Showing papers in "Clays and Clay Minerals in 1994"
TL;DR: In this article, the absorption of H20 in montmorillonites was studied using reflectance spectroscopy in the infrared and band assignments were made for absorption in the 3 ~tm region due to structural OH vibrations, symmetric and asymmetric 1-120 stretching vibrations and the H20 bending overtone.
Abstract: Interlayer cations and moisture content greatly influence the molecular vibrations of H20 in montmoriUonite as shown through reflectance spectroscopy in the infrared. The absorptions due to H~O have been studied in montmorillonites exchanged with H, Na, Ca, Mg and Fe 3+ interlayer cations under variable moisture environments. Band assignments have been made for absorptions in the 3 ~tm region due to structural OH vibrations, symmetric and asymmetric 1-120 stretching vibrations and the H20 bending overtone. Changes in the energies of the absorptions due to H20 stretching vibrations were observed as the samples were dehydrated by reducing the atmospheric pressure. Absorptions near 3620 cm -1 and 3550 cm -1 have been assigned to water bound directly to cations (inner sphere) and surface- bonded H20 and absorptions near 3450 cm -1 and 3350 cm ~ have been assigned to additional adsorbed water molecules. Band assignments have been made for combination bands in the near-infrared as well. Absorptions near 1.41 ~m and 1.91/zm are assigned to bound H20 combination bands, while the shoulders near 1.46 izm and 1.97 ~*m are assigned to combinations of additional H20 molecules adsorbed in the interlayer regions and along grain surfaces.
389 citations
TL;DR: In this article, X-ray absorption fine structure (XAFS) spectra were collected on a series of ferrihydrite samples prepared over a range of precipitation and drying conditions.
Abstract: X-ray absorption fine structure (XAFS) spectra were collected on a series of ferrihydrite samples prepared over a range of precipitation and drying conditions. Analysis of the XAFS pre-edge structures shows clear evidence of the presence of lower coordination sites in the material. These sites, which are most likely tetrahedral, are believed to be at the surface and become coordination unsaturated (CUS) after dehydroxylation. With chemisorbed water molecules, the CUS sites become the crystal growth sites responsible for the phase transformation of ferrihydrite to hematite at low temperatures. On the other hand, when impurity anions such as SiO4−4 are present in the precipitation solution, the CUS sites may instead absorb the impurity anions, thereby blocking the crystal growth sites and inhibiting the formation of hematite.
209 citations
TL;DR: In this article, the thermally induced transformation of synthetic lepidocrocite (3,FeOOH) to Maghemite (a-Fe203) was studied using IR spectroscopy.
Abstract: Infrared (IR) spectroscopy, in combination with magnetic methods, was used to study the thermally induced transformation of synthetic lepidocrocite (3,-FeOOH) to maghemite (~-Fe203). Mag- netic analyses showed that the thermal conversion began at about 175"C with the formation of super- paramagnetic maghemite clusters. The overall structural transformation to ferrimagnetic'y-Fe203 occurred at 200"C and was complete around 300"C. At higher temperatures, the maghemite converted into hematite (a-Fe203). Observation of the transformation from 3,-FeOOH to "y-Fe203 using variable-temperature IR spectroscopy indicated that dehydroxilation on a molecular level was initiated between 145"C and 155*C. The lag time between the onset of the breaking of OH bonds and the release of H20 from lepidocrocite around 1750C can be explained by diffusive processes. Overall dehydroxilation and the subsequent break- down of the lepidocrocite structure was complete below 219*(3. The comparison of the magnetic and IR data provides evidence that the dehydroxilation may precede the structural conversion to maghemite.
121 citations
TL;DR: X-ray absorption spectroscopy (XAS) was used to determine the local molecular environment of Co(II) surface complexes sorbed on three different kaolinites at ambient temperature and pressure in contact with an aqueous solution as mentioned in this paper.
Abstract: X-ray absorption spectroscopy (XAS) was used to determine the local molecular environment of Co(II) surface complexes sorbed on three different kaolinites at ambient temperature and pressure in contact with an aqueous solution. Interatomic distances and types and numbers of backscattering atoms have been derived from analysis of the extended X-ray absorption fine structure (EXAFS). These data show that, at the lowest amounts of Co uptake on kaolinite (0.20--0.32 ~mol m-2), Co is surrounded by ~60 atoms at 2.04-2.08 A and a small number orAl or Si atoms (N = 0.6-1.5) at two distinct distances, 2.67-2.72 A and 3.38-3.43/~. These results indicate that Co bonds to the kaolinite surface as octahedrally coordinated, bidentate inner-sphere mononuclear complexes at low surface coverages, confirming indirect evidence from solution studies that a fraction of sorbed Co forms strongly bound complexes on kaolinite. In addition to inner-sphere complexes identified by EXAFS spectroscopy, solution studies provide evi- dence for the presence of weakly bound, outer-sphere Co complexes that cannot be detected directly by EXAFS. One orientation for inner-sphere complexes indicated by XAS is bidentate bonding of Co to oxygen atoms at two A1-O-Si edge sites or an A1-O-Si and AI-OH (inner hydroxyl) edge site, i.e., comer- sharing between Co octahedra and AI and Si potyhedra. At slightly higher surface sorption densities (0.51- 0.57/~mol m-2), the presence of a small number of second-neighbor Co atoms (average Nco < 1) at 3.10- 3.13/~ indicates the formation of oxy- or hydroxy-bridged, multinuclear surface complexes in addition to mononuclear complexes. At these surface coverages, Co-Co and Co-AI/Si distances derived from EXAFS are consistent with edge-sharing between Co and AI octahedra on either edges or (001) faces of the aluminol sheet in kaolinite. Multinuclear complexes form on kaolinite at low surface sorption densities equivalent to < 5% coverage by a monolayer of oxygen-ligated Co octahedra over the N2-BET surface area. These spectroscopic results have several implications for macroscopic modeling of metal ion uptake on kaolinite: 1) Primary binding sites on the kaolinite surface at low uptake are edge, non-bridging AI- OH inner hydroxyl sites and edge AI-O-Si bridging oxygen sites, not Si-OH sites typically assumed in sorption models; 2) specific adsorption of Co is via bidentate, inner-sphere complexation; and 3) at slightly higher uptake but still a small fraction of monolayer coverage, formation of Co multinuclear complexes, primarily edge-sharing with A1-OH octahedra, begins to dominate sorption.
