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Rapid and accurate determination of the surface area of three kaolinite clay standards, taking into account the complex microtopography of the particles, was achieved using atomic force microscopy images and computerized image analysis. All surface areas were determined to within 3%. Edge surface area is 18.2-30.0% of the total surface area depending on the particular kaolinite standard. Specific surface areas agree to within 4% of published values determined by the BET method. The approach can be applied to clay and nanoparticle samples too small in quantity for BET analysis, since it requires ~11 orders of magnitude less material.

The electronic structure and energies of Au atoms adsorbed on an atomic cluster of kaolinite were calculated using the self-consistent-field Discrete Variational (DV) method. A hexagonal ring of Si[O.sub.4] tetrahedra and three Al[O.sub.2][(OH).sub.4] octahedra, comprising 38 atoms, was used to model the kaolinite crystal flake. An Au atom was used to model the submicrometer Au. Calculations were performed with Au atoms adsorbed at different sites. The systems with lower total energy are those with Au adsorbed on the edge surfaces. The adsorption of Au atoms on the edges is more stable than Au atoms adsorbed on the basal plane. Bond order calculations suggest that significant shifting of atomic charge and overlap of electron clouds between Au and the surface atoms of kaolinite takes place in the systems with Au adsorbed on the edges, especially at sites near Al octahedra.

The theoretical infrared spectrum of kaolinite [A[l.sub.2]S[i.sub.2][O.sub.5][(OH).sub.4], triclinic] was computed using ab initio quantum mechanical calculations. Calculations were performed using the Density Functional Theory and the generalized gradient approximation. The low-frequency dielectric tensor of kaolinite was determined as a function of the light frequency using linear response theory. The IR spectrum was then calculated using a model that takes into account the shape and size of kaolinite particles. A remarkable agreement was obtained between theory and experiment, especially on the position of the stretching bands of OH groups. This agreement provides a firm basis for the interpretation of the IR spectrum of kaolinite in terms of structural parameters.

Al magic-angle spinning (MAS) NMR studies on kaolinite, metakaolinite and related materials are presented. Recording reliable Al MAS NMR spectra of disordered materials with distorted local Al environments needs the use of very rapid MAS.

A new study investigates the effects of ionic strength, pH and chemical composition of clays on the structure of aqueous laponite dispersions.

The 2H NMR spectra and 29Si, 1H, 13C and 2H spin-lattice relaxations in kaolinite and its intercalation compounds with formamide, formamide-N,N-d2, N-methylformamide and N,N-dimethylformamide were examined to study the dynamics of guest molecules. The spinning-lattice relaxation times and spectra were measured at different temperature conditions. Results showed that the interlayer guest molecules undergo librational motions.

Solid state NMR was used to examine the geometries and orientations of guest molecules in kaolinite intercalation compounds of kaolinite/formamide, kaolinite/formamide-N,N-d2, kaolinite/N-methylformamide and kaolinite/N,N-dimethylformamide. The interlayer guest molecules were determined by quantitative analysis of the 29Si and 13C NMR spectra. Insights into the conformation of the guest molecules and hydrogen bonding interactions between the guest and host were revealed by the 13 C and 1H chemical shifts of the guest molecules.

Adsorption of quinoline (pK(sub a) = 4.92) and background electrolyte (LiCl) onto specimen kaolinite was measured as a function of surface-bound humic acid (HA) concentration (foc = 0-0.5%), pH (3-10), and ionic strength (1-10 mM). Complexation of HA on the kaolinite surface (4.5 mg C kg(super -1)) reduced the point of zero net charge (pznc) for kaolinite by more than one pH unit and resulted in a significant increase in negative surface charge. Maximum sorption of quinoline occurred near its pK(sub a) for all sorbents. Below the pK(sub a) quinoline sorption increases with increasing pH and decreasing proton competition. Above the pK(sub a), sorption is reduced in parallel with (but offset to a higher pH from) the ionized fraction. Competition with Li+ for surface sites is apparent from diminished quinoline adsorption with increasing ionic strength, but sorption of the ionized form of quinoline is always favored and kaolinite exhibits selectivity for cationic quinoline over Li+ (K(sub exc) = 65 at pH 5). However, increasing foc diminishes quinoline sorption and selectivity (K(sub exc) = 32 at pH 5) and increases sorption reversibility relative to uncoated kaolinite. Humic acid along exhibits lower selectivity for quinoline (K(sub exc = 4 at pH 5). The results indicate that mineral-sorbed humic stubstances can diminish retention of cationic quinoline despite an increase in overall cation-exchange capacity.

