Zn- and Ca-adsorption equilibria of five Ca-saturated halloysite samples were measured at equilibrium Zn concentrations up to 10−2 M in 10−4 to 10−2 M CaCl2. The results were interpreted on KCaZn vs. [Zn]/CEC (%) plots, where KCaZn is the selectivity coefficient (= [Zn][Ca]/[Ca][Zn]), Zn and Ca represent the adsorbed species, and CEC is the cation-exchange capacity. All Zn adsorption occurred at cation-exchange sites, and 0.77 to 36.0 meq Zn/100 g clay, which constitutes 9 to 83% of the CEC, was adsorbed with “high selectivities” (KCaZn > 10). The higher values were found for two spherical and one “filmy” halloysites, whereas the lower values were found for two tubular halloysites. The magnitude of their 001 intensity, hydration in interlayer space, CEC, and “free” iron oxide content did not correlate with the selective Zn adsorption, but a good correlation was found between the proportion of “high selectivity” sites for Zn and proportion of “high affinity” sites for H+. The adsorption of Zn at the “high selectivity” sites was not completely reversible, and KCaZn values >1000 were recorded in 0.5 M CaCl2 for Zn which occupied 10–40% of the exchange sites. Selective Zn adsorption decreased with decreasing pH, and all adsorbed Zn was extracted with 0.1 M HCl.

Samples containing allophane with molar Al/Si ratios from 1.0 to 2.0 have been examined by infrared spectroscopy, X-ray fluorescence, and phosphate adsorption methods. The infrared spectra of allophane with Al/Si ratios close to 2.0 showed that the wall of the allophane spherules is made up of imogolite structural units similar to “proto-imogolite.” X-ray fluorescence gave no clear evidence of Al in tetrahedral sites (AlIV), while pyridine adsorption results suggested that a small number of Bronsted acid sites (AlIVOH) are present in silica-rich allophanes. Lewis acid sites (AlH2O) are present in both silica-rich and alumina-rich allophanes. The results suggest that the framework for the allophane structure is an Al octahedral sheet.

Allophanes with Al/Si ratios close to 1.0 contain condensed silicate units either on the outside surface of the Al octahedral sheet, giving rise to a halloysite-like structure, or bonded on the inside surface of an imogolite-like structure. Allophanes with Al/Si ratios between 1.0 and 2.0 appear to be mixtures of the “proto-imogolite” structure and the allophane (Al/Si = 1.0) structure.

The pH, Eh, electrical conductivity (EC), and the amounts and valency of replaceable iron were measured periodically on Fe2+- and Fe3+-saturated montmorillonite and cation-exchange resin at three temperatures. Differences in the pattern of change of pH, Eh, and EC with time appear to be related more to the histories and modes of preparation of the systems than to intrinsic differences in the hydrolysis of the iron in them. Electron transfer reactions involving crystal components of the clay can cause oxidation of adsorbed Fe2+ ions; the activation energy (Ea) for oxidation on the clay's surface was 6 kcal/mole, less than a third of the activation energy reported for Fe2+ oxidation in solution. In the Fe2+-resin, where Ea = 10.7 kcal/mole, perturbed surface-water molecules may act as electron acceptors enhancing Fe2+ oxidation.

Polymerization and precipitation of the adsorbed iron is affected by the necessity to maintain electroneutrality, the ability of the iron-hydroxy ions and small polymers to move about in the voids of the ion exchanger, and the steric hindrance posed by the matrix of the ion exchanger to the formation of large polymers. In resin, little or no iron precipitates, probably due both to steric hindrance and the inability of the resin to release ionic components to maintain electroneutrality. In clays, steric hindrance is small, and Al and Mg are released from the crystal to maintain electroneutrality, thus the precipitation of iron is abundant and is controlled by the rate of release of Al and Mg from the crystal.

Vernadite (MnO2·nH2O) is a mineral with a poorly ordered structure. Its synthetic analogue is designated δ-MnO2. Birnessite and vernadite are independent mineral species and cannot be described further under the same name. They have similar hexagonal unit-cell parameters, a0, but different c0 parameters. Rancieite has a structure similar to that of birnessite. Calcium bearing, 14-Å birnessite occurring in nature was first described by the authors. In addition to the todorokite having the parameters a0 = 9.75 Å, b0 = 2.84 Å, and c0 = 9.59 Å, other species of natural todorokite are known having a0 parameters that are multiples of 4.88 Å equal to 14.6 and 24.40 Å, the b0 and c0 parameters being the same.

