The reactivity of colloidal particles is regulated by their surface properties. These properties affect the wettability, flocculation-dispersion characteristics, ion exchange, sorption capacities and transport of inorganic colloids. Most studies have focused on hydrophilic, charged-particle surfaces, often ignoring the alterations in surface properties produced by the adsorption of natural organic matter, surfactants and other compounds. Adsorption of these substances can potentially render a surface substantially more hydrophobic. Nevertheless, comparatively little is known about changes in surface properties and reactivity of minerals upon sorption of hydrophobic organic compounds. In this study, the properties of four minerals (kaolinite, pyrophyllite, montmorillonite and Min-U-Sil®) and two inorganic materials (X-ray amorphous Al hydroxide and X-ray amorphous Si oxide) were compared before and after treatment with the common silylating agent, trimethylchlorosilane (TMCS). The samples were characterized by measurements of total carbon, cation exchange capacity (CEC), particle size, specific surface area (SSA), electrophoretic mobility, contact angle, particle aggregation, and by X-ray diffraction and diffuse reflectance infrared spectroscopy. For the layer silicates, surface coverage was limited to ∼2% trimethyl silane (TMSi). TMSi covered 7.5% of the Min-U-Sil® surface and 33% of the X-ray amorphous Si oxide. Treatment did not affect the structure of the minerals but reduced the CEC, SSA and electrophoretic mobilities. Water contact angles increased to between 18 and 114° with treatment. While the apolar characteristic of the surfaces decreased minimally with treatment, the Lewis acid/base properties were substantially reduced and interfacial free energy shifted from positive to negative values indicating a more hydrophobic surface character. For all the samples except kaolinite, these changes affected the stability of the colloids in suspension depending upon solution pH. Although the grafting of TMSi altered colloidal mineral surface properties and increased their hydrophobicity, these changes were not sufficient to predict colloid aggregation behavior.

Bentonites are candidate materials for encapsulating radioactive waste within barrier systems in crystalline rocks. In the ‘Alternative Buffer Material’ (ABM) test in the hard rock laboratory in Äspö, Sweden, six packages of eleven different buffer materials (mainly bentonites) with various exchangeable cation populations were packed vertically with an iron tube used as a heater in the center. After installation, the second ‘ABM package’ (ABM-II) was first allowed to saturate with water for approximately 2.5 years. The blocks were then exposed to a temperature of up to 141°C for approximately 3–4 years. The hypotheses for the present study were: (1) no horizontal gradient of the cation exchange population was present in the individual blocks of ABM-II because ABM-II had a longer reaction time in comparison to the ABM-I package, which did not have horizontal gradients; (2) the exchangeable cation Ca2+:Na+:Mg2+ ratio was equal in all blocks of ABM-II and was independent of block position in the package. As expected from ABM-I, all blocks in the ABM-II experiment showed large differences between the measured values of the reference materials and the reacted samples. The exchangeable Na+ and Mg2+ values in ABM-II were reduced by up to 55% to 59% in comparison to the reference material. Contrary to the first hypothesis, horizontal gradients were observed in ABM-II; and, contrary to the second hypothesis, the exchangeable cation ratios differed markedly in the different reacted buffer materials. The largest total Na+ loss was observed in the middle part (-67%), whereas Mg2+ values decreased by 79% in the upper part. The exchangeable Ca2+ values increased strongly in ABM-II, particularly in the upper part. The most useful parameter to distinguish between ion exchange equilibria of ABM-I and ABM-II was the Na+/Mg2+ ratio. This ratio was constant in ABM-I (3.0) and had a similar ratio (3.5) in the lower part of ABM-II; however, the ratio strongly increased (5–10) in the upper part of the ABM-II package. The large Na+/Mg2+ ratios in the upper part of ABM-II could possibly be explained by water loss into the rock (caused by a pressure drop and boiling) and subsequent water uptake.

