A combined powder X-ray diffraction (XRD) and X-ray absorption (XAS) study of Fe(III) cation ordering within pyroaurite is described. It is concluded that there is no correlation between Fe(III) cation positions over distances of a few tens of angstroms, but that there is a very high level of local ordering, involving the absence of Fe(III)-Fe(III) neighbors. These observations are rationalized in terms of a significant frequency of lattice defects in the form of cation vacancies or Mg for Fe(III) substitutions. These results are expected to be generalizable to other M(II)/M(III) layered double hydroxides (LDHs), but are in contrast to the long-range cation ordering observed in Li/Al LDHs. This raises the interesting possibility of differing properties and stabilities based on the degree of cation ordering.

This paper compares mechanisms of the reaction of smectite to illite, in light of structural models for interstratified illite/smectite (I/S). The crystal structure of I/S has been described previously by a nonpolar and polar 2:1 layer model. In a nonpolar model, individual 2:1 layers are chemically homogeneous, whereas a polar model assumes a 2:1 layer can have a smectite charge on one side and an illite charge on the other side. Several kinds of data support the polar model; however, more determinations of the negative charge of expandable sites in I/S are needed to confirm such a model.

Assuming a polar 2:1 layer model for I/S, we compare the mineralogical and geochemical consequences of several reaction mechanisms for smectite illitization: 1) solid-state transformation (SST), 2) dissolution and crystallization (DC) and 3) Ostwald ripening (OR). Features of an SST model are the replacement of smectite interlayers by illite interlayers, resulting in gradual changes in interlayer ordering, polytype, chemical and isotopic composition and crystal size and shape. Several SST models are possible depending on the nature of the reaction site (framework cations, polyhedra or interlayers). In contrast, DC models allow for abrupt changes in the structure, composition and texture of I/S as illitization proceeds. Several DC models are possible depending on the nature of the rate-controlling step, for example, diffusional transport or surface reactions during crystal growth. The OR model represents the coarsening of a single mineral where the smallest crystals dissolve and nucleate onto existing larger crystals, allowing for evolution in the overgrowth but not in the template crystal.

An SST mechanism, involving either reacting polyhedra or reacting interlayers, seems to best model illitization in rock-dominated systems such as bentonite. A DC mechanism seems to best model illitization in fluid-dominated systems such as sandstone and hydrothermal environments. Both DC and SST mechanisms can occur in shale. Differences in reaction mechanism may be related to permeability. An OR model poorly describes illitization because of the progressive mineralogical and chemical changes involved. For many geologic environments, it is important to consider alternate origins for I/S such as kaolinite illitization and detrital. Further work is needed to clarify the DC crystal growth process in terms of a structural model of I/S and to determine which specific SST or DC model best characterizes illitization in geologic systems.

A large and representative collection of clay-size dioctahedral mica minerals differing in their chemical compositions has been studied by infrared (IR) spectroscopy in the OH stretching vibration region. Decomposition of the IR spectra in the individual OH bands has provided unambiguous identification of the band positions for each defined pair of octahedral cations bonded to OH groups. The presence of pyrophyllite-like local structural environments in samples having a deficiency of K in interlayers has been established. A set of the relationships between the OH frequencies corresponding to pairs of cations having different valency and mass has been found.

Because of the large surface area and common occurrence of smectites in the clay fractions, they are important in sorption/desorption reactions of organic pollutants entering soils and sediments. Results of sorption and desorption of atrazine by Al- and Ca-saturated smectite reported here showed that Al-saturated smectite sorbed much higher amounts of atrazine than Ca-saturated smectite. Al-saturated smectite sorbed 3820 mg kg−1 as compared to 1902 mg kg−1 by the Ca-saturated smectite during 5 consecutive sorptions from 10 ppm atrazine solution. Sorption isotherms over 2–10 ppm atrazine concentration range were nonlinear in all cases. Freundich coefficients, Kf, obtained from the sorption isotherms were much higher for Al- than Ca-saturated smectite; Kf varied from 405 to 3035 for Al-saturated and 100 to 306 for Ca-saturated. The pH values of Al- and Ca-saturated smectites were 3–4 units higher than the pKa (1.68) of atrazine, which suggests that atrazine was sorbed as neutral molecules. Stronger H-bonding between the more polarized H2O associated with the trivalent Al ion than the divalent Ca ion is likely responsible for the greater sorption by Al-smectite.

