The terms “hydrophobic” and “hydrophilic” are typically used in a non-specific sense and, as such, they have a limited utility. Surface thermodynamic theory, as described here, allows a natural and potentially powerful definition of these terms. The boundary between hydrophobicity and hydrophilicity occurs when the difference between the apolar attraction and the polar repulsion between molecules or particles of material (1) immersed in water (w) is equal to the cohesive polar attraction between the water molecules. Under these conditions, the interfacial free energy of interaction between particles of 1, immersed in water (ignoring electrostatic interactions), exactly zero. When is positive, the interaction of the material with water dominates and the surface of the material is hydrophilic; when is negative, the polar cohesive attraction between the water molecules dominates and the material is hydrophobic. Thus, the sign of defines the nature of the surface and the magnitude of may be used as the natural quantitative measure of the surface hydrophobicity or hydrophilicity.

The smectite-to-illite conversion during shale diagenesis has recently been used to constrain the estimate of a basin's thermal history. We have systematically investigated the kinetics for the conversion of a Na-saturated montmorillonite (SWy-1) to a mixed-layer smectite/illite as a function of KCl concentration (from 0.1 to 3 moles/liter) over a temperature range of 250° to 325°C at 500 bars in cold-seal pressure vessels using gold capsules. The results show that the conversion rate can be described by a simple empirical rate equation

-dS/dt = A · exp(-Ea/RT) · [K+] · S2

where S = fraction of smectite layers in the I/S, t = time in seconds, A = frequency factor = 8.08 × 10-4 sec-1, exp = exponential function, Ea = activation Energy = 28 kcal/mole, R = gas constant, 1.987 cal/deg-mole, T = temperature (degree Kelvin), [K+] = K+ concentration in molarity (M) in the fluid.

The results also show that Ca2+ in solutions barely affects the illitization rate, whereas Mg2+ significantly retards the rate. The retardation, however, is not as severe as previously reported. Na+ ion can significantly retard the rate only if the concentration is high.

We found that by assuming a range 0.0026-0.0052 moles/liter (100-200 ppm) of K+, concentrations similar to the value typically reported in oil field brines, the present kinetic model can reasonably predict the extent of the smectite-to-illite conversion for a number of basins from various depths and age. This narrow range of potassium concentrations, therefore, is used to model the smectite-to-illite conversion in shale when the actual chemical information of pore fluid is not available.

The kinetic equation has been tested using field data from a large variety of geologic settings worldwide (i.e., the Gulf of Mexico, Vienna Basin, Salton Trough Geothermal Area, East Taiwan Basin, Huasna Basin, etc). The results show that the equation reasonably predicts the extent of the reaction within our knowledge of the variables involved, such as burial history, thermal gradients, and potassium concentration.

The adsorption of arsenate (As(V)) on kaolinite, montmorillonite and illite was investigated at varying pH and competing anion concentration while holding As(V) concentration (6.7 × 10−7M), clay suspension density (2.5 g L−1) and ionic strength (0.1 M NaCl) constant. The effects of 2 concentrations of phosphate (P) or molybdate (Mo) (6.7 × 10−7 and 6.7 × 10−6M) on As(V) adsorption envelopes (adsorption vs. pH) gave evidence for direct competitive adsorption (in the case of As(V) + P) and possibly site-specific non-competitive adsorption (As(V) + Mo). Distinct As(V) adsorption maxima occurred at approximately pH 5.0 for kaolinite, 6.0 for montmorillonite and 6.5 for illite, and ranged from 0.15 to 0.22 mmol As(V) kg−1. When both As(V) and P were present at equimolar concentrations (6.7 × 10−7M), As(V) adsorption decreased slightly, whereas As(V) adsorption substantially decreased in binary As(V)/P systems when the P concentration was 6.7 × 10−6M, which was 10 times greater than As(V). The presence of Mo at equimolar (6.7 × 10−7 M) and 10 times greater (6.7 × 10−6M) concentrations than As(V) caused only slight decreases in As(V) adsorption because the Mo adsorption maximum occurred at pH < 4. The constant capacitance surface complexation model was applied to As(V) and P adsorption data and was used to predict As(V) adsorption at varying P concentrations. The model gave reasonable descriptions of As(V) adsorption on the 3 clay minerals at varying pH and in the presence of a competing oxyanion (P), indicating that surface complexation modeling may be useful in predicting As(V) adsorption in soils.

