2H NMR spectra of synthetic deuterated kaolinite have been collected in the temperature range from 150 K to 350 K. Hydroxyl groups show a Pake doublet pattern with an asymmetry factor of 0. They are almost fixed spatially, and undergo a wobbling motion with increasing temperature. The quadrupole coupling constant is 273 ± 3 kHz at 150 K, which indicates that interlayer hydrogen bonding is relatively weak.

The long-term impermeability of clay barriers in waste disposal facilities and hydraulic structures is of critical importance to environmental, agricultural, and industrial concerns. Changes in the oxidation state of Fe in the constituent clays of compacted clay barriers may degrade the hydraulic conductivity of these structures because other properties related to hydraulic conductivity, such as swelling, gel microstructure, and particle size, are greatly altered by the oxidation state. Two Na-saturated smectites (SWa-1 and API 25) were reduced by sodium dithionite (Na2S2O4), both in suspension and in situ after consolidation, to examine the effects of structural Fe reduction on hydraulic conductivity. Results indicated that the hydraulic conductivity depended on both the oxidation state and the consolidation history of the clay. The hydraulic conductivity of clay reduced in suspension before consolidation was lower than that of oxidized clay. Initially reduced smectite, thus, may be compactable to a less-permeable material with higher bulk density. But reduction of smectite in situ after consolidation increased the hydraulic conductivity and its variability. The oxidized state of clay liners should, therefore, be preserved.

The chemical and the mineralogical composition of a group of pumiceous tuffs associated with recent salic volcanic episodes from Tenerife (Canary Islands) have been studied. The investigation focused on the two main types of pyroclastic deposits of the zone: ash-flows and ash-falls. The samples can be classified chemically as trachytic and phonolitic rocks with an intermediate silica content and a high percentage of alkali cations (Na+ and K+). The mineralogical composition, determined by X-ray diffraction, scanning electron microscopy, and optical microscopy, shows the occurrence of zeolites (mainly phillipsite, with lesser chabazite and analcime), associated with the parent glass. K-feldspar (sanidine) and calcite are accessory minerals. Zeolites are significantly more abundant in the ash-flow deposits. Zeolite formation by hydrothermal weathering in closed-system conditions varies according to the nature and the origin of the pyroclastic deposits. Tenerife phillipsites differ from typical diagenetic, lacustrine, and deep-sea phillipsites, both in chemical and mineralogical features. Alkali cations exceed divalent cations in the unit-cell that, assuming a monoclinic symmetry, has the following parameters: a = 8.46–10.55 Å, b = 14.21–14.40 Å, c = 7.80–8.70 Å, and β = 105°–110°.

The hydrolytic behavior of Fe solutions at room temperature under acidic conditions was investigated. In the presence of Al ions, with Cl and NO3 as associated anions, the Fe hydrolysis began almost instantaneously and a crystalline β-FeOOH (akaganeite) was formed in the AlCl3/FeCl3 system within a short period. Initially the particles were small with large surface area. However, with time the particles grew in size and the surface area decreased. After about 42 days of equilibration, the akaganeite particles grew to 60–300 nm long, 10–50 nm wide and with a surface area of 55 m2/g, which is similar to other reports for akaganeite prepared at higher temperatures. In the NO3 system [Al(NO3)3/Fe(NO3)3], lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) were formed. In a mixed anion system (Cl/NO3) solid phases identified were akaganeite (β-FeOOH) and lepidocrocite (γ,-FeOOH). The introduction of poly-nuclear hydroxy-Al along with monomer Al in Cl and NO3 systems of Fe affected the quantity and quality of the solid phase. The crystallinity of β-FeOOH formed in the presence of polynuclear hydroxy-Al ions in a Cl-system was more disordered than when it formed in the presence of monomer Al-ions alone. In NO3 systems, polynuclear hydroxy-Al hindered the formation of goethite (α-FeOOH). Our experiments showed that Fe oxyhydroxides crystallize readily under acidic conditions in the presence of Al ions and the data also indicated that the Cl was essential for the crystallization of akaganeite, whereas goethite was formed in those systems when Cl was absent.