Adsorption studies of aromatic hydrocarbons of various molecular sizes on organo-clays in aqueous solution were carried out for characterizing the surface heterogeneity of organo-clays. Benzene, toluene, p-xylene, ethylbenzene and n-propylbenzene adsorption by a smectite with 5 different exchange degrees of trimethylphenylammonium (TMPA) cations for Ca2+ was measured. The Langmuir isotherm equation did not adequately describe the experimental data, especially for small molecules, whereas the Dubinin-Radushkevich (DR) equation combined with a gamma-type adsorption energy distribution function described all experimental data well, suggesting the surface and structural heterogeneity of TMPA-smectites. The calculated adsorption energy distributions indicated that the apparent heterogeneity depends on the molecular size of adsorbates. Small adsorbate molecules such as benzene explore a highly heterogeneous surface of TMPA-smectites while large molecules such as n-propylbenzene detect a relatively homogeneous surface. The surface fractal dimension was dependent on the extent of TMPA exchange for Ca2+. When TMPA content is less than 75% of the cation exchange capacity (CEC) of the smectite, the heterogeneity decreases as TMPA content increases; it increases with TMPA content thereafter. These results are related to the size distribution of micropores in TMPA-smectites, which are defined by the 2 semi-infinite aluminosilicate sheets and the interlayer cations. The micropore size distributions and, hence, heterogeneity are created in part by the inhomogeneity of the charge density of clay surfaces and the tendency for cation segregation in these systems.

The micropore volumes of 2 montmorillonites (SAz-1 and SWy-1), each exchanged with Ca, Na, K, Cs and tetramethylammonium (TMA) ions, were calculated from the measured vapor adsorption data of N2 and neo-hexane by use of t- and αs-plots. The corresponding surface areas of the exchanged clays were determined from Brunauer-Emmett-Teller (BET) plots of N2 adsorption data. Micropore volumes and surface areas of the samples increased with the size of exchanged cation: TMA > Cs > K > Ca > Na. The SAz-1 exchanged clays showed generally greater micropore volumes and surface areas than the corresponding SWy-1 clays. The vapor adsorption data and d(001) measurements for dry clay samples were used together to evaluate the likely locations and accessibility of clay micropores, especially the relative accessibility of their interlayer spacing. For both source clays exchanged with Na, Ca and K ions, the interlayer spacing appeared to be too small to admit nonpolar gases and the accessible micropores appeared to have dimensions greater than 5.0 Å, the limiting molecular dimension of neo-hexane. In these systems, there was a good consistency of micropore volumes detected by N2 and neo-hexane. When the clays were intercalated with relatively large cations (TMA and possibly Cs), the large layer expansion created additional microporosity, which was more readily accessible to small N2 than to relatively large neo-hexane. Hence, the micropore volume as detected by N2 was greater than that detected by neo-hexane. The micropore volumes with pore dimensions greater than 5 Å determined for clays exchanged with Na, Ca and K likely resulted from the pores on particle edges and void created by overlap regions of layers. The increase in micropore volumes with pore dimensions less than 5 Å determined for clays exchanged with TMA and possibly Cs could be caused by opening of the interlayer region by the intercalation of these large cations.