Surface adsorption mechanisms of dissolved inorganic carbon species on soil minerals are not well understood. Traditional infrared (IR) study of adsorbed species of inorganic carbon using air-dried samples may not reveal true species in the solid/water interface in suspension. The purpose of this study was to obtain information on interfacial carbonate speciation between solid and aqueous phases. The interaction of bicarbonate and carbonate ions with X-ray amorphous (am) Al and Fe oxides, gibbsite (γ−Al(OH)3) and goethite (α-FeOOH) was examined by electrophoresis and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The presence of carbonate lowered the electrophoretic mobility and decreased the point of zero charge (PZC) of all minerals, implying specific adsorption. Inner-sphere complexation of bicarbonate and carbonate was supported by a lowering in the anion symmetry due to the interaction with Al and Fe oxide surfaces. Only complexed monodentate carbonate was identified in am-Al(OH)3/aqueous solution at pH 4.1-7.8 when the solid was reacted with either NaHC03 or Na2CO3 solutions. Am-Al(OH)3 was transformed to a crystalline sodium aluminum hydroxy carbonate, dawsonite [NaAl(CO3)(OH)2], and bayerite (α-Al(OH)3) after reacting with 1.0 M Na2CO3 for 24 h. Gibbsite adsorbed much less carbonate than am-Al(OH)3 such that adsorbed carbonate on gibbsite gave weak IR absorption. It is probable that monodentate carbonate is also the complexed species on gibbsite. Evidence suggesting the presence of both surface complexed bicarbonate and carbonate species in the interfacial region of am-Fe(OH)3 in suspension and the dependence of their relative distribution on solution pH is shown. Only monodentate carbonate was found in the interfacial region of goethite in 1.0 M NaHCO3. A ligand exchange reaction was proposed to describe the interaction of bicarbonate and carbonate with the surface functional groups of Al and Fe oxides.

The cation exchange capacity (CEC) of fine-grained materials, and especially clay minerals, is a fundamental property of these materials, and can be determined routinely. A search of the recent literature illustrates the great interest of this property to researchers. For example, a search of the GeoRef database for references to “cation exchange capacity” for the years 1980 to 1999 yields 2559 citations.

The standard form of the Scherrer equation, which has been used to calculate the mean thickness of the coherent scattering domain (CSD) of illite crystals from X-ray diffraction (XRD) full width data at half maximum (FWHM) intensity, employs a constant, Ksh, of 0.89. Use of this constant is unjustified, even if swelling has no effect on peak broadening, because this constant is valid only if all CSDs have a single thickness. For different thickness distributions, the Scherrer “constant” has very different values.

Analysis of fundamental particle thickness data (transmission electron microscopy, TEM) for samples of authigenic illite and illite/smectite from diagenetically altered pyroclastics and filamentous illites from sandstones reveals a unique family of lognormal thickness distributions for these clays. Experimental relations between the distributions’ lognormal parameters and mean thicknesses are established. These relations then are used to calculate the mean thickness of CSDs for illitic samples from XRD FWHM, or from integral XRD peak widths (integrated intensity/maximum intensity).

For mixed-layer illite/smectite, the measured thickness of the CSD corresponds to the mean thickness of the mixed-layer crystal. Using this measurement, the mean thickness of the fundamental particles that compose the mixed-layer crystals can be calculated after XRD determination of percent smectitic interlayers. The effect of mixed layering (swelling) on XRD peak width for these samples is eliminated by using the 003 reflection for glycolated samples, and the 001, 002 or 003 reflection for dehydrated, K-sa-turated samples. If this technique is applied to the 001 reflection of air-dried samples (Kubler index measurement), mean CSD thicknesses are underestimated due to the mixed-layering effect.

The technique was calibrated using NEWMOD©-simulated XRD profiles of illite, and then tested on well-characterized illite and illite/smectite samples. The XRD measurements are in good agreement with estimates of the mean thickness of fundamental particles obtained both from TEM measurements and from fixed cations content, up to a mean value of 20 layers. Correction for instrumental broadening under the conditions employed here is unnecessary for this range of thicknesses.

