The cation-exchange capacities (CEC) of a previously well-characterized beidellite have been estimated nondestructively from X-ray photoelectron spectroscopy (XPS) data following exchange by various cations. The CEC obtained as an atomic ratio to silicon from Na- and Ca-clays are close to that derived chemically, i.e., 0.10(2), and 0.10(1), respectively, compared with 0.12(1). However, the apparent CEC obtained following K-, Pb-, and Ba-exchange are all about 50% greater. These measurements are complemented by XPS studies of clay exchanged in a solution containing both Ca and Ba ions where XPS gave a total apparent exchange capacity relative to Si of 0.16, with 0.10 from Ca and 0.06 from Ba. Bulk chemical analysis has, however, shown that the true Ba value is only 0.01. It is concluded that the cation excess detected by XPS—a technique with a total sampling depth of only about 100 Å—is concentrated at the surfaces of the clay particles.

Several dioctahedral clay minerals are related through reaction series. These series can be produced hydro-thermally from beidellite gel and montmorillonite by making simple changes in interlayer and solution chemistry. The series are:

• gel-K-beidellite-random illite/smectite-K-rectorite-illite

• K-montmorillonite-K-rectorite-illite

• Na-montmorillonite or beidellite-Na-rectorite-paragonite

• Li-montmorillonite-Li-tosudite-Li-rectorite-cookeite(?)

• Mg-montmorillonite-Mg-rectorite-tosudite-sudoite(?)

• Ca-montmorillonite-Ca-rectorite-margarite(?)

• Al-Ca-montmorillonite-kaolinite/smectite-kaolinite (150°C)

• Al-Ca-montmorillonite-pyrophyllite/smectite-pyrophyllite (320°C).

Assuming stability for the mixed-layer phases, paragenesis is a function of P, T, and X conditions. If the phases are considered to be metastable, paragenesis is a function of the speed and path of reaction.

Clay fractions of eight vertisols and vertisolic soils from Israel were found to consist principally of a Fe-rich beidellite. Sediment volumes of Na-clay suspensions, obtained in measuring cylinders and read every 24 hr for as long as 720 hr, ranged from 3.8 to 8.4 cm/l00 mg clay and were as much as 19 times larger than corresponding suspensions of Ca-clays. Optical density data for all clay suspensions showed absorption curves typical of smectite. The relative number of platelets per tactoid, calculated from optical density measurements, ranged between 1.4 and 5.4 for the Na-clays and between 7.4 and 14.1 for the Ca-clays. In the Ca-clays, the sediment volume decreased with an increase in the relative number of platelets per tactoid. With increase in the major dimension of particles (calculated also from optical density curves), sediment volume tended to increase for the Na-clays and decrease for the Ca-clays. These relationships can be explained on the basis of particle arrangement patterns: face-to-face arrangements dominated the Ca-clays and edge-to-edge and edge-to-face arrangements dominated the Na-clays. The amount of iron extractable in dithionite-citrate-bicarbonate (DCB) correlated positively with the relative number of plates per tactoid and with the major dimensions of the particles in the Ca-clay suspensions. This correlation suggests that DCB-extractable iron affects the tactoid dimensions of Ca-clays from vertisols and, therefore, may also affect structural properties of vertisols.

Pillaring of montmorillonite and beidellite with aluminum polyhydroxypolymer takes place first by the saturation of the cation-exchange capacity by monomeric and/or dimeric aluminum hydroxide species and then the intercalation of the so-called Al13-polyhydroxypolymer. The clay slurry must have a solid concentration greater than 0.01% (w/w) to produce a basal spacing of about 18 Å. Sizeable clay tactoids must therefore exist in the slurry in order to produce a turbostratic structure ordered along the c axis. The main difference between pillared montmorillonite and pillared beidellite seems to be a more ordered distribution of pillars within the interlamellar space of the clays that are rich in tetrahedral substitutions. Recent 27Al and 21Si high-resolution nuclear magnetic resonance data suggest that this higher degree of ordering results from the reaction of the aluminic pillars and the clay sheet near the sites of the tetrahedral substitutions.

