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.

The clay mineralogy of tuffs from Yucca Mountain, Nevada, the potential site of the nation's first high-level radioactive waste repository, has been studied in order to understand the alteration history of the rocks and to predict potential future alterations. Bulk-rock samples and clay-mineral separates from three drill holes at Yucca Mountain (USW G-1, USW G-2, and USW GU-3/G-3) were studied using X-ray powder diffraction, and supporting temperature information was obtained using fluid inclusion data from calcite. Twelve K/Ar dates were obtained on illite/smectite (I/S) separated from the tuffs from the two northernmost drill holes, USW G-1 and G-2. The predominant clay minerals in the Yucca Mountain tuffs are interstratified I/S, with minor amounts of chlorite and interstratified chlorite/smectite. The I/S reactions observed as a function of depth are similar to those observed for pelitic rocks; I/S transforms from R = 0 interstratifications through R = 1 and R ≥ 3 interstratifications to illite in USW G-2 and to R ≥ 3 I/S in USW G-1. The R = 0 I/S clays in USW GU-3/G-3 have not significantly transformed. K/Ar dates for the I/S samples average 10.4 my. These data suggest that the rocks at depth in the northern portion of Yucca Mountain were altered 10.0-11 my ago, soon after creation of the Timber Mountain caldera to the north. Both I/S geothermometry and fluid inclusion data suggest that the rocks at depth in USW G-2 were subjected to postdepositional temperatures of at least 275°C, those in USW G-1 reached 200°C, and rocks from USW GU-3/G-3 probably did not exceed 100°C. These data suggest that no significant hydrothermal alteration has occurred since Timber Mountain time, ~ 10.7 my ago.

Estimates of the temperature of formation of illite/smectites yield probable stability limits for several minerals at Yucca Mountain. Clinoptilolite apparently became unstable at about 100°C, mordenite was not a major phase above 130°C, and analcime transformed to albite above 175°-200°C. It appears that cristobalite transformed to quartz at 90°-100°C in USW G-2 but must have reacted at considerably lower temperatures (and for longer times) in USW GU-3/G-3. The reactions with increasing depth appear coupled, and clinoptilolite and cristobalite disappear approximately simultaneously, supporting aqueous silica activity as a controlling variable in the clinoptilolite-to-analcime reaction. The reaction of clinoptilolite to analcime also coincides with the appearance of calcite, chlorite, and interstratified chlorite/smectite. Although the hydrothermal fluids may have been a source for some cations, breakdown of clinoptilolite (and mordenite) probably provided the source of some of the Ca for calcite, Mg for chlorite, K for the I/S found deeper in the section, and Na for analcime and albite.

Using the rocks in USW G-1, G-2, and GU/G-3 as natural analogs to repository-induced thermal alteration suggests that the bulk of the clinoptilolite- and mordenite-bearing rocks in Yucca Mountain will not react to less sorptive phases such as analcime over the required lifetime of the potential repository. The zeolites in zeolite interval I, directly underlying the proposed repository horizon, may transform at the predicted repository temperatures. However, the reaction of clinoptilolite to analcime in interval I may require the transformation of all of the abundant opal-CT and glass to quartz, an unlikely scenario considering the unsaturated nature of these rocks and the predicted temperatures of <100°C.

The full structure of dickite from St. Claire, Pennsylvania, including hydrogen atoms, was refined in space group Cc using time-of-flight neutron powder diffraction data obtained at 12 K and Rietveld refinement/difference-Fourier methods (Rwp = 2.62%, reduced χ2 = 1.915, 113 variables, a = 5.1474(6)Å, b = 8.9386(10)Å, c = 14.390(2)Å, and ß = 96.483(1)°). The non-hydrogen structure is essentially identical to published structures for dickite, but the hydrogen positions are distinct. The inner hydroxyl group is approximately parallel to the (001) plane, inclined by 1.3° towards the tetrahedral sheet. Contrary to published low-temperature infrared (IR) spectra, there is no evidence that dickite possess lower symmetry at low temperatures although there is tentative evidence for statistical occupancy of H3 on more than one site. Low-temperature IR spectra of St. Claire and Wisconsin dickites do not show evidence for more than four hydroxyl groups and are consistent with the reported structure. Upon cooling from 300 to 15 K, the position of the OH3 stretching band increased from 3710 to 3731 cm−1. This large, positive shift in frequency was attributed to the increase in the internuclear O-H3 ⋯ O distance upon cooling. The frequency of the 3655 cm−1 band initially decreased by 2 cm−1 to 3653 cm−1 upon cooling from 300–125 K; however, the band increased in frequency by 1 cm−1 upon further cooling to 15 K. This unusual change in frequency upon cooling is consistent with the assignment of this band to OH2 and OH4. The position of the OH1 stretching band decreased from 3622 to 3620 cm−1 upon cooling, which was attributed, in part, to the observed increase in the Al-O(H1)-Al angle at low temperature.