Clay mineralogy (including illite crystallinity) was studied in Pennsylvanian synorogenic sediments (Atoka Formation) in the subsurface of the Arkoma Basin and the adjacent Ouachita thrust belt. Vitrinite reflectance values range from ≥0.8% at the surface up to as high as 4.7% Ro at the base of the Atoka Formation. The mineralogy of the <2 µm fraction of the mudrocks is fairly monotonous and composed of illite (<10% interstratified smectite), Fe-chlorite, kaolinite, quartz, and traces of feldspars. Kaolinite is common at shallow levels and “disappears” in most wells at a thermal maturity of 1.9–2.1% Ro, suggesting its possible use as an independent paleothermal indicator in this basin. Illite crystallinity (IC) values are fairly high (0.3–0.5° 2θ) and show little variation throughout the entire maturity range. In addition, no relation was observed between vitrinite reflectance and illite crystallinity, indicating that IC is not a useful paleothermal indicator in these rocks. Illite is almost exclusively of the 2M1 polytype, suggesting a predominantly detrital origin. Incipient metamorphic and low-grade metamorphic mudrocks in the Ouachita thrust belt to the east of the Arkoma Basin are regarded as the source rocks for the clays of the Atoka Formation. Rapid transportation and deposition by turbidity currents probably played a key role in protecting these unweathered micas from pervasive alteration in the terrestrial environment.

In the Pohang area of Korea, heulandite occurs as cements in conglomerate and sandstone of a Miocene marine fan-delta system resting on Eocene dacitic volcanics. Three types of heulandite cements are distinguishable in the fan-delta sediments on the basis of texture, chemical composition and authigenic mineral association. The earliest type I heulandite (Si/(A1+Fe): 3.5–3.8) occurs as microcrystalline (10–30 μm) in situ crystallites that replace volcanic matrix and are intermixed with early-formed smectite. Type II heulandite (Si/(A1+Fe): 3.2–3.6) occurs as medium-grained (30–60 μm) crystal aggregates rimming intergranular cavities. Type III heulandite (Si/(A1+Fe): 3.6–4.1) is the last to form and is a composite phase of heulandite-clinoptiloite, which occurs as unusually coarse (50–200 μm) single-crystal cement associating with late-formed smectite and hematite.

These characteristic heulandite cements were formed by alteration of volcaniclastic sediment during shallow burial (burial temperature: 40–60 °C) and uplift in marine pore fluid diluted by meteoric water. Sr isotope data for heulandite II (87Sr/86Sr: 0.706565–0.706598) and heulandite III (87Sr/86Sr: 0.707347–0.707432) indicate that the pore fluid was progressively mixed with meteoric water during burial and uplift but the Ca in the pore-filling heulandites has been derived mainly from dissolution of carbonate cements. Heulandite III, heulandite-clinoptilolite, was formed with an unusual coarsening and chemical zoning at somewhat diluted and disequilibrium conditions caused by the migration of oxygen-rich meteoric water during or after uplifting.

Upper Cretaceous-Tertiary marine clayey-calcareous rocks of the Hekimhan basin contain fibrous clay minerals in significant amounts. Ophiolitic rocks in the provenance area have contributed the elements to form the clay minerals. XRD, SEM, major, trace and REE analyses were applied to samples taken from several stratigraphic sections. Diagenetic minerals such as smectite, dolomite, calcite, gypsum, celestite and quartz/chalcedony are associated with sepiolite-palygorskite group clays. Trace and rare earth elements (REE) are more abundant in palygorskite than sepiolite. REE abundances in the sepiolite-palygorskite are characterized by negative Eu and positive Nd anomalies when normalized with respect to chondrite and shale. Sepiolites with sharp XRD peaks are formed by diagenetic replacement of dolomite and diagenetic transformation of palygorskite, or by direct crystallization from solution. The average structural formula of the sepiolite is:

(Mg7.15Al0.13Fe0.31Cr0.06Ni0.04)(Si11.98Al0.02)O30(OH)4(OH2)4Ca0.03Na0.02K0.02.8H2O

Palygorskite appears to be authigenic by direct precipitation from solution. It exists in both monoclinic and orthorhombic forms with the mean structural formula given below

(Mg2.22Al1.00Ti0.04Fe0.77Cr0.01Ni0.02)(Si7.68Al0.32)O20(OH)2(OH2)4Ca0.07Na0.05K0.10.4H2O

