its higher iron content, although additional data are necessary to confirm that it is not related also to the platy structure. Furthermore, morphological differences between the cylindrical and spherical varieties appear to have had little effect on the energy required to dehydroxylate these halloysite structures. Exceptionally high values obtained for the dehydroxylation enthalpies using the van't Hoff equation, compared with values derived using other methods, may be explained by a 10–15-bar excess in the intracrystalline H2O fugacity during dehydroxylation. Intracrystalline fugacity is defined here as the H2O fugacity within crystallites and is not related to the partial pressure of H2O around individual particles.

Samples of Na-saturated, Upton montmorillonite were prepared with different contents of water (H2O or D2O) by: (1) adsorption of water from the vapor phase at a specific value of p/p°, the relative humidity, (2) adsorption of water from the vapor phase at p/p° = 1.0 followed by desorption of the water into the vapor phase at a specific p/p° < 1.0, and (3) adsorption of water from the liquid phase followed by desorption of the water into the vapor phase at a specific p/p° < 1.0. Water adsorbed initially from the vapor phase was called V-adsorbed water, and water adsorbed initially from the liquid phase was called L-adsorbed water. The water contents of these samples were determined by gravimetric analysis, the c-axis spacings by X-ray powder diffraction, the O-D stretching frequencies by IR spectroscopy, and the heats of immersion by differential microcalorimetry. No difference was found between V-adsorbed and L-adsorbed water; however, if the final water content was established by adsorption, the system was in a different state than if the final water content was established by desorption. In particular, hysteresis was observed in the following properties: the relative humidity of the adsorbed water, the O-D stretching frequency in this water, and the degree of order in the stacking of the clay layers. The only property that did not exhibit hysteresis was the heat of immersion. Apparently, hysteresis occurred because the orderliness of the system was not reversible, and, thus, any property that depended on orderliness was hysteretic.

Water-infiltration characteristics of soil can be improved by preventing clay dispersion. The present study determined the adsorption properties of hydroxy-Al polycations (Al-p) and their relation to the destabilization of clay suspensions. Al-p was synthesized and fractionated into nominal molecular weights between 104 and 5 × 104. The reactions of Al-p with Na-illite and Na-montmorillonite indicated a very strong affinity of Al-p to the clay surfaces. The maximum adsorptions of Al-p by Na-illite and Na-montmorillonite were found to be 0.37 and 1.7 mmole Al/g, and very close to the cation-exchange capacity of the two clays, suggesting that the adsorption was chiefly controlled by the mechanism of charge screening. Adsorption of Al-p increased the points of zero charge (PZCs) and the apparent points of zero salt effect (PZSEs) of illite and montmorillonite. PZSEs for both clays were 4.7 at their maximum Al-p adsorption, and PZCs ranged from 5.3 to 6.4, depending on solution ionic strength and the individual clay minerals. The differences in PZCs were probably due to outer-sphere complex formation between Al-p-treated illite and montmorillonite and the swamping electrolyte. Critical flocculation concentrations (CFCs) of Al-p for Na-illite and Na-montmorillonite were at 0.28 and 1.0 mmole Al/g, whereas zero electrophoretic mobilities were at about 0.36 and 1.67 mmole Al/g Al-p additions. Excessive addition of Al-p reversed this surface charge of clay colloids and restabilized the illite but not montmorillonite suspensions. This difference was probably due to the stronger and more extensive interparticle bridging of montmorillonite particles by Al-p than those of illite. The CFCs for the two clays were also found to be dependent on sodium adsorption ratio (SAR), pH, and ionic strength. Increases in SAR and pH significantly increased the CFC, whereas an increase in ionic strength decreased the CFC.

Spherical halloysite aggregates have been identified for the first time in mineral matter isolated from bituminous coals. The spherules, found in Permian coals of the Sydney basin, New South Wales, range from 0.4 to 0.6 µm in diameter and have a delicate ring-like structure that helps to confirm the halloysite identification. They appear from their location to be related to influxes of pyroclastic debris, either directly or from nearby soils, into the original peat accumulation. Analytical electron microscopy indicates higher proportions of Si and Fe than coexisting particles of hexagonal platy kaolinite, and electron diffraction reveals a typical disordered halloysite structure. The aggregates are larger than those normally reported in soils, and comparison to growth rates in soils suggests development over a significantly longer time than that expected for accumulation of the host coal seams. The buckled structure in the ring-like pattern and the related crude polyhedral outlines probably reflect shrinkage with dehydration during the coalification process, but it may also be due to the different sample preparation techniques.

Fourier-transform infrared (FTIR) spectroscopic studies were carried out on 1,6-hexanediamine hydrochloride (HDA)-treated synthetic fluorhectorite to determine the orientation of functional groups within the structure. Oriented crystal layers were prepared by flocculating the smectite slurry with glass fibers to obtain a 100-µm-thick paper. Orientations were determined by measuring integrated IR band intensities at various incident beam angles (≤60°), inasmuch as absorption occurred only if the oscillating dipole of the functional group interacted with the electric vector of the incident radiation. The H-N-H plane in amine groups was aligned parallel to the lamellar plane. The H-O-H plane of the small amount of sorbed water was inclined 45° or more to the interlamellar layer, and the OH groups were inclined 45° to this layer.