117 citations
TL;DR: In this paper, the acid hydrolysis products of trioctahedral fluorohectorite and phlogopite have been assessed 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.
Abstract: The acid hydrolysis products of trioctahedral fluorohectorite and phlogopite have been inves- tigated 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 Q" SiO4 sites accompanied the depletion of Mg 2+ from the octahedral sheet of both 2:1 layered structures Depending on the extent of hydrolysis, the products derived from fluo- rohectorite exhibited surface areas up to 208 mVg, whereas phlogopite hydrolysis products gave values <20 mY/g The dramatic differences in surface areas were not related to differences in hydrolysis mech- anisms 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 Mg 2+ 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
113 citations
TL;DR: The electrochemical properties of kaolinite before and after modification with chlorodimethyl octadecylsilane have been studied by electrophoretic mobility, surface charge titration, and extrapolated yield stress measurements as a function of pH and ionic strength as mentioned in this paper.
Abstract: The electrochemical properties of kaolinite before and after modification with chlorodimethyl- octadecylsilane have been studied by electrophoretic mobility, surface charge titration, and extrapolated yield stress measurements as a function of pH and ionic strength. A heteropolar model of kaolinite, which views the particles as having a pH-independent permanent negative charge on the basal planes and a pH- dependent charge on the edges, has been used to model the data. The zeta potential and surface charge titration experimental data have been used simultaneously to calculate acid and ion complexation equi- librium constants using a surface complex model of the oxide-solution interface. The experimental data were modeled following subtraction of the basal plane constant negative charge, describing only the edge electrical double layer properties. Extrapolated yield stress measurements along with the electrochemical data were used to determine the edge isoelectric points for both the unmodified and modified kaolinite and were found to occur at pH values of 5.25 and 6.75, respectively. Acidity and ion complexation constants were calculated for both sets of data before and after surface modification. The acidity constants, pKat = 5.0 and pKa2 = 6.0, calculated for unmodified kaolinite, correlate closely with acidity constants determined by oxide studies for acidic sites on alumina and silica, respectively, and were, therefore, assigned to pH-dependent specific chemical surface hydroxyl groups on the edges of kaolinite. The parameters calculated for the modified kaolinite indicate that the silane has reacted with these pH- dependent hydroxyl groups causing both a change in their acidity and a concomitant decrease in their ionization capacity. Infrared data show that the long chain hydrocarbon silane is held by strong bonding to the kaolinite surface as it remains attached after washing with cyclohexane, heating, and dispersion in an aqueous environment.
107 citations
TL;DR: In this article, infrared spectroscopy is used to distinguish between octahedral and tetrahedral substitutions in an interstratified illite-smectite clay, and the Hofmann-Klemen (Li) test suggests that the A1Mgr7 and FeMgD OCTAhedral vacancies are preferentially occupied by Li after thermal treatment at 250#C.
Abstract: Infrared spectroscopy is used to distinguish between octahedral and tetrahedral substitutions in an interstratified illite-smectite clay. The Hofmann-Klemen (Li) test suggests that the A1Mgr7 and FeMgD octahedral vacancies are preferentially occupied by Li after thermal treatment at 250#C. The ammonium (Chourabi-Fripiat) test reveals the beidellitic character by the formation of two OH stretching modes upon deammonation. The illitic layers are not affected since K is not exchangeable.
105 citations
TL;DR: In this paper, the substitution of Fe3+ in the kaolinite structure is studied by EPR spectrometry and by FTIR spectrometers on a large set of kaolins from different origins.
Abstract: The substitution of Fe3+ in the kaolinite structure is studied by EPR spectrometry and by FTIR spectrometry on a large set of kaolins from different origins (sedimentary and primary ores, soil kaolins). The IR bands at 3598 and 875 cm−1, observed in the literature only in the case of disordered kaolins or in Fe-rich environments (synthetic, lateritic), are revealed by high-resolution IR analysis, whatever the origin and the total Fe content of the samples. The EPR bands corresponding to Fe3+ substituted in sites II of the octahedral sheet increase when the IR absorbance near 3600 cm−1 increases. Two IR absorption bands near 4465 cm−1 and 7025 cm−1 are observed for the first time, both in transmission and diffuse reflectance on all samples. These bands are assigned to the combination of the 3598 and 875 cm−1 bands and to the first harmonic of the band at 3598 cm−1, respectively. The area of the band at 4465 cm−1 in diffuse reflectance is quantitatively correlated to the abundance of Fe3+ located in centers II as measured by ESR. This directly confirms the assignment of the two IR bands at 3598 and 875 cm−1 to OH stretching and deformation vibration bands in octahedral FE3+ environment in the kaolinite structure, respectively. Effects due to the size of particles and to the main kaolins impurities on the near infrared spectra, are also discussed.