Surface adsorption mechanisms of dissolved inorganic carbon species on soil minerals are not well understood. Traditional infrared (IR) study of adsorbed species of inorganic carbon using air-dried samples may not reveal true species in the solid/water interface in suspension. The purpose of this study was to obtain information on interfacial carbonate speciation between solid and aqueous phases. The interaction of bicarbonate and carbonate ions with X-ray amorphous (am) Al and Fe oxides, gibbsite (γ−Al(OH)3) and goethite (α-FeOOH) was examined by electrophoresis and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The presence of carbonate lowered the electrophoretic mobility and decreased the point of zero charge (PZC) of all minerals, implying specific adsorption. Inner-sphere complexation of bicarbonate and carbonate was supported by a lowering in the anion symmetry due to the interaction with Al and Fe oxide surfaces. Only complexed monodentate carbonate was identified in am-Al(OH)3/aqueous solution at pH 4.1-7.8 when the solid was reacted with either NaHC03 or Na2CO3 solutions. Am-Al(OH)3 was transformed to a crystalline sodium aluminum hydroxy carbonate, dawsonite [NaAl(CO3)(OH)2], and bayerite (α-Al(OH)3) after reacting with 1.0 M Na2CO3 for 24 h. Gibbsite adsorbed much less carbonate than am-Al(OH)3 such that adsorbed carbonate on gibbsite gave weak IR absorption. It is probable that monodentate carbonate is also the complexed species on gibbsite. Evidence suggesting the presence of both surface complexed bicarbonate and carbonate species in the interfacial region of am-Fe(OH)3 in suspension and the dependence of their relative distribution on solution pH is shown. Only monodentate carbonate was found in the interfacial region of goethite in 1.0 M NaHCO3. A ligand exchange reaction was proposed to describe the interaction of bicarbonate and carbonate with the surface functional groups of Al and Fe oxides.

The chemical composition of mixed-layer serpentine/chlorite (Sp/Ch) in Tuscaloosa Formation sandstone was analyzed by energy dispersive X-ray spectroscopy (EDX) in the scanning electron miscroscope (SEM) and by X-ray diffraction (XRD). EDX results indicate little depth-controlled variation in composition, whereas XRD results suggest distinct decreases in octahedral Fe and tetrahedral Al. XRD-determined compositions appear to be erroneous and actually reflect progressive changes in Sp/Ch unit-cell dimensions caused by polytype transformations of Ibb layers to Iaa layers in a mixed-layer Ibb/Iaa polytype. The relative lack of variation in Sp/Ch composition, especially when compared to other studies of chlorite minerals over similar temperature ranges, is attributed to a reaction mechanism whereby mineralogic transformations (serpentine layers to chlorite layers and Ibb layers to Iaa layers) occur on a layer-by-layer basis within coherent crystallites, rather than by dissolution-precipitation crystal growth.

The lack of titanium in chlorite minerals is attributed to high levels of octahedral Al3+ that prohibit inclusion of the highly charged Ti4+ in the octahedral sheet. Anatase (TiO2) in the Tuscaloosa Formation apparently formed when Ti was liberated during crystallization of Sp/Ch following the breakdown of a Ti-bearing precursor (detrital ultramafic clasts and/or odinite). Odinite, an Fe-rich 7-Å phyllosilicate that forms in some shallow marine sands, apparently existed as a short-lived, poorly crystallized intermediary between dissolution of the ultramafic clasts and formation of Sp/Ch.

A new model for the interpretation of dioctahedral mica infrared (IR) spectra in the OH stretching vibration region is proposed. It is based on the analysis of the main factors responsible for the observed sequence of the OH frequencies. In terms of this model, the simple analytical dependence between the OH frequencies and the mass and valency of cations bonded to OH groups has been found. The specific character of the interaction between octahedral Al and OH groups in the mica structures is assumed.

Integrated optical densities of the OH bands determined by the decomposition of the studied mica IR spectra are used for the quantitative analysis, that is, for the determination of a number of each type of octahedral cations per unit cell of the sample under study. A good agreement between the octahedral cation contents of 2:1 layers found from the IR spectra decomposition and the chemical analysis has shown that this technique may be used to study the local order-disorder of isomorphous cation distribution in mica structures.

The essential result obtained by the quantitative analysis of the mica IR spectra is the determination of Fe3+ cations in the tetrahedral sites of some samples. This means that the conventional presentation of structural formulae for Al-Fe3+-containing 2:1 layer silicates is unacceptable without consideration of tetrahedral Fe3+ by spectroscopic techniques and by the quantitative analysis of the IR spectra, in particular.

