The 16-8-, 8-5-, 5-2-, 2-1-, 1-0.5-, 0.5-0.3-, 0.3-0.1-, and <0.1-μm size fractions were centrifuged from a Georgia (U.S.A.) sedimentary kaolin and a hydrothermal kaolin from the Sasso mine (Italy) and analyzed by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), infrared spectroscopy (IR), differential thermal analysis (DTA) and thermogravimetry (TGA) together with the corresponding whole rocks. All size fractions of the Georgia sample consisted dominantly of well-crystallized, fine-grained kaolinite, associated with minor quantities of smectite. Some halloysite-like elongate particles were noted by SEM in the intermediate size fractions, minor amounts of quartz were identified in the coarsest size fractions, and < 1% noncrystalline material and traces of organic material were suspected in the finest size fraction. The crystallinity of the kaolinite as measured by XRD and IR varied moderately with size. IR suggested that nacrite-like stacking disorder increased with decreasing size for particles < 5 μm in size.

In the Sasso sample kaolinite dominated all size fractions and was accompanied by dickite in the coarse and by halloysite in the fine size fractions. Regular mixed-layer illite/smectite (I/S) was present in all size fractions and dominated in the finest. Abundant quartz and traces of alunite were identified in the whole rock and coarsest size fractions. The kaolinite in this sample showed marked variation in stacking order and crystallinity, as shown by changes in XRD, IR, and DTA patterns.

The observed compositional and structural variations in the size fractions of the Georgia sedimentary kaolin are small, as expected from formational environment, which was characterized by low temperatures and relative stable genetic conditions. The much more marked differences in composition within the size fractions of the Sasso hydrothermal kaolin are likely a result of the broad range of temperatures and fluid chemistry of its formational environment. The sequence dickite-well-crystallized kaolinite-kaolinite-halloysite is probably temperature-dependent.

The rate of sorption of methanol (MeOH), propan-2-ol (i-PrOH), and 2-methyl-propan-2-ol (t-BuOH) onto a Wyoming montmorillonite saturated with Al3+-, Cr3+-, or Fe3+-cations has been studied by isothermal gravimetry in the temperature range 18°–105°C using samples of differing weights and grain-size distributions. The rate of sorption for all the alcohols increased with decreasing sample and grain size, demonstrating that inter-, rather than intraparticle mass transfer was the rate-limiting process. Optimization of the sample parameters (2 mg sample of < 45-μm grain size, pretreated at 120°C yielded integral diffusion coefficients at 18°C of 1.1 × 10−4 m2/s for t-BuOH for the Cr3+-form and 2.0 × 10−14 m2/s for MeOH and i-PrOH for the Al3+-form. In general, the rate of alcohol sorption decreased as MeOH ≥ i-PrOH > t-BuOH, but no temperature dependence of the sorption rate was observed. The alcohol sorption rate was dependent on the cation present, with Fe3+ < Cr3+ < Al3+.

The rate of vapor-phase sorption of tetrahydrofuran (THF), tetrahydropyran (THP), and 1,4-dioxan (DIOX) onto a Wyoming montmorillonite saturated with Al3+ or Cr3+ has been studied by isothermal gravimetry in the temperature range 18° to 105°C using samples of different weights and grain-size distributions. The rate of sorption for all the cyclic ethers increased with decreasing sample and grain size, demonstrating that inter-, rather than intraparticle mass transfer was rate-determining. Optimization of the sample parameters (2 mg sample of <45-μm grain size, pretreated at 120°C yielded integral diffusion coefficients at 18°C of 0.5 × 10−14 m2/s for DIOX for the Cr3+-clay to 3.5 × 10−4 m2/s for THF for the Al3+-clay; however, no temperature or cationic dependence of the cyclic ether uptake was observed. In general, the rate of sorption of the cyclic ethers increased as THF ≥ THP > DIOX indicating that the sorption rate of THF and THP was dependent on concentration or that DIOX sorption was retarded by bidentate coordination to aluminum ions at the edges of the clay platelets.