105 citations
TL;DR: In this article, a new, well-ordered, thermally robust ethylene glycol intercalate of kaolinite was formed by refluxing the dimethyl sulfoxide intercalates of Kao-DMSO with dry E.G., which is characterized by a dool of 9.4 A.
Abstract: A new, well-ordered, thermally robust ethylene glycol intercalate of kaolinite was formed by refluxing the dimethyl sulfoxide intercalate of kaolinite (Kao-DMSO) with dry ethylene glycol (EG). This new phase (Kao-EG 9.4 A) which is characterized by a dool of 9.4 A is distinct from a previously reported ethylene glycol intercalated phase of kaolinite (Kao-EG 10.8/k) which has a dool of 10.8 ,~. The char- acterization of these two phases was studied by XRD, NMR, FTIR, and TGA/DSC. It was found that the concentration of water in the ethylene glycol reaction media played a crucial role in governing which of the phases predominated. Water favored Kao-EG 10.8 A formation, while anhydrous conditions favored the formation of Kao-EG 9.4 A. It is hypothesized that Kao-EG 9.4 A is a grafted phase resulting from the product of the condensation reaction between an aluminol group on the interlamenar surface of kaolinite and the alcohol group of ethylene glycol. Ethylene glycol units would be attached to the inter- lamellar surface of kaolinite via A1-O-C bonds. The Kao-EG 9.4 A phase was found to be resistant to both thermal decomposition up to 330~ and also, once formed, in the absence of interlamellar water molecules, to decomposition by hydrolysis in refluxing water.
104 citations
TL;DR: It is argued that none of the five mechanisms mentioned can, in and of itself, account fully for the observed exchange hysteresis and a conceptual model is proposed that combines elements of these five mechanisms and is able to describe, at least qualitatively, the effects of factors such as clay type, electrolyte concentration, and extent of dehydration.
Abstract: The binary exchange of cations on clays and soils is generally regarded as a thermodynamically reversible process. The literature on soil chemistry and geochemistry, however, abounds with reports on cation exchange reactions that appear to have only limited reversibility, i.e., that exhibit hysteresis. A satisfactory explanation of this phenomenon is still lacking, even though a number of mechanisms have been advocated, e.g., charge or site heterogeneity at the surface, differential hydration of cations, dehy- dration of the exchanger, crystalline swelling hysteresis, and inaccessibility of sites caused by domain or quasi-crystal formation. In the present article, the relevant literature is reviewed and analyzed critically. On the basis of available evidence, it is shown that exchangeable cations can be classified into three groups, defined in such a way that hysteresis has, in the literature, generally not been observed when exchange reactions involved cations belonging to the same group, but has often been found when the reactions involved cations from different groups. Furthermore, it is argued that none of the five mech- anisms mentioned can, in and of itself, account fully for the observed exchange hysteresis. A conceptual model is proposed that combines elements of these five mechanisms and is able to describe, at least qualitatively, the effects of factors such as clay type, electrolyte concentration, and extent of dehydration.
99 citations
TL;DR: The large contents of octahedral vacancies in published formulae of chlorite from hydrothermal systems and clastic sequences are largely caused by inclusion of other minerals.
Abstract: The large contents of octahedral vacancies in published formulae of chlorite from hydrothermal systems and clastic sequences are shown to be largely caused by inclusion of other minerals. Verification is provided by analytical electron microscope (AEM) analyses of chlorite in pelitic rocks from the Gasp6 Peninsula, Quebec and the Gulf Coast, Texas. The Gasp6 chlorite occurs as discrete crystals locally coexisting with corrensite, and the Gulf Coast chlorite is free of mixed layers other than local serpentine- like 7-~ layers. Unlike most electron microprobe analyses (EMPA), but like other AEM analyses, the reported chlorite formulae do not have significant octahedral vacancies, are not Si-rich and (Fe + Mg)- poor relative to classic metamorphic chlorite, and have nearly equal amounts oftetrahedral and octahedral AI. The studied chlorites and those in metabasites and clastic rocks that could be positively identified as containing no or minimal mixed layering or submicroscopic intergrowths have little or no Ca or alkalis. In contrast, EMPA of chlorite reported for other clastic sequences show variable amounts ofNa + K + 2Ca that exhibit a poorly defined positive correlation with the proportion of octahedral vacancies. The EMPA of chlorite from the Salton Sea and Los Azufres geothermal fields that were suggested to contain temperature-dependent amounts of tetrahedral A1 (and thus used as "'chlorite geothermometers") show compositional characteristics similar to those reported for several saponite-chlorite transition series in metabasites. Continuous increases in octahedral occupancy and tetrahedral Al with increasing metamorphic grade are attributed to decreases in abundance of mixed layers or fine-grained intergrown minerals that com- monly occur as a result of increasing crystal size and homogeneity in prograde sequences. Use of"chlorite geothermometry'" based on the proportion of apparent octahedral vacancies or tetrahedral A1 is therefore unwarranted and leads to inaccurate temperature estimates.
