TheAu-Cu (±Pb, Zn, Ag) prospects of Lavras do Sul, southernmost Brazil, arehosted in Neoproterozoic granitic and volcanogenic rocks. Mineralization occurs in structurally controlled N40°W to E-W quartz veins; sericite (±chlorite) and sulfides are the main secondary minerals in associated wall rocks.

In the present contribution we use petrography (optical microscopy and scanning electron microscopy (SEM)), mineralogy (X-ray diffraction (XRD) with polytypes, FWHM, decomposition of diffraction patterns), and crystal chemistry of samples from several prospects to document the spatial and temporal evolution of sericitic alteration of veins and wall rocks associated with gold.

Hexagonal, coarse-grained 2M1 phengite-rich alteration (± illite) is best developed with coarse-grained primary growth (comb) quartz + pyrite ± Au veins and altered wall rock from the western portion of the granitic complex (phyllic alteration). Pure phengite was recognized by narrow XRD profiles (FWHM ⩽ 0.2°2θ CuKα) of the <5 µm particle-size fraction, non-expandable d001 X-ray reflections and interlayer charge (IC) >0.9 per O10(OH)2.

Towards the eastern zones of the granitic complex and in the volcanogenic rocks, wider XRD profiles (FWHM values ⩾ 0.2°2θ CuKα) were decomposed. They contain mixtures of coarse- to fine-grained, lath-like crystals of both 2M1 and 1M illite (non-expandable d001 X-ray reflections, IC between 0.85 and 0.89 per O10(OH)2) with expandable d001 reflections associated with lath-like, fine-grained crystals of ordered (R ⩾ 1) illite-rich I-S (80–90% of illite; IC of ∼0.8 per O10(OH)2), and minor amounts of regularly ordered (R = 1), illite-rich I-S mixed layers (75% of illite; IC of ∼0.74 per O10(OH)2). The dioctahedral clay association of illite + illite-rich I-S mixed layers (intermediate argillic alteration) is best developed in quartz + pyrite ± Au veins, breccias, and wall-rock alteration from the eastern portion of the granitic complex and in the volcanic area. Quartz from veins and breccias has fine-grained primary growth, recrystallization, and replacement textures, similar to those in epithermal deposits.

The overall distribution of the dioctahedral clays indicates that the study area represents a fracture-controlled, tilted, porphyry to epithermal deposit, with telescoping alteration features observed in the east of the mining district. Deeper levels of exposure of a large hydrothermal system are observed in the west of the mining district, as shown by higher-rank dioctahedral minerals (phengite) that crystallize at relatively high temperatures (Tphe ≈ 300°C, phyllic alteration) associated with coarse-grained, primary-growth quartz veins, similar to those observed in porphyry deposits. On the other hand, shallower levels of exposure are observed in the east of the study area, associated with abundant, lower-rank dioctahedral clay minerals (illite + illite-rich I-S mixed layers, intermediate argillic alteration) that crystallize at relatively lower temperatures (TI-S ≈ 120–200°C).

Available data show that gold is associated with phengite, but that lower-rank, overprinting alteration characterized by illite-I-S may have locally modified the original gold grades.

Increasing use of irrigation in India has exacerbated the problems of soil salinity and sodicity. The present study was undertaken on shrink-swell soils from Maharastra State to determine if changes in soil chemistry due to irrigation have affected the clay mineralogy. Twenty six samples (15 locations) of irrigation-induced, saline-sodic, shrink-swell soils and 27 samples (22 locations) of normal un-irrigated (rain-fed) shrink-swell soils were studied using X-ray powder diffraction (XRPD), infrared spectroscopy (FTIR), and scanning and transmission electron microscopy (SEM, TEM). The XRPD analysis of the <0.2 µm fraction of rain-fed, shrink-swell soils indicates a predominance of dioctahedral smectite with minor to trace amounts of kaolinite and chlorite. Traces of palygorskite (1–4%) were detected in three samples. In contrast, palygorskite is a common component (1–20%) of the fine-clay fraction of saline-sodic soils. Quantitative analysis of palygorskite by XRPD in whole-soil (<2 mm) samples showed that saline-sodic soils contain up to 20 wt.% of palygorskite, whereas palygorskite was only detectable (1.5 wt.%) in one sample of the rain-fed set. The SEM, TEM, and FTIR confirm the presence of Fe-rich palygorskite in saline-sodic soils and demonstrate that the fibrous palygorskite crystals are exceedingly small (∼0.5 µm long). Delicate palygorskite fibers radiate from the margins of smectite plates suggestive of a pedogenic origin and a close genetic relationship between smectite and palygorskite. The compositions of saturation-paste extracts display a shift from the stability field of smectite in rain-fed soils to that of palygorskite in saline-sodic soils. Thus the occurrence and formation of palygorskite appears to be related to the change in land management from rain-fed to irrigated agriculture. This change has occurred over a period of no more than 40–50 y, implying that palygorskite formation in the irrigated, saline-sodic soils has been an extremely rapid process.

