A review is presented of work carried out over the last 40 years on non-porous sorbents and on microporous shape-selective sorbents derived from palygorskite, smectite, and vermiculite. Among such materials four kinds of behavior were observed. In some systems uptake was restricted to external surfaces; in others, intercalation also occurred but only above threshold pressures. If the interlayer region was completely filled by long-chain organic cations, imbibition was possible, but in amounts which were very strongly dependent upon the cohesive energy density of the sorbate. Finally, in certain permanently expanded derivatives of layer silicates intercalation proceeded without any threshold pressure, just as in zeolites.

In this latter group, micropores existed which sometimes resulted in shape-selective sorption and molecule sieving. The micropores in clay minerals were modified by varying the size and shape of the interlayer cations, their charge, and the charge density of the siliceous layers. The interplay of these factors was investigated and the micropore sorbents were shown to be highly effective in the separation of mixtures.

Hydrotalcite-like compounds, described by the formula [Zn1-xAlx(OH)2][(CO3)x/2 · nH2O], were prepared by coprecipitation methods at 80°C and characterized by bulk chemical analysis, X-ray powder diffraction (XRD), nuclear magnetic resonance (NMR), and scanning-transmission electron microscopy (STEM). An x value of 0.33 was previously assumed to be an upper limit, but recently, Al-rich hydrotalcite-like compounds have been prepared with x as large as 0.44 by hydrothermal synthesis. In the Zn-Al system, Al-rich hydrotalcite was synthesized at normal pressure by coprecipitation. Zn-Al hydrotalcite-like compounds were obtained in the range of x = 0.3 to 0.4. An Al-rich hydrotalcite-like compound with x = 0.44 was formed in mixtures containing large amounts of a poorly crystalline Zn-Al phase. A continuous contraction of the hydrotalcite-like structure occurred as x increased, both the a and c lattice parameters decreasing for x values as large as 0.44. This study illustrates the advantages of using quantitative analytical electron microscopy with high spatial resolution to complement conventional (and bulk) characterization techniques for correlating structural and compositional characteristics of finely divided materials.

Observation of quadrupolar splittings for 2H and 17O nuclei from D2O solvent molecules in suspensions of the Ecca Gum BP bentonite points unambiguously to the ordering of water molecules with respect to a static magnetic field. The application of the magnetic field apparently orients the clay platelets as tactoids. Some water molecules adhere to the platelets and are oriented. The magnitude of the quadrupolar splitting depends strongly on the nature of the interstitial cations. The sign of the splitting changes as the ratio of divalent to monovalent cation increases, reflecting a switch in the microdynamics of water molecules next to the plates. With the entry of divalent cations, the clay becomes more cohesive due to the vertical electrostatic attraction of the counterions to the charged plates. More of these hydrated divalent ions compared with monovalent ions condense onto the charged sheet, thereby increasing metal coordination to those water molecules in contact with the clay surface. The dominant reorientation mode of the water molecules switches from rotation of water molecules around hydrogen bonds to the charged sheets to rotation around the electrostatic bond to the divalent metal cation. The effect is spectacular and important in understanding the chemical properties of cation-exchanged montmorillonites.

The oxygen isotopes of albite, quartz, and zeolites from the Hokonui Hills, New Zealand, constrain crystallization temperatures and the type of pore fluids present during diagenesis. A section of altered vitric tuffs in this region contains an extremely sharp reaction boundary between a heulandite-chlorite assemblage containing fresh detrital plagioclase and a laumontite-albite-quartz assemblage. A laumontite vein follows the local joint pattern and forms the reaction boundary, suggesting that laumontitization occurred as a result of fracturing and increased fluid flow during uplift. The albite (δ18O = +15.0)-quartz (δ18O = +19.9 to +20.5) geothermometer constrains the temperature of alteration between 145° and 170°C with a pore water δ18O of +1.8 to +3.5. The tuff was buried to an estimated maximum temperature of about 225°C, indicating that alteration occurred after maximum burial.

Framework oxygen was extracted from zeolites by reaction with ClF3 after the zeolites were thermally dehydrated in a vacuum. Laumontite was dehydrated at 300°C, and stilbite at 150°C. The precision of the method is typically about ±0.45‰. Fractionation curves for dehydrated zeolites are based on a general expression from the literature for feldspars, which depends only on the Si/Al ratio of the mineral. Measured δ18O values for laumontite in the groundmass of the altered tuff were +14.4‰. The laumontite-quartz pair constrains the temperature to between 139° and 162°C, in excellent agreement with the albite-quartz pair, and supporting the petrographic observation of co-crystallizing albite-laumontite.

