Secondary mineral formation associated with respiration of nontronite,NAu-1 by iron reducing bacteria

Experimental batch and miscible-flow cultures were studied in order to determine the mechanistic pathways of microbial Fe(III) respiration in ferruginous smectite clay, NAu-1. The primary purpose was to resolve if alteration of smectite and release of Fe precedes microbial respiration. Alteration of NAu-1, represented by the morphological and mineralogical changes, occurred regardless of the extent of microbial Fe(III) reduction in all of our experimental systems, including those that contained heat-killed bacteria and those in which O₂, rather than Fe(III), was the primary terminal electron acceptor. The solid alteration products observed under transmission electron microscopy included poorly crystalline smectite with diffuse electron diffraction signals, discrete grains of Fe-free amorphous aluminosilicate with increased Al/Si ratio, Fe-rich grains, and amorphous Si globules in the immediate vicinity of bacterial cells and extracellular polymeric substances. In reducing systems, Fe was also found as siderite. The small amount of Fe partitioned to the aqueous phase was primarily in the form of dissolved Fe(III) species even in the systems in which Fe(III) was the primary terminal electron acceptor for microbial respiration. From these observations, we conclude that microbial respiration of Fe(III) in our laboratory systems proceeded through the following: (1) alteration of NAu-1 and concurrent release of Fe(III) from the octahedral sheets of NAu-1; and (2) subsequent microbial respiration of Fe(III).     

Modeling the kinetics of silica nanocolloid formation and precipitation in geologically relevant aqueous solutions

The kinetics of the formation and precipitation of nanocolloidal silica from geologically relevant aqueous solutions is investigated. Changes in monomeric (SiO_2(mono)), nanocolloidal (SiO_2(nano)) and precipitated silica (SiO_2(ppt)) concentrations in aqueous solutions from pH 3 to 7, ionic strengths (IS) of 0.01 and 0.24 molal, and initial SiO₂ concentrations of 20.8, 12.5 and 4.2 mmolal (reported in [Icopini, G.A., Brantley, S.L., Heaney, P.J., 2005. Kinetics of silica oligomerization and nanocolloid formation as a function of pH and ionic strength at 25 °C. Geochim. Cosmochim. Acta69(2), 293-303.]) were fit using two kinetic models. The first model, termed the concentration model, is taken from Icopini et al. (2005) and assumes that the rate of change of SiO_2(mono) as a function of time has a fourth-order dependence on the concentration of SiO_2(mono) in solution. The second model, termed the supersaturation model, incorporates the equilibrium concentration of amorphous silica and predicts that polymerization will be a function of the degree of silica supersaturation in solution with respect to amorphous silica. While both models generally predicted similar rate constants for a given set of experimental conditions, the supersaturation model described the long-term equilibrium behavior of the SiO_2(mono) fraction more accurately, resulting in significantly better fits of the monomeric data. No difference was seen between the model fits of the nanocolloidal silica fraction. At lower pH values (3-4), a metastable equilibrium was observed between SiO_2(mono) and SiO_2(nano). This equilibrium SiO_2(mono) concentration was found to be 6 mmolal, or three times the reported solubility of bulk amorphous silica under the experimental conditions studied and corresponds to the predicted solubility of amorphous silica colloids approximately 3 nm in diameter. Atomic force microscopy was used to determine the average size of the primary nanocolloidal particles to be ∼3 nm, which is in direct agreement with the solubility calculations. Larger aggregates of the primary nanocolloids were also observed to range in size from 30 to 40 nm. This work provides the first kinetic models describing the formation and evolution of nanocolloidal silica in environmentally relevant aqueous solutions. Results indicate that nanocolloidal silica is an important species at low pH and neutral pH at low ionic strengths and may play a more important role in geochemical cycles in natural aqueous systems than previously considered.     