Montmorillonite modified with the cationic biopolymer, chitosan, has, in weak acidic solutions, protonated amine groups which act as anion-adsorption sites. Due to the specific surroundings of the adsorption sites and diffusion paths in the interlayer of chitosan-montmorillonite, preferential adsorption of certain anions is likely. In the present study, the adsorption properties for the inorganic anions Cr(VI) and As(V) were determined, taking into account solution pH and competitive adsorption in the presence of Cl− and ${\text{SO}}_4^{2-}$${\rm{SO}}_4^{2 - }$. Chitosan-montmorillonite was prepared by adding an amount of chitosan equivalent to 500% of the cation exchange capacity (CEC) at pH 5 and 75°C. The resulting anion exchange capacity was ∼0.34 molc/kg. The adsorption properties for As(V) and Cr(VI) were determined with the batch technique at pH 3 to 9. Adsorption isotherms were fitted to the Langmuir and Dubinin-Radushkevich equations and judged quantitatively by the correlation coefficient. To describe the competitive adsorption, the selectivity (S) was determined by the ratio of amounts of anions A and B adsorbed (qA/qB) in a binary system. The ionic species adsorbed, i.e. either Cr(VI) or As(V), depended on the pH, as did the degree of protonation of the amine groups, and this played a decisive role in the amount of anions adsorbed. The maximum amount of Cr(VI) adsorbed was 180 mmol/kg at pH 3.5, whereas for As(V) it was 120 mmol/kg at pH 4.0 to 5.0. The adsorption process of Cr(VI) and As(V) fit well to the Langmuir isotherm. By increasing the concentration of the competitive anion, Cl−, in solution, the amount of Cr(VI) and As(V) adsorbed remained almostconstant, whereas ${\text{SO}}_4^{2-}$${\rm{SO}}_4^{2 - }$ had a more pronounced competitive effect. At concentration ratios of 0.5 and 1 for ${\text{SO}}_4^{2-}$${\rm{SO}}_4^{2 - }$ to Cr(VI) and As(V), respectively, the sorption capacity decreased by 10 and 25%, respectively. The sequence of the selectivity was: $\text{Cr}\left(\text{VI}\right)>{\text{SO}}_4^{2-}>\text{As}\left(\text{V}\right)>{\text{Cl}}^{-}$${\rm{Cr}}\left( {{\rm{VI}}} \right) > {\rm{SO}}_4^{2 - } > {\rm{As}}\left( {\rm{V}} \right) > {\rm{C}}{{\rm{l}}^ - }$. Chitosan-montmorillonite showed a high selectivity for Cr(VI), which adsorbed chemically. Despite the lower affinity for As(V) and physical adsorption, the adsorption capacity was relatively high.