Oxygen isotope values for fracture-filling laumontite in the vitric tuff, as well as those for groundmass and vein laumontite from other parts of the stratigraphic section, cluster around +14.5, suggesting that laumontite probably crystallized under similar conditions throughout much of the section. Oxygen isotope values for stilbite veins from various parts of the section indicate that this mineral crystallized at lower temperatures than the laumontite, for a given fluid isotopic composition, in agreement with the observed cross-cutting of laumontite by stilbite.

X-Ray powder diffraction and thin section analyses indicate that marginal lacustrine mudstones of the Green River Formation in the south-central Uinta basin, Utah, contain abundant analcime. The analcime has a low Si/Al ratio (<2.31) and occurs as very fine grained disseminated crystals and, to a lesser extent, as coarser-grained pore-filling cement. Analcime-rich mudstones and associated sandstones, siltstones, and carbonates lack volcanic detritus and zeolites other than analcime, thus making it difficult to support the concept that the analcime formed from precursor zeolites derived from volcanic glass altered in saline, alkaline-lake water. Abundant dolomite, syneresis cracks, and the absence of freshwater pelecypods and gastropods suggest that the lake (Lake Uinta) was moderately saline and alkaline. The restricted illite-illite/smectite clay mineral suite in the analcime-rich mudstones suggests that detrital clays significantly altered in a moderately saline and alkaline environment, thereby providing a source of Si and Al for the formation of analcime.

Red mudstones contain twice as much analcime as green mudstones (14 vs. 7 wt. %). Green mudstones have a day mineral suite consisting of illite (44 wt. %), mixed-layer illite/smectite (35 wt. %), smectite (12 wt. %), and minor kaolinite (4 wt. %) and chlorite (5 wt. %), whereas red mudstones have a more restricted day mineral suite consisting ofillite (68 wt. %) and mixed-layer illite/smectite (26 wt. %) with very minor smectite, chlorite, and kaolinite. Periodic minor fluctuations in lake level probably exposed large areas of shallow lacustrine-interdistributary green mud. Evaporative pumping on the exposed mudflats concentrated the moderately saline and alkaline-lake water, thereby producing Na-rich brines that enhanced the formation of analcime by accelerating the alteration of detrital clays and, perhaps, other minerals. Oxidation of iron from altered iron-bearing minerals stained the analcime-rich mud red with iron hydroxide or oxide (perhaps hematite). The overall reaction from green to red mud (mudstones) was probably: detrital phyllosilicates + Na-brine + iron-bearing minerals + oxygen → analcime + iron hydroxide or iron oxide.

A method using a finite, first-order Markov chain is presented to estimate rate constants for zeolite formation from experimental nuclear magnetic resonance (NMR) data on the abundance of different silica oligomers. An experimental design is suggested by which this method can be implemented. The method uses weighted least squares to estimate transition probabilities from aggregate NMR data. Rate constants, equilibrium constants, and free energies of elementary zeolite-forming reactions can be estimated. Hypothetical zeolite-forming reactions can also be modeled. An example of modeling, using hypothetical data, shows how zeolite formation can result from reactions involving mainly silica cyclic tetramers.

The structural damage produced by dry grinding and acid leaching of chrysotile was studied by transmission and scanning electron microscopy, infrared spectroscopy, X-ray powder diffraction, and thermogravimetric analysis. Severe dry grinding converted the chrysotile fibers into fragments having strong potential basic reaction sites. These sites were immediately neutralized by molecules present in the atmosphere (e.g., H2O, CO2). Acid leaching transformed the chrysotile fibers into very porous, non-crystalline silica, which was easily fractured into short fragments. The damage produced in the chrysotile structure by grinding or leaching was assessed by monitoring the intensity of various infrared absorption bands.

Volatiles formed on heating clay minerals contain a variety of cations and anions and are highly reactive. Gas chromatograph-mass spectrometer analysis of long-chain n-alkanes exposed to such volatiles showed that some decomposition occurred even <250°C. Reactions with anions donated by the volatiles led to compounds containing atoms other than C and H, e.g., Cl. The products obtained from n-alkanes heated in a stream of clay volatiles to 500°C resembled those produced by corresponding alkane-clay mixtures heated to 250°C. At higher temperatures contact between the alkanes and montmorillonite or halloysite led to changes in the assemblages formed, whereas kaolinite or sepiolite had no effect. Thus, the first reactions that occurred when clays were heated with n-alkanes were apparently induced by the volatiles; at higher temperatures catalysis due to direct contact came into play with some of the clay minerals.