During photochemical dissolution of goethite in acid/oxalate solution, Fe3+, Fe2+, and CO2 were released and towards the end of the reaction ferrous oxalate precipitated. The dissolution process involved an initial slow stage followed by a much faster reaction. The slow stage was eliminated by addition of 20 ppm Fe2+ to the system at the start of the reaction. The presence of this Fe2+ did not accelerate the secondary dissolution process. Both protons and oxalate ions appear to have been involved in the dissolution process. Dissolution was accelerated by an increase in oxalate concentration (from 0.0025 to 0.025 M) in the system and also depended on pH, reaching a maximum rate at pH 2.6. Highly substituted (15.9 mole % Al) goethite dissolved more slowly per unit area than unsubstituted goethite. Lepidocrocite (γ-FeOOH) dissolved faster than goethite. The first stage of the dissolution process probably proceeded by slow release of Fe3+ through complexation with oxalate adsorbed on the goethite surface. The faster, secondary step appears to have been a reductive dissolution reaction involving adsorbed ferrous oxalate.

Saponites and vermiculites may assume at least 11 ordered or semi-ordered layer stacking sequences. For a given relative humidity, the layer stacking type assumed is a function of the nature of the interlayer cation, the layer charge density, the mean size of the particles, and the di- or trioctahedral character of the sheets. For each interlayer cation, a succession of layer stacking types can be observed as relative humidity increases. For high relative humidity, some particular layer stacking types exist, but only for low-charge minerals. No other differences have been found for saponites and vermiculites in each successive layer stacking type. The degree of order that these layer stacking types imply is probably due to the existence of electrostatic bonds between hydrated interlayer cations and surface oxygens of the substituted tetrahedra. For octahedrally substituted 2:1 phyllosilicates, however, the disorder of the layer stacking sequences is related to a highly delocalized distribution of negative charges on the surface oxygens of the layers.

A study of the superstructures detected in saponites and vermiculites indicates that the interlayer cations tend to be located as far as possible from one another. The superstructures exist only with some cations and some layer stacking types and if the layer charge density is compatible with the charge produced by the cation distribution in this kind of superstructure.

A new approach to the interpretation of Mössbauer spectra of Fe3+-phyllosilicates having vacant trans-octahedra is based on (1) crystal structure simulation methods that allow for the size and the shape of a Fe3+-octahedron as a function of the nearest surrounding cations; and (2) calculations of electric field gradients (EFG) on Fe3+ in terms of the ionic point-charge model. Calculations were performed by direct summation within the region of radius ≤50 Å. Coordinates for the anions in the coordination octahedra have been assigned to take into account the nearest cationic environment. Atomic coordinates for the rest of the summation volume are those for the average unit cell. EFG calculations for cation combinations responsible for the visible quadrupole splitting Δvis in the spectra of nontronite, “red” muscovite, and celadonite have led to good agreement between Δvis and Δcalc. Computer fitting of the nontronite and celadonite spectra based on EFG calculations for the rest of the possible cation combinations suggests that the distribution of tetrahedral cations in nontronite obeys the Loewenstein rule, and in celadonite, the distribution of R3+ and R2+ over cis-octahedra is predominantly ordered, in agreement with electron diffraction and infrared spectroscopy data. The Mössbauer spectrum of one of the glauconites suggested the presence of celadonite-like and muscovite-like domains in its 2:1 layers.

The effects of chemical reduction of structural Fe3+ in nontronite SWa-1 (ferruginous smectite) on intervalence electron transfer (IT) and magnetic exchange were investigated. Visible absorption spectra in the region 800-400 nm of a chemical reduction series of the SWa-1 nontronite revealed an IT band near 730 nm (13,700 cm−1). Both the intensity and position of this band were affected by the extent of Fe reduction. The intensity increased until the Fe2+ content approached 40% of the total Fe, then decreased slightly with more Fe2+. The position of the band also shifted to lower energy as the extent of reduction increased.