A fundamental property of electrostatic potentials is their additivity. This study demonstrates that the electrostatic potential of a negatively charged, cation-substituted phyllosilicate layer can be represented as the sum of two potentials. Viewing cation substitution as a defect, one potential is derived from the atoms in a charge-neutral, unsubstituted layer such as pyrophyllite or talc. The “neutral-layer” potential rapidly decays to zero with distance from the layer and is determined primarily by the atoms in the first two atomic planes parallel to the (001) surface, i.e., the basal oxygens and tetrahedral cations. The second component, characterized as a “defect” potential, is a long-range potential derived from cation-substitution. The model used to compute the electrostatic potentials, a two-dimensional Ewald lattice sum, represents the atoms of a single phyllosilicate layer as point charges.

The rehydration properties of Ca-, Mg-, Na-, and K-saturated homoionic beidellites after heating at various temperatures were compared with those of montmorillonites. The behavior of interlayer Na+ during dehydration and rehydration was also investigated by means of one-dimensional Fourier analysis. The K- and Mg-saturated specimens exhibited fast and slow rehydration rates, respectively, during exposure to air of 50% RH after heating at 800°C. These homoionic specimens showed strong rehydration properties on saturation with deionized water after heating <900°C for 1 hr. On the basis of Fourier analysis, the interlayer cations appeared to have migrated into the hexagonal holes of SiO4 network on thermal dehydration, and the migrated cations returned to the interlayer space on rehydration. This behavior of the interlayer cations appears to have been strongly dependent on value of the octahedral negative charge and on the sizes of interlayer cations. The small octahedral negative charge of beidellite produced a weaker attractive electrostatic force between the octahedral sheets and the migrated interlayer cations. Therefore, the migrated interlayer cations in beidellite were easily extracted from the hexagonal holes, and rehydration was rapid. The small cation migrated easily into hexagonal holes and was fixed to the holes. On the contrary, large cations were probably difficult to fix and were easily extracted from the hexagonal holes. Consequently, the rehydration rate of K-saturated beidellite was fast, and that of Mg-saturated beidellite was slow.

Beidellite was synthesized hydrothermally from a noncrystalline gel at 320°C and 130 bar pressure. The beidellitic character of the product was verified by infrared spectroscopy on the NH4+-exchanged form. Intercalation was achieved with hydroxy-aluminum solutions having different OH/Al molar ratios. The solutions were investigated by several methods, including 27Al nuclear magnetic resonance. Essentially, two Al species were detected: monomelic Al and a polymerized form containing Al in four-fold coordination. This latter species was found to be selectively fixed in the interlamellar region, which resulted in a stable spacing of 18 Å at 110°C and 16.2 Å at 700°C. The pillared beidellites had specific surface areas of > 300 m2/g, mainly due to micropores. Both Brönsted and Lewis acid sites were evidenced by infrared spectroscopy using pyridine as a probe molecule.

Na-beidellite, a member of the smectite group, was grown hydrothermally from a gel of composition 0.35Na2O·2.35Al2O3·7.3SiO2 in NaOH solutions at a pH between 7.5 and 13.5, a pressure of 1 kbar, and a temperature of 350°C. The synthetic Na-beidellite was characterized by means of scanning electron microscopy, X-ray powder diffraction, infrared spectroscopy, electron microprobe, inductively coupled plasma-atomic emission spectroscopy, and thermogravimetric analysis. The unit-cell parameters of the orthorhombic cell are: a = 5.18, b = 8.96, and c = 12.54 Å. The cation-exchange capacity was determined to be 70 meq/100 g. A maximum of 40 wt. % water was present and reversibly lost by heating to about 55°C. The loss of water caused a decrease of the basal spacing to 9.98 Å. At temperatures ≥600°C, the Na-beidellite started to dehydroxylate, reaching its maximum in the range 600°–630°C. At 1100°C the remaining solid recrystallized to Al6Si2O13 (mullite) and SiO2 (cristobalite).