TL;DR: In this article, 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 were compared.
Abstract: 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.
TL;DR: In this article, the quantity of reduced Ni is determined as a function of the calcination temperature, and the solid phases remaining after reduction are determined with XRD, where the Ni/A1 ratio, pH of precipitation, and calcination temperatures are important preparation conditions.
Abstract: Takovi te, Ni6A12(OH)t6CO 3. 5H20 , with molar Ni/A1 ratios of 2.5 and 3 is prepared by pre- cipitation at pH levels of 7 and 10, periods of reaction of 3 and 20 hr, and calcination temperatures varying between 120 ~ and 853~ The quantity of reduced Ni is determined as a function of the calcination temperature, and the solid phases remaining after reduction are determined with XRD. The Ni/A1 ratio, pH of precipitation, and calcination temperature are important preparation conditions. A precursor of a Ni/A1 ratio of 2.5 precipitated at a pH-level of 10 exhibits on reduction metallic Ni particles of about 15 nm irrespective of the drying or calcination temperature. After reduction, a considerable amount of NaA102 is present. Metallic Ni particles of 6 nm are present in a reduced precursor of Ni/A1 ratio of 2.5 precipitated at a pH-level of 7. The size of the Ni particles present in a reduced precursor of Ni/A1 ratio of 3 precipitated at a pH-level of 10 rises from 4-8 nm to 16 nm after calcination at temperatures increasing from 120* to 853"C. The last precursor contains much less sodium, and shows after reduction a disordered NiO phase containing some alumina. Especially, the takovite with the molar Ni/A1 ratio of 2.5, thoroughly washed to remove Na, and calcined at T < 260"C before reduction of Ni provides a promising catalyst for the production of hydrogen-carbon monoxide flows out of methane and steam.
TL;DR: In this paper, a study was conducted to examine the mineralogical alteration of birnessite after treatment with various concentrations of MnSO4 and solution pH, and specific Mn minerals formed were governed by the pH of the reaction, and the rate of the transformation was determined by Mn2+ concentration and pH.
Abstract: The amount of Mn2+ adsorbed or removed from solution by birnessite is several times greater than its reported cation exchange capacity. Extractability of the sorbed Mn2+ decreases with aging. It is uncertain whether the sorbed Mn2+ is oxidized on the surface or incorporated into the structure of birnessite. Using X-ray powder diffractometry and transmission electron microscopy, a study was conducted to examine the mineralogical alteration of birnessite after treatment with various concentrations of MnSO4 and solution pH. The sorbed Mn2+ was not directly oxidized and remained on the birnessite surface. The sorption of Mn2+ was followed by alteration of birnessite with the formation of new Mn minerals. The specific Mn minerals formed were governed by the pH of the reaction, and the rate of the transformation was determined by Mn2+ concentration and pH. Nsutite and ramsdellite were identified at pH 2.4, crypto-melane at pH 4, groutite at pH 6, and manganite at pH 8. Other Mn minerals formed at these and other pH levels could not be identified. As the concentration of Mn in the solution decreased, the time required to form new minerals from the birnessite increased. The newly formed phases were the result of structural conversion since dissolution of birnessite and reprecipitation of new phases were not observed.
TL;DR: In this article, the potential for surface diffusion of Sr2+, Ca2+ and Na+ on three clays compacted to dry bulk densities of 1.25 and 1.60 Mg/m3 was examined.
Abstract: Surface diffusion, or migration within the electrical double layer next to mineral surfaces, is often invoked as a significant contributor to the overall diffusion coefficient in compacted clays, particularly where model predictions underestimate measured diffusion coefficients. The potential for surface diffusion of Sr2+, Ca2+ and Na+ on three clays compacted to dry bulk densities of 1.25 and 1.60 Mg/m3 was examined. The clays were a bentonite, an illite/smectite, and a glacial lake clay (composed mainly of smectite, illite, kaolinite and quartz). The clays were saturated with a Na-Ca-Cl-dominated synthetic groundwater solution with an effective ionic strength of 220 mol/m3. Total intrinsic diffusion coefficients for the cations were determined from their steady-state flux through compacted clays, and apparent diffusion coefficients were obtained from the time-lag technique. Models of diffusive transport in compacted clays, based only on diffusion in the pore solution, adequately described the diffusion data for all clays and diffusants, and there was no need to invoke other transport mechanisms, like surface diffusion. The data indicate that surface diffusion is not a significant transport mechanism in compacted clays at least to a clay density of 1.60 Mg/m3.
TL;DR: In this article, a new appraisal of radiation-induced defects (RID) in natural kaolinite, i.e., positive trapped holes on oxygen atoms, has been undertaken using Q-band EPR spectra, recorded at 93 K, of irradiated annealed and oriented Kaolinite samples originating from various environments.