Unmodified and surfactant-modified clinoptilolite-rich tuff (referred to here as “clinoptilolite”) and muscovite mica were examined with tapping-mode atomic force microscopy (TMAFM) and high-resolution thermogravimetric analysis (HR-TGA) in order to elucidate patterns of hexadecyltrimethylammonium bromide (HDTMA) sorption on the treated surface and to understand the mechanisms of this sorption. TMAFM images were obtained to a scale of 50 nm by 50 nm. The images of unmodified clinoptilolite showed a framework pattern on the ac plane, comparable to previously reported images. Images of modified clinoptilolite at 12.5% and 25% of external cation exchange capacity (ECEC) coverage by HDTMA showed evidence of the HDTMA molecules arranged as elongated, topographically raised features on the ac plane. At 50% HDTMA coverage, the images contained what appeared to be agglomerations of surfactant tail groups. The z-directionthickness of the raised features on the 12.5% coverage sample corresponded to the thickness of the carbon chain of the surfactant tail-group (0.4 nm), whereas the z-thicknesson the 25% coverage sample was between 0.4 and 0.8 nm, indicating crossing or doubling of tail groups. Repulsive forces between the modified clinoptilolite and the silicon TMAFM probe increased with increasing HDTMA coverage. HR-TGA showed a 100 °C increase in HDTMA pyrolysis temperatures at coverages of less than 50%, probably due to an increased stabilization of the HDTMA due to direct tail interactions with the clinoptilolite surface at lower coverages versus smaller stabilization due to surfactant tail-tail interactions at higher coverages. Our results indicate that buildup of HDTMA admicelles or some form of a bilayer begins before full monolayer coverage is complete.

The crystal structure of a natural, ordered IIb-4 triclinic clinochlore has been refined in space group C1̄ from 4282 unique X-ray intensity measurements of which 3833 are greater than 3 times the statistical counting error (3σ). Unit cell parameters are a = 5.3262(6) Å; b = 9.226(1) Å; c = 14.334(3) Å; α = 90.56(2)°; β = 97.47(2)°; and γ = 89.979(9)°, which represents the greatest deviation from mono-clinic symmetry yet recorded for a triclinic chlorite. The final weighted R is 0.059 for reflections with I > 3σ and 0.064 for all reflections. The chemical formula is (Mg0.966Fe0.034)MI(Mg0.962Fe0.038)M22(Si2.96Al1.04)O10 (OH)2(Mg0.996Fe0.004)M32(Al0.841FeIII0.102Cr0.004Ti0.004)M4(OH)6, which is consistent with electron microprobe (EMP), wet chemical analyses, Mössbauer spectroscopy and X-ray structure refinement. The high degree of ordering of the divalent versus trivalent octahedral cations in the interlayer is noteworthy, with FeIII and Al in M4 and virtually no Fe in M3. In the 2:1 layer, M1 and M2 each contain similar amounts of Fe. The 2 tetrahedral sites have nearly identical mean oxygen distances and volumes, and thus show no evidence of long-range cation ordering.

Smectite and mixed-layer illite/smectite (I/S) in Triassic heulandite-rich bentonite from Kaka Point, New Zealand, have been investigated by scanning electron microscopy (SEM), transmission electron microscopy/analytical electron microscopy (TEM/AEM) and X-raydiffraction (XRD) for comparison with matrix phyllosilicates in closely associated siltstones and analcimized tuff. Samples that were treated to achieve permanent expansion showed that some smectite in bentonite occurs as curved packets of wavy 10.5- to 13-Å layers enveloping relict glass shards, the centers of which consist of an amorphous clay precursor. The dominant clay minerals in bentonite are smectite-rich randomly disordered (R0) I/S with variable proportions of 10-Å illite-like interlayers, only locally organized as 1:1 ordered (R1) I/S. R0 I/S was also observed in separate packets retaining the detailed texture of packets that replaced shards. Such relations are consistent with a “solid-state”-like, layer-by-layer replacement of original smectite layers by illite-like layers with partial preservation of the primary smectite texture, in contrast to textures observed elsewhere, such as in Gulf Coast mudstones. The smectite, as in other examples in marine sediments, has K as the dominant interlayer cation, suggesting that precursor smectite may be a major K source for reaction to form illite.

