Several detailed studies have been done on the characterization of organoclays and the type of structures developed when they interact with alkylammonium molecules. Few published contributions exist, however, on the distribution of surfactant within the organoclays and the mechanism by which they are intercalated. Also, although X-ray photoelectron spectroscopy (XPS) is a suitable technique for the study of the surface characteristics of organoclays, very few such XPS studies have been carried out. With the aim of contributing to a better understanding of the intercalation process, a series of organoclays was synthesized using a montmorillonite and the cationic surfactant hexadecyltrimethylammonium bromide (HDTMABr), with an increasing surfactant load of between 0.2 and 4.0 times the cation exchange capacity of the starting clay. By means of XPS, zeta potential, and thermal analysis techniques, distinguishing the strongly interacting fraction from the weakly interacting fraction of the adsorbed surfactant molecules was possible. Adsorption isotherms of each of these processes were constructed and then adjusted using the Langmuir and Dubinin-Radusquevich adsorption models. Three types of interaction between the surfactant and the clay were identified and described qualitatively and quantitatively. Two of these interactions, strong and weak, involved the hexadecyltrimethylammonium cation (HDTMA+). The third was a weak interaction involving the ion pair HDTMA+Br−. The results of this study may be useful for the comprehensive design of organoclays with specific physicochemical properties according to the application for which they are destined.

The reaction of gibbsite in various organic solvents at 250°–300°C under spontaneous vapor pressure of the solvents was examined. Glycols and aminoalcohols afforded the organic derivatives of boehmite in which one of the oxygen atoms of the glycol molecule or the alcoholic oxygen atom of aminoalcohol was incorporated into the boehmite layers. By increasing the molecular size of the solvent, the yield of the boehmite derivative decreased, and, at the same time, the basal spacing of the boehmite derivative increased. The product had a honeycomb texture on the surface of the particle, which suggests a dissolution-recrystallization mechanism for the formation of the boehmite derivatives. A hydroxyl group and a functional group, such as hydroxyl, methoxyl, or amino group having the ability to donate its lone pair electrons, were apparently necessary for the organic solvent molecules to form the boehmite derivative by this mechanism.

The pillaring of Na-montmorillonite with cationic oligomers of hydroxyaluminum (COHA) in the presence of an aqueous solution of polyvinyl alcohol resulted in the formation of a clay having a large surface area and pore volume. The pore-size distribution determined from a N2 adsorption/desorption hysteresis was narrow and centered at about 25 Å. The peak width at half height in the distribution curve was < 5 Å. As a result of delamination, the layer structure of the prepared clay was found from X-ray powder diffraction measurements to be lost. Short-range ordering, however, still existed in this delaminated clay, because exchangeable cations in the montmorillonite completely exchanged with Al3+, a requisite step for pillaring.

The order of adding the starting materials (Na-montmorillonite, polyvinyl alcohol, and COHA) greatly affected the surface area and the pore volume of the delaminated clay. Two orders of addition (Namontmorillonite, then COHA, then polyvinyl alcohol; and COHA, then Na-montmorillonite, then polyvinyl alcohol) gave no measurable surface area and pore volume. Two other orders of addition (polyvinyl alcohol, then COHA, then Na-montmorillonite; and polyvinyl alcohol, then Na-montmorillonite, then COHA) gave surface areas of 107 and 160 m2/g and pore volumes of 0.13 and 0.29 cm3/g, respectively. The amounts of the COHA solution and polyvinyl alcohol added greatly influenced the surface area and pore volume of the delaminated clay. Both properties increased monotonically with increasing amount of added polyvinyl alcohol, and increased to a maximum and then decreased with increasing amount of added COHA solution. The maximum surface area of the prepared delaminated clay was 330 m2/g.