Even with the incorporation of HDA in the interlamellar structure, at high humidity, additional water sorbed. The sorbed water competed with and displaced amine groups from the surface, resulting in randomly oriented amine groups. Many of the amine groups were ionized, whereas the additional sorbed water showed little orientation.

This study demonstrated that the orientation of intercalated amines in fluorhectorite can be determined by following the intensity changes in infrared-active bands as a function of the incident beam angle. With intercalated HDA, the orientations were influenced by the presence of interlayer water.

The conversion of Malawi vermiculite into K-vermiculite by treatment with bi-ionic K-Mg solutions of 1 N total ion concentration (KCl and MgCl2 mixed solutions of ionic strength equal to 0.5) was studied by following the 00l X-ray powder diffraction (XRD) reflections. Flakes of Mg-saturated samples were treated at 160°C during 24 hr with bi-ionic solutions, with the K concentration varying from zero to pure 1 N KCl solution. The K-Mg interlayer exchange began at a critical value xK = .0196 (K/Mg = 1/100) of the molar fraction of K in the solution. Above the critical concentration and extending to pure 1 N KCl, the XRD diagrams were characteristic of a 10-Å/14-Å interstratification that had a marked tendency towards regularity. Experiments with KCl and MgCl2 mixed solutions of ionic strength equal to 0.75 and 1.0 showed that the exchange began at the same critical value xK as the experiments with ionic strength equal to 0.5, if the K added was equivalent. X-ray fluorescence analysis further showed that the amount of K adsorbed was proportional to the molar fraction xK and to the proportion of K-saturated layers (10 Å) in the interstratification. To explain the mechanism of this quasi-regular interstratification, a crystallochemical rather than a thermodynamic mechanism is proposed.

Maps of the electrostatic potentials at the basal plane of talc and pyrophyllite, computed using a two-dimensional Ewald lattice-sum, reveal the effects caused by structural distortion of the phyllosilicate layer. Rotation and tilting of basal tetrahedra in phyllosilicates dramatically perturb the electrostatic potential near the (001) surface. A potential high exists at the center of each six-fold ring of the talc (001) surface. Concerted counter-rotations of basal tetrahedra by 10°, as are typical in pyrophyllite, cause the potential lows above basal oxygens rotated into the ring to overlap, eliminating the ring-centered potential highs. Expansion of the vacant site in dioctahedral minerals tilts the basal tetrahedra by 4° and moves one-third of the basal oxygens about 0.2 Å toward the center of each phyllosilicate layer and away from the (001) surface, thereby producing corrugations of the basal surface. This shift dramatically reduces the contribution of these displaced basal oxygens to the (001) surface potential. Rotation and tilting of basal tetrahedra may influence the arrangement of interlayer water molecules on smectites and other swelling phyllosilicates by the effect that these distortions have on the (001) surface potential.

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.

Secondary minerals formed during simulated weathering of nuclear waste glasses have been identified by analytical electron microscopy. A complete description of the reacted glass, from the outermost surface in direct contact with the leachant solution to the reacting front that migrates into the bulk glass, was obtained. Manganese and iron oxyhydroxide phases and saponite were found to have precipitated onto the residual glass surface from the leachant solution. Iron-bearing smectite, serpentine, and manganese and uranium-titanium oxyhydroxides formed in situ in the glass in several distinct bands at different depths beneath the original surface. This sequential development of secondary phases displays a clear trend toward more order and crystallinity in the phases farthest from the reaction front and indicates that complete restructuring of the glass into crystalline phases did not occur at the interface with fresh glass. Additionally, the formation of a discrete uranium-bearing phase, as opposed to uranium uptake by precipitated phases, suggests that stable actinide phase formation rather than ion exchange may be a possible mechanism for retarding radionuclide release to the environment.

The oxidation of As(III) to As(V) by K-birnessite was examined at different temperatures, pHs, and birnessite/As(III) ratios. Experiments ranged in duration from 5 to 64 hr, and solution and solid products were determined at several intervals. All experiments showed that the reaction produced large amounts of K+ to solution and very little Mn2+. As(V) was released to solution and incorporated into the K-birnessite. The oxidation was initially rapid and then slowed. The oxidation of As(III) was probably facilitated initially by autocatalytic Mn-As(V) reactions occurring mostly in the interlayer, in which large amounts of As(V) and K+ could be easily released to solution. The reaction also slowed when interlayer Mn was exhausted by forming Mn-As(V) complexes. Mn(IV) could only be acquired from the octahedral sheets of the birnessite. The two-stage reaction process proposed here depended on the layered structure of birnessite, the specific surface, and presence of exchangeable cations in K-birnessite.