We report the first application of 39K solid-state NMR to the study of tecto- and phyllosilicates. Under high field (11.7 Tesla) and with the application of a spin-echo sequence, informative 39K spectra can be obtained for several compounds of interest to the geologist and the agronomist. Tectosilicates and phyllosilicates can be distinguished from the uncorrected frequency (δCG) of the observed NMR peak. A series of montmorillonites submitted to increasing numbers of wetting and drying cycles was studied in order to discriminate between mobile and “fixed” forms of K+: when the spectra are run on hydrated samples, two different signals are observed corresponding to K+ in different hydration states, and NMR data can be correlated with the amount of exchangeable K+ measured by ion exchange. Thus, it appears that NMR can provide useful information on K fixation complementary to classical chemical methods.

X-ray diffraction studies of the pore-lining chloritic mineral from the Tuscaloosa Formation disclose a marked pattern of odd-order line broadening for the 001 to the 00,16 reflections. The odd-order peaks are approximately twice as broad as the even orders after correction for instrumental broadening effects. These results are consistent with a randomly interstratified 7-Å/14-Å structure, which is most likely serpentine/chlorite. Quantitative analysis of line broadening and model calculations indicate that the serpentine/chlorite contains 7% serpentine layers.

A simplified method is proposed for quantifying randomly interstratified serpentine/chlorite. Residual line broadening (βr) is obtained from the half-height widths of uncorrected diffraction profiles for the 004 and 005 “chlorite” reflections by means of the following:

${\beta }_r=({\beta }_{005}^{1.25}-{\beta }_{004}^{1.25}{)}^{\frac{1}{1.25}}.$

$\mathrm{\%}S=-0.51+24.27{\beta }_r,$

57Fe Mössbauer spectra of two synthetic samples of ferrihydrite, recorded at 4.2 K in applied fields of up to 9 T, have been analysed by a mean-field model. The samples exhibit two and six X-ray diffraction peaks. It is shown that only one ferric ion site is present in the mineral, and that in this site the ions are octahedrally coordinated. The spectra show the presence of different magnetic states: ferrimagnetism in two-line ferrihydrite, and antiferromagnetism in six-line ferrihydrite. The ferrimagnetism in two-line ferrihydrite is analysed in terms of random fluctuations arising from the small numbers of ferric ions per particle, and it is shown that the different magnetic states may arise purely as a result of these fluctuations.

Thin section, XRD, SEM, and isotopic techniques have been used to study authigenic kaolinite occurring in reservoir sandstones of the Lower Permian Aldebaran Sandstone. Where the unit is no longer an active aquifer, kaolinite is an intermediate-stage phase, and is highly depleted in deuterium (δDSMOW = −115 to −99‰) and 18O (δ18OSMOW = +7.8 to +8.9‰), indicating that precipitation must have been from meteoric water. Deep penetration of this water is linked to Late Triassic deformation and uplift of the Denison Trough sequence, an event which led to exposure of the Aldebaran Sandstone by the Early Jurassic prior to its re-burial beneath Jurassic and Cretaceous sedimentary rocks. The same water was probably involved in the creation of secondary porosity in the interval.

Where the Aldebaran Sandstone is presently undergoing meteoric flushing, kaolinite is relatively enriched in deuterium (δDSMOW = −104 to −93‰) and 18O (δ18OSMOW = +11.7 to +14.6‰), reflecting precipitation largely from post-Mesozoic meteoric water which was isotopically heavier than the Mesozoic water involved in intermediate-stage kaolinite precipitation. This temporal shift in meteoric water isotopic composition is related to the northward drift of the Australian continent to lower latitudes since the Mesozoic Era.