A number of organobentonites were synthesized by exchanging the metal ions in bentonite with the cationic surfactant, cetyltrimethylammonium bromide. Samples of natural bentonite and organobentonites were analyzed for their organic carbon contents, examined by X-ray diffraction (XRD) for interlayer spacings, and viewed using a scanning electron microscope (SEM) for surface morphology. The sorption isotherm of benzene vapor from ambient air (relative humidity (RH) = 45 ± 5%) on natural bentonite was nonlinear; however, the isotherms of benzene from ambient air onto organobentonites were virtually linear over a large range of relative vapor concentrations. The sorption capacities of air-dried organobentonites were far greater than that of the natural bentonite. For air-dried organobentonites, the sorption coefficients correlated positively with the sample organic carbon contents and negatively with the sample BET-N2 surface areas. The heats of benzene vapor sorption onto air-dried organobentonites were less exothermic than the heat of benzene-vapor condensation. These findings suggest that benzene vapor sorption by air-dried organobentonites occurs essentially by vapor partition into the sample organic-matter fractions. This offers a potential application of organobentonites for the removal of organic vapors from flue gases and for assessing the efficiency of vapor removal.

The proposed method is a modification of one by Alexiades and Jackson (1965). Calcium exchange capacity (CaEC) and potassium exchange capacity (KEC) are determined, after removal of organic matter and free iron oxides, by saturating the exchange complex with centrifuge washings of pH 7 acetate solutions of Ca or K, respectively. Excess salt in solutions remaining in contact with the soil after saturation is determined by measuring the weight and concentration of the excess solution. The exchangeable cations and excess salt are then replaced by centrifuge washings with 1 N acetate solutions of Mg (for CaEC) or NH4 (for KEC), after overnight 110°C oven-drying to enhance K fixation for KEC. The replaced cations are determined and CaEC and KEC values are calculated. Per cent ‘vermiculite’ is based on the difference between CaEC and KEC (expressed in m-equiv/100 g) and an assumed ‘vermiculite’ in-terlayer exchange capacity of 154 m-equiv/100 g; percentage Vr = (CaEC-KEC/154) × 100. The ‘vermiculite’ interlayer fraction (VIF) of the CaEC may also be calculated; VIF= CaEC-KEC/CaEC. The measured ‘vermiculite’ is shown in quotation marks since the method is open to criticism regarding exactly what is being measured, the assumptions made, etc. and to emphasize that the determination procedure is an operational one for the characterization of cation exchange complexes.

Removal of free iron oxides increased both CaEC and KEC values of several soils but percentage Vr was little affected. The amount of K fixation was affected by the drying treatment employed after K saturation (none vs air-drying vs oven-drying). Thoroughly crushing Montana and African vermiculites dramatically increased their CEC and measured ‘vermiculite’ values, but had little effect with two samples of saprolite from chloritic metabasalt.

Sudoites exhibit different crystal-chemical and textural properties which may be related to the structural and valence state of Fe. Mössbauer spectroscopic analysis shows that all Fe previously analyzed using a microprobe (from 1 wt.% to 7.2 wt.% total Fe as Fe2O3) is structural and occurs in both oxidation states (40% Fe2+ and ∼60% Fe3+). Electron microprobe analyses from ∼200 sudoites indicate that Fe occurs in both octahedral sheets according to three main types of substitution: Fe3+ = octahedral Al; Fe2+ = Mg; and Fe3+ + Fe2++ □ = 3Mg. Decreasing tetrahedral substitution balances Fe3+ substitution in the trioctahedral sheet. Increasing octahedral Fe results in a more dioctahedral character of sudoite.

X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, differential thermal analysis (DTA) and scanning electron microscope-transmission electron microscope (SEM-TEM) analyses showed that increasing octahedral Fe is associated with decreased stacking order and thermal stability due to the greater number of defects. In addition, with increasing octahedral Fe in sudoite, particles became smaller and more anhedral and consequently less stable with increasing Fe content. These structural and textural variations are interpreted as a result of the distortion of the sudoite structure by substitutions of Fe3+ with larger ionic radii for Al and Mg octahedral cations and by the formation of octahedral vacancies.