Particle-size distribution (PSD) is a fundamental soil property usually reported as discrete clay, silt, and sand percentages. Models and methods to effectively generate a continuous PSD from such poor descriptions using another property would be extremely useful to predict and understand in fragmented distributions, which are ubiquitous in nature. Power laws for soil PSDs imply scale invariance (or selfsimilarity), a property which has proven useful in PSD description. This work is based on two novel ideas in modeling PSDs: (1) the concept of selfsimilarity in PSDs; and (2) mathematical tools to calculate fractal distributions for specific soil PSDs using few actual texture data. Based on these ideas, a random, multiplicative cascade model was developed that relies on a regularity of scale invariance called ‘log-selfsimilarity.’ The model allows the estimation of intermediate particle size values from common texture data. Using equivalent inputs, this new modeling approach was checked using soil data and shown to provide greatly improved results in comparison to the selfsimilar model for soil PSD data. The Kolmogorov-Smirnov D-statistic for the log-selfsimilar model was smaller than the selfsimilar model in 92.94% of cases. The average error was 0.74 times that of the selfsimilar model. The proposed method allows measurement of a heterogeneity index, H, defined using Hölder exponents, which facilitates quantitative characterization of soil textural classes. The average H value ranged from 0.381 for silt texture to 0.838 for sandy loam texture, with a variance of <0.034 for all textural classes. The index can also be used to distinguish textures within the same textural class. These results strongly suggest that the model and its parameters might be useful in estimating other soil physical properties and in developing new soil PSD pedotransfer functions. This modeling approach, along with its potential applications, might be extended to fine-grained mineral and material studies.

The use of porous filters is indispensable in laboratory- and field-scale diffusion studies, where sample confinement is needed for mechanical reasons. Examples are diffusion studies with compacted swelling clays or brittle clay stones. Knowledge of the diffusion properties of these filters is important in cases where they contribute significantly to the overall diffusive resistance in the experimental setup. In the present study, measurements of effective diffusion coefficients (Db) in porous, stainless steel filter discs are reported for tritiated H2O (HTO), 22Na+, Cs+, and Sr2+ before and after use of the filters in diffusion experiments with different clay minerals. The Db values for used filters were found to be less than those of the as-received filters by ∼30–50%. The Db values measured for the diffusion of HTO, 22Na+, Cs+, and Sr2+ in unused and used stainless steel filter discs correlated fairly well with the respective molecular diffusion coefficients in bulk water. Although such correlations are inherently associated with some uncertainties, they allow reasonable estimates to be made for diffusants for which no Db values are available. For the first time, a procedure is outlined that allows an integrative assessment to be made for the impact of the uncertainties in the filter diffusion properties on the combined standard uncertainties of the diffusion parameters obtained from through-diffusion experiments. This procedure can be used in the design and optimization of through-diffusion experiments in which the diffusive resistance of the porous filters must not be ignored. Shown here, as a general rule of thumb, is that, if the effective diffusion coefficient in the porous filter is at least three times larger than that in the clay, the choice of geometrical boundary conditions is rather uncritical, as long as the thickness of the clay sample is greater than that of the porous filters.

Low-temperature FTIR spectroscopy was used to characterize the v(OH) region of kaolin-group minerals including well ordered to poorly ordered kaolins from Georgia, Brazil, and England, along with samples of discrete dickite and nacrite. Low-temperature FTIR spectra were useful in resolving dickite- and nacrite-like features present in the spectra of kaolins when cooled to <30 K. These features were not resolved at room temperature and only partially resolved at liquid N2 temperature (77 K). The room-temperature and low-temperature positions of the ν(OH) bands of kaolinite, dickite, and nacrite were linearly correlated with the interatomic OH⋯O distances and this relationship served as the basis for polytype/disorder identification. Dickite or dickite-like disorder was found in high Hinckley-Index kaolinite from Keokuk, Iowa, and from Cornwall, England. Dickite- and nacrite-like features were observed in both high- and low-Hinckley-index kaolinite and the amounts of these stacking sequences generally increased with decreasing Hinckley Index.

Caliche in various forms, namely powdery, nodule, tube, fracture-infill, laminar crust, hard laminated crust (hardpan), and pisolitic crust, is widespread in the Mersin area in southern Turkey. It generally occurs within and/or over the reddish-brown mudstone of the Kuzgun Formation (Tortonian, Miocene) and alluvial red soils of the Quaternary. The mineralogical distribution along representative caliche profiles was examined by X-ray diffraction, scanning electron microscopy, differential thermal analysis-thermal gravimetry, and chemical techniques. Calcite is the most abundant mineral associated with minor amounts of palygorskite in caliche samples, whereas smectite is prevalent mainly in the reddish-brown mudstone and alluvial red soils of the caliche parent materials and is associated with appreciable amounts of palygorskite. These minerals are also accompanied by trace amount of illite, quartz, feldspar, and a poorly crystalline phase. Palygorskite fibers and fiber bundles were developed authigenically on euhedral or subhedral calcite crystals of the caliche units and at the edges of smectite flakes in the caliche host-rocks or sediments. Intense, continuous evaporation of subsurface soil-water resulted in an increase in pH and the dissolution of detrital smectite within the red mudstones and alluvial red soils that enclose the isolated caliche forms, and caused an increase in the Al+Fe and Mg/Ca ratio, favoring the formation of palygorskite under alkaline conditions. The calcium required for caliche formation may have originated from eolian dust, detrital carbonate minerals, and/or other caliche materials, which are dissolved by carbonic acid.