Deposition rates of polysilicic acid with up to 10 M calcium ions

Cementitious materials used for radioactive waste repository construction complicate the performance assessment of radioactive waste systems because the use of cement may greatly alter the pH (8–13) of groundwater and release constituents such as calcium ions. Under such conditions, it is important to clarify also the dynamic behavior of silica (silicic acid), in order to evaluate the alteration in the chemical and physical properties of the fractured layer or the host rock surrounding the repository. Since silica undergoes polymerization, precipitation or dissolution depending on the pH and/or temperature, the behavior of silica would be greatly complicated in the presence of other ions. This study is focused on the deposition rates of polysilicic acid and soluble silicic acid with up to 10⁻³ M Ca ions. In the experiment, Na₂SiO₃ solution (250 mL, pH > 10, 298 K) was poured into a polyethylene vessel containing amorphous silica powder (0.5 g), and a buffer solution, HNO₃, and CaNO₃ as Ca ions were sequentially added into the vessel. The pH of the solution was set to 8. The silica, initially in a soluble form at pH > 10 (1.4 × 10⁻² M), became supersaturated and either deposited on the solid surface or changed into the polymeric form. Then the concentrations of both poly- and soluble silicic acid were monitored over a 40-day period. The decrease of polysilicic acid became slow with an increase in the concentration of Ca ions in the range of up to 10⁻³ M. In general, the addition of electrolytes to a supersaturated solution accelerates the aggregation and precipitation of polymeric species. However, the experimental result showed that polysilicic acid in the presence of Ca ions is apparently stable in solution, compared with that under a Ca-free condition. On the other hand, the concentration of soluble silicic acid in the presence of Ca ions immediately became metastable, that is, slightly higher than the solubility of soluble silicic acid. Its dynamic behavior was similar to that in the Ca-free condition.     

Interaction between dissolved silica and calcium carbonate: 1. Spontaneous precipitation of calcium carbonate in the presence of dissolved silica

The kinetics of spontaneous precipitation of CaCO₃ from aqueous solution in the presence of dissolved silica was investigated by recording pH as a function of time. The presence of dissolved silica, at concentrations below saturation with respect to the amorphous phase, decreases induction time for CaCO₃ nucleation, but does not affect CaCO₃ polymorphism. For a “pure” system without silica, the surface free energy, σ, determined from classical nucleation theory is 42 mJ m⁻². This agrees well with values reported in the literature for vaterite and indicates some degree of heterogeneous nucleation, which can occur because of the relatively low degree of supersaturation used for the experiments. In the presence of 1 and 2 mM silica, σ is 37 and 34 mJ m⁻², indicating an increasing degree of heterogeneous nucleation as the amount of polymeric silica increases. The ratio of Ca²⁺ to CO₃²⁻ activity was a governing parameter for determining which CaCO₃ polymorph precipitated. At high Ca²⁺ to CO₃²⁻ activity ratios, almost all initial solid was vaterite, whereas at low ratios, a mixture of vaterite and calcite was observed. In solutions with low Ca²⁺ to CO₃²⁻ activity ratios, the presence of silica at concentrations above saturation with respect to amorphous silica led to formation of only calcite and strongly influenced the crystalline structure and morphology of the precipitates. At high Ca²⁺ to CO₃²⁻ ratios, system behaviour did not differ from that without silica.     

Carbonate- and silicate-rich globules in the kimberlitic rocks of northwestern Tarim large igneous province,NW China: Evidence for carbonated mantle source