The vitric component of the silicic pyroclastic flows and surge deposits (Prassa ignimbrite unit), from the northwestern exposures of the pyroclastics formation of Polyegos Island, South Aegean Sea Volcanic Arc in Greece, is replaced by authigenic zeolite and clay minerals. Mordenite dominates, and clinoptilolite (heulandite type 3), illite and illite-smectite (I-S) are subordinate. Opal-CT, quartz, feldspar, biotite and halite complete the mineralogical suite. In the southwestern part of the pyroclastics formation (Myrsini pyroclastics unit), kaolinite, halloysite, alunite and amorphous silica are the major mineralogical constituents as a result of a strong hydrothermal alteration by solutions enriched in SO42-. Scanning electron microscope (SEM) examination proved that the 1st type of zeolites formed within the area were heulandite minerals, following the formation of smectite, as a result of the activity of pore fluids within the volcaniclastic pile. There was no substantial evidence to support a hypothesis that mordenite was formed with the heulandite minerals after the initial stages of glass dissolution, other than some very minor mordenite that was formed from a gel-like type of material. The majority of mordenite present is most often draped over and formed from the crystals of heulandite minerals. Heulandite minerals often show advanced dissolution effects. The mineral dissolution was due to the emplacement of rhyolite lava domes and flows and the associated temperature rise and circulation of hydrothermal fluids, which had also mixed with seawater. The volcanic activity raised the temperature and changed the pore fluid chemistry, and the more unstable members of the heulandite minerals group were transformed to mordenite. Clinoptilolite (heulandite type 3), which was found within a few samples, was thermally more stable than any heulandite type 1 or 2 phases initially present. Therefore, clinoptilolite was either transformed more slowly or within a very few cases, it has not been affected at all. A heulandite-minerals-derived material acted as the major precursor for the formation of mordenite. The temperature increase within the area and the later hydrothermal alteration effect are also indicated by the illitization of the smectite. Mordenite is also found to have formed from I-S clays. Overall, mordenite formed as a result of elevated temperature and high Na+ concentration.

Monte Carlo simulations based on tested water-water, cation-water, and water-clay potential functions were applied to calculate radial distribution functions for O-O, O-H and H-H spatial correlations in the interlayer region of the two-layer hydrates of Na- and K-montmorillonite. The simulated radial distribution functions then were used to compute the total radial distribution function for interlayer water, a physical quantity that can be determined experimentally by H/D isotopic-difference neutron diffraction. The simulated total radial distribution functions were compared with that for bulk liquid water, and with a total radial distribution function determined experimentally for the two-layer hydrate of Na-montmorillonite by Powell et al. (1997). This comparison suggested that water molecules in the two-layer hydrate of montmorillonite have nearest-neighbor configurations which differ significantly from the tetrahedral ordering of nearest neighbors that characterizes bulk liquid water.

Experimental alteration of albite in deionized-distilled water at 150° to 225°C for various times up to 30 days was performed to elucidate formation processes for alteration products of albite in aqueous solution. The alteration products were examined by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDX). The surface compositions of albite before and after alteration were investigated by X-ray photoelectron spectroscopy (XPS). TEM clearly showed that an amorphous leached layer was produced on the albite surface at the earliest alteration stage together with small amounts of allophane. The leached layer increased successively in thickness and tended to be detached from the albite surface as alteration proceeded. Noncrystalline fibers less than 0.5 µm in length appeared within the leached layer matrix and transformed into thin flaky smectite and small amounts of K-mica. The leached layer gave electron diffraction patterns with a diffuse halo, whereas the flaky smectite displayed rings at 4.51, 2.61, and 1.54 Å. EDX confirmed that the flaky smectite consisted mainly of Si and Al, and small amounts of Na. The smectite was formed in the stability field of Na-smectite for the system of Na2O-Al2O3-SiO2-H2O.

This paper compares the results of 27Al nuclear magnetic resonance spectroscopy (NMR) and Al-K-edge X-ray Absorption Near Edge Structure (XANES) of natural imogolite and allophanes and some crystalline reference minerals. All soil allophanes studied contain 4-coordinated Al (AlIV). The highest relative proportion of AlIV, 21% of the total Al, was found in Si-rich allophane. This value is close to that found in spring allophanes, which were previously considered to be different from soil allophanes. For a quantitative determination of the AlIV/Altotal ratio, NMR is more reliable than XANES, because of the sensitivity of the chemical shift to low AlIV concentrations, but XANES may be used even if paramagnetic impurities (mostly Fe) are present. Al-K XANES also yields more information than NMR on the local environment of AlVI and especially site multiplicity. AlVI XANES of imogolite and allophanes are similar regardless of the Al/Si ratio. They yield two well-resolved resonances with maxima near 1568 and 1570 eV, which indicates the presence of a unique AlVI site by comparison with crystalline references. The presence of only one AlVI site indicates that imogolite and allophanes have an octahedral sheet with a structure similar to 2/1 dioctahedral phyllosilicates but different from gibbsite or kaolinite, previously considered as structural analogues. The 27AlIV MAS NMR peak maxima of allophanes are between 58.6 and 59.8 ppm, in the range observed for crystalline and amorphous framework alumino-silicates, and less positive than those of sheet silicates, which are typically in the range 65–75 ppm. 27Al-H1 CPMAS NMR spectra suggest that both AlIV and AlVI have Al-O-H linkages.