The effect of natural clay-mineral properties on the rheological behavior of dispersion is very important in new geotechnical and industrial applications. The colloidal behavior of natural clay minerals with various octahedral structures was investigated using macro- and microrheological measurements and compared with the behavior of synthetic hectorite. In the present study montmorillonite (dioctahedral smectite of Volclay), natural hectorite (trioctahedral smectite of SHCa-1 Source Clay), and the synthetic trioctahedral smectite Laponite®, with lateral layer dimensions of 277, 100, and 30 nm, respectively, were used. The structure formation, kinetics of aging, and broad bandwidth viscoelastic response (10-2 — 106 rad/s) of their dispersions were obtained using mechanical shear and squeeze flow rheometry combined with diffusing wave spectroscopy (DWS) and multiple particle tracking (MPT) microrheology. State diagrams were determined at inherent pH considering the clay-mineral and NaCl concentrations as well as the kinetics of structure formation and sample aging. Due to the larger mean layer diameter and greater layer-charge density of natural clay-minerals, their sol—gel transitions occurred at higher solid and NaCl concentrations than those of Laponite®. Structure formation was faster at pH < pHPZC,edge than at pH > pHPZC,edge (point of zero charge at the edge). The long-term aging of natural clay-mineral samples was less pronounced in the glass state than in the gel state, in contrast to the findings for Laponite®. The storage modulus, G’, of clay-mineral dispersions in arrested states remained essentially constant in a wide frequency range (up to 100 rad/s), as expected. The corresponding plateau value of G’ depends on the number of particle contacts per volume and, hence, increased with decreasing particle size at a given concentration. The dissipation mechanisms determining the high-frequency loss modulus, G", however, are independent of particle size and, accordingly, the high-frequency crossover of G’ and G" shifted to higher values when the particle size decreased. The MPT data revealed structural refinement on the submicrometer length scale during the aging of weak hectorite gels, which was similar to the results for Laponite®. No refinement, however, occurred for montmorillonite in the glass or strong gel state.

Fire resistance performance is one of the most important requirements in geological storage conditions in order to improve the resistance of storage packages to high thermal constraints (in the case of a fire for example). With the need to develop new fire-resistant materials, the aim of the present study was to develop fire-resistant geopolymer binders based on Callovo-Oxfordian (COx) argillite. Two types of kaolin with different degrees of purity were mixed with argillite in various proportions. These mixtures were calcined at 600 or 750°C. In order to assess the fire resistance of activated materials, thermal treatment at 1000°C was performed. The compressive strength and mineralogical composition of the samples were investigated before and after heat treatment. The results showed that the addition of argillite improved significantly the thermomechanical properties of kaolin-based geopolymers containing impurities, especially the mixture containing 67% argillite and calcined at 750°C. This phenomenon was not observed for the pure-kaolin geopolymer. Improvement of fire resistance was due to the formation in situ of leucite and zeolite-type phases (KAlSi2O6 and KAlSiO4) and of wollastonite (CaSiO3) at high temperature, which is linked to the Ca available in the raw materials.

The ordering conformation of surfactant molecules in intercalated montmorillonite prepared at various concentrations was investigated by 13C MAS NMR. The 13C MAS NMR study demonstrates the coexistence of ordered and disordered chain conformations. Two main resonance peaks are associated with the backbone alkyl chains: the resonance at 33 ppm corresponds to the ordered conformation (all-trans), and the resonance at 30 ppm corresponds to the disordered conformation (mixture of trans and gauche). Deconvolution of 13C MAS NMR spectra indicates that the ordering conformation of surfactant molecules within the gallery of montmorillonite depends very much on their orientation and packing density. When amine chains are oriented parallel to the silicate layers, the amount of all-trans conformer decreases with the increase of amine concentration. However, the amount of all-trans conformer increases with the increase of amine concentration when amine chains radiate from the silicate layers. Furthermore, 13C MAS NMR spectra show that the intercalated surfactant molecules in the clay minerals never attained the complete liquidlike or solidlike behavior.

To provide structural and diffraction criteria for the identification of trans-vacant (tv) and cis-vacant (cv) mica varieties with different layer stackings, powder X-ray diffraction (XRD) patterns have been simulated for 1M, 2M1, 2M2, 3T and 2O structural models consisting of either tv or cv layers. The differences in the unit-cell parameters resulting from the specific structural distortions of tv and cv layers lead to the differences in the positions of reflections having the same indices in the XRD patterns for tv and cv 1M, 2M1 and 2M2 mica varieties. The tv 1M, 2M1 and 2M2 varieties of Al-rich micas can therefore be distinguished from the corresponding cv varieties using powder XRD diffraction provided that the d values are measured with high precision and accurately compared with those calculated from the unit-cell parameters for the corresponding hkl indices. The differences in reflection positions for these tv and cv varieties should decrease with increasing Mg and/or Fe contents, thus complicating their identification.