The influence of Mn2+ on the formation of Fe oxides at pHs of 6.0 and 8.0 and varying Mn/Fe molar ratios (0, 0.1, 1.0, and 10.0) in the FeCl2-NH4OH and FeSO4-NH4OH systems was studied by X-ray powder diffraction (XRD), infrared absorption, transmission electron microscopic, and chemical analyses. In the absence of Mn2+, lepidocrocite (γ-FeOOH) and maghemite (γ-Fe2O3) were the crystalline species formed at pHs of 6.0 and 8.0, respectively, in the FeCl2 system, whereas lepidocrocite and goethite (α-FeOOH) and lepidocrocite were the crystalline species formed at pHs of 6.0 and 8.0, respectively, in the FeSO4 system. The amount of Mn coprecipitated with Fe (as much as 8.1 mole % in the FeCl2 system and 15.0 mole % in the FeSO4 system) increased as the initial solution Mn/Fe molar ratio increased from 0 to 10.0, resulting in the perturbation of the crystallization processes of the hydrolytic products of Fe formed. At pH 6.0, the perturbation led to the formation of poorly ordered lepidocrocite, as reflected in the increasing broadening of its characteristic peaks in the XRD patterns. At pH 8.0, poorly ordered iepidocrocite and a honessite-like mineral (Mn-Fe-SO4-H2O) formed in the FeCl2 and FeSO4 systems, respectively.

The effects of relative humidity (RH) on the expansion of Mg-smectite, equilibrated with low vapor pressure of ethylene glycol (EG), was studied. Four smectite samples were equilibrated with EG vapor from: (1) saturated vapor of pure EG, (2) an EG-CaCl2 solvate, and (3) 0.05 relative EG vapor pressure at 65°C. X-ray powder diffraction (XRD) analyses of the clays under different RH conditions indicated that without EG, the basal spacing of Mg-smectite samples could only be expanded to 16 Å, even at 0.97 RH. The basal spacing of Mg-smectite samples equilibrated with saturated EG vapor was expanded to 17.1 Å and was not significantly affected by RH in the range 0.1–0.9. The basal spacing of the Mg-smectite samples equilibrated with vapor from the EG-CaCl2 solvate and 0.05 relative EG pressure at 65°C was expanded by an amount that depended on the RH during the XRD analysis. The basal spacing increased from 14.2 to 17.1 Å as the RH increased from 0.6 to 0.75, except for sample API 23, which expanded to 17.1 Å at RH > 0.9. This sample did not expand beyond 16 Å when it was equilibrated with 0.05 EG relative vapor pressure. Sorption of moisture from air caused the one-layer EG-Mg-smectite complex (basal spacing = 14.1 Å) to rearrange itself to a double-layer EG-Mg-smectite complex (basal spacing = 17.1 Å). A small amount of the adsorbed EG in Mg-smectite, much less than was needed to cover a one-layer of the interlayer surfaces, caused an expansion of the basal spacing to 17.1 Å at high RH during the XRD analysis. The minimum amount of adsorbed EG which caused the one-/two-layer EG complex conversion was about 20–30 mg EG/g. The conversion was fast (<5 min) and was relatively reversible.

The resolution of the infrared spectra of structural OH in kaolin-group minerals increases with decreasing temperature. Poorly crystalline kaolinite gives typical infrared spectra, which suggest the existence of kaolinite-, dickite-, and nacrite-like configurations. The relative amount dickite- and nacrite-like configurations compared with kaolinite-like configurations increases with decreasing crystallinity. Low-temperature infrared spectroscopy is a more sensitive means of estimating the degree of order of poorly crystalline kaolinite than is X-ray powder diffraction.