We report carbonate- and silicate-rich globules and andradite from the Wajilitage kimberlitic rocks in the northwestern Tarim large igneous province, NW China. The carbonate-rich globules vary in size from 1 to 3 mm, and most have ellipsoidal or round shape, and are composed of nearly pure calcite. The silicate-rich globules are elliptical to round in shape and are typically larger than the carbonate-rich globules ranging from 2 to several centimeters in diameter. They are characterized by clear reaction rims and contain several silicate minerals such as garnet, diopside and phlogopite. The silicate-rich globules, reported here for the first time, are suggested to be related to the origin of andradite within the kimberlitic rocks. Our results show that calcite in the carbonate-rich globules has a high X_Ca (>0.97) and is characterized by extremely high concentrations of the total rare earth elements (up to 1500 ppm), enrichment in Sr (8521–10,645 ppm) and LREE, and remarkable depletion in Nd, Ta, Zr, Hf and Ti. The calcite in the silicate-rich globules is geochemically similar to those in the carbonate-rich globules except the lower trace element contents. Garnet is dominantly andradite (And_{59.56–92.32}Grs_5.67–36.03Pyr_0.36–4.61Spe_0–0.33) and is enriched in light rare earth elements (LREEs) and relatively depleted in Rb, Ba, Th, Pb, Sr, Zr and Hf. Phlogopite in the silicate-rich globules has a high Mg^# ranging from 0.93 to 0.97. The composition of the diopside is Wo_{45.82–51.39}En_{39.81–49.09}Fs_0.88–0.95 with a high Mg^# ranging from 0.88 to 0.95. Diopside in the silicate-rich globules has low total rare earth element (REE) contents (14–31 ppm) and shows middle REE- (Eu to Gd), slight light REE- and heavy REE-enrichment with elevated Zr, Hf and Sr contents and a negative Nb anomaly in the normalized diagram. The matrix of the kimberlitic rocks are silica undersaturated (27.92–29.31 wt.% SiO₂) with low Al₂O₃ (4.51–5.15 wt.%) and high CaO (17.29–17.77 wt.%) contents. The samples are characterized by incompatible element enrichment with high (La/Yb)_N values (41–58) and remarkable negative anomalies in HFSEs (e.g. Ta, Zr, Hf). Our new data suggest that the carbonate-rich globule most likely crystallized at high-temperature and does not represent immiscible liquids, whereas the silicate-rich globules are related to carbonate-rich deuteric hydrothermal fluids during the later-stage of melt evolution. The fluids reacted with the surrounding silicate melts resulting in the formation of skarn minerals such as phlogopite, diopside and andradite. The presence of the carbonate-bearing globules indicates that the Wajilitage kimberlitic rocks are carbonate-rich and most likely derived from an enriched mantle with abundant carbonate. We correlate the carbonated mantle to metasomatism by the migration of deep-seated fluids (carbonate-rich) in response to the impingement of the early Permian mantle plume.     

Hydrogeochemical factors effecting the scaling problem in Balçova geothermal field,İzmir, Turkey

Reservoir fluid compositions have been assessed from analytical data on water samples collected from thermal and cold waters in Balçova geothermal field. The results of mineral equilibrium modelling indicate that the waters, with some exceptions, are systematically supersaturated with respect to calcite, aragonite, dolomite, chalcedony and quartz, but undersaturated with respect to amorphous silica, celestite, anhydrite and gypsum and undersaturated or supersaturated with respect to barite, low-albite, K-feldspar, gibbsite and Fe(OH)₃(a). Calculation of mineral saturation states and geochemical analyses of scale and field observations show that carbonate minerals (calcite, aragonite and dolomite) are most likely to be precipitated as a scale type. Besides carbonates, scale formation risk of amorphous silica, Fe(OH)₃(a), anhydrite, barite and celestite minerals should be taken into account in some wells and surface equipment. Most of the waters, with some exceptions, have carbonate scaling risk at all temperatures, whereas the other scaling risks only exist over a limited temperature range. While silica, Fe(OH)₃(a) and barite show a scaling tendency at low temperatures, anhydrite and celestite scaling occurs at higher temperatures.     

The effect of Fe on Si adsorption by Bacillus subtilis cell walls: insights into non-metabolic bacterial precipitation of silicate minerals

Si adsorption onto Bacillus subtilis and Fe and Al oxide coated cells of B. subtilis was measured both as a function of pH and of bacterial concentration in suspension in order to gain insight into the mechanism of association between silica and silicate precipitates and bacterial cell walls. All experiments were conducted in undersaturated solutions with respect to silicate mineral phases in order to isolate the important adsorption reactions from precipitation kinetics effects of bacterial surfaces. The experimental results indicate that there is little association between aqueous Si and the bacterial surface, even under low pH conditions where most of the organic acid functional groups that are present on the bacterial surface are fully protonated and neutrally charged. Conversely, Fe and Al oxide coated bacteria, and Fe oxide precipitates only, all bind significant concentrations of aqueous Si over a wide range of pH conditions. Our results are consistent with those of Konhauser et al. [Geology 21 (1993) 1103; Environ. Microbiol. 60 (1994) 49] and Konhauser and Urrutia [Chem. Geol. 161 (1999) 399] in that they suggest that the association between silicate minerals and bacterial surfaces is not caused by direct Si–bacteria interactions. Rather, the association is most likely caused by the adsorption of Si onto Fe and Al oxides which are electrostatically bound to the bacterial surface. Therefore, the role of bacteria in silica and silicate mineralization is to concentrate Fe and Al through adsorption and/or precipitation reactions. Bacteria serve as bases, or perhaps templates, for Fe and Al oxide precipitation, and it is these oxide mineral surfaces (and perhaps other metal oxide surfaces as well) that are reactive with aqueous Si, forming surface complexes that are the precursors to the formation of silica and silicate minerals.     