The coarse (0.4–2 µm) clay fraction of an Albian black shale collected in the Atlantic Ocean (Deep Sea Drilling Project leg 11) consists chiefly (90–95%) of smectite and 5–10% illite. Both minerals are locally surrounded by overgrowth structures, such as fine laths about 0.05–0.4 µm long and 0.02–0.1 µm wide. Individual laths or assemblages of laths protrude from the center of smectite flakes at angles of about 60° to each other. Laths occur around illite crystals in a similar manner or coalesce into a rim that consists of 0.05–0.1-µm-size particles. On the basis of scanning transmission electron microscopy: (1) the center of individual illite crystals consists of a dioctahedral mineral, but the overgrowth structures are Al-Fe beidellites; and (2) the smectite flakes have highly variable compositions, but correspond chiefly to Fe-Al-beidellite, whereas the overgrowths are compositionally close to montmorillonite.

The overgrowth structures seem to have formed during early diagenesis. The chemical composition of overgrowths around illite and smectite tend to be similar in response to the new environment, implying an addition of silica to both materials.

Multivariate analysis of variance and discriminant analysis were used to establish the crystal chemistry of several Al-rich smectites. The statistical analyses were carded out on 78 samples taken from the literature which were classified on the basis of their physicochemical properties. A strong discrimination exists between beidellites and montmorillonites, ‘non-ideal’ montmorillonites and ‘ideal’ montmorillonites, and Wyoming-type and Cheto-type montmorillonites. Of the Cheto-type montmorillonites, the Tatatilla-type samples are strongly discriminated, whereas the distinction between Chambers- and Otay-types is not strong. AlIV, AlVI, Fe, Mg, and Ca are generally important discriminating variables, whereas the tetrahedral portion of the layer charge, commonly used as a discriminating factor among these minerals, is only moderately significant.

A method using Li saturation and heating to 250°C to differentiate montmorillonite from beidellite and nontronite has been developed. The test utilizes three washings with 3 M LiCl and two washings with 0.01 M LiCl in 90% methanol to prevent dispersion. An 'infinitely thick’ sample (6–8 mg/cm2) on a glass slide is used to avoid the effects of the reaction of a thin clay film with sodium of the slide when it is heated at 250°C. Solvation with glycerol rather than ethylene glycol is used, because all of the Li smectites studied expanded to some extent in ethylene glycol after the heating. The smectites included several montmorillonites, a nontronite, and saponites. The presence of interstratified montmorillonite and beidellite layers was clearly shown by the test for several smectite samples, including the so-called beidellites from Beidell, Colorado, and Chen-yuan, Taiwan, and several soil clays. The test thereby provides more mineralogical information than the often-used arbitrary dividing point between montmorillonite and beidellite at 50% tetrahedral charge. Heating the Li-saturated clays at 250°C caused substitution of 35 to 125 meq/100 g of nonexchangeable Li. These amounts exceeded the changes in cation-exchange capacity plus Li by 4 to 21 meq/100 g, except for the end-member beidellite from the Black Jack mine, Idaho. Fusion with LiNO3 at 300°C could not be used to differentiate between smectites instead of washing with LiCl solution and heating to 250°C, because fused montmorillonite subsequently expanded to 18 Å with glycerol. Large increases in nonexchangeable Li were caused by the fusion of smectites, a vermiculite, and two partially expanded micas.