Abstract: A new appraisal of radiation-induced defects (RID) in natural kaolinite, i.e., positive trapped holes on oxygen atoms, has been undertaken using Q-band EPR spectra, recorded at 93 K, of irradiated annealed and oriented kaolinite samples originating from various environments. Three different centers were identified. Two of the centers, A- and A'-centers, are trapped holes on oxygen from Si-O bonds. They have a distinct signature and orthogonal orientation, i.e., perpendicular and parallel to the (ab) plane, respectively. The third center, the B-center, is a hole trapped on the oxygen bonding A1 in adjacent octahedral positions (Alv~-O--Alw bridge). This confirmed some previous assignments from the literature, some others are no longer considered as valid. A least squares fitting procedure is proposed to assess the RID concentration in any kaolinite. It allows a quantitative approach of the thermal stability of RID. Isochronal annealing shows that the thermal stability of the centers decreases in the order A, A', B over the temperature range 0-450~ (1) B-center is completely annealed above 300~ (2) A'-center can be annealed by heating at 400~ for more than two hours; (3) A-center is stable up to 450~ The activation energy and the magnitude of the mean half- life for A-center is evaluated through isothermal annealing at 350, 375 and 400*(2, with Ea = 2.0 eV + 0.2, and tv, > l0 ~2 years at 300 K. The stability of A-center seems to decrease with increasing crystalline disorder. Nevertheless, it is high enough for radiation dosimetry using kaolinites from any environment on the Earth's surface.
TL;DR: In this paper, the authors collected greenish-collected clay samples from submarine hydrothermal chimneys of the Galapagos Rift and Mariana Trough, showing that Fe oxidizing, sheath forming bacteria played a decisive role in nontronite formation at these sites.
Abstract: Clay samples of greenish colour were collected from submarine hydrothermal chimneys of the Galapagos Rift and Mariana Trough. Mineralogical and chemical investigations of the clay by scanning and transmission electron microscopy, X-ray diffraction, differential thermal analysis, infrared-spectros-copy, X-ray fluorescence, and determination of specific surface area, and oxygen isotope composition identify it as a well crystallized nontronite. This nontronite of hydrothermal origin has a nearly monomin-eralic character, a low Al-content, and a formation temperature of 21.5 to 67.3°C. The most remarkable characteristic, however, of the nontronite deposit is its microstructure, a network of microtubes composed of fine frequently folded clay sheets. These delicate filaments show close similarity in size and form to sheath forming bacteria. The correlation between clay mineral and chemical characteristics, as well as biological conditions at marine hydrothermal smoker chimneys, let us suggest that Fe oxidizing, sheath forming bacteria are playing a decisive role in nontronite formation at these sites.
TL;DR: In this paper, the second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, ferrihydrite and Fe-polymer) were used to distinguish between the Fe-oxides occurring as coatings and as occluded phases.
Abstract: Diffuse reflectance spectra of kaolins have been recorded in samples from different environ- ments. They show the systematic presence of Fe-oxides, even in bleached kaolins, with no contribution from the Fe 3+ ions substituted in kaolinite. Second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, ferrihydrite and Fe-polymer) may be differentiated by the position of a diagnostic band corresponding to the 2QA0 ~ 2(4T~(4G)) transition. The systematic com- parison of diffuse reflectance spectra of unbleached and bleached kaolins has demonstrated the differences between the Fe-oxides occurring as coatings and as occluded phases. The features observed in second derivative spectral curves are consistent with assignments of crystal field transitions to goethite, hematite, akaganeite, and aged hydrous ferric oxides. The optical determination of the Fe-phases associated to kaolins assists in the interpretation of the formation conditions of these minerals.
TL;DR: In this article, the results showed that the Al13 complexes having the Keggin structure were dominant only in fresh, laboratory-hydrolyzed OH-A1 solutions of OH/A1 molar ratio = 1.8 and above.
Abstract: Laboratory-hydrolyzed and commercial OH-Al solutions were characterized using kinetics of Al-ferron color development, kinetics of structural OH neutralization with H+, 27A1 NMR spectroscopy, and sulfate precipitation. The results showed that the Al13 complexes having the Keggin structure were dominant only in fresh, laboratory-hydrolyzed OH-A1 solutions of OH/A1 molar ratio = 1.8 and above. These species gradually converted to other polynuclear forms that reacted with ferron slowly, were not detectable by 27A1 NMR spectroscopy, and yielded different basic Al sulfates following Na2SO4 addition. These more stable complexes can best be interpreted to have a Al(OH)3-fragment structure. In the three commercial aluminum chlorohydrate (ACH) solutions studied, Al13 complexes accounted for a small portion of the total Al present. More than 80% of the Al was present as species that were not detectable with NMR spectroscopy and resembled the slow-reacting complexes in aged, laboratory-hydrolyzed OH-Al solutions. Small portions of the slow-reacting complexes appeared to be submicron particulates that acted as nuclei for gibbsite formation or aggregates of Al13 complexes that dispersed to Al13 upon dilution. Polyaluminum chloride (PA) solution resembled the moderately aged laboratory-hydrolyzed OH-Al solutions.
TL;DR: In this article, a stable 8.4 A hydrate of kaolinite was prepared by exchanging ethylene glycol for water in the 10.8 A intercalate of glycol in Kaolinite.
Abstract: A stable 8.4 A hydrate of kaolinite was prepared by exchanging ethylene glycol for water in the 10.8 A intercalate of ethylene glycol in kaolinite. The hydrate of kaolinite was characterized by XRD, FTIR and TGA/DSC. From the TGA data, one can estimate that there is 0.60 water molecule per Al2Si2O5(OH)4 units. The IR data suggest a similarity of the local environment of the intercalated water in this 8.4 A hydrate of kaolinite and the 8.4 A hydrate of nacrite previously described by Wada (1965).