The peak positions and intensity distributions in the XRD pattern for the tv 3T variety are similar to those for the cv 3T structure with the vacancy in the right-hand cis site (3T-cv1), and both XRD patterns are similar to that for the 1M-cv mica. The simulated XRD pattern for the cv 3T structure with the vacancy in the left-hand cis site (3T-cv2) is similar to that for the 1M-tv variety. The similarities and dissimilarities in intensity distribution between the XRD patterns simulated for the 1M and 3T varieties in question may be associated with the differences in the mutual arrangement of cations and anions in successive layers.

Possible interstratification of tv and cv layers within the same structure should seriously complicate the identification of dioctahedral mica polytypes and polymorphs.

Bulk mineralogical and chemical properties of a weathering profile derived from serpentinite were studied using classical pedological methods (Munsell soil colors, particle-size distribution, density, cation exchange capacity, exchangeable bases, among others) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) results. Bulk clay fractions were characterized using X-ray diffraction, thermal analysis, electron microprobe, Mössbauer and infrared spectroscopies. Bulk geochemical mass-balance calculated from ICP-AES results shows leaching of both Mg and Si which reflects the early weathering of serpentine minerals. As a consequence, newly formed clay minerals are enriched with the least mobile elements, i.e. Fe and Al, producing dioctahedral smectites. These dioctahedral smectites are complex, heterogeneous and consist mainly of two populations. One population is an Fe-rich montmorillonite with little or no tetrahedral charge and Fe3+ as the dominant octahedral cation whereas the second population exhibits tetrahedral charge. Both populations occur as interstratified layers in the lower horizon of the weathering profile but show increasing segregation into well-defined end-members towards the surface horizons. Considering total Al and Fe contents, these clays differentiate into two chemical composition domains, Fe-rich clays in the lower profile and Al-rich clays towards the surface horizons.

During recent decades, the search for possible repositories for high-level nuclear waste has yielded large amounts of sorption data for actinides on different minerals. Clays and clay minerals are of special interest as potential host-rock formations and backfill materials, by virtue of their good retardation properties. Neptunium (Np) is one of the actinides which is considered in long-term scenarios due to its long-lived nuclide 237Np (t1/2 = 2.1 × 106 y). Because neptunium sorption is heavily dependent on the experimental conditions, comparison of sorption data from different experiments is challenging. Normalizing reported data with respect to the surface area of the sorbent enables conversion of conventional distribution coefficients (Kd) to normalized (Ka) values, which improves comparability among the results of different experiments. The present review gives a detailed summary of sorption data of Np on clays and clay minerals and examines critically the applicability of the Ka approach.

Waste brownfield-site soils contaminated with heavy metals such as Zn and Cr are of critical environmental concern because of the rapid urbanization and industrialization that is occurring in China. Thermal treatment can fix heavy metals in specific mineral structures, which might be a promising technology for remediation and reutilization of the metal-contaminated soils. The objective of the present study was to elucidate the stabilization mechanisms of Zn and Cr through thermal treatment of mixtures of ZnO + Cr2O3 to form ZnCr2O4 and to confirm that Zn and Cr were incorporated simultaneously into the spinel structure. The incorporation efficiency for Zn was quantified, with the value ranging from 70.6 to 100% over the temperature range 700–1300°C. Leaching results further confirmed that ZnCr2O4 spinel was a superior product for Zn and Cr immobilization. Then, by artificially sintering Zn- and Cr-enriched soils, both Zn and Cr were immobilized effectively (with three orders of magnitude reduction in Zn leachability) in the ZnCr2O4 spinel as the predominant product phase. In addition, multiple heavy metals such as Zn, Cu, and Cr in the actual brownfield-site soils were well immobilized after sintering, which confirmed the potential for practical application of the thermal treatment technology utilized in this study.

