ALBERT

Description: The intercalating growth of new silicate layers or metal hydroxide layers in the interlayer space of other clay minerals is known from various mixed-layer clay minerals such as illite-smectite (I-S), chlorite-vermiculite, and mica-vermiculite. In a recent study, the present authors proposed that smectite-group minerals can be synthesized from solution as new 2:1 silicate layers within the low-charge interlayers of rectorite. That study showed how oxalate catalyzes the crystallization of saponite from a silicate gel at low temperatures (60ºC) and ambient pressure. As an extension of this work the aim of the present study was to test the claim that new 2:1 silicate layers can be synthesized as new intercalating layers in the low-charge interlayers of rectorite and whether oxalate could promote such an intercalation synthesis. Two experiments were conducted at 60ºC and atmospheric pressure. First, disodium oxalate solution was added to a suspension of rectorite in order to investigate the effects that oxalate anions have on the structure of rectorite. In a second experiment, silicate gel of saponitic composition (calculated interlayer charge –0.33 eq/O 10 (OH) 2 ) was mixed with a suspension of rectorite and incubated in disodium oxalate solution. The synthesis products were extracted after 3 months and analyzed by X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). The treatment of ultrathin sections with octadecylammonium ( n C = 18) cations revealed the presence of 2:1 layer silicates with different interlayer charges that grew from the silicate gel. The oxalate-promoted nucleation of saponite and talc crystallites on the rectorite led to the alteration and ultimately to the destruction of the rectorite structure. The change was documented in HRTEM lattice-fringe images. The crystallization of new 2:1 layer silicates also occurred within the expandable interlayers of rectorite but not as new 2:1 silicate layers parallel to the previous 2:1 silicate layers. Instead, they grew independently of any orientation predetermined by the rectorite crystal substrate and their crystallization was responsible for the destruction of the rectorite structure.

Description: Bentonites are candidate materials for encapsulation of radioactive waste. The cation exchange capacity (CEC) has proven to be one of the most sensitive parameters for detecting changes in mineral properties in bentonite-alteration experiments. An interlaboratory study of CECs and exchangeable cations for three reference bentonite buffer materials that were used in the Alternative Buffer Material test (ABM) project in Äspö, Sweden, was conducted to create a suitable database. The present study focused on CEC accuracy and compared CEC methods where care was taken to prevent dissolution of soluble minerals such as calcite and gypsum. The overall quality of the CEC and exchangeable cation values measured using non-Cu-trien CEC methods were good, with CECs of 74–91±0.5–3.3 meq/100 g and exchangeable cation values of 22–61±1.2–3.9 meq/100 g Na + , 7–23±0.8–1.5 meq/100 g Mg 2+ , and 19–39±0.8–1.6 meq/100 g Ca 2+ . The precision was comparable to the standard Cu-trien method even for exchangeable Ca 2+ , although the laboratories used different solution/solid ratios and reaction-time parameters for Cu-trien which affect carbonate dissolution. The MX80 and Dep.CAN bentonite exchangeable Ca 2+ values were more accurate than standard-Cu-trien values. Using the optimized methods of this study, MX80 and Dep.CAN exchangeable Ca 2+ values averaged 20.2±1.6 and 38.8±1.4 meq/100 g which amounts to ~70% of the inflated Cu-trien values. A more complex analysis of the CEC data using different methods, anion analyses, and mineralogical analysis is necessary to obtain plausible and accurate CEC values. Even with a more complicated analytical procedure, the CEC and exchangeable cation values were still not accurate enough because of excess anions. Chloride, sulfate, and dolomite might have increased the exchangeable Na + , Mg 2+ , and Ca 2+ values.