The reaction of hydroxy-Al interlayers in montmorillonite with monosilicic acid was studied by chemical analysis, X-ray diffractometry (XRD), and Fourier-transform infrared (FTIR) spectroscopy. Hydroxy-Al interlayers in montmorillonite was prepared by treating Ca-montmorillonite (<2) μm) with hydroxy-Al solutions at an initial Al concentration of 0.5 mM and OH/Al molar ratios of 1.0, 2.0, and 2.5. The resultant partially interlayered montmorillonite was reacted with 0.5, 0.9, and 1.4 mM monosilicic acid solutions.

A substantial amount of monosilicic acid was adsorbed by the interlayer hydroxy-Al polymers through the reaction of Si-OH groups with mainly Al-OH groups, resulting in the formation of silicated Al-interlayers with Si/Al molar ratios up to 0.19. The adsorption of silicic acid by interlayer hydroxy-Al polymers increased the basal spacings of the partially interlayered montmorillonite. This effect was most pronounced for the hydroxy-Al interlayered montmorillonite formed in the systems at an OH/Al ratio of 2.0, in which the d(001) reflections (110°C) shifted from 13.6 Å before silication to 14.1 Å with a shoulder at 17.0 Å after silication. The differential IR spectrum of the silicated hydroxy-Al interlayers in montmorillonite exhibited absorption bands at 935 and 3730 cm−1, which can be assigned to Si-O and Si-OH, respectively, of the adsorbed Si(OH)4. An interlayer structure analogous to that of “proto-imogolite” was, thus, proposed for the silicated interlayer hydroxy-Al polymers in montmorillonite. This study revealed a new mechanism through which Si enters the interlayer spaces of smectite. The silication of hydroxy-Al-interlayered clays in natural environments and the related modifications in surface properties of the clays should receive increasing attention.

Polytype diversity of hydrotalcite-like minerals is mainly a function of the nature of the interlayer anion. Among the varieties with CO32− anions, only two- and three-layer polylypes having the same structure as manasseite and hydrotalcite have been confirmed. Stichtite and reevesite, which have been previously identified as six-layer polytypes, are in fact three-layer polytypes.

Among SO42− varieties, one-layer and three-layer polytypes have been identified, but the one-layer types are only present in more hydrated minerals with larger interlayer spacings. The three-layer varieties are of three different polytypes, with both P- and O-types of interlayers. Both rhombohedral and hexagonal varieties exist. Interlayer type may change during hydration-dehydration or anion exchange. Thus, in contrast with the CO32−-bearing minerals, a complete description of the polytype of the SO42−-bearing minerals cannot be made by simply indicating the number of the brucite-like layers in the unit cell.

The two-layer unit cell seen in refined crystal structures of some minerals with SO42− interlayers is not due to a doubled periodicity of alternation of brucite-like layers but to periodicity of interlayer anions, or layer cations.

Investigation of a naturally occurring mixture of dioctahedral micas prompted an examination of X-ray diffraction (XRD) techniques for obtaining quantitative estimates of 1M and 2M 1 mica proportions. A method for determining quantitative estimates has been developed that includes the effect of preferred orientation of mica particles as described by the March function. A diagram is presented from which the percentage of 2Mı and the March parameter (r) can be obtained from two peak-area ratios: 2.80 Å/5.0 Å and 2.80 Å/2.58 Å, which are measured on observed XRD patterns of mica mixtures. The ability of this peak-area ratio diagram to predict reasonably accurate proportions of 1M and 2Mı micas was tested by comparing calculated and observed XRD patterns over a range of 2θ values from 16° to 40° for size-fractionated mixtures. Lack of accurate crystal-structure data for dioctahedral 1M mica proved an impediment to obtaining improved quantitative estimates of mica proportions.