Gibbs free energies for phases in the system Al2O3-SiO2-H2O have been calculated from reversed experiments in order to correct earlier values and to calculate a phase diagram consistent with more recent experiments. An internally consistent diagram could not be calculated that agreed with all published experiments, and choices of preferred data were made. The following Gibbs free energies, relative to the elements at STP (298.15 K, 1 bar), have been derived

The above values were calculated assuming literature values for corundum, quartz, and H2O (v).

Examination of available thermodynamic, experimental, and observational data on the aluminum hydroxides gibbsite, boehmite, bayerite, and nordstrandite suggests that these minerals are metastable with respect to diaspore + water at STP and at higher temperatures. Similarly, halloysite and dickite are metastable with respect to kaolinite at these conditions. The occurrence of these minerals in soils must therefore be ascribed to nonequilibrium processes, and the use of equilibrium phase diagrams to explain their occurrence is inappropriate.

Oxygen isotope ratios of <0.1-μm smectite in bottom sediments of the Mississippi River and the Gulf of Mexico near the mouth of the river have been determined to investigate diagenesis of land-derived clay minerals during sedimentation in the sea. No difference was detected in δ18O (SMOW) between the river and the Gulf samples indicating that no smectite alteration or addition of neoformed smectite to the river samples took place during sedimentation. Thus, authigenic minerals in the river sediments cannot make up more than a few tenths of a percent of the bulk sediments.

Similar results were obtained from 3 × 106-yr b.p. sediments buried to 80 to 600 m at Deep Sea Drilling Project site 323, Bellingshausen Abyssal Plain. No significant change with depth was noted in the δ18O of the <0.3-μm size fraction, mostly smectite, of these land-derived sediments. On the basis of the δ18O of the deepest sample, the maximum amount of authigenic minerals in the land-derived sediments during burial in the sea cannot be more than one or two percent of the bulk sediments. Hence, the alteration at seafloor temperatures of 25-45% of the <0.1-μm size clays in 3 × 106 yr b.p. sediments reported in a previous study is not substantiated. The data demonstrate that land-derived smectite is stable in the sea, and that oxygen isotopes can be used to investigate the modes and the temperatures of formation of authigenic smectites in marine sediments that are younger than 25 × 106 yr and that formed below 25°C.

The tropical weathering of sedimentary kaolin deposits from the plateaux surrounding Manaus (Alter do Chao formation, Amazon basin, Brazil) leads to the in situ formation of thick kaolinitic soils. The structural changes of kaolinite have been investigated quantitatively by infrared spectroscopy and electron paramagnetic resonance. Both techniques consistently show that each sample contains two types of kaolinite in various proportions. The progressive decrease in kaolinite order from the bottom to the top of the profile results from the gradual replacement of an old population of well-ordered kaolinite, typical of the underlying sedimentary kaolin, by a more recent generation of poorly ordered soil kaolinite. The vertical pattern of kaolinite replacement differs from that of the transformation of Fe oxides and oxyhydroxides previously observed in the same profile. The inherited fraction of well-ordered kaolinite ranges from 60% at a depth of 9 m to 30% in the upper levels of the soil. The persistence of sedimentary kaolinite in the upper horizons suggests that the rate of kaolinite transformation is relatively slow at the time scale of lateritic soil formation. Kaolinite inheritance unlocks the lateritic record of past weathering conditions.