Only a small proportion of illite (35%) occurs in mixed-layer smectite-rich I/S in bentonite and the dominant trioctahedral phyllosilicate is disordered high-Fe berthierine, implying that little mineralogical change has occurred with burial. This contrasts with observations of closely associated siltstones and analcimized tuff, which contain well-defined packets of illite and chlorite but which have no detectable matrix smectite component. These data imply that the rate of transformation of smectite to illite is much slower in bentonites than in associated sediments of the same burial depth and age. Such relations emphasize the significance of factors other than temperature, (e.g., organic acids, permeability and pore fluid compositions) in affecting the rate and degree (and perhaps mechanism) of transformation of smectite to illite.

Inverse gas chromatography (IGC) at zero surface coverage was used for determining the surface characteristics of a slate sample. Previously this sample had been characterized by means of X-ray diffraction (XRD), chemical analysis, infrared (IR) spectroscopy and nitrogen adsorption. Quartz, muscovite and chlorite were the minerals foming the slate. A well-defined mesoporous structure with pores of 15 Å of interlayer distance was observed. Chemical analysis and IR spectroscopy corroborated the X-ray results. The surface characteristics of the slate were defined in relation to nonspecific and specific interactions with organic molecules. In accordance with the nonspecific interactions, the London component (γSD) of the surface free energy gave an estimation of the surface energy of the slate sample. The obtained values for γSD were 140.0, 124.8, 108.2 and 96.8 mN m-1 at 100, 110, 120 and 130 °C, respectively. These values were characteristic of an inorganic sample of high surface energy. The values of the thermodynamic variables—differential heat of adsorption, free energy and entropy of adsorption—were in accordance with these results. At the same time, specific interactions were characterized by the ηπ parameter and the acid-base (KA, KB) indices. The positive value of ηπ (0.09 kJ mol-1) indicated the presence of Lewis acidic sites on the slate surface. The value of KA (acid index) of 1.17 was higher than the value of KB (base index) of 0.37; this result confirmed the acidic nature of the studied slate's surface.

The enthalpy of hydration of natural clinoptilolite was determined by isothermal immersion calorimetry on Ca-, Na- and K-exchanged clinoptilolite (Fish Creek Mountains, Nevada). Heats of immersion of clinoptilolite were determined at initial H2O contents ranging from θ = 0.02 to 0.85 (where θ is the ratio [H2O content]/[maximum H2O content]). The heat of immersion (liquid H2O reference state) of Ca-clinoptilolite ranged from -7.5 (θ = 0.87) to -25.7 kJ/mol-H2O (θ = 0.19); values for Na-clinoptilolite ranged from -6.3 (θ = 0.85) to -21.8 kJ/mol-H2O (θ = 0.11); and values for K-clinoptilolite ranged from -7.7 (θ = 0.80) to -24.6 kJ/mol-H2O (θ = 0.02). Linear regression of the calorimetric data provided the following values for the complete heat of immersion (from θ = 0): Ca-clinoptilolite, -30.3 ± 2.0; Na-clinoptilolite, -23.4 ± 0.6; and K-clinoptilolite, -22.4 ± 0.8 kJ/mol-H2O.

The heat of immersion measurements were compared with the enthalpy of hydration results of Carey and Bish (1996) determined in a thermogravimetric study of the same samples. The heat of immersion data are similar but of smaller magnitude than the values of enthalpy of hydration and are believed to be more accurate because they represent direct measurements of this thermodynamic property.

The effect of dehydration of clinoptilolite on the thermal evolution of the potential high-level radioactive waste repository at Yucca Mountain was considered by comparing the amount of energy consumed by clinoptilolite dehydration with the amount of energy necessary to heat rocks lacking hydrous minerals. The extra energy consumed on heating clinoptilolite from 25 to 200 °C ranges between 70 and 80% in excess of that required for nondehydrating materials (that is, clinoptilolite acts as a heat sink). These results indicate that accurate thermohydrologic modeling of rock units at Yucca Mountain should consider the thermal effect of dehydration/hydration processes in clinoptilolite and other hydrous minerals, in addition to the water produced/adsorbed during heating/cooling.