TL;DR: In this paper, the authors show that hectorite and saponite are exchanged with [Al13O4(OH)24(H2O)12]7+ and the amount of Al3+ adsorbed and Na+ released are followed as a function of the exchange conditions.
Abstract: Hectorite and saponite are exchanged with [Al13O4(OH)24(H2O)12]7+ and the amount of Al3+ adsorbed and Na+ released are followed as a function of the exchange conditions. On saponite the reaction is a pure ion exchange with 2–2.15 mmol Al3+/g adsorbed and release of 0.80 mmol Na+/g. On hectorite the ion exchange is accompanied by supplementary hydrolysis-polymerization of Al13. When excess Al is offered in the form of Al13, ion exchange is incomplete and is accompanied by precipitation and polymerization of Al13 on the surface of both hectorite and saponite. The typical spacing of 1.8 nm is developed after washing, when at least 1.3–1.4 mmol Al3+/g is adsorbed. Above a loading of 2.2–2.5 mmol/g the 1.8 nm spacing is obtained without washing. Only pillared saponite with a loading of at least 1.9 mmol Al3+/g is thermally stable up to 550°C.
TL;DR: Two orientations of white micas with subordinate chlorite have been observed in a fine-grained (50 A to 2μm) matrix of a Silurian lower anchizonal mudrock from central Wales.
Abstract: Two orientations of white micas with subordinate chlorite have been observed in a fine-grained (50 A to 2μm) matrix of a Silurian lower anchizonal mudrock from central Wales: one parallel to bedding and one parallel to cleavage that is approximately 30°-50° to bedding. Bedding-parallel micas consist of small (50-200 A thick) deformed packets (1Md polytype) and larger (100 A-2 μm) strain-free grains (2M1 polytype). All strained micas and some strain-free grains have compositions varying from Mu86Pg14 to Mu58Pg42, intermediate to muscovite and paragonite, and falling within the Mu-Pg solvus. Individual packets of layers are chemically homogeneous and some of them give only one set of 00l reflections (d ≈ 19.6 A). Micas with such intermediate compositions are metastable. Some packets of coarse, strain-free micas have compositions of approximately Mu93Pg7 or Mu11Pg89. Split pairs of 00l reflections with d-values of 20 A and 19.6 A, and 20 A and 19.2 A, respectively, were observed in some SAED patterns, suggesting coexistence of muscovite and intermediate Na/K mica (∼Mu60Pg40), and of discrete muscovite and paragonite, consistent with the splitting of the basal reflections of micas as observed in bulk-rock XRD patterns. Cleavage-parallel micas (2M1 and 3T polytypes) occur as strain-free large grains (200 A to 2 μm) of discrete muscovite (Mu100Pg0) and paragonite (Mu6Pg94), often with subhedral to euhedral cross-sections.
TL;DR: In this paper, the authors showed that hexacyanoferrate(II) is held rigidly in the interlayer by electrostatic forces and is characterized by a singlet at 0.01 mm s−1 at 80 K. X-ray diffraction shows an increase of the basal spacing from 0.78 to 1.1 nm following double ion exchange.
Abstract: Hydrotalcite (HT) interlayered with hexacyanoferrate(II) ions ([Mg4.62Al1.52(OH)12][(Fe(CN)6)0.30-(CO3)0.29]) has been synthesized from the carbonate-HT ([Mg4.52Al1.50(OH)12][(CO3)0.77]) through double ion exchange reactions, and the products have been studied by chemical and physical methods. Mossbauer spectroscopy shows that hexacyanoferrate(II) is held rigidly in the interlayer by electrostatic forces and is characterized by a singlet at 0.01 mm s−1 at 80 K. X-ray diffraction shows an increase of the basal spacing (d003 from 0.78 to 1.1 nm following exchange. Hexacyanoferrate(II) in the interlayer was oxidized to hexacyanoferrate(III) (up to 20%) by dioxygen on dehydrating the interlayer either by drying at 70°C or by washing with nonaqueous solvents like acetone or ethanol. The CN (ν6) band of hexacyanoferrate(II) and (III) is found at 2036 cm−1 and 2112 cm−1, respectively. The presence of an absorption band at 2080 cm−1, assigned to free cyanide anions in the interlayer, suggests that the hexacyanoferrate(II,III) complexes are not inert in the interlayer, cyanide ligands being substituted with either water or hydroxyls. Oxidation and ligand exchange of the hexacyanoferrate(II) are also indicated by Mossbauer spectroscopy.
TL;DR: The occurrences of sepiolite beds and nodules in alkaline and saline Miocene Eskisehir lake deposits provide unique examples of ancient lacustrine environments as discussed by the authors, and they were deposited by direct precipitation from Si-supersaturated lake water under alkaline/salinity conditions.