Molecular dynamics simulations of kaolinite structures with 8.5 and 10 Angstrom clay layer spacings were performed to study the geometry and short-time dynamics of the interlayer water molecules in these structures. Results revealed two kinds of adsorbed water molecules that have different orientations with respect to the clay sheet surface. An analysis of water molecule dynamics in the interlamellar space revealed a significant decrease of the diffusion coefficient and increase of orientational relaxation times.

Intercalation of an ordered kaolinite with potassium acetate (KC[H.sub.3]COO) under a pressure of 20 bars and 220 [degrees] C, induced new Raman bands at 3590, 3603, and 3609 [cm.sup.-l] in addition to the normal kaolinite bands. These bands are attributed to the inner surface hydroxyls hydrogen bonded to the acetate. It is proposed that the intercalation under 20 bars pressure at 220 [degrees] C caused the differentiation of the inner surface hydroxyl groups, resulting in the appearance of these additional bands. Diffuse reflectance infrared spectra of the potassium acetate intercalated kaolinite that was formed at 20 bars and at 220 [degrees] C showed new bands at 3595 and 3605 [cm.sup.-1]. Upon formation of the intercalate at 2 bars and at 120 [degrees] C additional infrared (IR) bands were found at 3592, 3600, and 3606 [cm.sup.-1]. These IR bands correspond well with the observed Raman spectra. It is proposed that the effect of intercalation of the highly ordered kaolinite under pressure caused the kaolinite to become disordered and this disordering was dependent on the temperature of intercalation. It is suggested that when pressure is applied to the kaolinite crystal in the presence of an intercalating agent, the hydrogen bonds between adjacent layers are broken to create space for the intercalating agent between the layers. A direct result is that the order of the kaolinite crystals shows a decrease resulting in more defect structures. This is evidenced by the additional spectroscopic bands in both the Raman and IR spectra.

Intercalation of an ordered kaolinite with potassium acetate (KC[H.sub.3]COO) under a pressure of 20 bars and 220 [degrees] C, induced new Raman bands at 3590, 3603, and 3609 [cm.sup.-l] in addition to the normal kaolinite bands. These bands are attributed to the inner surface hydroxyls hydrogen bonded to the acetate. It is proposed that the intercalation under 20 bars pressure at 220 [degrees] C caused the differentiation of the inner surface hydroxyl groups, resulting in the appearance of these additional bands. Diffuse reflectance infrared spectra of the potassium acetate intercalated kaolinite that was formed at 20 bars and at 220 [degrees] C showed new bands at 3595 and 3605 [cm.sup.-1]. Upon formation of the intercalate at 2 bars and at 120 [degrees] C additional infrared (IR) bands were found at 3592, 3600, and 3606 [cm.sup.-1]. These IR bands correspond well with the observed Raman spectra. It is proposed that the effect of intercalation of the highly ordered kaolinite under pressure caused the kaolinite to become disordered and this disordering was dependent on the temperature of intercalation. It is suggested that when pressure is applied to the kaolinite crystal in the presence of an intercalating agent, the hydrogen bonds between adjacent layers are broken to create space for the intercalating agent between the layers. A direct result is that the order of the kaolinite crystals shows a decrease resulting in more defect structures. This is evidenced by the additional spectroscopic bands in both the Raman and IR spectra.

