Showing papers on "Nontronite published in 2001"

In situ atomic force microscopy study of hectorite and nontronite dissolution: Implications for phyllosilicate edge surface structures and dissolution mechanisms

Abstract: The dissolution behavior of two smectite minerals, hectorite (trioctahedral) and nontronite (dioctahedral), was observed in situ, in acid solutions, using atomic force microscopy. As expected, the crystallites dissolved inward from the edges, and the basal surfaces appeared to be unreactive during the timescale of the experiments. The hectorite (010) faces appeared to dissolve about 6 × more slowly than the lath ends, usually broken edges. The edges visibly dissolved on all sides, and appeared to roughen somewhat. On the other hand, the (010), (110), and (11 ‐ 0) faces on nontronite crystals were exceptionally stable, so that any dissolution fronts originating at broken edges or defects would quickly become pinned along these faces, after which no more dissolution was observable. These observations can be explained by using periodic bond chain theory to predict the topology of the surface functional groups on the edge faces of these minerals. If a certain amount of predicted surface relaxation is allowed on the (110) and (11 ‐ 0) faces of nontronite, an important difference between the exceptionally stable faces and the others becomes apparent. That is, the oxygen sites connecting the octahedral and tetrahedral sheets are all fully bonded on the nontronite (010), (110), and (11 ‐ 0) edge faces, whereas all hectorite edge faces and nontronite broken edges would have coordinatively unsaturated connecting O atoms. This explanation for the differential reactivity of these crystal faces implies that the rate limiting step of the dissolution process is the breaking of bonds to connecting O atoms.

Possible Role of Microbial Polysaccharides in Nontronite Formation

Abstract: Nontronite and microbes were detected in the surface layers of deep-sea sediments from Iheya Basin, Okinawa Trough, Japan. Nontronite, an Fe-rich smectite mineral, was embedded in acidic polysaccharides that were exuded by microbial cells and electron microscopy showed that the nontronite layers were apparently oriented in the polysaccharide materials. We propose that the formation of nontronite was induced by the accumulation of Si and Fe ions from the ambient seawater and that extracellular polymeric substances (EPS) served as a template for layer-silicate synthesis. Experimental evidence for this hypothesis was obtained by mixing a solution of polysaccharides (dextrin and pectin) with ferrosiliceous groundwater. After stirring the mixture in a sealed vessel for two days, and centrifuging, Fe-rich layer silicates were identified within the precipitate of both the dextrin and pectin aggregates, whereas rod-shaped or spheroidal Si-bearing iron hydroxides were found in the external solution. Microbial polysaccharides would appear to have affected layer-silicate formation.

Microbial biomineralization in weathered volcanic ash deposit and formation of biogenic minerals by experimental incubation

Abstract: Microbial biomineralization in a weathered volcanic ash deposit from the 1914 to 1915 a.d. eruption of Sakurajima volcano was investigated by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). The solution chemistry of pore water was also analyzed to elucidate saturation conditions. In addition, experimental incubations of bacteria collected from the volcanic ash were performed to confirm bacterial mineralization. TEM revealed that the weathered volcanic ash contains significant amounts of spherical to rod-shaped bacteria ranging from 1.3 × 108 to 2.6 × 108 cell/g, most of which have cell wall surfaces that are completely covered or decorated by either massive aggregates of allophane-like granular materials or irregular aggregates of smectite-like fibers and/or flakes. EDX confirmed that the granular minerals have chemical compositions similar to proto-imogolite allophane, whereas the smectite-like fibers and/or flakes show a wide range of chemical compositions corresponding to the compositional field between allophane and nontronite. The volcanic ash contains about 22 wt% of pore water, which is slightly acidic, relatively low redox potential, and enriched in Si, Na, Cl−, and SO42– ions. The saturation indices (SI) calculated by the PHREEQC geochemical code indicate that the pore water is almost saturated with respect to amorphous Al(OH)3, ferrihydrite, amorphous silica, and cristobalite, and significantly oversaturated with respect to silicate minerals in the order: halloysite < kaolinite < montmorillonite < allophane < nontronite. The allophane-like granular minerals seems to be preferentially precipitated by bacterial interaction with Al and Si ions in the pore water as a metastable phase. The poorly ordered smectite-like fibers and/or flakes may be transformed from the allophane-like materials as a intermediate phase between allophane and nontronite by the driving force originated from the greatest SI value of nontronite. The experimental incubation confirmed that amorphous silica containing a small amount of Fe is formed on the bacterial cell surfaces in liquid media with both Fe and Si ions. Likewise, beidellite-like smectite associated with the bacterial surfaces is produced in liquid media containing both Al and Si ions. However, no minerals are produced in the same media containing no metal ions or no bacteria. These results imply that bacteria play an important role in the accumulation of metal ions and in the formation of silicate minerals during weathering of volcanic ash.