Ionic liquids intercalated in kaolinite constitute a novel class of nanostructured material. Kaolinite-pyrrolidinium halide intercalates have been prepared successfully by reacting the pyrrolidinium salts with kaolinite which was preintercalated with dimethyl sulfoxide (DMSO) using the melt condition under N2. X-ray diffraction, 13C magic angle spinning nuclear magnetic resonance, differential thermal analysis (DTA)-thermal gravimetric analysis, and Fourier transform infrared spectroscopy confirm the displacement of DMSO during the intercalation process. Based on results from the various characterization techniques, a structural model is proposed in which one mole of the pyrrolidinium salt covers two or three structural units of kaolinite, depending on the structure and size of the salt. The thermal stability was improved remarkably after intercalation of the pyrrolidinium salts, compared to the pre-intercalate. The DTA-TGA data show that the largest number of organic units released and decomposed, occurs under N2 flow, at temperatures ranging from 260 to 340°C, depending on the nature of the intercalated organic salts.

A method for the determination of the water-uptake capacity of powders which is said to be applicable to viscose silk, specific celluloses, soap, paint, glue, gelatine, adhesives based on starch, biological samples, and soils, was proposed by Enslin in 1933 and has been improved several times since then. Today in Germany, the so-called Enslin-Neff method is used by the clay industry, in civil engineering, and in soil science. Many authors have identified the influence of evaporation on the results obtained using this method and the latest modification of the Enslin-Neff method was introduced by Dieng in 2005 where a balance was used instead of a burette to record the water-uptake capacity. It is proved here that the Dieng method actually operates correctly, independent of relative humidity. Therefore, a significantly improved reproducibility of the Dieng method compared to the traditional method was expected. However, it was found that the Dieng apparatus has specific sources of error (e.g. constance of the balance over 24 h) and varying the relative humidity no longer has a systematic affect on the results.

The reproducibility of the traditional Enslin-Neff method is strongly influenced by variations in ambient conditions in the laboratory (particularly temperature and relative humidity). Application of the Dieng method in different laboratories with varying ambient conditions will lead to improved reproducibility and comparability of results. In addition, results from the Dieng apparatus can be collected using a connected computer and this represents an important advance also.

Clayey soils are essential materials used to reduce hydraulic conductivity and pollutant migration, common at sites of waste disposal. This study investigates the possible use of a Tunisian soil as a lining material for disposal sites for acidic-fluoride wastes. A permeability test on a waste-solution sample (pH = 2.7) obtained from a disposal site in southern Tunisia was conducted over a period of about 2 years. The test results show that the permeability decreased with time until stabilized at 8.33 × 10−11 m/s. After the permeability test, the samples retrieved from the permeameter show a degradation state which varied with the thickness of the specimen. These samples can be classified into three zones (Z1: unaffected, Z2: moderately affected; and Z3: extensively affected). Physicochemical characterization of the three samples (Z1, Z2, and Z3), and of the original argillaceous soil, was by X-ray diffraction, Fourier transform infrared spectroscopy, differential thermal and thermal gravimetric analysis, 29Si and 27Al nuclear magnetic resonance, and N2-adsorption techniques. The original sample consists essentially of palygorskite, kaolinite, and quartz. Sample Z3 underwent complete dissolution of kaolinite which supports the precipitation of fluoroaluminate and the appearance of an X-ray amorphous silica phase. In samples Z1 and Z2, the soil adsorbs fluoride at a rate of ∼68.5 mg/g and is highly resistant to acidic attack.