The dissolution kinetics of Ni(II) sorbed to kaolinite at pH 7 were examined as a function of initial aqueous Ni(II) concentrations; the local molecular structure of the Ni surface complexes were determined using extended x-ray absorption fine structure (EXAFS) spectroscopy. The dissolution of kaolinite was nonstoichiometric with a preferential release of Si over Al. The dissolution rate, [R.sub.Si], increased with an increase in the aqueous concentration of Ni. A rate law was developed representing a fractional-order dissolution reaction with respect to the surface concentration of Ni. Absorption spectra were obtained from kaolinite samples washed with 0.10 M NaN[O.sub.3] adjusted to pH 3.4 to remove amorphous Al-hydroxide surface deposits or adsorbed Al and from kaolinite that was untreated. For all samples, EXAFS results revealed the presence of multinuclear Ni surface complexes with a similar, but not identical, local structural environment to pure crystalline Ni[(OH).sub.2]. The Ni-Ni bond distances were shorter (3.06-3.11 [Angstrom]) than in Ni[(OH).sub.2](s) (3.13 [Angstrom]) and increased with an increase in surface coverage. Additionally, Al was present in all but the highest surface coverage as a second neighbor backscatterer. At the lower surface coverages ([Gamma] [less than or equal to] 0.406 [micro]mol [m.sup.-2]), Al is present at 2.69 [Angstrom], indicating the formation of a bidentate Ni surface complex. With increased surface coverage, the d(Ni-Al) increases to 2.96 [Angstrom], which is believed to result from a Ni,Al-hydroxide precipitate. No significant structural differences were found between kaolinite pretreatments. based on these results, a hypothesized mechanism for Ni sorption on kaolinite is proposed.

Natural solutions containing clay extracts and synthetic solutions with varying concentrations of [Al.sup.3+], [Ca.sup.2+], and [Na.sup.+] were used as anodic flushing solutions, and investigated for their enhanced in-situ removal of Pb from clay by electrokinetics (E-K). The experiments were conducted in a horizontal soil column apparatus with kaolin clay at an initial concentration of 340-410 [[micro]gram] Pb/g clay (dry) and 80-83% moisture content. Natural flushing solutions were prepared by mixing water and clay in ratios varying from 2:1 to 40:1 by weight, and the supernatant was used as an anodic flushing solution. The 7:1 natural solution was the most effective in Pb removal. Synthetic solutions were prepared from the salts of [Al.sup.3+], [Ca.sup.2+], [Na.sup.+], [Mg.sup.2+], and [K.sup.+] ions. Pb removal was highest at a solution ionic strength of [approximately]0.001 M for each element group, and with trivalent [Al.sup.3+] and divalent [Ca.sup.2+] ions at concentrations of [approximately]6.4 x [10.sup.-5] M and 3.1 x [10.sup.-4] M, respectively. The 5.5 X Ca synthetic solution had the highest overall Pb removal efficiency. Energy expenditure/unit volume of soil for all experiments was 8-31 kW [multiplied by] h/[m.sup.3].

Most rain-induced landslides on Hong Kong's volcanic rocks are shallow and small-scale. However, two major landslides that occurred on Hong Kong Island in 1995 highlight relationships between primary volcanic foliations and kaolin accumulations that may have general significance in determining loci of larger failures. In the first case, a bedding-parallel shear zone was the locus for kaolin concentration in a shallowly dipping tuff. This formed a weak, planar, low-permeability layer which acted as the surface of rupture of a translational failure. In the second case, steeply inclined fabrics (eutaxitic foliations) and subparallel, closely spaced joints occurred within a zone of syn-volcanic folding and shearing. Deep weathering developed along the zone, with kaolin concentrations present mainly above the soil-rock interface. The irregular and variably concave to planar rupture surface was closely associated with kaolin-infilled relict joints towards the base of the soil profile. Kaolin distribution is largely associated with the weathering profile, and may have been both physically and chemically redistributed within it. Halloysite is more abundant than kaolinite in most clay samples at and near both rupture zones, and may have developed from kaolinite in response to strain. Detailed mapping of volcanic fabrics around the major landslides suggests that kaolinized bedding-parallel shear zones are uncommon, but that deep weathering zones associated with shearing and faulting are common, mappable features. Keywords: landslides, eutaxitic foliation, halloysite, kaolinite, shearing.

This technical note describes the use of the modified Langmuir isotherm (MLI) model recently introduced by Gu and coworkers to describe the adsorption and desorption of natural organic matter (NOM) onto Kaolinite. The MLI was able to describe the adsorption of two different NOM samples with the use of three adjustable parameters. A commercial humic acid adsorbed to a much larger extent than a NOM sample collected from a swamp. The hysteresis coefficient was used to describe adsorption-desorption reversibility. Both samples had similar hysteresis coefficients. Such affinity parameters, however, determined from adsorption and desorption experiments, varied significantly. This study concludes that the new MLI model is better than the traditionally used Langmuir isotherm in describing adsorption-desorption of NOM to Kaolinite and desorption parameters may be considered to describe dissolved organic carbon concentrations in subsurface environments.