Dark Fe oxides and sulfides are major discoloring impurities in mined commercial white kaolin clay. In order to evaluate the potential influence of Fe-cycle bacteria on Fe cycling during post-depositional clay-weathering alteration, Fe(III)-reducing and/or Fe(II)-oxidizing microorganisms were examined in open-pit, subsurface mine samples from kaolin lenses and smectite formations collected from sites in central Georgia. Samples of varying age were examined, including late Eocene smectite overburden, hard kaolin of Middle Eocene age, soft gray kaolin from the late Paleocene, and soft tan kaolin of late Cretaceous age. These clays contained 0.06–5.33% organic carbon, which included various potential organic electron donors for bacterial metabolism: formate (1.1–30.6 mmol/kg), acetate (0–40.5 mmol/kg), lactate (0–12.1 mmol/kg), pyruvate (0.4–78 mmol/kg), oxalate (0–141.7 mmol/kg), and citrate (0–1.4 mmol/kg). All clay samples studied had small concentrations of ‘bio-available’ Fe(III) (0.5 M HCl-extractable Fe, 0.5–2.8 mmol/kg) compared to total Fe (HF-extractable, 25–171.9 mmol/kg). The highest Fe(III)/[Fe(II)+Fe(III)] ratio and the lowest organic carbon content were in kaolin samples in which Fe(III) reduction was determined to be the dominant terminal electron accepting process by hydrogen analysis. All clay samples showed greater numbers of Fe(II)-oxidizing bacteria (22–22,000 cells/g) than Fe(III)-reducing bacteria (3–410 cells/g) as determined by MPN analysis. The Fe(III)-reducing activity in clays could be stimulated with the addition of 1 mM of the Fe(III) chelator, nitrilotriacetic acid. The addition of nitrate stimulated anaerobic Fe(II) oxidation. These results suggest that anaerobic bacteria involved in both oxidation and reduction of Fe exist in these subsurface clay formations, and might have had an influence on post-depositional weathering reactions.

Adsorption of p-hydroxybenzoate anion on synthetic Fe oxides, hydroxides, and oxyhydroxides (hereafter referred to as oxides) was measured at pH 5.5, and the organic-oxide interaction was characterized using diffuse-reflectance Fourier-transform infrared (DRIFT) spectroscopy. Surface complexes with ferrihydrite, hematite, goethite, and noncrystalline Fe oxide were investigated. Infrared (IR) spectra of the oxides after separation from p-hydroxybenzoate solutions showed the organic to be coordinated by an inner-sphere mechanism to the oxide surface through the carboxylate group. Bidentate binding of the carboxylate was identified to be the dominant type of complexation on the oxides. Although the IR study suggested that goethite formed the strongest surface iron-carboxylate bond, the total amount of organic adsorbed was the least on this oxide. This result suggested that bond energy was less important than adsorption site density in determining the amount of organic anion adsorption. Increasing ionic strength had little effect on adsorption by the noncrystalline Fe oxide, but dramatically decreased adsorption on hematite and goethite. This difference might have been due to anion competition for binding sites. At pH 5.5, the amount of organic adsorbed per unit weight of oxides (in 0.05 M NaClO4) followed the order: ferrihydrite > noncrystalline Fe oxide > hematite ≫ goethite. Adsorption per unit of surface area however, followed the order: ferrihydrite > hematite > noncrystalline Fe oxide ≫ goethite. The hematite adsorption reaction appeared to be driven by entropy, inasmuch as the reaction was endothermic. The difference among the oxides in surface reactivity toward p-hydroxybenzoate is hypothesized to have been caused by differences in both quantity and structural arrangement of reactive sites on the oxide surface.

Single grain X-ray and electron diffraction patterns of weathered biotite flakes exhibit groupings of 001 and hk reflections of biotite, vermiculite, mixed-layer clay minerals, and kaolinite indicating that the secondary minerals are in parallel crystallographic orientation to the parent biotite. Asterism of biotite reflections is enhanced by weathering. Gibbsite crystals developed in parallel basal orientation to biotite flakes. Most goethite in weathered biotite occurs as aggregates of randomly oriented crystals in cleavages and on grain surfaces. Some goethite is present on micaceous fragments as 0.05-μm size, lathlike crystals in a hexagonal arrangement with their (100) face resting on the (001) biotite face. Selected area electron diffraction patterns of aggregates of lathlike goethite crystals contain 0kℓ, 1kℓ, and 2kℓ reflections due to undulation of the aggregates and the extreme thinness of the crystals. These patterns indicate that the close packed anion layers in goethite coincide with the brucite-like layer of the micaceous minerals.