Using 19F magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, we show that most of the fluoride present in the KGa-lb reference kaolinite from Washington County, Georgia, occurs as a surface-adsorbed species bonded to Al. This surface fluoride can be removed from the <2 µm fraction by acid wash, but is largely retained in the coarse fraction. Correlation of integrated 19F NMR peak intensities with fluoride sorption experiments indicates a bulk F content of ∼144 ppm for KGa-1b, of which ∼30% substitutes for hydroxyl sites in the mineral structure and the remaining 70% occurs adsorbed on particle surfaces, corresponding to an edge surface fluoride density of ∼0.7 F− nm−2. 19F{27Al} TRAPDOR (TRAnsfer of Populations in DOuble Resonance) NMR data for the original kaolinite and for products of F− sorption experiments at pH 4 show that all of the observed 19F signals arise from fluoride bonded to Al atoms. Furthermore, bridging Al-F-Al sites and terminal Al-F give distinctly different TRAPDOR fractions allowing assignment of resolved peaks based on the number of Al in the first coordination sphere. This result was confirmed for fluoride adsorbed to the surface of gibbsite from aqueous suspension. No evidence was found for Si-F-type environments on the kaolinite surfaces.

The properties that might influence the sequestration of aflatoxin B1 (AfB1) were examined. Laser-diffraction, particle-size analysis (LDPSA) indicated that the particle size of the smectite influences the amount of AfB1 adsorbed. Effective adsorbent smectites disperse well under combined sodium hexametaphosphate solution and ultrasonic agitation. Particle size explained 66% of the variability for most of the samples investigated in an ‘as-received’ state. One effective adsorbent smectite was especially well aggregated and required additional physical dispersion, thus raising the correlation to 73%. Transmission electron microscope (TEM) images show typical smectites and reveal the very diverse morphology of smectites in bentonites. Thin, cloud-like smectite, in TEM images, related positively to AfB1-adsorption capacity. Particles that often fold and are usually ∼0.5 µm across seem to be optimal. The selection of criteria for evaluating these smectites provides a scientific basis for their selection to obtain reliable performance. Particle size is of particular importance as outlined below, and use of LDPSA makes it possible to perform the analysis efficiently and with precision.

The fungus Aspergillus flavus Link ex Fries can infect grains and oil seeds and develop Aflatoxin B1 (AfB1) in the fieldor in storage. Aflatoxin contamination is a serious health hazard — it is extremely toxic and hepatocarcinogenic for animals and humans. A practical approach to solve this problem is to use smectite clay as an amendment to animal feed. The objective of this research is to investigate smectite clay—AfB1 interactions by employing Fourier transform infrared (FTIR) spectroscopy to determine how clay composition influences AfB1 adsorption by smectites. When AfB1 was present in the clay, the spectral region from 1800 to 1300 cm−1 was altered, and the regions between 4000 and 1800 and 1300 and 400 cm−1 were unchanged except for the intensity in the broad region near 3400 cm−1 related to the abundance of water. The 1300–400 cm−1 region is attributed only to smectite clay properties, and it relates to the relative adsorption potential of the different smectites. Bonding between AfB1 and smectite clay appears to be in the furan rings. Other possible bonding is with the two oxygens in the coumarin ring of AfB1 and interlayer cations or their associated water molecules. The FTIR evidence of octahedral Fe in smectite and amorphous silica in the clays both indicate greater AfB1 adsorption potential. Other smectites with spectral absorption indicating predominantly Al in the octahedral positions adsorbed less AfB1.