Description: Clay minerals are abundant in soils and sediments and often contain Fe. Some varieties, such as nontronites, contain as much as 40 wt.% Fe2O3 within their molecular structure. Several studies have shown that various Fe-reducing micro-organisms can use ferric iron in Fe-bearing clay minerals as their terminal electron acceptor, thereby reducing it to ferrous iron. Laboratory experiments have also demonstrated that chemically or bacterially reduced clays can promote the reductive degradation of various organics, including chlorinated pesticides and nitroaromatics. Therefore, Fe-bearing clays may play a crucial role in the natural attenuation of various redox-sensitive contaminants in soils and sediments. Although the organochlorinated pesticide p,p'-DDT is one of the most abundant and recalcitrant sources of contamination in many parts of the world, the impact of reduced Fe-bearing clays on its degradation has never been documented. The purpose of the present study was to evaluate the extent of degradation of p,p'-DDT during the bacterial reduction of Fe(III) in an Fe-rich clay. Microcosm experiments were conducted under anaerobic conditions using nontronite (sample NAu-2) spiked with p,p'-DDT and the metal-reducing bacteria Shewanella oneidensis MR-1. Similar experiments were conducted using a sand sample to better ascertain the true impact of the clay vs. the bacteria on the degradation of DDT. Samples were analyzed for DDT and degradation products after 0, 3, and 6 weeks of incubation at 30{degrees}C. Results revealed a progressive decrease in p,p'-DDT and increase in p,p'-DDD concentrations in the clay experiments compared to sand and abiotic controls, indicating that Fe-bearing clays may substantially contribute toward the reductive degradation of DDT in soils and sediments. These new findings further demonstrate the impact that clay materials can have on the natural attenuation of pollutants in natural and artificial systems and open new avenues for the passive treatment of contaminated land.

Description: The crystal growth of NH 4 -illite (NH 4 -I) from the hydrothermal system of Harghita Bãi (Eastern Carpathians) was deduced from the shapes of crystal thickness distributions (CTDs). The 〈2 μm and the 〈2–0.2 μm fractions of clay samples collected from the argillized andesite rocks consist of NH 4 -illite-smectite (I-S) interstratified structures (R1, R2, and R3-type ordering) with a variable smectite-layer content. The NH 4 -I-S (40–5% S) structures were identified underground in a hydrothermal breccia structure, whereas the K-I/NH 4 -I mixtures were found at the deepest level sampled (–110 m). The percentage of smectite interlayers generally decreases with increasing depth in the deposit. This decrease in smectite content is related to the increase in degree of fracturing in the breccia structure and corresponds to a general increase in mean illite crystal thickness. In order to determine the thickness distributions of NH 4 -I crystals (fundamental illite particles) which make up the NH 4 -I-S interstratified structures and the NH 4 -I/K-I mixtures, 27 samples were saturated with Li + and aqueous solutions of PVP-10 to remove swelling and then were analyzed by X-ray diffraction. The profiles for the mean crystallite thickness ( T mean ) and crystallite thickness distribution (CTD) of NH 4 -I crystallites were determined by the Bertaut-Warren-Averbach method using the MudMaster computer code. The T mean of NH 4 -I from NH 4 -I-S samples ranges from 3.4 to 7.8 nm. The T mean measured for the NH 4 -I/K-I mixture phase ranges from 7.8 nm to 11.7 nm (NH 4 -I) and from 12.1 to 24.7 nm (K-I). The CTD shapes of NH 4 -I fundamental particles are asymptotic and lognormal, whereas illites from NH 4 -I/K-I mixtures have bimodal shapes related to the presence of two lognormal-like CTDs corresponding to NH 4 -I and K-I. The crystal-growth mechanism for NH 4 -I samples was simulated using the Galoper code. Reaction pathways for NH 4 -I crystal nucleation and growth could be determined for each sample by plotting their CTD parameters on an α–β 2 diagram constructed using Galoper . This analysis shows that NH 4 -I crystals underwent simultaneous nucleation and growth, followed by surface-controlled growth without simultaneous nucleation.