The fine silt (2–5 μm) coarse clay (0.1–2 μm) and fine clay (<0.1 μm) fractions of a Vertisol from South Italy were studied with X-ray diffraction. The most reactive fine clay (<0.1 μm) fraction was investigated in detail using a curve decomposition method analysis of X-ray diffraction diagrams, FTIR spectroscopy and chemical analysis. In the soil parent material, the fine clay fraction was dominated by low-charge smectites (beidellite and montmorillonite) whereas, high-charge beidellite was the dominant clay mineral in the fine clay from the upper soil horizons. This suggested that high-charge beidellite was produced through alteration of the preexisting low-charge smectites and was the stable clay phase in this soil environment, characterized by high pH (>8.0). After K-saturation and 25 wetting and drying cycles, the high-charge beidellite from the soil horizons lost expandability far more than the original low-charge smectites.

A number of smectitic samples from Almeria (SE Spain) were studied by chemical analysis, DTA, TG, XRD (oriented aggregates with ethylene glycol treatment and Greene-Kelly test), FTIR and MAS NMR. Chemically they resembled a beidellite-montmorillonite series, displaying DTA/TG characteristics already quoted in the literature in beidellite descriptions. They did not swell after the Greene-Kelly test, as it has also been reported for some beidellites. Nevertheless XRD of the oriented, glycolated samples, FTIR, MAS NMR and revision of the chemical analysis demonstrated that they were mixed-layered kaolinite/montmorillonite. It is possible that some of the reported beidellites in the literature are kaolinite/montmorillonite. Beidellite characterization must be supported by several different techniques.

A new method for the prediction of Gibbs free energies of formation for hydrated clay minerals is proposed based on the parameter ΔGO= Mz+(clay) characterizing the oxygen affinity of the cation Mz+. The Gibbs free energy of formation from constituent oxides is considered as the sum of the products of the molar fraction of an oxygen atom bound to any two cations multiplied by the electronegativity difference defined by the ΔGO= Mz+(clay) between any two consecutive cations. The ΔGO= Mz+(clay) value, using a weighting scheme involving the electronegativity of a cation in a specific site (interlayer, octahedral, or tetrahedral) is assumed to be constant and can be calculated by minimization of the difference between experimental Gibbs free energies (determined from solubility measurements) and calculated Gibbs free energies of formation from constituent oxides. Results indicate that this prediction method compared to other determinations, gives values within 0.5% of the experimentally estimated values. The relationships between ΔGO= Mz+(clay) corresponding to the electronegativity of a cation in either interlayer or octahedral sites and known ΔGO= Mz+(aq) were determined, thereby allowing the prediction of the electronegativity of transition metal ions and trivalent ions in hydrated interlayer sites and octahedral sites. Prediction of Gibbs free energies of formation of any clay mineral with various ions located in the interlayer and with different cations in octahedral sites is possible. Examples are given for Al-rich montmorillonite from Aberdeen, transition element-exchanged montmorillonite, and Ni-rich stevensite, and the results appear excellent when compared to experimental values.

Recent pedological evidence of the widespread distribution of beidellites in soils indicates the need for a greater knowledge of the effect of charge location on the microstructural organization of Na+ smectite in gels. After equilibration at a suction pressure of 3.2 kPa before and after desiccation, TEM observations showed large differences between a beidellite and both a low and high charge montmorillonite. Monolayers were rare; individual layers were instead organized in particles with larger interparticle distances. This has implication for theories relating swelling pressures to interlayer distances or surface areas and implies the need for a geometrical approach to the study of swelling in smectites. Location of isomorphous substitution in the tetrahedral sheet of smectites results in an increased lateral extension of overlapping layers. This was reflected in a greater capacity to rehydrate after desiccation. Increased number of layers in particles were found with increasing surface charge density. The geometric organization of the particles is critical to the understanding of the ability of Na+ smectite to hold water against an applied suction.