Silicate globules in kyanite from grospydites of the Zagadochnaya kimberlite pipe,Yakutia: The problem of the origin

The results of complex study of silicate globules and α-quartz paramorphs after coesite in kyanite from grospydites from the Zagadochnaya kimberlite pipe, Yakutia, using optical and scanning electron spectroscopy, electron and ion microprobes, LA ICP MS and Raman spectroscopy, are presented. The existence of radial fractures diverging from silicate globules into the matrix (kyanite) attests to the fact that the content of the globules is extremely condensed. A zonal structure is usually typical for globules: a coat and a core, which can be explicitly distinguished under the electron microscope, can be differentiated in them. Compositionally, the coat of the globule corresponds to potassium feldspar (wt %: 66.4 SiO₂; 16.9 Al₂O₃; 0.4 FeO; 0.1 CaO; 0.2 Na₂O; 14.7 K₂O). The globules were also detected in which along with K, a high content of Na and Ca was also ascertained in the silicate coat. The globule coat is considerably enriched with Ba, La, Ce, Nb, and a number of other noncompatible elements as compared with xenolith minerals. The water content in globules is ∼0.6 wt %. As compared with the host mineral (kyanite), the core part of the globules is also enriched with Co, Ni, Zn, and Cu; their content in kyanite is negligibly low. The entire data collection attests to the fact that the formation of silicate globules could have been caused by interaction of the conservated fluid and/or water-silicate melt with the host mineral and crystalline inclusions of clinopyroxene and garnet with decreasing pressure during the transportation of grospydite xenoliths by the kimberlite melt to the Earth’s surface.     

Control of Fe(III) site occupancy on the rate and extent of microbial reduction of Fe(III) in nontronite

A quantitative study was performed to understand how Fe(III) site occupancy controls Fe(III) bioreduction in nontronite by Shewanella putrefaciens CN32. NAu-1 and NAu-2 were nontronites and contained Fe(III) in different structural sites with 16 and 23% total iron (w/w), respectively, with almost all iron as Fe(III). Mössbauer spectroscopy showed that Fe(III) was present in the octahedral site in NAu-1 (with a small amount of goethite), but in both the tetrahedral and the octahedral sites in NAu-2. Mössbauer data further showed that the octahedral Fe(III) in NAu-2 existed in at least two environments- trans (M1) and cis (M2) sites. The microbial Fe(III) reduction in NAu-1 and NAu-2 was studied in batch cultures at a nontronite concentration of 5 mg/mL in bicarbonate buffer with lactate as the electron donor. The unreduced and bioreduced nontronites were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, and transmission electron microscopy (TEM). In the presence of an electron shuttle, anthraquinone-2,6-disulfonate (AQDS), the extent of bioreduction was 11%-16% for NAu-1 but 28%-32% for NAu-2. The extent of reduction in the absence of AQDS was only 5%-7% for NAu-1 but 14%-18% for NAu-2. The control experiments with heat killed cells and without cells did not show any appreciable reduction (<2%). The extent of reduction in experiments performed with a dialysis membrane to separate cells from clays (without AQDS) was 2%-3% for NAu-1 but 5%-7% for NAu-2, suggesting that cells probably released an electron shuttling compound and/or Fe(III) chelator. The reduction rate was also faster in NAu-2 than that in NAu-1. Mössbauer data of the bioreduced nontronite materials indicated that the Fe(III) reduction in NAu-1 was mostly from the presence of goethite, whereas the reduction in NAu-2 was due to the presence of the tetrahedral and trans-octahedral Fe(III) in the structure. The measured aqueous Fe(II) was negligible. As a result of bioreduction, the average nontronite particle thickness remained nearly the same (from 2.1 to 2.5 nm) for NAu-1, but decreased significantly from 6 to 3.5 nm for NAu-2 with a concomitant change in crystal size distribution. The decrease in crystal size suggests reductive dissolution of nontronite NAu-2, which was supported by aqueous solution chemistry (i.e., aqueous Si). These data suggest that the more extensive Fe(III) bioreduction in NAu-2 was due to the presence of the tetrahedral and the trans-octahedral Fe(III), which was presumed to be more reducible. The biogenic Fe(II) was not associated with biogenic solids or in the aqueous solution. We infer that it may be either adsorbed onto surfaces of nontronite particles/bacteria or in the structure of nontronite. Furthermore, we have demonstrated that natural nontronite clays were capable of supporting cell growth even in medium without added nutrients, possibly due to presence of naturally existing nutrients in the nontronite clays. These results suggest that crystal chemical environment of Fe(III) is an important determinant in controlling the rate and extent of microbial reduction of Fe(III) in nontronite.     