The present report is a review of uses of quaternary ammonium cations (QACs) as free monomers or immobilized in micelle-clay complexes in bacteria removal from water. The removal of bacteria from water by filtration through a bed of a granulated QAC-clay micelle was improved by minute concentrations of QAC that were released from the complex during filtration, which exerted biostatic or biocidal effects on the bacteria that emerged from the filter. The relationships between antibacterial activity (minimum inhibition concentration, MIC; minimum lethal concentration, MLC) and structural parameters of the QACs (head group size and alkyl chain length) are discussed. The antibacterial activity of QACs in aqueous phases is mainly due to the free monomeric species. Bacterial inactivation is enhanced by QACs with longer alkyl chains. In most recorded cases, however, minimum MIC and MLC values occurred at n = 14–16 and mostly at n = 16, where n is the number of C atoms in the alkyl chain. This outcome is explained by the combination of two antagonistic effects: (i) An increase in alkyl chain length (i.e., QAC hydrophobicity) enhances QAC binding, penetration, and destabilization of bacterial membranes; and (ii) an increase in alkyl chain length lowers the critical micelle concentration (CMC) of QACs and, thus, reduces QAC monomer concentrations, which more efficiently inactivate bacteria than the micelles. The octadecyltrimethylammonium (ODTMA, n = 18) MLC value (0.25 μm) for the cyanobacterium genus Aphanizomenon is significantly lower than the CMC (300 mm) value. Hence, a test to determine the minimum MLC value at n = 16 is of interest. Removal of bacteria from water by filtration is expected to be made more efficient by small increases in the ODTMA/clay ratio in the complex, which will act to increase the concentrations of ODTMA cations released during filtration.

Çayırbağı magnesite is one of numerous magnesite deposits occurring throughout Turkey. In this deposit, sepiolite and newly found stevensite occur locally as two daughter minerals formed from magnesite. The sepiolite and stevensite show distinctive compositions and modes of formation compared to those described in the literature. The objective of the current study was to characterize these minerals by means of mineralogic, thermal, structural, geochemical, and textural analyses and to describe their mechanisms of formation. The geology, mineralogy, and geochemistry were examined by field work followed by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential thermal (DTA), and thermogravimetric (TG) analyses. Chemical analyses were performed by means of electron microprobe (EMPA), inductively coupled plasma-optical emission spectrometry (ICP-OES), and laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). The XRD analyses showed that the d110 of the sepiolite was at 12.64 Å and at 13 Å after air-drying and ethylene-glycol solvation, respectively. Identification of the sepiolite as sepiolite-13 Å was supported by FTIR and TG-DTA data. Chemical analyses showed an ideal composition with a structural formula of (Ca0.05K0.02)(Mg7.79Al0.10□0.11)Si12O30(OH)4. Stevensite displayed distinctive results for XRD, FTIR, and thermal characteristics. The structural formula of stevensite was: (Ca0.01Na0.20K0.04)(Mg1.90Al0.30Fe3+0.37Ti0.01□0.43)(Si3.93Al0.07)O10(OH)2, indicating a layer charge arising mainly from octahedral sheets. Field and SEM observations demonstrated that sepiolite was formed from magnesite by transformation via a dissolution–precipitation mechanism. Descending surface waters were responsible for this transformation. Thick magnesite veins were partly replaced whereas in thin veins sepiolite replaced the overall mass. Both surface waters with high Si, low Al and Fe activities, and pH values of 8–9.5 were responsible for sepiolite formation. Stevensite was formed similarly to sepiolite with respect to the mechanism and parent mineral under permanent groundwater; where both Si, Fe, Al activities and pH (>9.5) were high.

Secondary Fe(III) oxyhydroxides play a key role in controlling the mobility and bioavailability of trace metals in acidic, sulfate-rich soils, such as mining and smelter sites. Schwertmannite, jarosite, goethite, and ferrihydrite are the most common mineral phases identified in such soils. A good understanding of the precipitation and transformation of these minerals in soils is very important for predicting the mobility and long-term stability of trace metals associated with them. In the present study, bulk powder X-ray diffraction (XRD), scanning electron microscopy (SEM), synchrotron based micro- X-ray diffraction (µ-XRD), and micro X-ray fluorescence (µ-SXRF) spectroscopy were used to investigate precipitates fromthe surface horizon of an organic soil (Histosol) at a site that once contained a lead smelter. Soil samples were collected from 0 to ≈10 cm depth during both wet and dry seasons. Goethite and akaganeite were identified as the major mineral phases in this soil. Schwertmannite and jarosite were also occasionally identified, particularly in the soil samples from dry periods. The peaks in the akaganeite XRD pattern were significantly broadened and the relative intensities of some major peaks were distinctly different compared with the diffraction pattern of synthetic akaganeite, possibly due to the effects of pH and the incorporation of sulfate. The SEM and µ-XRD data support the hypothesis that the goethite in the precipitates is not the product of direct precipitation froms olution but the transformation of previously precipitated schwertmannite or akaganeite.