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 clay mineralogy and major element geochemistry of four distinct layers within the Cretaceous/Tertiary (K/T) boundary marl (i.e., II, IIIa, IIIb, and IV) at Stevns Klint, Denmark, including “impact layer” (layer IIIa), were examined, and there was not a marked change in the clay mineralogy throughout this K/T boundary marl. A magnesium smectite (i.e., Mg-smectite) was the predominant clay mineral at the K/T boundary not only at Stevns Klint and at Nye Kløv; it was also found to be the predominant clay mineral in the K/T boundary at Karlstrup Quarry. In addition, Mg-smectite was found in a smectitic marl 32 meters below the K/T boundary at the Limhamn Quarry (near Malmö, Sweden), and it did not have anomalous concentrations of iridium or other siderophile trace elements. Given its occurrence in a Maastrichtian marl, it is therefore argued that the Mg-smectite is not derived from meteorite impact.

The rare earth element (REE) signatures of the Mg-smectites ranged from being comparable to the North American Shale Standard (NASC) to being one-half an order of magnitude depleted relative to NASC. One Mg-smectite collected from layer IIIb, immediately above the “impact/red layer,” was depleted in REE by one order of magnitude relative to NASC, and these levels of REE are comparable to those of smectite and illite/smectite (I/S) formed authigenically in bentonites and K-bentonites, respectively. Thus, the REE data suggest this Mg-smectite in all likelihood was formed authigenically from a glassy precursor. The presence of the low levels of REE of the Mg-smectite in the layer IIIb has no particular significance other than to suggest that this Mg-smectite separate was the least contaminated with illite or apatite having higher REE levels. With better separation, the other Mg-smectites would be expected to have comparably low levels of REE. Given the presence of the Mg-smectite throughout the K/T boundary and in Maastrichtian and Danian marls, the Mg-smectite is thought to be of volcanic origin. However, is not certain whether the Mg-smectite formed from volcanic glass deposited at the K/T boundary or whether it was formed from volcanic glass as young as late Cretaceous.

This study assesses the distribution of Al and Fe in mixed-layer illite/smectites (I/S) in shales undergoing burial diagenetic changes, using evidence from 27Al NMR, XRD, and chemical analyses. Samples studied include a sequence of mixed-layer I/S (ranging from 40% to 68% illite layers) in shales from a well located in the Caillou Island Oil Field, Terrebonne Parish, Louisiana, as well as synthetic mica-montmorillonite (Syn-1), Silver Hill illite (IMt-1), K-benonite (ISMt-1), an Fe-bearing muscovite, phengitic muscovite, and a randomly interstratified mixed-layer I/S with 50% illite layers. Using a simplified model, where Fe3+ isomorphously substitutes randomly for [6]Al in the dioctahedral 2:1 structure, the 27Al NMR signal intensities are examined with regard to the paramagnetic deshielding effect of the Fe3+. The rapid decrease in paramagnetic deshielding with distance allows for a spherical “wipeout” model with a radius of 6 Å, over which there is complete effective paramagnetic line broadening (i.e., Al within the sphere is not “seen”). Using the average dimensions of a dioctahedral mica, the expected relative intensities of the octahedral and tetrahedral Al signal are determined as a function of Fe2O3 content.

Observed 27Al signals, normalized per unit weight of Al2O3 and relative to the lowest Fe-bearing phase, show a clear trend of decreasing intensity with increasing Fe2O3 content. Normative fitting of oxide data to structural formulae reveals a similar trend of decreasing 27Al intensity with increasing fraction of dioctahedral site occupied by Fe3+. Agreement between the observed 27Al intensities of low Fe-bearing 2:1 phyllosilicates and 27Al intensities predicted using the wipeout model indicate regular ordering of Fe and Al within the low Fe-bearing phases. However, observed 27Al intensities for the I/S specimens fall into a region where the amount of Al seen is in excess for the given XFe, thus indicating segregation of Al and Fe domains.

The second order quadrupole effect for the [6]Al site in the I/S fraction of shales decreases very slightly with increasing depth and percent of illite in the I/S, but not enough to effect site quantitation. Quantitative apportionment of elements into the I/S phase of the <0.2 µm fraction using NMR constraints shows directly a trend of increasing number of [4]Al sites and no change in the number of [6]Al sites with increasing degree of illitization for samples from the Gulf Coast diagenetic environment. Stoichiometry indicates an approximate 1:1 substitution of tetrahedral Al for Si over the 40–68% range of illite in I/S examined.