The dealumination of kaolinite by the solid/solution 'interfacial' reaction with HCl(aq) was studied. Solid state (super1H) combined rotation and multiple-pulse spectroscopy (CRAMPS) and single pulse/magic-angle spinning (SP/MAS)(super29) silicon nuclear magnetic resonance (Si NMR) spectroscopy were used to analyze the reaction mechanism. The SP/MAS(super29) Si NMR and (super1H) CRAMPS studies led to the development of a structural model for dealuminated kaolinate solids.

The efficiency of electrokinetic injection of a cation (ammonium) from the anode and an anion (sulfate) from the cathode into a fine-grained sand bed and a kaolinite bed is investigated. Electrodes are placed in chambers across 80 cm of soil beds in a flume. The electrical conductivity of the kaolinite bed was 124.1 [+ or -] 6.6 [micro]S/cm approximately an order of magnitude higher than the fine sand bed while the hydraulic conductivity of the same was 2 x [10.sup.-7] cm/s about three orders of magnitude lower than the fine-grained sand bed. The electrical gradients of 1 V/cm or less constituted the predominant driving force for transport under constant current densities of 15 [[micro]ampere]/[cm.sup.2] and 123 [[micro]ampere]/[cm.sup.2] in the sand and kaolinite beds, respectively. An electrolyte conditioning scheme where the co-ions (hydroxide ion in the ammonium hydroxide used at the anode and the hydronium ion in the sulfuric acid used at the cathode) depolarized the electrode reactions maintained the pH value across the beds between 6.5 and 7.4. This novel conditioning scheme prevented formation and introduction of species formed by the electrode reactions and avoided unnecessary increase in the electrical conductivity in the electrolytes. Transport rates on the order of 8-20 cm/d were achieved for sulfate and ammonium ions in both the fine-grained sand bed and the kaolinite bed.

Geological mapping of existing and redundant kaolin workings within the St Austell Granite has identified a suite of granitic rocks which show evidence of complex late-stage magmatic and hydrothermal processes. Coarse porphyritic biotite granites, like those which predominate in southwest England, occur much more widely than previously acknowledged, and are intruded by an apparently cogenetic suite of lithium-mica granites and tourmaline granites. The tourmaline granites characteristically exhibit very variable textures, with coarse quartz grains set within a fine grained, tourmaline-rich matrix. A highly evolved fine-grained tourmaline granite represents the most evolved of this suite. Topaz granites intrude the earlier granite varieties, and all are intruded by rhyolite porphyry dykes (elvans). Major and trace element chemical data suggest that the biotite granite - lithium-mica granite - tourmaline granite suite represents the product of crystallization of a granitic magma within which B (but not F) became progressively enriched until water saturation was achieved. Water exsolution effectively quenched any remaining granitic melt, resulting in the very variable textures shown by the tourmaline granites. The topaz granites are chemically distinct from their predecessors, showing marked enrichment in F, Li and [P.sub.2][O.sub.5] (but not B). Instead of being products of differentiation of biotite granite magma, the topaz granite melts may have been derived separately in a later episode of partial melting of the same source. Kaolinization is widespread throughout the western part of the St Austell Granite, and deposits worked at present tend to be located in granite varieties other than biotite granite. The geochemical parameters used to distinguish the primary granite types (particularly Nb v. Zr and Ga-Nb-Zr plots) are sufficiently robust to permit the parent granitic rock type to be identified for heavily kaolinized material. Keywords: Cornwall England, granites, kaolinization.

The mechanical properties of a kaolinite/fiber soil composite were evaluated by a series of laboratory unconfined-compression, splitting-tension, three-point-bending, and hydraulic-conductivity tests. The inclusion of randomly distributed fibers significantly increased the peak compressive strength, ductility, splitting tensile strength, and flexural toughness of kaolinite clay. The increase in strength and toughness was a function of fiber length and content, and the water content of the composite. Increasing fiber content increased the compressive and tensile strength, and the toughness index of kaolinite clay, with the effect being more pronounced at lower water contents. The contribution of fibers to peak compressive and tensile strengths were reduced, and ductility increased, with increasing fiber length. The fiber inclusion increased the hydraulic conductivity of the composite and the increase was more pronounced at higher fiber contents. Despite the increase, the hydraulic conductivity of the composite was still low enough to be considered for some landfill applications.