Electro-osmotic chemical treatment is an innovative method to improve the strength of soft clays for geotechnical engineering purposes; the effectiveness of the treatment may be related to treatment time, the concentration of the solutions injected, and to variation of pH in the clay. The objective of this study was to investigate the relationship between the above-mentioned factors and the improvement in strength when calcium chloride solution was used as an injection material. A series of tests was carried out by injecting different concentrations of calcium chloride solution into a kaolin suspension, for different treatment times, during electro-osmosis. After the tests, the pH, cone resistance, water content, and concentration of Ca2+ in the kaolin at different locations were measured and analyzed. The results show that the concentration of Ca2+ in the kaolin, the pH, and the strength were increased near the cathode with increases in concentration of CaCl2 and treatment time. An insignificant increase in strength, due to ion exchange over the entire specimen, for short treatment times of 2 to 24 h, was observed because of a small increase in concentration of Ca2+ and in pH. During long-term treatment (120 h), a considerable increase in concentration of Ca2+ (137.0 mg/g) and pH (pH = 10) was observed near the cathode. This led to a pozzolanic reaction, which in turn caused a significant increase in the mechanical strength of the kaolin.

Four Na2S2O4-reduced Na-vermiculites, each with some trioctahedral mica interstratified, were oxidized with H2O2 at pH 6·5 and again reduced with Na2S2O4 in suspensions at pH 7·5–8·0. The layer charge (CEC + K+), measured at pH 6·50, did not change significantly when octahedral Fe was oxidized (7–92 mmole 100g−1) or reduced (6–71 mmole 100 g−1). Electroneutrality was maintained within the octahedral sheet when Fe was oxidized or reduced. When Fe(II) was oxidized, electroneutrality was maintained by deprotonation of octahedral OH− groups,

(a)

${\left\{{\left[Fe\left(II\right)\right]}_{2}M{g}_4{O}_4{\left(OH\right)}_4\right\}}^{\pm 0}\leftrightarrows {\left\{{\left[Fe\left(III\right)\right]}_{2}M{g}_4{O}_4{\left(OH\right)}_2{O}_2^{\ast }\right\}}^{\pm 0}+2e^{-}+2H^{+}$${\left\{ {{{\left[ {Fe\left( {II} \right)} \right]}_2}M{g_4}{O_4}{{\left( {OH} \right)}_4}} \right\}^{ \pm 0}}\leftrightarrows{\left\{ {{{\left[ {Fe\left( {III} \right)} \right]}_2}M{g_4}{O_4}{{\left( {OH} \right)}_2}O_2^*} \right\}^ {\pm 0}} + 2{e^ - } + 2{H^ + }$

and by ejection of (dissolution of structural) octahedral metallic cations,

(b)

${\left\{{\left[Fe\left(II\right)\right]}_{5}Mg{O}_4{\left(OH\right)}_4\right\}}^{\pm 0}\to {\left\{{\left[Fe\left(III\right)\right]}_4{O}_4{\left(OH\right)}_4\right\}}^{\pm 0}+5e^{-}+F{e}^{3+}+M{g}^{2+}.$${\left\{ {{{\left[ {Fe\left( {II} \right)} \right]}_5}Mg{O_4}{{\left( {OH} \right)}_4}} \right\}^{ \pm 0}} \rightarrow {\left\{ {{{\left[ {Fe\left( {III} \right)} \right]}_4}{O_4}{{\left( {OH} \right)}_4}} \right\}^{ \pm 0}} + 5{e^ - } + F{e^{3 + }} + M{g^{2 + }}.$

When Fe(III) was reduced, electroneutrality was maintained by reprotonation of the deprotonated sites (O*, equation a). Reaction (b) was not reversible. Thus, reversibility of the reaction, Fe(II) ⇄ Fe(III) within the octahedral sheet decreased with increasing amount of ejected metallic cations. The amount of Fe(III) and Mg2+ ejected per Fe(II) oxidized was related to the degree of vermiculitization, being greatest with Na-degraded biotite [0·03 Fe3+ and 0·11 Mg2+ per Fe(II) oxidized] and lowest (nearly zero) with South African vermiculite. The number of deprotonated (O*) and reversible sites increased from 0·69 per Fe(II) oxidized with the K-depleted biotite to approximately 1·0 with South African vermiculite. The weathering increment was small since, of the total amount of Fe + Mg, less than 1·3 per cent was ejected from any of the four vermiculitic materials. When biotite was K-depleted, about 20 m-equiv of layer charge per 100g (300°C basis) was lost, while 51 mmole of Fe(II) per 100g was oxidized in the presence of Na2S2O4 and 82 mmoles in its absence in the aqueous suspensions. Since sequential reduction-oxidation-reduction treatments of K-depleted biotite and mica-containing vermiculites did not cause significant changes in layer charge (r2 = 0·04), the layer charge changes were concluded to be entirely independent of the oxidation or reduction of Fe in these minerals.