The structure of kaolinite (non-hydrogen atoms only) from Keokuk, Iowa, was refined in space group C1 using CuKα X-ray powder diffraction data and Rietveld refinement techniques (Rwp = 12.3%). A distance least-squares (DLS) model for the initial atomic coordinates avoided a false minimum and unrealistic results characteristic of previous Rietveld refinements of kaolinite. All Keokuk samples examined contain small amounts of dickite. The profile of the mixture was calculated using a fixed set of dickite atomic parameters and by constraining the dickite profile parameters to those refined for kaolinite. Results indicate that the individual kaolinite layer is very similar to the dickite layer; bond distances, which are similar to dickite and nacrite, are: Si-O, 1.60-1.63 Å; Al-O, 1.87-1.97 Å. The tetrahedral rotation angle is 6.9°, compared with 7.3° for dickite and 7.4° for nacrite. Previous Rietveld refinements apparently suffered from poor preferred orientation models, failure to consider the presence of dickite as a second phase, and, most importantly, refinement to a false-minimum structure. The conclusions based on the earlier refinements must therefore be reevaluated, especially regarding the kaolinite space group and hydrogen positions. Existing X-ray and electron diffraction and spectroscopic data support the presence of C-centering in kaolinite.

Goethites were synthesized from ferrihydrite in 0.7 M KOH between 4° and 90°C. As temperatures increased, the goethite crystals became larger and of less domainic character, and surface areas decreased from 153 to 9 m2/g. Surface area, oxalate-soluble Fe to total Fe ratios, chemisorbed water, Mössbauer parameters, and dissolution rate in 6 M HCl at 25°C are particle-size controlled, whereas mean crystallite dimensions, a-dimension of the unit cell, differences between the two OH-bending modes, and dehydroxylation temperatures suggest the existence of a low-temperature (high-a-dimension) and a high-temperature (low-a-dimension) goethite, with a narrow transition range at a synthesis temperature of 40°–50°C. Hydrothermal treatment at 125°–180°C of a low-temperature goethite led to a healing of the multidomainic, microporous high-a-dimension goethite into a monodomainic low-a-dimension goethite of similar overall crystal size with the properties of a low-a-dimension goethite.

Equilibrium studies on clay films exposed to NH3 and H2O vapors demonstrate the effect of exchangeable cations on surface acidity and its relationship to hydration. At a relative humidity of 98 per cent the order of acidity on the clay surface as indicated by protonation of NH3 was Al > Mg > Ca = Li > Na = K for Wyoming bentonite and Al > Mg > Li > Ca = Na = K for nontronite. At a relative humidity of 20 per cent, however, the order was Al = Mg > Ca > Li > Na > K for the bentonite and Al = Mg > Li > Ca > Na > K for nontronite. The largest change in proton donation properties due to hydration effects was in the calcium clays. For Ca-bentonite the NH4+ formation was 16 me/100 g at 98 per cent and 80 me/100 g at 20 per cent relative humidity. In Ca-nontronite, the NH4+ formation was 14 and 64 me/100 g for the wet and dry systems respectively. The differences in proton donation between the bentonite and nontronite clays are believed to be due to charge site location and its effects on ion hydration. The NH4+ formed by the protonation process seemed to exist in different environments in the bentonite and nontronite as indicated in the i.r. absorption spectra.

Chemical analysis is an essential step to establish the nature of minerals (Newman, 1987). The techniques used in rock and mineral analyses are generally valid for the analyses of clays. Additional information from other analytical techniques, which are mentioned here, is needed for accurate interpretation of the chemical analysis results of major elements (Gabis, 1979). In traditional chemical analyses, the aim is to obtain accurate analyses for all elements present in the sample, in such a way that the sum of elements expressed as oxides, including hydration and structural water, approaches the sample weight as closely as possible.

The 10-Å clay components of sedimentary rocks (“illites”) are commonly mixtures of 100% nonexpandable illite and an ordered illite/smectite mixed-layer mineral. If the proportion of the illite/ smectite in a mixture is sufficient to produce a measurable reflection between 33–35°2θ (CuKα radiation) that is noncoincident with an illite reflection, the ratio of component layers and type of interstratification for the mixed-layer mineral can be determined. The identification technique developed in this study rests upon the following experimental findings for ordered illite/smectites of diagenetic origin: (1) the thickness of the illite layer in illite/smectites is 9.97 Å; (2) the thickness of smectite-ethylene glycol complex ranges from 16.7 to 16.9 Å; (3) illite/smectites form a continuous sequence of interstratification types—random, random/IS, IS, IS/ISII, ISII—and each type is related to a specific range of expandability.

The new technique broadens the computer simulation method developed by R. C. Reynolds and J. Hower to include those sedimentary materials which are dominated by the presence of discrete illite, are low in illite/smectite, and, as such, have been described previously only by an “illite crystallinity index.”