The presence of multiphase region under shock effects near 30 giga Pascals is revealed by the XRD, TEM, thermoponderal and infrared spectra of aluminous smectite and postshock kaolinite. A fraction of the crystalline water of the clays is transferred to a new glass phase. Under certain conditions, some of the completely nonalkali composition are capable of forming glass, which does not devitrify when subjected to postshock conditions. The thermal stability of clays at atmospheric and low pressure conditions causes the continuous stability of shock events.

Results of resonant column tests were used to determine the effects of overconsolidation ratio (OCR) and of aging on the normalized rate of secondary increase N(sub G) of low-amplitude shear modulus G(sub 0) of a remolded kaolinite clay. The value of N(sub G) decreased approximately linearly with the logarithm of OCR and with the logarithm of duration of aging of the clay. The similarity of behavior provided a basis for establishing an equivalency between age-and-stress - induced equivalent overconsolidation for the clay in question. It was concluded that up to a certain limiting duration of aging the age-induced OCR increases linearly with elapsed time, whereas the effect starts diminishing for longer durations of aging. It is suggested that the results of similar studies on natural clay soils be used for predicting the long-term behavior of clays from the results of short-duration tests.

Electro-osmosis tests are conducted on saturated kaolinite specimens loaded with phenol at a level of 500 [mu]g/g of dry kaolinite. This amount was below the phenol adsorption capacity of this mineral. The flow behavior and the electrochemistry (voltage, current, resistance, pH gradients, and conductivity) in these experiments are presented. Electroosmotic flow with open electrodes is time-dependent. Acid front generated at the anode sweeps across the specimen neutralizing the base at the cathode. The adsorbed phenol was removed by 85% - 95% by the process. Phenol concentration profile in the effluent did not display any retardation due to desorption. The energy expenditure for removal was of the order of 18-39 kWh/[m.sup.3] of oil processed. The study displays that it may be feasible to remove low-concentration organic contaminants from cohesive soils by application of a direct current., Tests were conducted to investigate the removal of phenol adsorbed on saturated kaolinite using electrokinetic soil processing. The flow behavior, electrochemistry and energy requirements are described. Results show that the process removed 85% to 95% of the phenol with energy expenditures between 18 to 39 k Wh/cubic meter. In addition, phenol removal did not exhibit any retardation in effluent concentration profiles.

The Fire Clay tonstein is a kaolinized, air-fall volcanic ash bed that was deposited in a widespread late Carboniferous peat-forming mire. Eleven samples from Kentucky and West Virginia, spanning a distance of 200 km, and two samples from Tennessee and Virginia indicate a characteristic mineralogical signature, as compared with other Appalachian tonsteins, consisting of well-crystallized kaolinite, beta-quartz crystal paramorphs, sanidine, ilmenite, zircon, and brookite. Detrital illite and quartz are rarely present or are in very small amounts, which indicates rapid deposition in a mire. Several normal graded cycles in this tonstein suggest repeated episodes of pyroclastic activity that produced a composite ash layer. A high-silica alkalic rhyolitic source is suggested by the geochemistry of immobile elements and by electron-probe analyses of glass inclusions in volcanic quartz from the Fire Clay tonstein. The rare-earth-element plots (chondrite normalized) of the tonstein show a pronounced negative Eu anomaly and relatively high concentrations of Zr and Th, which are both indicative of a rhyolitic source. Probe analyses of the Fire Clay glass inclusions from four states indicate a chemically identical high-silica rhyolite with peraluminous affinities. 40Ar/39Ar sanidine plateau dating indicates an age of 312 plus or minus 1 Ma for the Fire Clay tonstein, which is consistent with previous 40Ar/39Ar dates for this tonstein. This age is in agreement with a late Westphalian B age in the European Carboniferous chronostratigraphy on the basis of an age of 311 Ma for the Westphalian B/C boundary. A new isopachous map of the Fire Clay ash-fall deposit indicates an area of 37,000 km squared and a probable source to the present-day southwest. The deposit has a minimum preserved compacted volume of 2.8 km cubed, which corresponds to an original uncompacted volume of about 20 km cubed. This preserved volume indicates an ultraplinian volcanic explosion. Pindell and Dewey (1982) proposed an Andean-type arc in this block during the late Carboniferous, prior to South American-North American plate collision. We hypothesize an associated back-arc caldera system in the Yucatan block to explain the high-silica, potassic rhyolitic ash that gave rise to the Fire Clay tonstein.