The occurrence of kerolite in association with various secondary Ca-Mg carbonate mineral deposits (speleothems) was identified in basaltic sea caves on the island of Kauai, Hawaii. Kerolite is the dominant clay mineral in the deposits. X-ray diffraction (XRD) peaks of the kerolite are characteristically broadened indicating its extremely poor crystallinity. Few changes were observed in the XRD patterns of this kerolite when it was subjected to various humidity, temperature and ethylene-glycol treatments. The crystals appear as flaky masses with irregular or jagged edges in scanning (SEM) and transmission electron microscopy (TEM). Electron probe and energy dispersive X-ray (EDX) microanalysis show that the clay material is dominated by Mg-Si-O, with minor amounts of Al and Ca in some samples. The chemical composition, thermal analysis and TEM observations suggest that smaller amounts of an amorphous serpentine-like phase are mixed with the kerolite. Kerolite is often the only mineral associated with poorly mineralized, actively-growing microbial mats in these caves and it is common in completely lithified microbial mats. The latter commonly have microstromatolitic structures with kerolite as a dominant phase. These features suggest that kerolite formation is at least in part a result of microbial activity. The abundant extracellular polymers of the mat-forming bacteria bind and concentrate ions (Mg2+, silica) from solution and serve as nucleation sites for kerolite precipitation. Conditions within the mats also probably lead to formation of Mg-Si-gels, amorphous Mg-silicate precursors and ultimately kerolite. Evaporation of the cave solutions may also contribute to kerolite formation.

Four sepiolite deposits of different origin, which occur near Las Vegas, Nevada, are compared and described. They include a playa deposit, a valley terrace deposit, and two types of vein deposits. The physical and chemical characteristics of the four materials are described, the engineering and commercial features are considered, and the varied origins of the sepiolite are discussed. X-ray powder diffraction data, infrared spectra, thermal characteristics, and chemical analyses are given for the Two Crows sepiolite. The vein deposit is proposed for use as a clay mineral reference standard because of its purity and availability. The apparent diverse morphology of the sepiolite deposits is shown with its common, lathlike basic structure.

The mineralogy of the clay fraction was studied for soils and saprolite on two Blue Ridge Front mountain slopes. The clay fraction contained the weathering products of primary minerals in the mica gneiss and schist parent rocks. Gibbsite is most abundant in the saprolite and residual soil horizons, where only chemical weathering has been operable. In colluvial soil horizons, where both physical and chemical weathering have occurred, the clay fraction consists largely of comminuted primary phyllosilicates —muscovite, chlorite, and possibly biotite—and their weathering products: vermiculite, interstratified biotite/vermiculite (B/V), and kaolinite. The clay-size chlorite contains Fe2+ as indicated by Mössbauer spectroscopy, and is more resistant to weathering than biotite. The vermiculite and B/V, both weathering products of biotite, contain Fe3+. Vermiculite in colluvial soils and, especially, surface horizons is weakly hydroxy-interlayered. The kaolinite in the clay fraction resulted at least partly from the comminution of kaolinized biotite in coarser fractions.

The hematite content ranged from 0 to 8% of the clay fraction and strongly correlates (r =.95) with dry clay redness, as measured by the redness rating: RR = (10 - YR hue) × (chroma) ÷ (value). The hematite is largely a product of the weathering of almandine; thus, the soil redness is dependent on the amount of almandine in the parent materials and its degree of weathering in the soils. Goethite (13–22% of the clay fraction) imparts a yellow-brown hue to soils derived from almandine-free parent rocks. The release of Fe in relatively low concentrations during the weathering of Fe-bearing primary minerals, particularly biotite, appears to have promoted the formation of goethite.