Fe sorption onto nontronite (NAu-2)

The sorption of ferrous iron to a clay mineral, nontronite (NAu-2, a ferruginous smectite), was investigated under strictly anoxic conditions as a function of pH (3-10), Fe²⁺ concentration (0.01-50 mM), equilibration time (1-35 days), and ionic strength (0.01-0.5 M NaClO₄). The surface properties of NAu-2 were independently characterized to determine its fixed charge and amphoteric site density in order to interpret the Fe²⁺ sorption data. Fe²⁺ sorption to NAu-2 was strongly dependent on pH and ionic strength, reflecting the coupled effects of Fe²⁺ sorption through ion exchange and surface complexation reactions. Fe²⁺ sorption to NAu-2 increased with increasing pH from pH 2.5 to 4.5, remained constant from pH 4.5 to 7.0, increased again with further increase of pH from pH 7.0 to 8.5, and reached a maximum above pH 8.5. The Fe²⁺ sorption below pH 7.0 increased with decreasing ionic strength. The differences of Fe²⁺ sorption at different ionic strengths, however, diminished with increasing equilibration time. The Fe²⁺ sorption from pH 4.5 to 7.0 increased with increasing equilibration time up to 35 days and showed stronger kinetic behavior in higher ionic strength solutions. The kinetic uptake of Fe²⁺ onto NAu-2 is consistent with a surface precipitation mechanism although our measurements were not able to identify secondary precipitates. An equilibrium model that integrates ion exchange, surface complexation and aqueous speciation reactions reasonably well describes the Fe²⁺ sorption data as a function of pH, ionic strength, and Fe²⁺ concentration measured at 24 h of equilibration. Model calculations show that the species Fe(OH)⁺ was required to describe Fe²⁺ sorption above pH 8.0 satisfactorily. Overall, this study demonstrated that Fe²⁺ sorption to NAu-2 is affected by complex equilibrium and kinetic processes, likely caused by surface precipitation reactions.     

Iron reduction and alteration of nontronite NAu-2 by a sulfate-reducing bacterium

Iron-rich clay minerals are abundant in the natural environment and are an important source of iron for microbial metabolism. The objective of this study was to understand the mechanism(s) of enhanced reduction of Fe(III) in iron-rich 2:1 clay minerals under sulfate-reducing conditions. In particular, biogenic reduction of structural Fe(III) in nontronite NAu-2, an Fe-rich smectite-group mineral, was studied using a Desulfovibrio spp. strain G-11 with or without amended sulfate. The microbial production of Fe(II) from NAu-2 is about 10% of total structural Fe(III) (30 mM) when Fe(III) is available as the sole electron acceptor. The measured production of Fe(II), however, can reach 29% of the total structural Fe(III) during sulfate reduction by G-11 when sulfate (50 mM) is concurrently added with NAu-2. In contrast, abiotic production of Fe(II) from the reaction of NAu-2 with Na₂S (50 mM) is only ca. 7.5% of the total structural Fe(III). The enhanced reduction of structural Fe(III) by G-11, particularly in the presence of sulfate, is closely related to the growth rate and metabolic activities of the bacteria. Analyses by X-ray diffraction, transmission electron microscopy, and energy dispersive spectroscopy reveal significant changes in the structure and composition of NAu-2 during its alteration by bacterial sulfate reduction. G-11 can also derive nutrients from NAu-2 to support its growth in the absence of amended minerals and vitamins. Results of this study suggest that sulfate-reducing bacteria may play a more significant role than previously recognized in the cycling of Fe, S, and other elements during alteration of Fe-rich 2:1 clay minerals and other silicate minerals.     