Nuclear magnetic resonance (NMR) spectroscopy, variable-temperature powder X-ray diffraction and Fourier-transform infrared spectroscopy were conducted to determine the structure and behavior of the guest dimethyl sulphoxide (DMSO) molecules in the kaolinite:DMSO intercalation compound. Data showed that a hydrogen-bonded oxygen atom linked the DMSO guest molecule to the kaolinite hydroxyls. Analysis of the H NMR spectra indicated that there were two sites for the DMSO molecule, wherein at least one was capable of motion.

The application of Si, Na and H MAS-NMR spectroscopy, thermal analysis, X-ray diffraction and infrared spectroscopy has proven effective in analyzing the phase compositions of the basic hydrosodalite (Na4 O2H3)2(SiAlO4)6 derived from the hydrothermal reaction of kaolinite with aqueous NaOH solution. The results show that five distinct phases represented by the structures Na8(SiAlO4)6(OH)2.2H2O, Na8(SiAlO4)6(OH)2, Na6(SiAlO4)6.8H2O, Na6(AlSiO4)6.4H2O and Na6(SialO4)6 comprise the basic hydrosodalite. Furthermore, an intracage NaOH/H2O transfer occurs during washing of the primary basic hydrosodalite forming basic and nonbasic sodalite mixtures.

Highly weathered kaolinitic soils in Brazil often have adequate levels of K and Mg to support plant growth. The source of K and Mg in these soils and their relationship with the dominant mineral, kaolinite, is addressed in this article. Crystallographic, chemical, and morphological properties of kaolinite, from selected highly weathered Brazilian soils, were investigated by x-ray diffraction (XRD), analytical electron microscopy (AEM), chemical, and thermal methods. Kaolinite properties showed significant variations such as d(001) spacing ranging between 0.713 to 0.728 nm, width at half height (WHH) between 0.30 to 0.97 [degrees] 2 [Theta], and dehydroxylation temperature between 489 to 518 [degrees] C. Kaolinite in the clay fraction has relatively poor crystal order with a mean crystallinity index value(CI) of 12.7. The dominant forms of the clay-fraction kaolinite were elongated and rounded, with relatively lower proportions of hexagonal particles. The silt-fraction kaolinite showed a tendency to form subspherical large aggregates with high stability. The average [Fe.sub.2][O.sub.3] level in the kaolinite of the clay fraction (19.1 g [kg.sup.-1]) was higher than that obtained for the silt fraction (6.6 g [kg.sup.-1]). The smaller kaolinite particles of the clay fraction showed a lower degree of crystal order, higher K and Mg levels, and lower dehydroxylation temperatures. From the strong relationship between the asymmetry index (AI) of the (001) diffraction line and the level of K in kaolinite from the younger soils, we believe that both K and Mg in kaolinite are part of residual micaceous layers interleaved in kaolinite crystals.

Six kaolinite [[Al.sub.2][Si.sub.2][O.sub.5][(OH).sub.4]] samples were synthesized under different conditions of temperature, pressure, and pH from two different starting materials. Chemical composition and properties of the samples were characterized using classical methods (electron microprobe, atomic absorption spectrometry, X-ray diffraction, differential and thermal analyses, and Fourier transform infrared spectrometry). All synthetic kaolinite samples contained various amounts of a boehmite impurity. The defect density was different for each kaolinite, ranging from high to low. The enthalpy of formation of these kaolinites at 25 [degrees] C was investigated by drop solution calorimetry into molten lead borate at 700 [degrees] C. All data were corrected for impurities. Whatever the synthesis conditions and the kaolinite properties, the enthalpy of kaolinite dissolution into molten lead borate at 700 [degrees] C and the standard enthalpy of kaolinite formation from the oxides and from the elements at 25 [degrees] C are constant: 372.3 [+ or -] 1.0 kJ/mol, -46.6 [+ or -] 2.6 kJ/mol, and -4115.3 [+ or -] 4.1 kJ/mol respectively. Using entropy data from the literature, the standard Gibbs free energy of kaolinite formation from the elements at 25 [degrees] C is -3793.9 [+ or -] 4.1 kJ/mol. This value is in excellent agreement with most of the literature data obtained for natural kaolinites. Furthermore, the standard Gibbs free energy of kaolinite formation at 25 [degrees] C and 1 atm is very close to that obtained using the same method for the San Juanito dickite, which is commonly used as a standard mineral, the value for kaolinite being slightly more negative than the value for dickite. This trend is also true for all the temperature and pressure range of kaolin minerals occurrences. Thus, dickite is a metastable phase relative to kaolinite, and kaolinite seems to be thermodynamically more stable than dickite, as already proposed by DeLigny and Navrotsky (1999) and Anovitz et al. (1991). The natural occurrence of dickite must result from specific reaction paths and be controlled by kinetic factors.