This paper describes in detail phosphate minerals occurring in refractory clays of Missouri and their effect on the refractory degree of the clays. The minerals identified include carbonate-fluorapatite (francolite), crandallite, goyazite, wavellite, variscite and strengite. It is emphasized that these phosphates occur only in local isolated concentrations, and not generally in Missouri refractory clays.

The Missouri fireclay region comprises 2 districts, northern and southern, separated by the Missouri River. In this region, clay constitutes a major part of the Lower Pennsylvanian Cheltenham Formation. The original Cheltenham mud was an argillic residue derived from leaching and dissolution of pre-Pennsylvanian carbonates. The mud accumulated on a karstic erosion surface truncating the pre-Cheltenham rocks. Fireclays of the northern district consist mainly of poorly ordered kaolinite, with variable but minor amounts of illite, chlorite and fine-grained detrital quartz. Clays of the southern district were subjected to extreme leaching that produced well-ordered kaolinite flint clays. Local desilication formed pockets of diaspore, or more commonly, kaolinite, with oolite-like nubs or burls of diaspore (“burley” clay).

The phosphate-bearing materials have been studied by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectral analysis (SEM-EDS) and chemical analysis. Calcian goyazite was identified in a sample of diaspore, and francolite in a sample of flint clay. A veinlet of wavellite occurs in flint clay at one locality, and a veinlet of variscite-strengite at another locality.

The Missouri flint-clay-hosted francolite could not have formed in the same manner as marine francolite. The evidence suggests that the Cheltenham francolite precipitated from ion complexes in pore water, nearly simultaneously with crystallization of kaolinite flint clay from an alumina-silica gel. Calcian goyazite is an early diagenetic addition to its diaspore host. The wavellite and variscite-strengite veinlets are secondary, precipitated from ion complexes in ground water percolating along cracks in the flint clay. The flint clay host of the variscite-strengite veinlet contains strontian crandallite. All of the phosphates contain significant amounts of strontium. The source of P, Ca and Sr was the marine carbonates. Dissolution of these carbonates produced the argillic residue that became the primordial Cheltenham paludal mud, which ultimately altered to fireclay.

Preliminary firing tests show that the presence of phosphates lowers fusion temperature. However, it is not clear whether poor refractoriness is due to the presence of phosphates, per se, or to Ca, Sr and other alkaline elements present in the phosphates.

A reservoir rock is a porous geological formation in contact with 2 liquids, brine and oil. An improved knowledge of rock wettability is of primary importance to estimate the amount of crude oil in underground resources. The petroleum industries have observed that wettability contrasts in sedimentary reservoir rocks are largely correlated to the presence of clays, illite and/or kaolinite in the rocks’ intergranular space.

More precisely, the grain surfaces of illite show a preference for brine. Kaolinite preferentially adsorbs oil, which imparts its hydrophobic characteristics to the mineral surface. Using X-ray absorption spectroscopy (XAS) and Fourier transform infrared (FTIR) spectroscopy, we studied the adsorption process of asphaltenes in the presence of water at the microscopic level. We demonstrate experimentally that the wettability contrasts observed in kaolinite and illite are related to structural differences between these 2 clays, and we show the role of the grain surface hydroxyls. With clay materials, the purity of the samples is the most important limitation of the quantitative use of extended X-ray absorption fine structure (EXAFS).