The mechanisms of palygorskite and sepiolite alteration to smectite under mild hydrothermal conditions were investigated by solid-state 27Al and 29Si magic-angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy, X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Palygorskite altered to smectite in the presence of NaOH at 150°C. 27Al MAS-NMR spectroscopy showed that the Al coordination changed from chiefly octahedral in palygorskite to chiefly tetrahedral in the smectite product. 29Si MAS-NMR spectroscopy showed that the nearest neighbor environment of Si also changed when palygorskite altered to smectite. The XRD data showed that the synthetic smectite is trioctahedral in nature with tetrahedral charge. The TEM results revealed that the needle-like morphology of palygorskite was preserved in the product smectite. The MAS-NMR results in conjunction with the above XRD and TEM studies suggest that the mechanism of palygorskite alteration was a dissolution and recrystallization process rather than a solid-state reorganization to form 2:1 layer silicate units from the preexisting chain structure. Sepiolite altered to smectite in the presence of 2 N salt solutions at 300°C. The trioctahedral nature of the product smectite as detected by XRD and the foil-like morphology of product smectite as shown by TEM suggest that the mechanism of sepiolite transformation to smectite was also a dissolution and recrystallization process. The tetrahedral Al coordination detected by 27Al MAS-NMR in the smectite altered from sepiolite corroborated the XRD and TEM results.

The osmotic swelling of an n-butylammonium vermiculite in a 0.1 M solution of n-butylammonium chloride has been studied as a function of temperature and hydrostatic pressure by neutron diffraction. On application of a pressure of 1050 bar the vermiculite swelled macroscopically at 20°C, the c-axis spacing changing from 19.4 to 126 Å. The phase transition was completely reversible with respect to both pressure and temperature, and a complete study of the temperature-pressure phase diagram was made at pressures as high as 2000 bar. The heat capacity change with temperature across the swelling transition was measured at atmospheric pressure, and the enthalpy and entropy of the change from crystalline to osmotic phases were found to be, respectively, 5.2 J/g and 0.0183 J/K·g of dry clay. The combination of the entropy change with the gradient of the pressure-temperature phase boundary gave the volume change accompanying the transition. The total volume of the swollen phase was less than that of the crystalline phase plus the appropriate amount of solution, corresponding to a fractional decrease of about 0.1% in the water volume from bulk solution to between the plates.

The rate and extent of oxidation of dihydroxybenzenes (DHB) to quinones in acetate-buffered suspensions of synthetic birnessite were studied using Mn dissolution to monitor reaction progress. Concentration of free Mn2+ in the aqueous phase was continuously monitored by electron spin resonance, and ultraviolet-visible (UV-VIS) spectroscopy was utilized to quantify dihydroxybenzene and quinone concentrations. Although dissolution of the oxide and release of Mn2+ to solution generally accompanied phenol oxidation, a threshold oxidation level had to be exceeded before Mn2+ appeared in solution. Once this threshold was surpassed, the mole quantity of Mn2+ dissolved equaled the mole quantity of organic oxidized for 1,4-DHB, but exceeded the quantity of organic oxidized for 1,2-DHB. Thus, the latter phenol was more efficient in dissolving the oxide. Soluble phosphate suppressed Mn2+ release without influencing the degree of organic oxidation, suggesting that phosphate chemically interacted with reduced Mn to hinder its dissolution. UV spectra provided tentative evidence for the transitory existence of Mn3+-1,4-DHB complexes in the solution phase.

Infrared spectra of the birnessite after reaction with 1,4-DHB indicated some new features, which may have been a result of the reduction of surface Mn atoms to the 3+ oxidation state. These features were not present after reaction with 1,2-DHB, confirming that the latter phenol efficiently dissolved the oxide to release Mn2+. Although the initial Mn dissolution was very rapid and was attributed to a surface reaction, further slow Mn release accompanied by more complete oxidation of the phenols suggests a process limited by the rate of dissolution of the solid.

The treatment of smectites with a hydroxy-Al polymer produced chemically modified clays that had much greater affinities for chlorinated dioxins and biphenyls than untreated clays. For hydroxy-Al-treated kaolinite, the high affinity for chlorinated dioxins is thought to have arisen from an interaction between the hydroxy-Al polymer bound to the clay and the chlorinated dioxin in solution. Thus, the adsorption of dioxins from aqueous solution by hydroxy-Al-treated clays is similar to the adsorption of reactants from the gas phase by metals supported on mineral oxides during catalysis. In both systems, the essentially inert mineral oxide support effectively disperses the active adsorptive agent. At the concentrations of octachlorodioxin used in this study (≤5 ppb), hydroxy-Al-montmorillonite had a distribution coefficient of 90,000 (ml/g), corresponding to about 95% removal efficiency of the dioxin from solution per batch. For hexachlorohiphenyl, the distribution coefficient was 30,000 (ml/g), and the removal efficiency was 85%.