A rapid and simple test to distinguish halloysite from kaolinite in mineral mixtures has been developed based on differences in the rate and extent of formamide intercalation. With halloysite, complex formation was both rapid (<1 hr) and complete, whereas no significant intercalation occurred with kaolinite until at least 4 hr after contact with formamide, and then the process may not have been complete. Unheated halloysite formed complete complexes with formamide regardless of the interlayer hydration state of the mineral. The test, however, was inconclusive for halloysite that had been oven-dried at 110°C, although some water may still have been present in the interlayer space. The extent of formamide intercalation by kaolinite was likely influenced by sample crystallinity, and the rate of complex formation was enhanced by the addition of up to 10% v/v water to the system. Nevertheless, the formamide test unambiguously differentiated halloysite from kaolinite. N-methylformamide, which yields complexes with a basal spacing of 10.9 Å, could be substituted for formamide (basal spacing = 10.4 Å) for samples containing appreciable amounts of illite-mica.

The intercalation of formamide, potassium acetate, and hydrazine by halloysite and/or ka-olinite-rich samples, with and without subsequent displacement of the interlayer species by water or glycerol/water, has been investigated. Halloysite, as such, or in mixtures with kaolinite is completely expanded by all the treatments used, thereby enabling halloysite concentrations to be determined from the basal X-ray powder diffraction (XRD) peak ratios of the appropriate complexes. The values so obtained are usually proportional to the abundance of tubes, laths, and spherules in the transmission electron micrographs of the samples. The analysis of kaolin samples (halloysite plus kaolinite) by intercalation methods is less straight forward because a proportion of the kaolinite component in the system may not expand, even after lengthy (≥ 18 days) contact of the sample with the intercalating agent. Only prolonged immersion in hydrazine produces complete or nearly complete expansion of this component. When allowance is made for the presence of non-clay mineral components, kaolin-mineral percentages estimated from XRD peak intensity ratios of the hydrazine complexes generally agree with values derived from differential thermal analysis to within ±10%. Kaolinite in mixtures with halloysite cannot be directly determined by intercalation procedures inasmuch as treatments which result in complex formation with kaolinite also expand halloysite. In such systems, kaolinite can be estimated by difference between the concentration of kaolin minerals and halloysite.

A kaolinite-rich bed (tonstein) and an associated bentonite in the upper part of Yegua Formation at College Station, east-central Texas, were formed by in situ weathering processes in a late Eocene swamp. X-ray powder diffraction, infrared spectroscopy, petrographic studies, and scanning and transmission electron microscopy not only show that dioctahedral smectite and coarsely crystalline kaolinite are the dominant minerals in the bentonite and tonstein, respectively, but that cryptocrystalline halloysite and kaolinite are localized along the weathering front (transitional zone) between the tonstein and the bentonite. As weathering progressed, the cryptocrystalline minerals gradually recrystallized to yield the coarse books and vermicular growths of kaolinite characteristic of the tonstein.

Small amounts of cristobalite, sanidine, and euhedral zircon crystals with liquid or gaseous inclusions accord with the formation of the bentonite by alteration of volcanic ash. Clinoptilolite in the lignitic layer and sandstone below the bentonite probably formed from ions that were released during alteration of the volcanic materials to smectite, but clinoptilolite in the tonstein and overlying strata appear to have formed after kaolinization of the bentonite.