We used high-resolution X-ray texture goniometry to quantify changes in the mm-scale orientation of phyllosilicate minerals in a suite of Paleogene mudstones from the Podhale Basin in southern Poland. The sample set covers an estimated range of burial depths between 2.4 and 7.0 km, corresponding to a temperature range of 60–160°C. Although mechanical compaction has reduced porosities to ∼10% in the shallowest samples, the phyllosilicate fabric is only modestly aligned. Coarser-grained (>10 µm) detrital chlorite and mica appear to be more strongly aligned with (001) parallel to bedding, suggesting their deposition as single grains rather than as isotropic flocs or aggregates. From 2.4 to 4.6 km, R0 illite-smectite with 40–50% illite layers changes to R1 illite-smectite with 70–80%) illite layers. At the same time kaolinite is lost and diagenetic chlorite is formed. The mineralogical changes are accompanied by a strong increase in the alignment of illite-smectite, chlorite, and detrital illite, parallel to bedding and normal to the presumed principal effective stress. We propose that the development of a more aligned I-S fabric results from the dissolution of smectite and the growth of illite with (001) normal to the maximum effective stress. Water released by illitization may act as a lubricant for the rotation of all platy minerals into nanoporosity transiently formed by the illitization reaction. At greater depths and temperatures, further illitization is inhibited through the exhaustion of K-feldspar. After the cessation of illitization, a further 2.4 km of burial only results in a small increase in phyllosilicate alignment. At such small values for porosity and pore size, increasing stress does not substantially reorient phyllosilicates in the absence of mineralogical change.

Upper Cretaceous—Danian chalk and interbedded clay-rich layers from wells of the South Arne Field and adjacent wells in the North Sea and from Stevns in Zealand have been investigated to determine the clay mineralogy in the chalk and the origin of the interbedded clay-rich layers. The mineralogy, with emphasis on clay mineralogy, was determined after removal of the calcite by dissolution at pH 4.5–5 in order to preserve the other minerals. Generally, mixed-layer minerals are the dominant clay minerals, except for two wells, Rigs-1 and Rigs-2, where a 3D ordered kaolinite prevails. A detailed structural characterization of the mixed-layer minerals was carried out by modeling of the X-ray diffraction (XRD) patterns. In most of the samples dominated by mixed-layer minerals, two mixed-layer phases, a high-smectite illite-smectite (I-S) and a low-smectite illite-smectite-chlorite (I-S-Ch), prevail, irrespective of depth or location of the samples. However, some samples contain I-S-Ch and ordered S-Ch, and others a chlorite-serpentine (Ch-Sr) phase, and these samples probably formed during diagenesis at higher temperatures. The clay-rich layers and the adjacent chalk have the same or quite similar clay mineralogy, both with respect to kaolinite vs. mixed-layer minerals and with respect to their detailed structure. In conclusion, the kaolinite is detrital and the I-S minerals formed in the chalk from volcanic ash. The main conclusion is that the clay-rich layers in the North Sea chalk formed by dissolution of the calcite in the chalk and that this dissolution took place at burial depths of >1 km, probably through migration of solutions through permeable chalk layers.

Thirty six bentonite samples from 16 different locations were examined in order to demonstrate the applicability of a new Rietveld description approach for quantitative phase analysis. X-ray diffraction patterns of the bulk material were obtained and analyzed by the Rietveld method. The samples contain up to ten different minerals, with dioctahedral smectite as the major component. A model for turbostratic disorder of smectites was formulated inside a structure-description file of the Rietveld program BGMN. The quality of the refinements was checked using an internal standard mineral (10.0 or 20.0 wt.% corundum) and by cross-checking results with X-ray fluorescence (XRF) data. The corundum content was reproduced with only small deviations from the nominal values. A comparison of the chemical composition obtained by XRF and the composition as re-calculated from quantitative Rietveld results shows a satisfactory agreement, although X-ray amorphous components such as volcanic glasses were not considered. As a result of this study, the Rietveld method combined with the new structure model for turbostratic disorder has proven to be a suitable method for routine quantitative analysis of bentonites with smectites as the dominant clay minerals.