Mössbauer spectroscopy of dioctahedral phyllosilicates showed that on dehydroxylation iron which originally occupied M(2) and M(l) sites became, respectively, 5- and 6-coordinated. The 6-coordinated sites are very distorted. No migration of cations occurs in the course of heating the specimens for 1–3 hr at 600°–700°C.

By using a combination of several physicochemical methods, different successive stages of the dehydroxylation process could be distinguished: (1) migration of protons; (2) localized dehydroxylation of individual associations without significant change in the overall configuration of the octahedral sheets; and (3) loss of most of the hydroxyl groups with concomitant changes in the cell dimensions. Penetration of Li into the octahedral sheets does not affect the course of the reaction, but reduces the dehydroxylation temperature and the stability of the products.

Dehydroxylation was preceded by or associated with the oxidation of any divalent iron present. Fe3+ derived from Fe2+ was indistinguishable by Mössbauer spectroscopy from iron initially present in the trivalent form. High concentrations of Fe lower the dehydroxylation temperature and reduce the stability of the dehydroxylate to the extent that partial disintegration may precede complete dehydroxylation.

The basal spacings of montmorillonites with Li, Na, K, Cs, Mg, Ca, Sr, Ba, and Pb interlayer cations were measured after immersion in dimethylsulfoxide (DMSO) and in water-DMSO mixtures. In DMSO alone, the spacings were in the range 18.3–19.3 Å and fell on or near a single curve when plotted against ionic field strength, q/r2, where q = valence and r = ionic radius. These spacings correspond to double layers of DMSO molecules between the silicate layers. Water had practically no effect on the spacings when the mole fraction of DMSO exceeded about 35–45%. Osmotic swelling of Li-, Na-, and K-mont-morillonite occurred up to mole percentages of DMSO 45%, 30%, and 10%, respectively. K- and Cs-mont-morillonite formed single-layer complexes in appropriate water-DMSO mixtures with spacings of 14.3 Å prior to development of double-layer complexes when the mole fraction of DMSO exceeded 35% and 15%, respectively.

Thirty two boehmites, synthesized at temperatures ranging from room temperature to 300°C, were examined by scanning electron microscopy, transmission electron microscopy, electron diffraction, X-ray powder diffraction, differential thermal analysis, and infrared spectroscopy. The results show that boehmite exhibits a continuous gradation in crystallite size ranging from single octahedral layers or a few unit cells to about 65 unit cells in the y-direction. This conclusion suggests that the term pseudoboehmite is inappropriate for finely crystalline boehmite. Finely crystalline boehmite contains more sorbed water than coarsely crystalline boehmite; this water is commonly intercalated between octahedral layers, usually randomly but sometimes regularly. The regularly interstratified boehmite gives rise to a diffuse “long spacing” X-ray diffraction reflection. Calculated 020 X-ray diffraction peaks approximate closely those observed experimentally when a range of crystallite sizes is taken into account.

Organic diacid (oxalic and succinic) adsorption onto montmorillonite is feasible, but weak (~1 mg/g). The comparison of chemical and radiochemical determinations reveals that 80% of the acid in contact with the smectite is used to attack the clay lattice. The pH is the main parameter involved in adsorption, and fixation passes through a minimum for pH 6 to 7. Polyacrylate adsorption is also weak (~1.5 mg/g). It changes with the nature of the exchangeable cation of smectite. Its pH-dependence displays a pronounced maximum for a value corresponding to the pKa of the acidic functions (pH ~6.8), and a minimum at about pH 8. On the assumption that a polyacrylate macromolecule is 100% hydrolyzed, it follows that the-COOH groups carried by 20% hydrolyzed Polyacrylamide molecules (such as those used in the tertiary recovery of petroleum) contribute at the very most to 10% of the total adsorption onto clay. Fixation, therefore, involves predominantly protonation of the amide functions at the edge surfaces of the clay. The acidic functions play a minor role in the adsorption phenomenon in that they affect the length of the macromolecule. The extent of this contribution, however, is virtually impossible to estimate.