The structure of dioctahedral true micas such as muscovite and celadonitic muscovite (2M1 polytype, space group C2/c) is mostly affected by variations of the octahedral Al (VIAl) content. Crystals with greater Mg, Fe substitutions (i.e. celadonitic muscovite) reduce the dimensional difference between the larger trans-oriented M1 site and smaller cis-oriented M2 octahedral site. The octahedral anionic position O4 is displaced from the center of the hexagon, defined by 031 and 032 oxygen atoms (i.e. ‘octahedral hexagon’), both on and off the (001) plane. The distance between interlayer cation A and O4 is smaller in more substituted species, thus providing different orientations of the O4−H vector, as a function of VIAl. Octahedral distances (<M2−O3> and <M2−O4> are expressed as a function of cell parameters and VIA1 content, thus allowing an approximate estimate of site dimensions. These approximations are useful when a detailed structural refinement is not available. In celadonitic muscovite, the octahedral hexagon mean edge (<O31−O32>Hex) is not significantly affected by VIA1 content. The VIA1 increase produces both a decrease in cell lateral dimensions and a distorted ‘octahedral hexagon’. The decrease in a and b is consistent with a decrease of <O31−O32>Hex, whereas the distortion of the’ octahedral hexagon’ is consistent with an increase of (<031–032>Hex), because an irregular hexagon produces a longer mean edge than a regular hexagon of equal area.

The tetrahedral mean basal edge (VI<O−O>bassal) is reduced as celadonitic substitution progresses. The tetrahedral rotation angle, α was thus found to increase from celadonite to muscovite. However, in muscovite with VIAl content between 1.8 and 2.0 atoms per formula unit (a.p.f.u.), α approaches a saturation value, thus showing a proportional increase of tetrahedral and octahedral sheet lateral dimensions. Furthermore, α variation allows a coarse approximation of the threshold VIAl content, below which celadonitic substitution may not progress.

Bentonites are commonly used as chemical containment barriers to minimize liquid flow and contaminant transport. However, chemicals can adversely affect bentonite performance to the extent that modified bentonites have been developed to improve chemical resistance relative to traditional (unmodified) bentonites. The present study focused on the diffusion of potassium chloride (KCl) through a bentonite-polymer composite, or BPC, that was known to behave as a semipermeable membrane. Specifically, the effective diffusion coefficients, D*, for chloride (Cl−) and potassium (K+) were measured and correlated with previously measured membrane efficiency coefficients, ω, for the BPC. The values of D* at steady-state for chloride ($D_{ss,C{l}^{-}}^{*}$\$\end{document}) and potassium ($D_{ss,K^{+}}^{*}$\$\end{document}) decreased as the ω values increased. The decrease in $D_{ss,C{l}^{-}}^{*}$\$\end{document} and $D_{ss,K^{+}}^{*}$\$\end{document} was approximately a linear function of (1 − ω), which is consistent with previous research performed on unmodified Na-bentonite contained within a geosynthetic clay liner (GCL). In contrast to the previous GCL tests, however, $D_{ss,C{l}^{-}}^{*}$\$\end{document} values for the BPC generally were greater than the $D_{ss,K^{+}}^{*}$\$\end{document} values, and the differences between $D_{ss,C{l}^{-}}^{*}$\$\end{document} and $D_{ss,K^{+}}^{*}$\$\end{document} decreased as KCl concentration increased. The apparent discrepancy between $D_{ss,C{l}^{-}}^{*}$\$\end{document} and $D_{ss,K^{+}}^{*}$\$\end{document} is consistent with excess sodium (Na+) in the BPC prior to testing and the requirement for electroneutrality during testing. Also, despite an apparent linear trend in diffusive mass flux for K+, lack of agreement between the ratio of the diffusive mass flux of K+ relative to that for Cl− as required on the basis of electroneutrality at steady state suggested that steadystate diffusive mass flux for K+ had probably not been achieved due to continual K+-for-Na+ cation exchange. Nonetheless, the excess Na+ and bentonite modification did not affect the fundamental correlation between D* and ω, which requires that D* approaches zero as ω approaches unity (D* → 0 as ω → 1).