Clay minerals in chilled or brecciated margins (altered glass) and massive inner crystalline parts (mesostasis) of three basalt-hawaiite bodies from Mururoa Atoll (French Polynesia) have been studied in order to compare their chemical and mineralogical compositions. Polyphase assemblages comprise di- and trioctahedral phases, both of which consist of non-expandable layers (chlorite, celadonite) and two types of expandable layers (saponite and Fe-rich smectite or ‘nontronite-like’ material). The presence of the Fe-rich clays is supported by the presence of the X-ray diffraction 060 peak at 1.51–1.52 Å and of the infrared absorption bands at 875 and 822 cm−1 (Fe3+-Al-OH and Fe3+-Fe3+-OH groups, respectively). The chemical composition of the Fe-rich smectites does not fit with the theoretical nontronite field. The layer charge averages 1 per Si4O10 making these Fe-rich smectites close to ‘celadonite-type’ clays. This could explain the presence of mixed-layer celadonite-smectite. Plotted in an M+/4Si vs. Fe/sum octahedral cations diagram, the chemical compositions of clay minerals in the mesostasis form a continuous field limited by the celadonite-high-charge nontronite-like smectite and chlorite end-members. The clay assemblages are different from those formed in hydrothermal systems or low-grade metamorphic conditions which are characterized by the sequence: saponite → randomly ordered chlorite-smectite mixed-layered minerals (MLMs) → corrensite → chlorite. The systematic presence of Fe-rich clays either in the altered chilled margins or in the massive inner parts of the basalt-hawaiite bodies (high-charge nontronite-like smectite and mixed-layer nontronite-celadonite) makes the Mururoa sea-mount a potential terrestrial analogue for Mars surface exploration.

Smectite formation in alkaline-saline environments has been attributed to direct precipitation from solution and/or transformation from precursor minerals, but these mechanisms are not universally agreed upon in the literature. The objective of this work was to investigate the mineralogy of smectites in the soils surrounding a representative alkaline-saline lake of Nhecolândia, a sub-region of the Pantanal wetland, Brazil, and then to identify the mechanisms of their formation.

Soils were sampled along a toposequence and analyzed by X-ray diffraction, transmission electron microscopy-energy dispersive X-ray analysis, and inductively coupled plasma-mass spectrometry. Water was collected along a transect involving the studied toposequence and equilibrium diagrams were calculated using the databases PHREEQC and AQUA.

The fine-clay fraction is dominated by smectite, mica, and kaolinite. Smectites are concentrated at two places in the toposequence: an upper zone, which includes the soil horizons rarely reached by the lake-level variation; and a lower zone, which includes the surface horizon within the area of seasonal lake-level variation. Within the upper zone, the smectite is dioctahedral, rich in Al and Fe, and is classified as ferribeidellite. This phase is interstratified with mica and vermiculite and has an Fe content similar to that of the mica identified. These characteristics suggest that the ferribeidellite originates from transformation of micas and that vermiculite is an intermediate phase in this transformation. Within the lower zone, smectites are dominantly trioctahedral, Mg-rich, and are saponitic and stevensitic minerals. In addition, samples enriched in these minerals have much smaller rare-earth element (REE) contents than other soil samples. The water chemistry shows a geochemical control of Mg and saturation with respect to Mg-smectites in the more saline waters. The REE contents, water chemistry, and the presence of Mg-smectite where maximum evaporation is expected, suggest that saponitic and stevensitic minerals originate by chemical precipitation from the water column of the alkaline-saline lake.

Air-dried and high-pressure frozen/freeze-etched samples of clinopyroxene and smectite in a saprolitized clinopyroxenite from Koua Bocca, Ivory Coast, West Africa, were compared to characterize textures developed during natural weathering of chain silicates. Comparison with air-dried material allowed evaluation of high-pressure cryofixation as a technique for preserving textures of hydrated clay minerals. Air-dried pyroxene surfaces appear very smooth. Small, flat lamellae, oriented parallel to the c axis, lend a distinct splintery appearance to pyroxene surfaces in fully hydrated samples. These lamellae often display a combination of straight (110) pyroxene edges and a crinkled border, suggestive of smectite. Narrow lenticular (110) open cleavages occur in both preparations and are not a pressurization artifact. Most often these openings contain no secondary minerals. Spaces between pyroxene denticles, lined with collapsed smectite in air-dried samples, are filled with thin packets of anhedral smectite crystallites oriented face to face when hydrated. Smectite microboxwork preserves original topotactic textures developed during isovolumetric pyroxene transformation, and smaller nanoporosity appears in hydrated cryofixed examples. Occasional regions of edge-to-face ‘house of cards’ texture also occur. Elimination of sample preparation artifacts induced by surface tension during air drying demonstrates that pores actually present during a hydration reaction-driven weathering episode are smaller and more numerous than would be inferred from examining air-dried materials.