Ab Initio Study of the Nonequivalence of Adsorption of D- and L-Peptides on Clay Mineral Surfaces

Abstract: Ab initio geometry optimizations were performed of the structures of the L and D enantiomers of the model dipeptide, N-formyl alanine amide (L-Ala and D-Ala, respectively) adsorbed on the mineral surface in the interlayer space of the 2:1 clay, nontronite. Density functional procedures were used as implemented in the ab initio program CASTEP, in fully periodic calculations in which the properties of a model unit cell are determined by including the effects of its neighboring cells in an infinite crystal. Geometry optimization included the atomic positions of the mineral and the adsorbates within the unit cell, in addition to unit cell lengths and angles, achieving full optimization of the entire crystal system. In agreement with previous studies of the adsorption of organic compounds on clay mineral surfaces, in the most stable arrangement of L-Ala and D-Ala found here, the two molecules reside flat on the mineral basal plane, forming a parallel monolayer. The resulting L-Ala/mineral cocrystal is more stable, by 6 kcal/mol, than D-Ala. A characteristic structural difference exists for the enantiomers in that, in the optimized structure of L-Ala, the C(α)–C(β) bond is directed away from one of the mineral basal planes toward the dioctahedral cavity of the other, while the side group of D-Ala is parallel to the mineral basal plane and pointing to the nearest neighbor in an adjacent unit cell. As expected, the reverse results are obtained for the adsorption of L-Ala and D-Ala on a nontronite surface that is the enantiomer of the one used above. The geometry optimizations illustrate the structural compatibility of clay mineral lattices with peptide structures. That is, balance of adsorption energies and peptide interaction energies, and mineral lattice structure and periodicity allow for adsorption structures, which involve the entire backbone of a single peptide molecule and can, at the same time, immobilize the adsorbates in such a way that stabilizing intermolecular interactions occur across unit cell boundaries. Compatibility of the repeat distances on the clay surface with repeat distances of peptides should be an important property when clay minerals serve as templates for protein synthesis.

Compositional changes of surface sediments and variability of manganese nodules in the Peru Basin

Abstract: Two types of manganese nodules were observed in the Peru Basin: large botryoidal nodules in basins and small ellipsoidal nodules on slope positions. The sediment in areas with large botryoidal nodules contains a thinner and weaker oxidation zone than the sediment under small ellipsoidal nodules, indicating that diagenetic processes in the sediment, which supply manganese nodules with metals for their growth, are stronger in sediments on which large botryoidal nodules grow. Organic matter, which activates remobilization of metals, occurs mostly in the form of refractory lipidic compounds in the inner capsule of radiolaria. This material needs bacterial degradation to act as a reducing agent. Easily oxidizable organic components could not be found in the sediments. Other changes in sediment composition do not have a link to manganese nodule growth. Biogenous components (radiolarians, organogenic barite and apatite) increase towards the equatorial high-productivity zone. Authigenous clay minerals (nontronite as well as montmorillonite with high Fe +3 incorporation on positions of ochtaedral Al) increase with distance from the continent. The assessment of environmental impacts will have to take into account the regional differences in sediment composition and the small-scale variability of manganese nodules.

Modeling of water -glass and water -granite interaction in Beishan Area, Northwestern China

Abstract: Water-glass and water-granite interactions in Beishan Area, a potential area for China HLW repository, have been studied using EQ3/6, a geochemical software package developed by the Lawrence Livermore Laboratory for use in modeling the complex geochemical processes that occur when aqueous solutions react with soil, rock, or solid waste materials, and the results of calculation are reported in this paper. The modeling shows that a lot of secondary minerals which contain the components of the glass could be formed due to the interaction between glass and groundwater of Beishan Wuyi well. The formation of the secondary minerals could not only decrease the concentrations of the radionuclides in the groundwater, but also fill and block the pores in the rock, and reduce the groundwater flow rate. These processes are very important to prevent the nuclides from migrating into the human’s environment. The most significant feature for the interaction between groundwater and granite from Beishan area is the precipitation of large amount of clay minerals, such as montmorillonite, smectite, nontronite, beidellite and so on, in the later stage of the reaction. All of them have very strong adsorptivity for the nuclides. So it is thought that the granite of Beishan Area is favorable to the construction of HLW repository.