Abstract: The occurrences of sepiolite beds and nodules in alkaline and saline Miocene Eskisehir lake deposits provide unique examples of ancient lacustrine environments. Stockwork-type magnesite deposits, which were formed very close to the Miocene lake, served as parent rocks for sepiolite nodules (meerschaum). The Miocene succession consists of calcareous clay, clayey carbonate, dolomite, a gypsum-bearing calcareous clay series, siliceous tuffs, sepiolite beds, sepiolite-bearing dolomite, and basal conglomerates of ultramafic rocks. Sepiolite beds were deposited by direct precipitation from Si-supersaturated lake water under alkaline and saline conditions. They are underlain by a gypsum series. Organic matter-rich sepiolites suggest the presence of water stratification with anoxic bottom waters, which developed due to high sulphate input at the base from the ascending hydrothermal solutions along the fracture systems with extensive fresh water input near to the surface. Sepiolite nodules resulted from diagenetic replacement of magnesite pebbles at shallow burial under alkaline conditions in the vicinity of paleo-shorelines. Sepiolite beds were deposited in three ways: 1) black (up to 2.8 wt. % TOC) sepiolite beds rich in organic matter accumulated in an anaerobic paleoenvironment; 2) brown (about 0.5 wt. % TOC) sepiolite beds poor in organic matter, containing minor amounts of white, 2–6 mm long, discontinuous, and very soft dolomite laminae formed in a dysaerobic paleoenvironment; and 3) white dolomitic sepiolite beds in which dolomite content is about 20–40% in an aerobic paleoenvironment. Cyclic dolomite and gypsum series indicate hypersaline-evaporative paleoenvironments with rapid changes in lake water chemistry. These cyclic evaporatic conditions are also related to cyclic changes in water depth. Based on X-ray powder diffraction data, except degree of crystallinity, no mineralogie difference was found between sepiolite in beds and nodules. SEM studies revealed fibers about 0.2 µm wide and up to 30 µm long in sepiolite beds; crystals less than 5 µm long with bent tips; and a more compacted appearance in sepiolite nodules.
TL;DR: This article measured the extent of Cs + sorption on bentonite compacted to a series of densities ranging from 0.50 to 1,50 Mg/m 3.
Abstract: Sorption parameters are important components of models used to predict mass transport through dense or compacted earthen materials. These parameters are, however, generally determined in batch tests with loose, unconsolidated materials. Here we directly measure, using a specially designed cell, the extent ofCs + sorption on bentonite compacted to a series of densities ranging from 0.50 to 1,50 Mg/m 3, and compare the results with those obtained from batch tests with loose bentonite. The clay was saturated with a Na-Ca-Cl-dominated solution with an effective ionic strength of 220 mol/m 3. The sorption data were expressed as distribution coefficients, I~. Over the clay density range examined, Ko values for Cs + with compacted clay are about one-half to one-third the value of those with loose clay. The lower sorption on compacted clay is attributed to small and occluded pores that Cs + cannot enter; thus it cannot access the entire volume, or all the sorption sites, of compacted clay. The data suggest that reasonable estimates of K~ with compacted clay can be obtained by scaling down the I~ values measured on loose clay by a factor na/n, where n a is the accessible porosity and n the total porosity of compacted clay.
TL;DR: In this paper, the hydration behavior of Na-smectite crystals synthesized at a pressure of 5.5 GPa and temper- atures of 1400"-1500~ was examined by X-ray powder diffraction at various relative humidities (RH) in the range of 0-100%.
Abstract: Hydration behavior of Na-smectite crystals synthesized at a pressure of 5.5 GPa and temper- atures of 1400"-1500~ was examined by X-ray powder diffraction at various relative humidities (RH) in the range of 0-100%. The basal spacing of the Na-smectite crystal increased stepwise with increase in RH. The reflections observed were only normal reflections of a single or dual hydration states of smectite. No irrational, intermediate, or asymmetrical reflections were observed. The simple hydration behavior, not known for natural smectite with fine particle sizes and low crystallinity, indicates that the Na-smectite crystals are as perfect as common inorganic crystals with an ordered structure.
TL;DR: In this article, the crystal structure of cronstedtite-2H2 was refined in a hexagonal cell, space group P63, Z = 2, using two acicular crystals from Wheal Maudlin, Cornwall, England, and from Pribram, Czech Republic.
Abstract: The crystal structure of cronstedtite-2H2 was refined in a hexagonal cell, space group P63, Z = 2, using two acicular crystals from Wheal Maudlin, Cornwall, England, and from Pribram, Czech Republic. The Wheal Maudlin sample has the chemical composition $\left( {{\rm{Fe}}_{2.291}^{2 + }{\rm{Fe}}_{0.709}^{3 + }} \right)\left( {{\rm{S}}{{\rm{i}}_{1.298}}{\rm{Fe}}_{0.707}^{3 + }{\rm{A}}{{\rm{l}}_{0.004}}} \right){{\rm{O}}_5}{\left( {{\rm{OH}}} \right)_4}$
and the Přibram sample has the composition $\left( {{\rm{Fe}}_{2.269}^{2 + }{\rm{Fe}}_{0.731}^{3 + }} \right)\left( {{\rm{S}}{{\rm{i}}_{1.271}}{\rm{Fe}}_{0.724}^{3 + }{\rm{A}}{{\rm{l}}_{0.005}}} \right){{\rm{O}}_5}{\left( {{\rm{OH}}} \right)_4}$
. The results of refinements are as follows: a = 5.500(1), c = 14.163(2) A, V = 371.08(8) A3, R = 3.83%, from 381 independent reflections, and a = 5.4927(1), c = 14.1481(2) A, V = 369.70(4) A3, R = 4.77%, from 1088 independent reflections for the Wheal Maudlin and Přibram samples, respectively. The best Fovs. Fc agreement was achieved when the structure was interpreted as merohedral twin; several possible twinning laws are discussed. The cronstedtite layer consists of one tetrahedral sheet and one octahedral sheet. There is one octahedral (M1) position, occupied by Fe only, and two tetrahedral (T1, T2) positions in the structure. Refinement of occupancy of tetrahedral sites led to values Si:Fe = 0.45:0.55(1) (Wheal Maudlin) and 0.432:0.568(8) (Přibram) in T1, and Si: Fe = 0.99:0.01(1) (Wheal Maudlin) and 0.888:0.112(7) (Přibram) in 72. Whereas the size of T1 is reasonable (average dT1-O = 1.693 A (Wheal Maudlin), 1.691 A (Přibram)), T2 is unusually large: (dT2-O= 1.740 A (Wheal Maudlin), 1.737 A (Přibram)) with respect to the small or almost zero Fe content. As an explanation, an alternative structure model comprising a certain amount of vacancies in T2 is presented. The tetrahedral rotation angle α is highly positive (+12.1° and +12.5° for the Wheal Maudlin and Přibram samples, respectively), and the layer belongs to the Franzini type A. Distortion parameters of octahedra and tetrahedra are given for both samples. One hydrogen atom engaged in the hydrogen bond was located in the Wheal Maudlin sample.