High-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), annular dark-field scanning transmission electron microscope (STEM) images, and electron nano-diffraction were used to examine structures of synthetic 2- and 6-line ferrihydrite specimens. HRTEM images of 2-line ferrihydrite (2LFh) show scattered small (~1-3 nm) areas with lattice fringes surrounded by areas free of fringes. All SAED patterns show two bright rings corresponding to d-values of ~0.15 and 0.25 nm; each ring has a conspicuous shoulder on each side. Faint rings corresponding to d-values of 0.08, 0.095, 0.100, 0.106-0.114 (very broad ring), and 0.122 nm are visible in strongly exposed SAED patterns. Nanodiffraction patterns show conspicuous streaks and a lack of superlattice formation.

HRTEM images of 6-line ferrihydrite (6LFh) display larger crystallites (typically ~5-6 nm) with lattice fringes visible in many thin areas. SAED patterns show rings corresponding to d-values of 0.148, 0.156, 0.176, 0.202, 0.227, and 0.25-0.26 nm and a shoulder extending between d-values of ~0.25 and 0.32 nm. Faint rings corresponding to d-values of 0.086, 0.093, 0.107, 0.112, 0.119, 0.125, and 0.135 nm are visible in strongly exposed SAED patterns. Small quantities of hematite, magnetite or maghemite, and an acicular material tentatively identified as goethite were observed in the 6-line ferrihydrite, but these quantities do not contribute significantly to the overall diffracted intensity from the sample.

Chromium(VI) in the environment is of particular concern because it is toxic to both plants and animals, even at low concentrations. As a redox-sensitive element, the fate and toxicity of chromium is controlled by soil reduction-oxidation (redox) reactions. In-situ remediation of chromium combines reduction of Cr(VI) to Cr(III) and immobilization of chromium on mineral surfaces. In this study, Fe-rich smectite, montmorillonite, illite, vermiculite, and kaolinite were examined to determine reactivity in sorption-reduction of Cr(VI). The clays were compared to forms that were reduced by sodium dithionite. Clays containing Fe(II) efficiently removed soluble Cr(VI) from solution. Chromium K-edge X-ray absorption near edge structure (XANES) suggested that clays containing Fe(II) reduced Cr(VI) to Cr(III), immobilizing Cr at the clay/water interface. Adsorption of Cr(VI) by the Fe(II)-containing clay was a prerequisite for the coupled sorption-reduction reaction. Sodium dithionite added directly to aqueous suspensions of non-reduced clays reduced Cr(VI) to Cr(III), but did not immobilize Cr on clay surfaces. The capacity of clays to reduce Cr(VI) is correlated with the ferrous iron content of the clays. For dithionite-reduced smectite, the exchangeable cation influenced the sorption reaction, and thus it also influenced the coupled sorption-reduction reaction of Cr(VI). The pH of the aqueous system affected both the amount of Cr(VI) reduced to Cr(III) and the partition of Cr(III) between aqueous and adsorbed species. A plot of pH vs. amount (adsorption envelope) adsorbed for the coupled sorption-reduction reaction of Cr by reduced smectite exhibited a similar pattern to that of typical anion-sorption.

The distribution and crystal-chemical characteristics of chlorite, corrensite, and mica in samples from a stratigraphic profile in the Cameros basin are controlled by changes in the sedimentary facies. The lacustrine marls and limestones from the base and the top of the profile contain quartz + calcite + illite ± dolomite ± chlorite ± albite ± paragonite ± Na, K-rich mica. Chlorite is rich in Mg, with Fe/(Fe + Mg) ratios ranging between 0.18–0.37. A formation mechanism involving reaction between Mg-rich carbonate and dioctahedral phyllosilicates is proposed for these Mg-rich chlorites, on the basis of the mutually exclusive relationship found between Mg-rich chlorite and dolomite, together with the relative increase in the proportion of calcite in samples containing chlorite.