NEWMOD was developed by R.C. Reynolds, Jr., for the study of two-component interstratifications of clay minerals. One-dimensional X-ray diffraction (XRD) profiles of an interstratified system of two clay minerals can be simulated using NEWMOD, given a set of parameters that describes instrumental factors, the chemical composition of the system (e.g. the concentration of Fe and interlayer cations), and structural parameters (e.g. proportions of the two components, the nature of ordering, and crystallite size distribution). NEWMOD has served as the standard method for quantitatively evaluating interstratified clay minerals for >20 y. However, the efficiency and accuracy of quantitative analysis using NEWMOD have been limited by the graphical user interface (GUI), by the lack of quantitative measures of the goodness-of-fit between the experimental and simulated XRD patterns, and by inaccuracies in some structure models used in NEWMOD. To overcome these difficulties, NEWMOD+ was coded in Visual C++ using the NEWMOD architecture, incorporating recent progress in the structures of clay minerals into a more user-friendly GUI, greatly facilitating efficient and accurate fitting. Quantitative fitting parameters (unweighted R-factor, Rp, weighted R-factor, Rwp, expected R-factor, Rexp, and chisquare, χ2) are included, along with numerous other features such as a powerful series generator, which greatly simplifies the generation of multiple simulations and makes NEWMOD+ particularly valuable for teaching.

With fossil-fuel consumption at an all-time high, air pollution is becoming one of the most prominent problems of the 21st century. In addition to their devastating effects on the environment, sulfur-based pollutants are problematic for infrastructure by undermining the structural stability of various oxide-based surfaces found in clays and clay minerals. Calcite (CaCO3) and alumina (α-Al2O3) are two such mineral oxides with surfaces that are potentially susceptible to damage by sulfur-based adsorbates. Their surface interactions with a wide range of sulfur-based pollutants, however, have yet to be studied adequately at the atomistic level. This problem can be addressed by utilizing density functional theory (DFT) to provide molecular-level insights into the adsorption effects of H2S, SO2, SO3, H2SO3, and H2SO4 molecules on calcite and alumina surfaces. DFT can be used to compare different types of adsorption events and their corresponding changes in the geometry and coordination of the adsorbates, as well as delineate any possible mineral-surface reconstructions. The hypothesis driving this comparative study was that the mineral-oxide surface structure will dictate the surface adsorption reactivity, i.e. the flat carbonate unit in calcite will behave differently from the Al–O octahedra in alumina under both vacuum and hydrated surface conditions. The set of sulfur-based adsorbates tested here exhibited a wide range of interactions with alumina and fewer with calcite surfaces. Events such as hydrogen bonding, sulfate formation, atom abstraction, and the formation of surface water groups were more prevalent in alumina than calcite and were found to be dependent on the surface termination. The results of this work will prove instrumental in the design of clay and mineral-based materials resilient to sulfur-based pollutants for use in construction and infrastructure such as smart building coatings and antifouling desalination membranes, as DFT methods can garner the atomistic insights into mineral-surface reactivity necessary to unlock these transformative technologies.

The assignment of the 29Si CP/MAS-NMR spectrum of naturally-occurring sepiolite clay was re-examined using 29Si COSY and 1H-29Si HETCOR pulse sequences. Each of the three main resonances at −92.1, −94.6 and −98.4 ppm has been attributed to one of the three pairs of equivalent Si nuclei in the basal plane, and the resonance at −85 ppm to Q2(Si-OH) Si nuclei. On the basis of the COSY experiment, the resonance at −92.1 ppm is unambiguously assigned to the intermediate, near-edge Si sites. The HETCOR experiment revealed that the resonance at −94.6 ppm cross-polarizes almost entirely from the Mg-OH protons, and therefore is assigned to the central Si position. The remaining resonance at −98.4 ppm correlates strongly to the protons of the structural water molecules and therefore is assigned to the edge Si sites. Nearly complete rehydration was achieved at room temperature by exposing sepiolite samples that had been partially dehydrated at 120°C to water vapor or to D2O vapor. The rehydration results support the 29Si NMR peak assignments that were made on the basis of the COSY and HETCOR experiments.

The 29Si CP/MAS-NMR spectrum corresponding to the folded sepiolite structure in which approximately one half of the structural water has been removed by heating to 350°C is reported for the first time. The chemical shift values and relative intensities are significantly different compared to the resonances that are observed in the corresponding spectrum of the true sepiolite anhydride. These observations support the earlier claim that sepiolite heated to ∼350°C exists as a distinct phase to be differentiated from that of the completely dehydrated state.