The crystal structure of synthetic deuterated kaolinite was refined by Rietveld analysis using time-of-flight (TOF) neutron powder diffraction data. For non-hydrogen atoms, Cl symmetry was assumed. Starting models were tested in which only the direction of O-D vectors was varied. The constraints were introduced to all Al-O, Si-O and O-D bonds. The refinement adopting the former gives Pl(C1), a = 5.169(1) Å, b = 8.960(2) Å, c = 7.410(2) Å, α = 91.26(2)°, ß = 104.99(2)°, γ = 89.93(1)°, Rwp = 3.17%, R1 = 5.78% and S = 1.34 with constraints of l(Al-O)= 1.93 ± 0.05 Å, l(Si-O)= 1.62 ± 0.03 Å and l(D-O) = 0.95 ± 0.15 Å. The inner O-D vector points toward the tetrahedral sheet. All inner-surface O-D groups form H bonding with basal O atoms in the next kaolinite layers. The results agreed with those obtained from natural kaolinite.

Fe(ClO4)3 solutions of different concentration and acidity were studied to search for the factors governing their appearance and stability during aging and to elucidate the mechanisms for the formation of amorphous and crystalline iron(III) hydroxides from slow hydrolysis. It was found that dilute solutions (0·001M or lower) rapidly hydrolyzed to clear sols after a brief induction period. With an increase in iron(III) concentration, one could notice gradual increases in the induction period, turbidity and particle size. Thus, solutions 0·006M to 0·01M in iron(III) developed into dense, cloudy, yellow suspensions impervious to light. The polymeric iron(III) species in the aged 0·02M solution were primarily large particles that settled under gravity, resulting in a suspension of low turbidity. It was also observed that in dilute solutions, the initial polymerization product was amorphous iron(III) hydroxide which yielded X-ray diffraction peaks for FeOOH only after prolonged aging or not at all. In 0·01 and 0·02M solutions, however, crystalline FeOOH was the major product from the very beginning of polymerization. The results suggest that the initial degree of supersaturation with respect to amorphous iron(III) hydroxide is the key factor governing particle size distribution, which in turn governs the appearance, stability and crystallinity of the hydrolyzed product during aging.

The addition of HClO4 to dilute Fe(ClO4)3 solutions decreased, whereas the addition of NaOH to concentrated solutions increased, the degree of supersaturation and, thus, induced changes in induction period, appearance, stability and crystallinity of the hydrolyzed product accordingly.

Particle–particle interactions in natural clays can be evaluated by their rheological behavior, but the results are often affected by the physicochemical properties of the clays. The behaviors of two fundamentally different types of clays (low-activity and high-activity) differ with respect to salinity and a time factor (duration of shearing at a given shear rate): illite-rich Jonquiere clay (low-activity clay, Canada) and montmorillonite-rich Wyoming bentonite (high-activity clay, USA). The purpose of the present study was to investigate these different behaviors. Most natural clays exhibit shear-thinning and thixotropic behavior with respect to salinity and the volumetric concentration of the solids. Natural clays also exhibit time-dependent non-Newtonian behavior. In terms of index value and shear strength, lowactivity and high-activity clays are known to exhibit contrasting responses to salinity. The geotechnical and rheological characteristics as a function of salinity and the shearing time for the given materials are compared here. The clay minerals were compared to estimate the inherent shear strengths, such as remolded shear strength (which is similar to the yield strength). Low-activity clay exhibits thixotropic behavior in a time-dependent manner. High-activity clay is also thixotropic for a short period of shearing, although rare cases of rheopectic behavior have been measured for long periods of shearing at high shear rates. The change from thixotropic to rheopectic behavior by bentonite clay has little effect at low shearing speeds, but appears to have a significant effect at higher speeds.