This paper describes structural models of four pyrophyllite edge faces: {010}, {110}, {100}, and {130}. Water molecules chemisorbed to Lewis acid sites stabilize edge faces both crystallochemically and electrostatically. The detailed assignment of protons to surface oxygens and the orientation of OH bond-vectors both influence the surface Coulomb energy.

The geometry chosen for the electrostatic calculations was infinite pyrophyllite ribbon the thickness of a single phyllosilicate layer and the width of 50 to 70 unit cells. Such a phyllosilicate ribbon has only two edges, a top and bottom, which were simulated using the edge-face models mentioned above. About 94% of the surface Coulomb energy is confined to the edge-face repeat unit. The surface Coulomb energies of the four edge faces are on the order of 2–3 nJ/m, varying ± 1 nJ/m with proton assignment. The Coulomb potential, measured either within the layer or parallel to the layer, has a distinct negative trend near the edge face that can be traced to chemisorbed water molecules. Finally, the correlation between proton Coulomb potentials at the edge face and the coordination environment of the protons is poor, obscured by long-range interactions.

Electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopy in combination with X-ray diffractometry and thermal methods were used to determine the coordination of residual exchangeable Mn(II) in an untreated sample of Wyoming montmorillonite. At room temperature, Mn(II) in a single-layer-hydrate interlayer was proposed to be coordinated directly with oxygen ions of the siloxane surface on one layer and to form water bridges to the oxygens on the siloxane surface of the opposite layer. Dehydration and collapse of the interlayer entrapped and thereby stabilized the partially solvated Mn(II) up to 600°C. A change to Mn(II) in highly symmetric coordination occurred during dehydroxylation of the montmorillonite structure between 600°C and 700°C. Manganese(II) remained coordinated at the surface but positioned in a bicapped trigonal antiprism formed by oxygens of the silicate structure. This coordination was metastable at 800°C when the structural decomposition of the clay mineral began.

The effect of three organic ligands on the adsorption of Cu on Ca-montmorillonite was studied. The results indicate that these effects include three different processes:

1. 1) Enhanced uptake of positively charged Cu-ligand complexes by ion-exchange.

2. 2) Formation of ternary surface complexes involving surface aluminol groups.

3. 3) Inhibited uptake due to competition between the surface ligands and the dissolved ligands for dissolved copper.

Ethylenediamine promotes Cu uptake by ion-exchange at low pH but tends to suppress adsorption at aluminol groups by ligand competition at high pH. The same mechanisms are operative for β-alanine; however, the uptake of Cu(β-ala)+ by ion-exchange is not promoted by the attached ligand. The influence of malonate includes both ligand competition and formation of ternary complexes. A quantitative interpretation based on the surface complexation model using the least-square programs FITEQL (Westall, 1982) and GRFIT (Ludwig, 1992) is presented. The obtained equilibrium constants are listed in Tables 2b and 3.

Solutions containing hydroxy-FeAl oligocations (HFA) were prepared by two procedures: (1) treatment of a mixture of FeCl3 and AlCl3 with aqueous Na2CO3, followed by aging of the product and (2) preliminary preparation and aging of hydroxy-Al13 oligocations followed by reaction of the latter with aqueous FeCl3. Ion-exchange of Na-montmorillonite with HFA yield pillared clay (designated as FeAl-PILC) with d(001) values of 1.98–1.56 nm and a surface area of 230 m2/g. The pillar structure, thermal stability, surface acidity, and reduction behavior of the pillared clays were determined by 27Al-NMR, XRD, DTA, Mössbauer spectroscopy, Py-IR, TPD, TPR. Fe/Al ratios greatly affect the pillar structure, surface area and thermal stability of FeAl-PILC. The pillar in FeAl-PILC with Fe/Al ratio <0.5 has a Keggin structure, similar to that of Al-PILC, but the pillar structures of FeAl-PILC with Fe/Al ≥0.5 are the ferric tripolymer species similar to those of Fe-PILC. The basal spacings, surface area, and thermal stability are decreased with increasing Fe/Al ratio. There is relatively strong interaction between Fe and Al in the pillars. The interaction is enhanced with decreasing Fe/Al ratio. Reduction of the Fe phase in FeAl-PILC was reduced by the interaction of Fe with Al.