The mineralogical and chemical characteristics of saprolites and soils developed from granulitic, monzonitic, and charnockitic gneisses and mylonites of the Blue Ridge Mountains and western Piedmont foothills of central Virginia, were studied. Micromorphological, x-ray diffraction (XRD), scanning electron microscopy (SEM), and microprobe analysis showed that the mineralogical and geochemical evolution of biotite in profiles developed on gneisses is consistent with pseudomorphic weathering of mica to kaolinite and halloysite, with or without a mica-vermiculite intermediate phase. On mylonitic rocks, saprolites and soils also contain muitimineral pseudomorphs after biotite, in which gibbsite crystals eventually fill the whole volume. There is evidence of topotaxial formation of halloysite after biotite. Although gibbsite is present at the first stages of the weathering of biotite, it is not clear if gibbsite forms directly from the mica or if it is a weathering product of kaolinite and/or halloysite. The source of aluminium may also be located outside the original biotite crystal. These multimineral assemblages suggest that microenvironments of weathering are controlling the formation of secondary products from the parent biotite rather than the so called anti-gibbsite effect, at least at this scale of investigation. It is suggested that the mylonitic fabric and subvertical foliation planes of these rocks are responsible for this mineralogical and geochemical evolution.

Kaolinite and dickite occur widely in central Scotland and Northern Ireland. Stable hydrogen and oxygen isotope compositions of both minerals are similar, suggesting that the formation of kaolinite occurred first at temperatures of [is less than] 50 [degrees] C from meteoric water, probably as a result of alteration of Lower Carboniferous volcanic rocks, and that dickitization followed locally as a result of local heating that accompanied the intrusion of dykes during Late Carboniferous--Permian times. This mechanism of dickite formation explains why the higher-temperature polytype dickite occurs in a region from the Firth of Clyde to Perthshire, in association with dyke swarms, whilst kaolinite occurs elsewhere. The original kaolinite precipitation was focused along the high permeability fault zone now marked by the Highland Boundary Fault Zone and its presumed trace in Northern Ireland. Keywords: Scotland, kaolin, kaolinite, dickite, palaeotemperature.

The anisotropy studies of Sivakugan, Chameau and Holtz on cuboidal shear devices provide very interesting results such as the positive relationship between the effective friction angle and the direction of the loading. However, the values derived for the friction angles need more detailed explanation as to the kinds of tests that were applied during the process of derivation and the reliability of those tests.

A multicomponent reactive solute-transport model was used to study the migration of dissolved heavy metals ([Cd.sup.2+], [Pb.sup.2+], [Cu.sup.2+], and [Zn.sup.2+]) in a clay barrier subject to two leachates having different pH values. This solute-transport model is capable of simulating simultaneous processes of water flow, advective-dispersive-solute transport, and chemical reactions. The migration of these metals was simulated in a kaolinite landfill liner, which was assigned realistic physical and chemical properties and boundary conditions to model one-dimensional contaminant transport. The leachate input properties to the model were those of an actual leachate containing the four heavy metals. The numerical simulations were focused on the concentration profiles of these metals in the simulated clay barrier and leachate pH affects their mobilities. The numerical results indicate that with a nearly neutral leachate, the heavy metals mobility follows: [Cd.sup.2+] < [Pb.sup.2+] < [Cu.sup.2+] < [Zn.sup.2+]. With an acidic leachate, the order changes to [Pb.sup.2+] < [Cu.sup.2+] < [Zn.sup.2+] < [Cd.sup.2+]. Leachate pH has a significant effect on [Cd.sup.2+] and [Pb.sup.2+] mobility and a small effect on [Cu.sup.2+] and [Zn.sup.2+].