A material resembling what has been described in the literature as hisingerite has been identified in the joints of Cenozoic grey basalt near Geelong, Victoria. The material gives an X-ray powder diffraction pattern with weak broad peaks at 4.49, 2.58, and 1.53 Å and a chemical analysis of SiO2 = 34.2, TiO2 = 0.28, A12O3 = 4.16, Fe2O3 = 17.10, FeO = 3.52, MnO = 0.04, MgO = 5.19, CaO = 0.68, Na2O = 0.26, P2O5 = 0.05, H2O+ = 5.3, H2O− = 25.4, total = 96.18% (3.82% siderite impurity). Assuming a smectite model with 44 negative charges per unit cell, the chemical composition gives a unit-cell formula with −1.14 charges, corresponding to a cation-exchange capacity (CEC) of 135 meq/100 g. The same unit-cell charge is obtained when the structural formula is calculated on the basis of (Si + Al) = 8. The measured CEC of 60 meq/100 g indicates that the smectite structure may not apply to this hisingerite-material, and that in the alternative structure, all of the Al atoms may not be in the tetrahedral positions. Although the specific surface area of this material is between 662 and 758 m2/g (comparable to that of smectite), the infrared spectra do not show the characteristic OH vibrations, and the thermal analysis curves lack the dehydroxylation peak at 550–600°C expected for a smectite. Transmission electron microscopy shows indications of a layer structure and apparently hollow spherical bodies with ‘onion’ structure of 140 to 200 Å internal diameter, which might represent the sites of the large amounts of water contained by the material. The mode of occurrence and the nature of the hisingerite-material suggest precipitation from solution at relatively low temperatures.

Adsorption isotherms and UV-visible and Mössbauer spectroscopic data point to specific interactions between flavomononucleotide (FMN) and Fe3+-smectite. The maximum amount of FMN adsorption was 0.3 mmole/g of Fe3+-smectite giving a 1:1 molar proportion of Fe3+ and FMN. The results suggest a Fe3+-FMN complex residing at the smectite surface. Other homoionic smectites (Cu2+, Zn2+, and Ca2+) exhibited lower levels of adsorption and less apparent specific interaction.

To evaluate the importance of oxides to the surface chemistry of acid mineral soils, clay fractions were separated from a surface and subsurface horizon of an Inceptisol representative of many of the acid soils of the Southern Tier of New York state. Portions of the clays were treated to remove selectively noncrystalline and microcrystalline Fe and A1 oxides (acid ammonium oxalate extraction), total free iron oxides (dithionite reduction in buffered citrate solution), and organic matter (hypochlorite oxidation). Charge and ion-adsorption characteristics of the treated and untreated clays were investigated by means of Ca2+- and Cl−-exchange capacities, potentiometric titrations, and electrophoretic mobility (zeta potential) measurements of the CaCl2-treated clays.

Based upon surface area and anion- and cation-exchange measurements, the Fe and A1 oxides or oxideorganic matter complexes were found to contribute a large part of the surface area and pH-dependent charge of these clays. Oxide removal increased the cation-exchange capacity (CEC) and virtually eliminated the anion-exchange capacity (AEC) at pH 3 and 5.5 while shifting the positive zeta potential (ZPC) of the B-horizon clay toward negative values. Organic matter oxidation increased the AEC at pH 3 and the CEC at pH 5.5 and markedly shifted the ZPCs of both A- and B-horizon clays toward more positive values, probably by the removal of adsorbed organics from oxide surfaces. Estimates of the ZPCs of the clays varied among the three methods used, Ca2+- and Cl−-exchange capacities giving the lowest, and electrophoresis giving the highest values.

The negative surface charge of synthetic allophanes with a range of Si/Al ratios decreased and positive charge increased with increasing alumina content at a given pH. The phosphate adsorption capacity also increased with increasing Al content. That this relationship between composition and chemical reactivity was not found for the soil allophanes is attributed to the presence of specifically adsorbed organic or inorganic anions on the natural material. Both synthetic and natural imogolites had a much lower capacity to adsorb phosphate than the allophanes and adsorbed anomalously high amounts of Cl− and ClO4− at high pH. It is proposed that intercalation of salt occurs in imogolite, although electron spin resonance studies using spin probes failed to reveal the trapping of small organic molecules in imogolite tubes. These spin probes in the carboxylated form did, however, suggest an electrostatic retention of carboxylate by imogolite and a more specific adsorption by allophane involving ligand exchange of surface hydroxyl. The results illustrate the inherent differences in charge and surface properties of allophane and imogolite despite the common structural unit which the two minerals incorporate.