The adsorption of the monovalent organic cations benzyltrimethylammonium (BTMA) and benzyltriethylammonium (BTEA) to montmorillonite was studied as a function of their concentrations and ionic strength. At low ionic strength the adsorbed amounts of BTMA and BTEA reached values of the cation exchange capacity (CEC) of the clay. An increase in the ionic strength resulted in reduction in the adsorbed amounts of the organic cations, unlike the pattern observed previously with organic monovalent cationic dyes. The reduction in adsorbed amounts of BTMA and BTEA depended on the inorganic cations according to the sequence Cs+ > Na+ > Li+, which follows the sequence of binding coefficients of these inorganic cations added. The type of the anion (that is, Cl−, ClO4−, SO42-) had no effect on the adsorbed amounts. An adsorption model which considers the electrostatic Gouy-Chapman equations, specific binding and closeness of the system could adequately simulate the adsorbed amounts of BTMA and BTEA and yield predictions for the effect of the ionic strength and concentration of electrolytes. The binding coefficient employed was K = 5000 M−1 for the formation of neutral complexes of BTMA and BTEA. This value is larger than those found for the inorganic cations but is several orders of magnitude below those found for the monovalent dyes. The binding coefficients for the formation of charged complexes of BTMA and BTEA were 20 and 5 M−1, respectively. The basal spacing of the clay did not change significantly with the adsorbed amounts of both BTMA and BTEA up to the CEC.

Phase and morphological changes during thermolysis of Mg4Al2(OH)12CO3·nH2O and Zn6Al2(OH)16CO3·nH2O layered double hydroxides (LDH) and their nanocomposites with poly(styrene sulfonate) (PSS) are studied by X-ray powder diffraction (XRD), scanning and transmission electron micrography (SEM and TEM) and thermal analyses. Mg4Al2(OH)12CO3·nH2O and Mg2Al(OH)6[CH2CH(C6H4SO3)]·3H2O show comparable thermal stabilities: the layered structure is lost above 300 °C with the nucleation of the MgO phase at approximately 400 °C and the MgAl2O4 phase at approximately 800 °C Zn3Al(OH)8[CH2CH(C6H4SO3)]·nH2O undergoes complete oxidative pyrolysis of the polyanion by 500 °C. Crystalline oxide products are obtained at a temperature approximately 300 °C lower than that of thermolysis of Zn6Al2(OH)16CO3·nH2O. The SEM and TEM images show that the thermolysis of LDH carbonates produces dense aggregates containing microcrystalline particles, whereas Mg2Al(OH)6[CH2CH(C6H4SO3)]·3H2O forms a macroporous solid.

A theoretical model describing the interaction between crystalline swelling and cation exchange selectivity is proposed for expanding 2:1 phyllosilicates. The model is based on the assumption that changes in basal spacing of a clay are phase changes, and that each phase of a clay has a different selectivity constant for a particular cation exchange reaction. Energy barriers stabilize the various phases over a limited range of interlayer ionic composition. These energy barriers cause hysteresis in crystalline swelling, which in turn causes hysteresis in cation exchange. Results are presented for an experiment involving Ba-Mg exchange on a synthetic fiuoro-hectorite. The results demonstrate key aspects of the proposed model, including a correlation between measured selectivity coefficients and basal spacings (R2 = 0.85), an abrupt change in basal spacing that corresponds with an abrupt change in selectivity and corresponding hysteresis in crystalline swelling and cation exchange selectivity. The results also demonstrate increased selectivity for the preferred cation (Ba) at high solution mole fraction of the preferred cation. This trend is opposite of that observed for heterogeneous natural smectites but consistent with predictions of the model for a homogeneous smectite.

The Raman spectra of a tubular halloysite originating from Matauri Bay, New Zealand, have been obtained using a Renishaw 1000 Raman microscope system. The Raman microprobe enables the Raman spectra of crystals as small as 0.8 μm diameter to be obtained over the complete wavelength range and allows spectral variations along the different crystal axes to be studied. Three bands in the hydroxyl stretching region were observed at 3616.5, 3623.4 and 3629.7 cm-1 and are attributed to the inner hydroxyls of the shared lower plane of the octahedral sheet of the halloysite. Two bands at 3698.2 and 3705 cm−1 were obtained for the outer hydroxyls of the unshared outer octahedral plane. The relative intensity of the 3629.7 cm−1 band varied according to the tube orientation. Lattice vibrations of the halloysite were also found to be orientation-dependent.