TL;DR: In this paper, it was found that there are two different converging models for fitting the zero-field spectra of the Maghemites with a superposition of two Lorentzian-shaped sextets, both resulting in inconsistent values for the hyperfine fields (Hhf) and/or the center shifts (6) of the tetrahedral (A) and octahedral (B) ferric ions.
Abstract: Synthetic, relatively well-crystallized aluminum-substituted maghemite samples, y-(Aly. Fe~_y)203, with y = 0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by X-ray diffraction and zero-field Mrssbauer spectroscopy in the range 8 K to 475 K, and also with an external field of 60 kOe at 4.2 K and 275 K. It was found that there are two different converging models for fitting the zero-field spectra of the maghemites with a superposition of two Lorentzian-shaped sextets, both resulting in inconsistent values for the hyperfine fields (Hhf) and/or the center shifts (6) of the tetrahedral (A) and octahedral (B) ferric ions. From the applied-field measurements it is concluded that there is a constant difference of 0.12 _+ 0.01 mm/s between 6B and 6A, regardless of the A1 content. For the Al-free sample the center shifts are found as: 64 = 0.370 mm/s and 6a = 0.491 mm/s at 4.2 K and 6n = 0.233 mm/s and fib = 0.357 mm/s at 275 K (relative to metallic iron), with an estimated error of 0.005 mm/s. Both 6A and fib are observed to decrease with increasing AI concentration. The effective hyperfine fields for the non-substituted maghemite sample are: Hefr.A = 575 kOe and Hofr.B = 471 kOe at 4.2 K and He~,A = 562 kOe and H~er.B = 449 kOe at 275 K, with an error of 1 kOe. The B-site hyperfine field remains approximately constant with A1 substitution, while for the A site a slight decrease with increasing A1 content was observed.
TL;DR: In this paper, the early alteration product is a hydrotalcite-like compound [Mg6Al2CO3(OH)16·4H2O] in which HPO42−, SO42− and Cl− substitutes for CO32−.
Abstract: Alteration experiments have been performed using RTT7 and synthetic basaltic glasses in MgCl2−CaCl2 salt solution at 190°C. The duration of experiments ranged from 0.25 to 463 days. The alteration products were studied by Scanning Electron Microscope (SEM), Scanning Transmission Electron Microscope (STEM), X-ray diffraction (XRD) and Electron Spectrometry for Chemical Analysis (ESCA). For both glasses, the early alteration product is a hydrotalcite-like compound [Mg6Al2CO3(OH)16·4H2O] in which HPO42−, SO42− and Cl− substitutes for CO32−. The measured basal spacing is 7.68 A for the hydrotalcite formed from R7T7 glass and 7.62 A for the hydrotalcite formed from basaltic glass which reflect the high Al/Al + Mg ratios x (0.34 ≤ x ≤ 0.46). The chemical microanalyses show that the hydrotalcite is subsequently covered by a silica-rich gel which evolves into saponite after a few months. These results support the use of basaltic glasses alteration patterns in Mg-rich solution, to understand the long-term behavior of R7T7 nuclear waste glass.
TL;DR: In this paper, a heterogeneous dehydroxylation mechanism is proposed in which micropores are formed in donor regions and magnesium and oxygen are concentrated in acceptor regions, which leads to a 27 A structure with micropore zones and enriched interlayer oxide zones alternating along the z-axis of the mineral.
Abstract: The “modified chlorite structure” forms by the dehydroxylation of the interlayer octahedral sheet of magnesian chlorite at around 500°C and results in a structure with a basal spacing near 27 Â (Brindley and Chang 1974). This process involves drastic textural modifications as indicated by gas adsorption experiments which reveal the formation of structural micropores. Infrared spectroscopy as well as thermogravimetry and mass spectrometric analysis show that these micropores are filled with molecular atmospheric water, carbon dioxide, nitrogen, argon and hydrocarbons which condense once the samples cool down. A high temperature treatment is needed in order to release the different phases. A heterogeneous dehydroxylation mechanism is proposed in which micropores are formed in donor regions and magnesium and oxygen are concentrated in acceptor regions. This leads to a 27 A structure with micropore zones and enriched interlayer oxide zones alternating along the z-axis of the mineral.