Solid products from hydrothermal experiments conducted at 250 [degrees] C and [P.sub.v,soln] were characterized by powder X-ray diffraction (XRD) and ATEM/HRTEM. Experiments were conducted with muscovite, kaolinite, and quartz or amorphous silica in 2M KC1 solutions for 43 to 176 d. Post-experiment solution compositions lie either within the illite(0.88 K) stability field or on the illite(0.88 K)-kaolinite or illite(0.88 K)-diaspore univariant boundaries in log [Mathematical Expression Omitted] vs. log [a.sub.[H.sub.4]Si[O.sub.4]] activity space. Transmission electron microscopy (TEM) observations of muscovite grain edges reveal the neoformation of illite crystals with a range of compositions (ATEM) from 0.31 to 0.89 K/[O.sub.10][(OH).sub.2]. The range of K-contents appears to narrow toward 0.88 K/[O.sub.10][(OH).sub.2] with increased experiment duration. HRTEM suggests the presence of 2 to 11 layer fundamental particles composed of illitic layers with 10 A periodicity. Fundamental particle thicknesses increase toward an average of 8 layers/particle with increased experiment duration. In the longer duration experiments, fundamental particle thicknesses were normally distributed about thicknesses of 4 and 8 layers, whereas fundamental particles with thicknesses [less than] 4 layers were common in a shorter duration experiment. The compositions and structure of the illites are consistent with the multiphase model, which states that the smectite-to-illite transition occurs through the step-wise formation of solubility-controlling phases consisting of fundamental particles with thicknesses of 1, 2, 4, and [greater than or equal to]8 layers. The increase in K-content and fundamental particle thickness with the extent of reaction suggests that the illite crystals underwent a prograde reaction culminating in the formation of end-member illite [0.88 K/[O.sub.10][(OH).sub.2]]. This reaction, in conjunction with the previously observed, retrograde reaction from muscovite to end-member illite, demonstrates the stability of end-member illite in the system [K.sub.2]O-[Al.sub.2][O.sub.3]-Si[O.sub.2]-[H.sub.2]O at 250 [degrees] C.

Solid products from hydrothermal experiments conducted at 250 [degrees] C and [P.sub.v,soln] were characterized by powder X-ray diffraction (XRD) and ATEM/HRTEM. Experiments were conducted with muscovite, kaolinite, and quartz or amorphous silica in 2M KC1 solutions for 43 to 176 d. Post-experiment solution compositions lie either within the illite(0.88 K) stability field or on the illite(0.88 K)-kaolinite or illite(0.88 K)-diaspore univariant boundaries in log [Mathematical Expression Omitted] vs. log [a.sub.[H.sub.4]Si[O.sub.4]] activity space. Transmission electron microscopy (TEM) observations of muscovite grain edges reveal the neoformation of illite crystals with a range of compositions (ATEM) from 0.31 to 0.89 K/[O.sub.10][(OH).sub.2]. The range of K-contents appears to narrow toward 0.88 K/[O.sub.10][(OH).sub.2] with increased experiment duration. HRTEM suggests the presence of 2 to 11 layer fundamental particles composed of illitic layers with 10 A periodicity. Fundamental particle thicknesses increase toward an average of 8 layers/particle with increased experiment duration. In the longer duration experiments, fundamental particle thicknesses were normally distributed about thicknesses of 4 and 8 layers, whereas fundamental particles with thicknesses [less than] 4 layers were common in a shorter duration experiment. The compositions and structure of the illites are consistent with the multiphase model, which states that the smectite-to-illite transition occurs through the step-wise formation of solubility-controlling phases consisting of fundamental particles with thicknesses of 1, 2, 4, and [greater than or equal to]8 layers. The increase in K-content and fundamental particle thickness with the extent of reaction suggests that the illite crystals underwent a prograde reaction culminating in the formation of end-member illite [0.88 K/[O.sub.10][(OH).sub.2]]. This reaction, in conjunction with the previously observed, retrograde reaction from muscovite to end-member illite, demonstrates the stability of end-member illite in the system [K.sub.2]O-[Al.sub.2][O.sub.3]-Si[O.sub.2]-[H.sub.2]O at 250 [degrees] C.