The surface excesses of Na and Cl on synthetic imogolite and allophanes with varying Al/Si molar ratios in 0.10 M and 0.01 M NaCl solutions were determined using 22Na and 36Cl as ion probes. The point of zero net charge (PZNC) values ranged from 4.1 to 8.4, increasing with the Al/Si molar ratio for the allophanes, and was highest for imogolite (Al/Si = 2.01). The PZNC values were significantly lower than the point of zero charge (PZC) values previously determined by microelectrophoresis for the same material, indicating that Na resided within the shear plane to a greater extent than Cl. The PZNC values of allophanes were lower than their PZSE values, indicating that permanent charge existed in allophanes, and increased as Al/Si decreased. Conversely, the PZNC of imogolite was higher than its point of zero salt effect (PZSE) determined by potentiometric titration. Adsorption of Cl on imogolite from 0.1 and 0.01 M NaCl solutions below pH 8.4 and of Na from 0.1 M NaCl solutions between pH 5 and 8.4 exceeded the proton charge determined by potentiometric titration. There was no direct evidence of permanent charge in imogolite and excess Cl adsorption could not be entirely explained by simultaneous intercalation of Na and Cl. Isomorphic substitution of Al in tetrahedral sites was shown to increase with decreasing Al/Si by 27Al high-resolution solid-state nuclear magnetic resonance (NMR) spectra of allophanes, and was absent in imogolite. The chemical shifts of Al(4) and Al(6) were similar in allophanes (63.0–64.7 ppm and 6.1–7.8 ppm, respectively) and the chemical shift of Al(6) was 9.4 in imogolite.

Adsorption of cetylpyridinium chloride (CPC) onto kaolinite can be followed using the electroacoustic effect. The dynamic mobility, measured at a frequency of 1 MHz, varies from about −2 to +1 × 10−8 m2 V−1 s−1 in a number of steps, reflecting the adsorption of two separate layers, with the bilayer being more obvious, especially at pH 5–8. The behaviour at different pHs reflects the different charge characteristics of the basal cleavage planes and the crystal edges. When the amount of added CPC is equal to the cation exchange capacity of the clay, the kinetic charge changes from negative to positive and there is a pronounced break in the conductivity curve. It is also possible to estimate the edge to face area from such measurements and so obtain a measure of the aspect ratio of the clay crystallites. The (minimum) value for this clay is about 5:1.

According to Lippmann (1977, 1982) wide compositional variations and excess enthalpies of mixing calculated with electrostatic models imply that clay minerals of variable composition are dis-equilibrium solids. However, recent ATEM analyses of illite samples indicate compositional homogeneity of single illite grains and limited compositional variations in sedimentary basins. Moreover, Lippmann's electrostatic model may be inadequate inasmuch as it neglects polarization energy which is known to be a significant component of lattice energy even in dominantly ionic structures. Contrary to the assumptions of Lippmann, I/S minerals have also been shown to undergo Ostwald ripening.

May et al. (1986) reported that smectites do not reversibly control equilibria and further argued that conceptual and experimental deficiencies inherent in the solubility method prevent the attainment and demonstration of equilibrium in experiments with complex aluminosilicates of variable composition. However, equilibrium may be assumed if: (1) steady states are approached from both under- and over-saturation, (2) the slopes of univariant lines representing mineral-solution equilibria are rational over a wide range of solution compositions and temperature, and (3) results are reproducible in experiments of long duration. Recent solubility studies of smectites, chlorites, and illites meet these criteria indicating that clay minerals of variable composition are true phases capable of attaining equilibrium.

Five montmorillonites, one hectorite, and two vermiculites were treated with OH-Al solutions containing rapid- and slow-reacting polymers of similar concentrations. With all smectites, both rapid- and slow-reacting OH-Al polymers were much more preferentially adsorbed than monomeric species. Relatively slow-reacting OH-Al polymers were more preferentially adsorbed than rapid-reacting ones. The average basicity of the adsorbed Al was 2.46, which was close to that of the OH-Al polymers in the original solution. The OH-Al polymers that enter the interlayer resemble those in original solutions.

With vermiculites, the solution concentration of rapid-reacting OH-Al polymers was much reduced after reaction. The average basicity of the adsorbed Al was 1.99, which was considerably lower than that in the original solution. It is postulated that OH-Al polymers break to monomeric Al ions and then enter the clay. The monomeric Al species that enter the clay interlayers hydrolyze and polymerize in situ and become fixed. The H+ ions released from hydrolysis convert the rapid-reacting OH-Al polymers to monomeric Al in solution. Limited amounts of slow-reacting polymers were adsorbed because of their resistance to acid depolymerization.