The clay mineralogy of an oxisol-saprolite overlying serpentinite and underlying basalt was studied with different techniques to evaluate the clay mineral transformation that occurred and to understand the origin of Fe3+-rich smectite. The saprolite zone of the oxisol, up to 30 m thick, contains smectites of the montmorillonite-beidellite and montmorillonite-nontronite series, as well as illite, chlorite, talc, and goethite or amorphous oxyhydrates. Illite is mainly concentrated in the upper 50 cm thick zone underlying the basalt layer and chlorite-content increases toward altered serpentinite at the base. Minor amounts of nontronite formed mostly toward westward exposures where the hot contact layer between serpentinite and basalt is only 20 cm thick. Greene-Kelly Li-tests revealed that all samples contain montmorillonite, but one sample shows the presence of a minor amount of beidellite.

Parent rocks are a mixture of mainly mica schist (the source of beidellite), and minor serpentinite in different percentages and laterally distributed. These rocks were intensely weathered under humid climatic conditions. Silica was concentrated as amorphous transparent (pure silica) cobbles and milky quartz pebbles, and originated from geothermal solutions rising through the Ovaclk thrust fault. The Mg partly formed chlorite. Ferrian smectites in serpentinites were derived obviously from the Mg-rich minerals but Mg is lost much more rapidly than Si during the formation of the clay deposit. The structural formula of the most Fe-rich smectite samples from the study area is (Si6.60−7.10Al1.40−0.90)(Al2.54−1.22Mg0.32−0.92Fe3+1.18−1.68−Ti0.06−0.04)(Ca0.16−0.10Na0.02K0.02−0.12)O20(OH)4. This composition is within the range recorded for the ferrian montmorillonite-beidellite series, with very little vermiculite forming the oxisol-vertisol horizon.

The dehydration and migration of the interlayer cation of the synthetic beidellite Na0.7Al4.7Si7.3O20-(OH)4·nH2O, were studied with solid-state 23Na and 27Al MAS-NMR, heating stage XRD, and thermogravimetric analyses (TGA, DTA). The 23Na MAS-NMR of Na-beidellite at 25°C displays a chemical shift of 0.2 ppm, which indicates a configuration comparable with that of Na+ in solution. Total dehydration proceeds reversibly in two temperature ranges. Four water molecules per Na+ are gradually removed from 25° to 85°C. As a result, the basal spacing decreases from 12.54 Å to 9.98 Å and the Na+ surrounded by the two remaining water molecules is relocated in the hexagonal cavities of the tetrahedral sheet. The chemical shift of 1.5 ppm exhibited after the first dehydration stage illustrates the increased influence of the tetrahedral sheet. The high local symmetry is maintained throughout the entire first dehydration stage. During the second dehydration, which proceeds in a narrow temperature range around 400°C, the remaining two water molecules are removed reversibly without any change of the basal spacing.

Ferruginous beidellites with Al:Fe atomic ratios up to 2.36 were obtained when solutions containing Al, Fe2+ and H4SiO4 were adjusted to pH 8.5 with Ca(OH)2 and incubated at 95 °C in the presence of CaCO3 as a pH buffer. Incubation took place under cyclic reducing and oxidizing conditions achieved by adding 2 mM hydrazine at 14–15-d intervals over a period of 10–13 weeks. During the 14–15-d cycle, atmospheric oxygen slowly diffused through the high-density polyethylene bottles used, causing a slow oxidation of Fe(II) to Fe(III). The infrared (IR) spectra of the products approached that of natural beidellite, but indicated little change in octahedral Al:Fe ratio in the products for starting Al:Fe ratios from 2.5 up to 3.5, which was the highest Al:Fe ratio at which a well-crystallized product was obtained. Chemical analysis showed the presence of more Al+Fe in the products than could be incorporated into a dioctahedral formula. After the excess was assigned to a hydroxy-aluminium interlayer, the formula of the most Al-rich beidellite was calculated to be 0.575Ca(Si6.85A1.15)(Al2.47Fe1.53)O20(OH)4. This composition lay within the range recorded for the ferruginous beidellites that form in Vertisols.