During an electron microscope study of the weathering of granite from southern New South Wales, Australia, an assemblage of minerals including florencite and rhabdophane was discovered replacing apatite. Light rare earth elements released from allanite early in weathering apparently combined with P released by leaching of primary apatite to form secondary phases exhibiting a range of morphologies and compositions. Chondrite normalized fractionation patterns La > Nd > Sm > Ce; La > Ce > Nd ≥ Sm and La = Sm = Nd > Ce were identified. The rare earth elements were present in very small crystals and aggregates of secondary minerals (< 10 μm) and coexisted with clays and secondary Fe-Ti oxides. The weathered granite was enriched about 6 to 10 times in all rare earth elements except Ce relative to fresh granite if the abundances were corrected for apparent enrichment due to reduction in density. The rare earth elements were probably derived from higher in the weathering profile, possibly by destruction of florencite and rhabdophane in very intensively weathered rock. Ce remained relatively immobile during weathering, probably due to its oxidation to Ce4+.

The surface charge components of 2 Georgia kaolinites of differing degrees of crystallinity (KGa-1 and KGa-2) were determined using procedures based on charge balance concepts. Permanent structural charge density (σ0) was determined by measuring the surface excess of Cs, which is highly selective to permanent charge sites. The values of σ0 determined were -6.3 ± 0.1 and -13.6 ± 0.5 mmol kg−1 for kaolinites KGa-1 and KGa-2, respectively. The net proton surface charge density (σH) was determined as a function of pH by Potentiometrie titration in 0.01 mol dm-3 LiCl. Correction from apparent to absolute values of σH was made by accounting for Al release during dissolution, background ion adsorption and charge balance. Lithium and C1 adsorption accounted for the remainder of the surface charge components. Changes in surface charge properties with time were measured after mixing times of 1, 3 and 15 h, the latter representing “equilibrium”. Time-dependent behavior is believed to be caused by mineral dissolution followed by readsorption or precipitation of Al on the mineral surface. Both the point of zero net charge (p.z.n.c.) and the point of zero net proton charge (p.z.n.p.c.) changed with mixing time, generally increasing. The “equilibrium” p.z.n.c. values were approximately 3.6 for KGa-1 and 3.5 for KGa-2, whereas the corresponding p.z.n.p.c. values were about 5.0 and 5.4. The p.z.n.c. results were in good agreement with previous studies, but the values of p.z.n.p.c. were higher than most other values reported for specimen kaolinite.

Empirical relationships between clay mineral transformations and temperature provide a basis for the use of clay minerals as geothermometers. Clay-mineral geothermometry has been applied mainly to diagenetic, hydrothermal, and contact- and burial-metamorphic settings to better understand the thermal histories of migrating fluids, hydrocarbon source beds, and ore and mineral formation.

Quantitatively, the most important diagenetic clay mineral reaction in sedimentary rocks is the progressive transformation of smectite to illite via mixed-layer illite/smectite (I/S). Changes in both the illite/smectite ratio and ordering of I/S, as determined from X-ray powder diffraction profiles, correlate with changes in temperature due to burial depth. Although the smectite-to-illite reaction may be influenced by several factors, reaction progress appears to be strongly controlled by temperature. Studies show that the model proposed by Hoffman and Hower in 1979 is applicable in burial diagenetic settings from about 5 to 330 Ma, and includes most rocks about Miocene to Mississippian in age. Reliability of the I/S geothermometer is, however, dependent upon a good understanding of the rock's original clay-mineral composition.

Changes in the ordering of I/S are particularly useful in the exploration for hydrocarbons because of the common coincidence between the temperatures for the conversion from random-to-ordered I/S and those for the onset of peak, or main phase, oil generation. Here, the utility of the I/S geothermometer is reviewed in hydrocarbon-bearing rocks of Miocene to Mississippian age. Using three common applications, the I/S geothermometer is compared to other mineral geothermometers, organic maturation indices, and grades of indigenous hydrocarbons. Good agreement between changes in ordering of I/S and calculated maximum burial temperatures or hydrocarbon maturity suggests that I/S is a reliable semiquantitative geothermometer and an excellent measures of thermal maturity.

The formation of hematite from amorphous Fe(III)hydroxide in aqueous systems at pH 6 and 70°C, both with and without oxalate, was followed by kinetic measurements, electron microscopy, i.r. spectroscopy and thermal analysis.

In the absence of oxalate, small amorphous particles coalesce into aggregates which eventually become single crystals of hematite. When oxalate is present, crystal growth is much faster and does not proceed through the intermediate stage of aggregation. Aggregates, when formed, consist of groups of single crystals. It is suggested that oxalate accelerates the nucleation of hematite crystals by acting as a template, the Fe-Fe distance in Fe-oxalate ions being similar to that in hematite.