Inverse gas chromatography at infinite dilution, employing alkanes and alkenes as probes, has been used to characterize the surface properties of a series of smectites of varying chemical composition. The results of this study show that the acidic centers and the interlayer distances have a great influence on the specific interaction of the smectite surface with π-bonds of alkenes. High values of the specific interaction parameter, ɛπ, are caused by the existence of strong acidic centers that are connected with interlayer cations as well as with the chemical structure of the mineral sheets. On the other hand, alkanes, whose interaction with the smectites is predominantly dispersive, are unaffected by changes in the clays’ composition and/or structure.

Hydroxyaluminosilicate (HAS) ions prepared from hydroxy-Al (HyA) ions and orthosilicic acid at different NaOH/Al molar and Si/Al atomic ratios were fixed in the interlayer spaces of vermiculite (Vt). The electric charge and surface characteristics of HAS-Vt and HyA-Vt complexes formed were investigated in the pH range of 4 to 8. At pH 4 to 6, the magnitude of negative charge (CEC) of HAS-Vt and HyA-Vt complexes was drastically reduced by a HAS- or HyA-interlayer formation of Vt. At pH 7 to 8, especially in NaOH/Al molar ratio of 2.5, the magnitude of negative charge was from 62 to 89% of CEC in untreated Vt, suggesting that part of HAS or HyA ions fixed on Vt was excluded from its interlayer spaces. The positive charge did not develop on HAS-Vt and HyA-Vt complexes at pH between 4 and 8. The fixation of HAS or HyA ions on Vt caused the significant reduction of its total and internal surface areas as well as the slight increase of its external surface area. The HAS- or HyA-fixation in the interlayer spaces of Vt was confirmed by X-ray diffraction analysis. Our results provided evidence of a possibility that Vt could fix HAS ions in the same way as HyA ions, transforming to chloritized-Vt. These interlayer materials could play a significant role in modifying the surface and mineralogical properties and cation exchange capacity of clays and soils.

The structure of a disordered IIb Mg-chamosite was studied using Rietveld refinement techniques and powder X-ray diffraction (CuKα, 18–120° 2θ in 0.02° 2θ increments). The refinement in space group CĪ yielded high precision lattice parameters (a = 5.36982(5)Å, b = 9.3031(9)Å, c = 14.2610(6)Å, α = 90.315(5)°, β = 97.234(7)°, γ = 90.022(9)°) and atomic coordinates very similar to previous studies. However, the presence of semi-random stacking in this specimen created a situation in which not all atoms could be precisely located: the positions of the octahedral cations and anions which repeat at intervals of ±b/3 could be uniquely determined in three dimensions whereas only the z parameter of the other atoms could be refined. The reasonable appearance of the final model, despite the fact that many of the atom positions could be located in only one dimension, may have resulted because all of the atoms in this structure except O(5) repeat at intervals which are very nearly ±b/3.

In a hitherto unpublished manuscript dated 1794–95, H.-B. de Saussure described three types of rudimentary laboratory experiments on weathering, each one lasting about 4 hours, undertaken on samples of jade, serpentine, feldspar, and mica schist. He submitted these samples to dry heating, heating with continuous addition of water, and heating with periodic addition of water. Experimentation was performed at temperatures of the order of 200–250°C, and Saussure seems to have realized that a reasonable increase of temperature was necessary for laboratory simulation of long-lasting geological processes. No changes were observed during dry heating whereas increasing alteration was noticed in the presence of water, particularly when heating was combined with periodic wetting. He concluded that these tests demonstrated his preconceived idea that weathering of minerals (mostly silicates) resulted from alternating periods of drying and wetting. This concept is basically very close, if not identical, to that underlying modern ideas on seasonal weathering.

The surface thermodynamic properties of a series of n-alkylammonium and quaternary ammonium treated clay films were determined by contact angle measurement of drops of test liquids using the Young equation for polar materials. The two clays were a Wyoming montmorillonite (SWy-1) and Laponite RD. For a series of primary n-alkyl (6 ≤ n ≤ 15) and several quaternary organic cations, the organo-clay (both SWy-1 and Laponite RD) showed very little change in the value of γLW compared to the equivalent ammonium-saturated clay. Also, γ⊕ remained small or increased slightly compared to the ammonium-saturated clay. For SWy-1 exchanged by both quaternary ammonium and primary n-alkylammonium cations, the value of γ⊖ was smaller (0.1 ≤ γ⊖ ≤ 15.8 mJ/m2) than for the ammonium-saturated clay (γ⊖ = 36.2 mJ/m2) and decreased linearly with the number of carbon atoms. The γ⊖ values for the organic cation-exchanged Laponite RD samples (24.2 ≤ γ⊖ ≤ 31.2 mJ/m2) were smaller than or comparable to the ammonium saturated clay (γ⊖ = 30.7 mJ/m2), and were relatively insensitive to the number of carbon atoms in the organic cation. Thus, for both clays the increased adsorption of organic molecules resulting from replacement of inorganic cations by organic cations is due primarily to the decrease in the value of the Lewis base parameter, γ⊖.