Description: The nano-aluminosilicate mineral allophane is common in soils formed from parent materials containing volcanic ash and often contains Fe. Due to its lack of long-range order, the structure of allophane is still not completely understood. In the present study, Fe K-edge X-ray absorption fine structure (XAFS) was used to examine Fe-containing natural and synthetic allophane and imogolite samples. Results indicated that Fe substitutes for octahedrally coordinated Al in allophane, and that Fe exhibits a clustered distribution within the octahedral sheet. Iron adsorbed on allophane surfaces is characterized by spectral features distinct from those of isomorphically substituted Fe and of ferrihydrite. Fe adsorbed on the allophane surfaces probably exists as small polynuclear complexes exhibiting Fe–Fe edge sharing, similar to poorly crystalline Fe oxyhydroxides. The XAFS spectra of natural allophane and imogolite indicate that the Fe in the minerals is a combination of isomorphically substituted and surface-adsorbed Fe. In the synthetic Fe-substituted allophanes, the Fe XAFS spectra did not vary with the Al:Si ratio. Theoretical fits of the extended XAFS (EXAFS) spectra suggest that local atomic structure around octahedral Fe in allophanes is more similar to Fe in a smectite-like structure than to a published theoretical nanoball structure.

Description: The adsorption mechanisms of divalent cations in zeolite nanopore channels can vary as a function of their pore dimensions. The nanopore inner-sphere enhancement (NISE) theory predicts that ions may dehydrate inside small nanopore channels in order to adsorb more closely to the mineral surface if the nanopore channel is sufficiently small. The results of an electron paramagnetic resonance (EPR) spectroscopy study of Mn and Cu adsorption on the zeolite minerals zeolite Y (large nanopores), ZSM-5 (intermediate nanopores), and mordenite (small nanopores) are presented. The Cu and Mn cations both adsorbed via an outer-sphere mechanism on zeolite Y based on the similarity between the adsorbed spectra and the aqueous spectra. Conversely, Mn and Cu adsorbed via an inner-sphere mechanism on mordenite based on spectrum asymmetry and peak broadening of the adsorbed spectra. However, Mn adsorbed via an outer-sphere mechanism on ZSM-5, whereas Cu adsorbed on ZSM-5 shows a high degree of surface interaction that indicates that it is adsorbed closer to the mineral surface. Evidence of dehydration and immobility was more readily evident in the spectrum of mordenite than in that of ZSM-5, indicating that Cu was not as close to the surface on ZSM-5 as it was when adsorbed on mordenite. Divalent Mn cations are strongly hydrated and are held strongly only in zeolites with small nanopore channels. Divalent Cu cations are also strongly hydrated, but can dehydrate more easily, presumably due to the Jahn-Teller effect, and are held strongly in zeolites with medium-sized nanopore channels or smaller.

Description: The objective of the present study was to investigate changes in the structural, textural, and surface properties of tubular halloysite under heating, which are significant in the applications of halloysite as functional materials but have received scant attention in comparison with kaolinite. Samples of a purified halloysite were heated at various temperatures up to 1400°C, and then characterized by X-ray diffraction, electron microscopy, Fourier-transform infrared spectroscopy, thermal analysis, and nitrogen adsorption. The thermal decomposition of halloysite involved three major steps. During dehydroxylation at 500–900°C, the silica and alumina originally in the tetrahedral and octahedral sheets, respectively, were increasingly separated, resulting in a loss of long-range order. Nanosized (5–40 nm) -Al 2 O 3 was formed in the second step at 1000–1100°C. The third step was the formation of a mullite-like phase from 1200 to 1400°C and cristobalite at 1400°C. The rough tubular morphology and the mesoporosity of halloysite remained largely intact as long as the heating temperature was 〈900°C. Calcination at 1000°C led to distortion of the tubular nanoparticles. Calcination at higher temperatures caused further distortion and then destruction of the tubular structure. The formation of hydroxyl groups on the outer surfaces of the tubes during the disconnection and disordering of the original tetrahedral and octahedral sheets was revealed for the first time. These hydroxyl groups were active for grafting modification by an organosilane (-aminopropyltriethoxysilane), pointing to some very promising potential uses of halloysite for ceramic materials or as fillers for novel clay-polymer nanocomposites.