Variable-temperature magnetic susceptibility measurements showed that the magnetic exchange in unaltered nontronite is frustrated antiferromagnetic, but ferromagnetic in reduced samples. Magnetic ordering temperatures are in the range 10–50 K, depending on the extent of reduction. The ferromagnetic component in the magnetization curve increased with increasing Fe2+ in the crystal structure. The positive paramagnetic interaction likely is due to electron charge transfer from Fe2+ to Fe3+ through such structural linkages as Fe2+-O-Fe3+ (perhaps following a double exchange mechanism), which is consistent with the visible absorption spectra.

Imogolite was synthesized at 25°C by aging partially neutralized solutions containing monomeric silicic acid and polymeric hydroxy-aluminum ions for 7 years. Solutions having an initial Si/Al molar ratio of about 0.5 and pHs of 4.0–4.5 produced the largest yields of imogolite, followed by those having an initial Si/Al ratio of about 1, although imogolite was not the principal product. Electron microscopic examination showed a small amount of imogolite fibers embedded in a noncrystalline gel-like substance. Traces of imogolite were detected in solutions having an initial Si/Al ratio of about 2, but no imogolite was found by electron microscopy in products from solutions having an initial Si/Al ratio of about 4. Only gibbsite formed from solutions having initial Si/Al ratios of <0.27. The diameter of the tubular structural unit of the imogolite produced in these experiments was 23 ± 2Å, close to that of natural imogolite.

Tobermorite, Ca5Si6O16(OH)2·4H2O, is a hydrous calcium silicate that has a layer-type of structure similar to that of the 2:1 clay minerals. In its natural form, tobermorite exhibits little or no exchange for alkali cations; synthetic tobermorites, however, exhibit reversible alkali cation exchange and selective cesium uptake upon a coupled substitution of (Al3+ + Na+) for Si4+. Substituted tobermorites were synthesized using aluminosilicate gels, NaOH, and CaO, in Parr bombs at 175°C for 4 days. Unsubstituted tobermorite was synthesized using quartz and CaO in a Parr bomb at 175°C for 20 hr. Two (Al + Na)-substituted tobermorites showed cation-exchange capacities (CEC) of 77 and 71 meq/100 g, whereas an unsubstituted tobermorite showed a CEC of 12 meq/100 g. The substituted tobermorites exhibited selective Cs exchange from either NaCl or CaCl2 solutions. For example, one substituted tobermorite showed a Cs-exchange coefficient (Kd) of 15,100, whereas unsubstituted tobermorite showed a Kd of only 90 from a 0.02 N CaCl2 solution containing 0.0002 moles/liter CsCl. Exchange isotherms for Na+ ⇋ Cs+ showed that Cs+ is preferred over Na+ throughout the exchange in the (Al + Na)-substituted tobermorites. This group of cation exchangers is expected to find applications in radioactive waste disposal.

Optically active cationic complexes adsorbed on montmorillonite can be used for the resolution of racemic mixtures. Montmorillonite-Cu-lysine systems were used as a solid phase in high-pressure liquid chromatography for the resolution of the optical isomers of α-amino acids. Selectivity constants > 1.5 were measured for phenylalanine and tryptophan. The selectivity constants for the amino acids containing saturated-hydrocarbon side chains were in the range of 1.25–1.44. The montmorillonite-Cu-l-lysine complex displayed a stronger affinity for the l-isomers of α-amino acids than for the d-isomers at pHs near neutrality. Inasmuch as surface-catalyzed peptide formation on clays has been proposed as a step in chemical evolution, this stronger affinity between the clay-Cu-l-amino acid complex and l-amino acids might have been significant in prebiotic evolution. The mechanism of optical resolution probably involved ligand exchange. Optimizing the choice of the optically active ligands and of the chelating cation in the chiral agent may improve the resolution of the optical isomers.