The mudrocks from the middle part of the profile are composed of quartz + albite + illite + corrensite (with a mean coefficient of variability of 0.60%) ± chlorite. Corrensite and chlorite are richer in Fe2+ than those from the base or top of the profile, with mean Fe/(Fe + Mg) ratios of 0.51 and 0.56, respectively. Textural and compositional features suggest a formation mechanism for the corrensite, chlorite, and chlorite-mica crystals through replacement of detrital igneous biotite. Whether or not corrensite occurs with chlorite appears to be related to redox conditions. The presence of corrensite alone is apparently favored by oxidizing conditions, whereas the occurrence of corrensite + chlorite is related to more reducing conditions. Corrensite shows higher Si and Na + K + Ca contents, and slightly lower Fe/(Fe + Mg) ratios than chlorite. The presence of corrensite and the lack of random chlorite-smectite interlayering is discussed in terms of the fluid/rock ratio; the occurrence is related to the hydrothermal character of metamorphism in the Cameros basin.

In a previous paper, we used powder X-ray diffraction and changes in magnetic susceptibility to argue the importance of pedogenic maghemite to soils and the efficacy of the chemical extractant citrate-bicarbonate-dithionite (CBD) to preferentially remove pedogenic maghemite from soil samples while not removing coarse-grained magnetite. Although X-ray diffraction provides strong support for this contention, Mössbauer spectroscopy is the method of choice for determining the oxidation state of iron in minerals and for inferring mineralogy of the iron oxide phases. Our objective in this work was to seek confirming evidence of the importance of maghemite as a pedogenic mineral and the usefulness of the CBD procedure in separating pedogenic maghemite from lithogenic magnetite. We present Mössbauer data on magnetic fractions from pre- and post-CBD treated soil samples. Six of the 10 samples had only maghemite as the sextet component and after CBD treatment, four lost between 96 and 100% of the magnetic susceptibility. Two samples were interpreted as highly oxidized magnetite or a mixture of magnetite and maghemite. We cannot distinguish between these with Mössbauer spectroscopy. In the remaining two samples, iron existed as hematite, ilmenite, magnetite and minor (<10%) amounts of maghemite. Our results provide additional support for pedogenic maghemite in soils and for the preferential removal of maghemite by the CBD procedure.

The acid hydrolysis products of trioctahedral fluorohectorite and phlogopite have been investigated by XRD, MAS NMR spectroscopy and nitrogen BET surface area analysis in an effort to assess the relative importance of edge attack and gallery access mechanisms. A dramatic loss of X-ray crystallinity and the formation of Q3 and Q4 SiO4 sites accompanied the depletion of Mg2+ from the octahedral sheet of both 2:1 layered structures. Depending on the extent of hydrolysis, the products derived from fluorohectorite exhibited surface areas up to 208 m2/g, whereas phlogopite hydrolysis products gave values <20 m2/g. The dramatic differences in surface areas were not related to differences in hydrolysis mechanisms. 19F MAS NMR studies indicated that the hydrolysis of fluorohectorite occurred primarily by an edge attack mechanism equivalent to the hydrolysis pathway for phlogopite. A gallery access mechanism contributed to the hydrolysis of fluorohectorite only at the later stages of octahedral Mg2+ depletion. Solvation effects appeared to be important in determining the surface areas of the reaction products derived from the swelling (fluorohectorite) and non-swelling (phlogopite) precursors.

We investigated the crystallization of ferrihydrite prepared by hydrolysis of Fe(NO3)3 solutions containing phosphate. Crystallization was studied at different pH (3–9), temperatures (298, 323, and 373 K), and initial P/Fe atomic ratios for periods to 730 d. Generally, crystallization was inhibited or only poorly crystallized lepidocrocite was formed at P/Fe > 2.5%. Phosphate favored the formation of hematite over goethite at all temperatures for most of the pH and P/Fe ranges investigated. This result is consistent with a model in which phosphate acts as a template for hematite formation, in analogy with other anions, such as oxalate. However, goethite was preferentially formed at alkaline pH and P/Fe > 1%, probably because high phosphate concentration resulted in a large increase in the negative charge of the ferrihydrite particles. This resulted in turn in less aggregation, a process that is known to precede dehydration to hematite. Phosphate greatly influenced the morphology of hematite and goethite. Hematite was often ellipsoidal or spindle-shaped. Twinned goethite crystals with a hematite core were formed at alkaline pH at P/Fe > 1%. Both hematite and goethite particles incorporated phosphate in an occluded form not desorbable by repeated alkali treatments.