Nonexchangeable polymers in interlayers of expansible phyllosilicates influence thermal dehydration in ways not completely understood. Thermal dehydration of hydroxy-interlayered vermiculite (HIV) from Florida soils, for example, results in irreversible d001 shifts. This study was conducted to characterize HIV dehydration as a function of time (t) and temperature (T), and to determine how reversibility of dehydration is affected by elevated T. Clay-sized HIV from 3 soils was heated incrementally and d-spacing shifts (Δd) were monitored by X-ray diffraction (XRD) at low relative humidity (RH). Samples were then mounted on a metal heating strip in the XRD focal plane and scanned repeatedly at constant T levels to monitor Δd with t. Finally, Δd in response to RH shifts from <5% to >85% was determined at 25°C and at elevated temperatures. Incremental heating revealed a Δd plateau roughly corresponding to the z dimension of hexameric octahedrally coordinated Al. The initial slope of Δd-vs-t curves increased with T. The same maximum Δd reached at 200°C was reached at 160°C, but more slowly. All samples exhibited reversible and irreversible dehydration, the former being attributable to sites in equilibrium with external vapor and the latter to sites requiring heat for desorption. Reversible sites were not perturbed by moderate heating, but were apparently eliminated by polymer dehydroxytation. The dehydration behavior of HIV could be explained by steric resistance of water vapor diffusion within a tortuous interlayer polymeric network. Alternatively, new polymer/oxygen-surface bonds exceeding the hydration energy of interlayer components could form via heat-induced re-articulation of polymer/oxygen-surface bonds at smaller basal spacings.

Six well-characterized soil kaolins from widely separated sites in south-western Australia and four reference kaolins were studied by 57Fe Mössbauer spectroscopy at room temperature after removal of non-structurally-bound iron with dithionite-citrate-bicarbonate solution. The soil kaolins and one of the reference kaolins were also studied at temperatures near 16 K. The soil kaolins were remarkably similar in crystal size, crystallinity, dehydroxylation temperature, cation exchange capacity, surface area and iron content. Müssbauer spectra of the soil kaolins at room temperature were also essentially identical consisting of a quadrupole-split doublet superimposed on a broad component which indicated that all of the iron was present as Fe(III) and that slow paramagnetic relaxation effects were present. Mean values for the chemical isomer shift and quadrupole splitting of the doublet for the soil kaolins were 0.33 and 0.55 mm/s respectively which indicates that the iron is in the octahedral sites of the kaolin lattice. The spectra of the soil kaolin samples at temperatures near 16 K showed a further slowing down of the paramagnetic relaxation and confirmed that no discrete iron oxide minerals were present.

Mössbauer spectra of the four reference kaolins at room temperature showed a doublet component similar to those for the soil kaolins. Three of them showed evidence for other spectral components including, in two cases, a component due to the presence of Fe(II).

Synthetic hematites with Al substitutions between 0 and 18 mol % were synthesized at different temperatures and water activities. The cell-edge lengths a for different synthesis conditions decreased linearly with increasing Al substitution. The regression lines, however, had different slopes and intercepts: the series with the highest synthesis temperature (1270 K) had the most negative slope. With increasing Al substitution, the hematites contained increasing amounts of non-surface water. Significant correlations were found between these chemically determined water contents and the deviations of the unit-cell parameters a, c, and V relative to the corresponding 1270 K regression lines. To explain the measured X-ray peak intensities, structural OH had to be included into the theoretical calculations. From intensity ratios normalized to I113, it is possible to determine the structural OH separately from the Al substitution, which can be assessed by the shift of the cell-edge lengths relative to the 1270 K regression lines. The incorporation of Al and OH into the hematite structure induces strain, which was quantified by X-ray diffraction.