Boxwork fabric in which numerous thin or thick halloysite walls are interconnected into a microscopically porous cellular pattern is widely developed in the halloysite-rich kaolin formed by weathering of anorthosite in Sancheong, Korea. Studies using optical microscopy, scanning electron microscopy, and transmission electron microscopy have been carried out in order to elucidate the detailed features and origin of the boxwork.

In the early stage of weathering, halloysite spheres formed in etch pits on the walls of microstructural discontinuities in the slightly weathered rock. With further weathering, the halloysite spheres grew to discs or flattened globules, which in turn coalesced to form large planar halloysite plates amid narrow fissures. The halloysite plates were detached by dissolution of the plagioclase in groundwater. Continued growth of the halloysite tubes in the plates resulted in the wrinkling of the plates. Finally, the plagioclase was completely dissolved by groundwater, leaving the boxwork of wrinkled halloysite walls and large pores. The relatively high rigidity of the boxwork is due to the compact agglomeration of halloysite tubes within the wrinkled halloysite walls.

Cation balance calculation shows that Al was significantly mobilized during the formation of the boxwork in the weathering environment. The well-developed microfissures, the high dissolution rate of the calcic plagioclase, and the rapid flow of groundwater in a mountainous topography with relatively steep (20°) slope have been the factors controlling the formation of the porous boxwork in the halloysite-rich kaolin of the Sancheong area.

Recent evidence of the occurrence of beidellite in many soils around the world necessitates a better understanding of the role of charge location on the colloidal behavior of this smectite as compared to the more frequently studied montmorillonites. Clay suspension stability and sorption of an organic polymer, two properties sensitive to surface charge characteristics, were selected to examine the differences in colloidal behaviors of these smectites. The Otay montmorillonite was shown to have a higher charge than either the beidellite or the SWy-1 montmorillonite. Even though structural formulae resulted in a higher permanent charge for the beidellite as compared to the SWy-1, effective charge of these two smectites is the same. The pH dependency of the critical coagulation concentration of the smectites could not be explained based only on edge charge considerations, and it is proposed that tetrahedral charge location enhances the pH effect on the CCC. Decreased poly(vinyl) alcohol sorption with either increasing surface charge or tetrahedral charge location was observed. Both parameters affect the ease of replacement of water molecules by PVA on the surface of smectites.

Comparing the state of reaction advancement of I/S (illite content of illite/smectite mixed layer minerals) and organic matter (vitrinite reflectance) in instances of high temperature gradients allows one to observe the evolution of geologic materials under extreme conditions. In a geothermally heated sedimentary series (Salton Sea area, California), both clays and organics have reacted to completion in the temperature gradient range of 200°-400oC/km during heating episodes of 104 years. We observed an instance of rapid heating, through magmatic intrusion into the Meso-Paleozoic eastern Paris Basin sedimentary series in late Permian time, which induced changes in the organic material, but where clays are apparently unaffected.

Authigenic kaolinite and illite are important diagenetic minerals in the Magnus Sandstone, a giant oil reservoir in the northern North Sea. These clay minerals, separated from three wells, show considerable ranges in their oxygen isotopic composition (δ8OSMOW = +9 to + 16%) and hydrogen isotopic composition (δDSMOW = - 55 to - 105%). The variations in δ18O and δD are positively linearly correlated with a high degree of statistical significance for both kaolinite and illite:

$\begin{array}{c}\mathrm{K}\mathrm{a}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}:\mathrm{n}=12;\delta \mathrm{D}=6.1{\delta }^{18}\mathrm{O}-169;\mathrm{r}=0.66(>95\mathrm{\%})\\ \mathrm{I}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{e}:\mathrm{n}=11;\delta \mathrm{D}=5.9{\delta }^{18}\mathrm{O}-159;\mathrm{r}=0.78(>99\mathrm{\%}).\end{array}$

Formation of the clays in a pore fluid of uniform isotopic composition over a range of temperatures appears unlikely. It is suggested that the observed relationships between clay mineral δ18O and δD are perhaps best explained by a model of precipitation at more or less constant temperature from pore fluids which varied isotopically across the oilfield. The isotopic composition of the formation waters would then lie along the line: δDw = 6.2 δl8Ow - 50. This is most plausibly interpreted as a mixing line with suggested minimal endmembers at (δ18O, δD) values of (+4, -24) and (-4, -76). The first of these represents reasonable isotopic values for Magnus Sandstone formation waters. Although δ18O of the second is compatible with an evolved Cretaceous meteoric water, its δD value is difficult to understand in the context of the model.