Adsorption of uranyl to SWy-1 montmorillonite was evaluated experimentally and results were modeled to identify likely surface complexation reactions responsible for removal of uranyl from solution. Uranyl was contacted with SWy-1 montmorillonite in a NaCIO4 electrolyte solution at three ionic strengths (I = 0.001, 0.01, 0.1), at pH 4 to 8.5, in a N2(g) atmosphere. At low ionic strength, adsorption decreased from 95% at pH 4 to 75% at pH 6.8. At higher ionic strength, adsorption increased with pH from initial values less than 75%; adsorption edges for all ionic strengths coalesced above a pH of 7. A site-binding model was applied that treated SWy-1 as an aggregate of fixed-charge sites and edge sites analogous to gibbsite and silica. The concentration of fixed-charge sites was estimated as the cation exchange capacity, and non-preference exchange was assumed in calculating the contribution of fixed-charge sites to total uranyl adsorption. The concentration of edge sites was estimated by image analysis of transmission electron photomicrographs. Adsorption constants for uranyl binding to gibbsite and silica were determined by fitting to experimental data, and these adsorption constants were then used to simulate SWy-1 adsorption results. The best simulations were obtained with an ionization model in which AlOH2+ was the dominant aluminol surface species throughout the experimental range in pH. The pH-dependent aqueous speciation of uranyl was an important factor determining the magnitude of uranyl adsorption. At low ionic strength and low pH, adsorption by fixed-charge sites was predominant. The decrease in adsorption with increasing pH was caused by the formation of monovalent aqueous uranyl species, which were weakly bound to fixed-charge sites. At higher ionic strengths, competition with Na+ decreased the adsorption of UO22+ to fixed-charge sites. At higher pH, the most significant adsorption reactions were the binding of UO22+ to AlOH and of (UO2)3(OH)5+ to SiOH edge sites. Near-saturation of AlOH sites by UO22+ allowed significant contributions of SiOH sites to uranyl adsorption.

A high-charge smectite from Arizona [cation-exchange capacity (CEC) = 120 meq/100 g] and a low-charge smectite from Wyoming (CEC = 90 meq/100 g) were used to prepare homoionic tetra-methylammonium (TMA)-clay complexes. The adsorption of benzene, toluene, and o-xylene as vapors by the dry TMA-clays and as solutes from water by the wet TMA-clays was studied. The adsorption of the organic vapors by the dry TMA-smectite samples was strong and apparently consisted of interactions with both the aluminosilicate mineral surfaces and the TMA exchange ions in the interlayers. In the adsorption of organic vapors, the closer packing of TMA ions in the dry high-charge TMA-smectite, compared with the dry low-charge TMA-smectite, resulted in a somewhat higher degree of shape-selective adsorption of benzene, toluene, and xylene. In the presence of water, the adsorption capacities of both samples for the aromatic compounds were significantly reduced, although the uptake of benzene from water by the low-charge TMA-smectite was still substantial. This lower sorption capacity was accompanied by increased shape-selectivity for the aromatic compounds. The reduction in uptake and increased selectivity was much more pronounced for the water-saturated, high-charge TMA-smectite than for the low-charge TMA-smectite. Hydration of the TMA exchange ions and/or the mineral surfaces apparently reduced the accessibility of the aromatic molecules to interlamellar regions. The resulting water-induced sieving effect was greater for the high-charge TMA-smectite due to the higher density of exchanged TMA-ions. The low-charge Wyoming TMA-smectite was a highly effective adsorbent for removing benzene from water and may be useful for purifying benzene-contaminated water.

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.

All possible polytypes of hydrotalcite-like minerals with a periodicity along the c axis of one-, two- and three-layers, as well as the simplest six-layer polytypes, were derived on the basis of the concept of closely packed brucite-like layers. Multilayer structures were found to be possible in several polytype modifications—three two-layer, nine three-layer, and a set of six-layer polytypes. The neighboring layers may be stacked in two different ways, building two kinds of interlayers: P-type where OH sheets lie one above another forming prisms and O-type where OH groups forms octahedra. Based on the kind of interlayer space, all polytypes may be separated into three groups: homogeneous interlayers of O-, or P-type, and alternating interlayers of both types. For the members of the first two groups, powder XRD patterns were calculated and criteria for distinguishing polytypes with the same number of layers per unit cell are suggested.