Description: A method to characterize and map both kaolinite and dickite polytypes in sandstone thin sections using infrared microspectroscopy (IRMS) was developed. Minerals identification using IRMS can be performed using the hydroxyl-stretching band of most clay minerals (3500–4000 cm –1 ) in spite of infrared (IR) interferences caused by the embedding resin and glass substratum. Emphasis was placed on determining the optimum analytical conditions for IR data acquisition. The best data-acquisition parameters for Fourier-transform infrared (FTIR) measurements ( i.e. spectra quality as a function of beam size and the number of scans) were obtained from a series of single spectra. Then, spatial resolution was explored as a function of the IR beam size (from 50 μm x 50 μm to 15 μm x 15 μm) and the step-scan interval ( i.e. the distance between two successive analysis points). The IRMS measurements were performed on thin sections of materials characterized previously using scanning electron microscopy (SEM) and chemical analysis. Using IRMS, locations on the thin sections containing nearly pure dickite or kaolinite polytypes were identified and mapped. Most spectra collected using IRMS represented kaolin mineral aggregates rather than individual crystals, however, and mixing of kaolin polytypes was common at the spatial resolution of the IR beam size used. The spatial resolution of the IRMS was comparable to optical petrography and made possible the identification of areas on the thin section for further ‘ in situ ’ investigation using other methods ( e.g. microprobe, Laser Ablation Inductively Coupled Plasma Mass Spectrometry – LA-ICP-MS, etc .). Also, the use of blocky crystal morphology to identify dickite was questioned, as kaolinite with blocky habit was identified. Mineral mapping using IRMS seems particularly suited for investigating petrographic relationships between kaolinite and dickite in sandstone diagenesis, but could also be used for clay minerals in other rock types or soils.

Description: Secondary surface layers form by replacement of almandine garnet during chemical weathering. This study tested the hypothesis that the kinetic role of almandine's weathering products, and the consequent relationships of primary-mineral surface texture and specific assemblages of secondary minerals, both vary with the solid-solution-controlled variations in Fe and Al contents of the specific almandine experiencing weathering. Surface layers are protective (PSL) when the volume of the products formed by replacement is greater than or equal to the volume of the reactants replaced. Under such circumstances, reaction kinetics at the interface between the garnet and the replacing mineral are transport controlled and either transport of solvents or other reactants to, or products from, the dissolving mineral is rate limiting. Beneath PSLs, almandine garnet surfaces are smooth, rounded, and featureless. Surface layers are unprotective (USL) when the volume of the products formed by replacement is less than the volume of the reactants replaced. Under such circumstances, reaction kinetics at the interface between the garnet and the replacing mineral are interface controlled and the detachment of ions or molecules from the mineral surface is rate limiting. Almandine garnet surfaces beneath USLs exhibit crystallographically oriented etch pits. However, contrary to expectations, etch pits occur on almandine garnet grains beneath some layers consisting of mineral assemblages consistent with PSLs. Based on the Pilling-Bedworth criterion, surface layers are more likely to be protective over a broad range of reactant-mineral compositions when they contain goethite, kaolinite, and pyrolusite. However, this combination requires specific ranges of Fe and Al content of the natural reacting almandine garnet. To form a PSL of goethite and kaolinite, an almandine garnet must have a minimum Al stoichiometric coefficient of ~3.75 a.p.f.u., and a minimum Fe stoichiometric coefficient of ~2.7 a.p.f.u. Product minerals also influence the mobility of the least-mobile major rock-forming elements. A PSL consisting of goethite, gibbsite, and kaolinite yields excess Al for export during almandine garnet weathering. As the quantity of kaolinite present in the PSL decreases, the amounts of Al available for export increases.