The purpose of this study was to investigate bonding mechanisms of representative inorganic anions and citrate with imogolite and allophane using electrophoresis. The electrophoretic mobility (EM) of synthetic imogolite and allophanes with Al/Si molar ratios of 2.02, 1.64, and 1.26 was determined in 0.001 and 0.01 M sodium solutions. The highest point of zero mobility (PZM) values for imogolite and the highest point of zero charge (PZC) values for allophane occurred in the presence of ClO4, NO3, Br, I, and Cl. Below the PZM and PZC, Cl and I lowered the EM relative to the other anions but did not shift the PZM and PZC significantly. This indicates that Cl and I formed more outer-sphere complexes than the other ions. The EM of imogolite and allophane was negative at pH < 6 in 0.001 and 0.01 M NaF probably due to a phase change. We observed the formation of cryolite (Na3AlF6) with transmission electron microscopy (TEM) and X-ray diffraction (XRD) in the NaF systems at low pH. Conversely, phosphate at 0.001 and 0.01 M concentrations lowered both the PZM and the EM in imogolite and both the PZC and the EM in allophane compared with ClO4. Phosphate-treated allophane had the same PZC as a synthetic amorphous aluminum phosphate. The PZM values of imogolite and allophane with 2:1 Al/Si in 0.0001 M Na-citrate were 10.9 and 5.9, respectively. At pH 7.3, Na-citrate lowered allophane EM more than it lowered imogolite EM relative to ClO4.

The EM in NaClO4 and Na2SO4 was reversible by forward- and back-titration with NaOH and HClO4, indicated that ClO4 and SO4 were not specifically adsorbed. Chloride likely formed more outer-sphere complexes than ClO4. Imogolite EM and allophane EM in dilute NaF and NaH2PO4 solutions were not reversible, indicating either surface inner-sphere complexes or surface precipitates of aluminum fluoride and amorphous aluminum phosphate-like materials on these minerals. Sulfate gave a lower EM than the monovalent anions, implying a greater tendency to form outer-sphere complexes. Citrate appeared to form inner-sphere complexes on both imogolite and allophane, but formation was concentration-dependent. The tendency of anions to form surface complexes with imogolite and allophane is consistent with the tendency of anions to form soluble aluminum complexes.

Highly tectonized contact between serpentinite and younger pegmatite in the magnesite mine of Wiry contains various layer silicates. Vermiculite, chlorite, smectite, and interstratified mica-vermiculite were recognized by means of routine XRD examination. Two three component interstratifications of mica-vermiculite-chlorite and chlorite-swelling chlorite-smectite were identified by a combined procedure of deconvolution of the XRD patterns and simulation of XRD tracings. A mineral with large diffraction maxima, displaying “chlorite intergrade” characteristics, appeared to be a mixture of chlorite, mixed layer chlorite-smectite, and vermiculite. Polytypes of phyllosilicates were determined by the X-ray transmission method. Due to the heritage of parent mineral polytype structure by transitional products of alteration, two distinct sequences of layer silicates were observed: one formed from trioctahedral mica (vermiculite, mixed layer mica-vermiculite); and one evolved from chlorite (e.g., mixed layer chlorite-swelling chloritesmectite). A tentative scheme of the primary contact zone structure, not obscured by subsequent brittle tectonics either by transformation of layer silicates, is proposed.

Illite from the Potsdam Sandstone (lower Ordovician) of Northwestern New York was studied by powder X-ray diffraction, scanning electron microscopy, and chemically analyzed and dated by the K/Ar method. The texture and ages of 360 and 392 Ma on two samples establish that the mineral is authigenic and relatively uncontaminated by Precambrian detritus. Random powder X-ray diffraction patterns show sharp and relatively intense 02l and 11l reflections, indicating an ordered structure. Comparisons with calculated patterns demonstrated that the mineral is not the common 1M or 2M1 polytype. Instead, the experimental pattern is very similar to that of a 3T polytype, but it agrees better with the calculated pattern of the octahedral cis-vacant, noncentrosymmetric (space group C2) structure found by Méring and Oberlin (1967) and Tsipursky and Drits (1984) in smectites, proposed for mica by Drits et al. (1984) and found by Zvyagin et al. (1985).