Adsorption of Cu2+ and Co2+ by synthetic imogolite, synthetic allophanes with a range of SiO2/ Al2O3 ratios, and allophanic clay fractions from volcanic ash soils was measured in an ionic medium of 0.05 M Ca(NO3)2. The effect of pH (and metal concentration) on adsorption was qualitatively similar for the synthetic and natural allophanes with relatively minor changes in behavior caused by variable SiO2/Al2O3 ratios. Cu and Co were chemisorbed by allophane at pH 5.0–5.5 and 6.9–7.2 (pH values for 50% adsorption level), respectively, with concomitant release of 1.6–1.9 protons/metal ion adsorbed. Quantitatively, adsorption by imogolite was less than that by the allophanes, presumably because of fewer sites available for chemisorption on the tubular structure of imogolite. Electron spin resonance studies of the imogolite and allophanes revealed that Cu2+ was adsorbed as a monomer on two types of surface sites. The preferred sites were likely adjacent AlOH groups binding Cu2+ by a binuclear mechanism; weaker bonding occurred at isolated AlOH or SiOH groups. These chemisorbed forms of Cu2+ were readily extracted by EDTA, CH3COOH, and metals capable of specific adsorption, but were not exchangeable. In addition, the H2O and/or OH− ligands of chemisorbed Cu2+ were readily displaced by NH3, with the formation of ternary Cu-ammonia-surface complexes.

The exchange of Na, K, and Ca ions in vermiculite has been studied between 50 and 150°C at a total normality of 0.1. The free energy changes were negative in the Na → K, Ca → K, and Na → Ca exchange reactions, and the cation preference in the vermiculite was Na < Ca < K in the temperature range examined. The enthalpy changes, calculated by application of the van't Hoff equation to the relation between equilibrium constant and temperature, were positive. The cation preference in the vermiculite seems to have been largely controlled by the increase in entropy which was due to the positive values of the entropy change term in the solid for the Na-K and Ca-K exchanges and the positive values of the entropy change in the solution for the Na-Ca exchange. From a thermodynamic analysis of the cation-mixing properties in vermiculite, the observed free energy change in the Na-K exchange was determined solely by the differences of the intrinsic electrostatic binding energy of the cations on the clay surface. That in the Na-Ca exchange was interpreted by taking into account an extra interaction energy of Ca-Ca pairs in addition to the intrinsic energy change. Furthermore, an additional interaction energy between K ions and the clay surface appears to have been added to the free energy change in the Ca-K exchange. In the Na → K and Ca → K exchange reactions a structural modification occurred in the K-equivalent fraction (X̄K) range 0.05–0.6, and a regularly interstratified phase of 15-Å and 10-Å members was formed at 0.05 < X̄K < 0.4.

An X-ray diffraction study of aqueous emulsions of a Na-montmorillonite shows that: (1) At low water content, the d-spacings of the montmorillonite increased stepwise with increasing water content; (2) At high water content, sharp (001) peaks due to regular stacking of montmorillonite layers were not detectable, but broad, halo patterns were observed in the low-angle scattering region; and (3) The addition of Ca2+ or H+ to the aqueous emulsions caused Ca-montmorillonite or H-montmorillonite crystals to form. A zig-zag column model of montmorillonite layers fits the observed data for aqueous emulsions of Na-montmorillonite.

The reaction of benzene with exchangeable Cu(II) and Fe(III) in hectorite clay films was studied by electron paramagnetic resonance (EPR). The reaction when carried out in a sealed tube between 60° and 100°C produced a variety of organic radical products. The nature of these products depended on the concentration of water in the reaction medium and the reaction time. The free radicals were accompanied by a reduction in the oxidation state of the metal ions; this process had a zero-order dependence on the metal ion concentration. The activation energies determined from the zero-order rate constants are 67 ± 11 kJ/mole (Cu(II)) and 46 ± 6 kJ/mole (Fe(III)). The free radicals initially formed under anhydrous conditions appeared to populate sites in the interlayer region; the activation energies determined for this first order process are 57 ± 9 kJ/mole (Cu(II)) and 42 ± 3 kJ/mole (Fe(III)). These activation energies are within experimental error of those determined from the metal ion kinetics. The EPR signal due to organic radicals showed little temperature dependence; thus, exchange processes involving organic free radicals were probably not important in determining EPR lineshapes.