Thermodynamic properties of solutions in metastable systems under negative or positive pressures

Metastable systems are created when the interface between the atmosphere (in which P_atm = 1 bar) and water forms a spherical meniscus either concave toward the air (water filling capillaries, wherein P_water < P_atm) or convex toward the air (fog water droplet, wherein P_water > P_atm). Soil water, undergoing negative pressure (“capillary potential”) remains bound to the solid matrix (instead of flowing downward) by the capillary meniscus, concave toward the undersaturated dry atmosphere. The positive counterpart of tensile water in soils is the pressurized water contained in fine droplets suspended in oversaturated humid air, as in clouds. All these systems are anisobaric domains the phases of which have different pressures. Geochemical consequences of such characteristics are assessed here by calculating the consequences of the positive or negative water potential on the equilibrium constants of reactions taking place in stretched or pressurized aqueous solutions. Thermodynamic properties of aqueous species are obtained by using the TH model, used explicitly for positive pressures but extrapolated to negative ones for soil solutions. It appears that soil water dissolves gases, offering an alternative explanation of the observed enrichment of atmospheric noble gases in groundwater and of carbonic gas in the unsaturated zone below the root zone. Water droplets obviously show the opposite behavior, that is, a decreasing dissolutive capability with decreasing droplet size (water pressure increases), inducing some climatic consequences. An application of this approach to the solid-solution equilibria is performed by comparing experimental solubility of amorphous silica in unsaturated media on the one hand, to theoretical calculations taking account of the negative water pressure on the other hand. This comparison outlines the potential complexity of anisobaric situations in nature and the necessity to develop a suitable approach for solid pressure.     

Catalytic peptide hydrolysis by mineral surface: Implications for prebiotic chemistry

The abiotic polymerization of amino acids may have been important for the origin of life, as peptides may have been components of the first self-replicating systems. Though amino acid concentrations in the primitive oceans may have been too dilute for significant oligomerization to occur, mineral surface adsorption may have provided a concentration mechanism. As unactivated amino acid polymerization is thermodynamically unfavorable and kinetically slow in aqueous solution, we studied mainly the reverse reaction of polymer degradation to measure the impact of mineral surface catalysis on peptide bonds.Aqueous glycine (G), diglycine (GG), diketopiperazine (DKP), and triglycine (GGG) were reacted with minerals (calcite, hematite, montmorillonite, pyrite, rutile, or amorphous silica) in the presence of 0.05 M, pH 8.1, KHCO₃ buffer and 0.1 M NaCl as background electrolyte in a thermostatted oven at 25, 50 or 70 °C. Below 70 °C, reaction kinetics were too sluggish to detect catalytic activity over amenable laboratory time-scales. Minerals were not found to have measurable effects on the degradation or elongation of G, GG or DKP at 70 °C in solution. At 70 °C pyrite was the most catalytic mineral with detectible effects on the degradation of GGG, although several others also displayed catalytic behavior. GGG degraded ∼1.5-4 times faster in the presence of pyrite than in control reactions, depending on the ratio of solution concentration to mineral surface area. The rate of pyrite catalysis of GGG hydrolysis was found to be saturable, suggesting the presence of discrete catalytic sites on the mineral surface. The mineral-catalyzed degradation of GGG appears to occur via a GGG → DKP + G mechanism, rather than via GGG → GG + G, as in solution-phase reactions. These results are compatible with many previous findings and suggest that minerals may have assisted in peptide synthesis in certain geological settings, specifically by speeding the approach to equilibrium in environments where amino acids were already highly concentrated, but that minerals may not significantly alter the expected solution-phase equilibria. Thus the abiotic synthesis of long peptides may have required activating agents, dry heating at higher temperatures, or some form of phase separation.     

Iron biomineralization by anaerobic neutrophilic iron-oxidizing bacteria

Minerals formed by bio-oxidation of ferrous iron (Fe(II)) at neutral pH, their association with bacterial ultrastructures as well as their impact on the metabolism of iron-oxidizing bacteria remain poorly understood. Here, we investigated iron biomineralization by the anaerobic nitrate-dependent iron-oxidizing bacterium Acidovorax sp. strain BoFeN1 in the presence of dissolved Fe(II) using electron microscopy and Scanning Transmission X-ray Microscopy (STXM). All detected minerals consisted mainly of amorphous iron phosphates, but based on their morphology and localization, three types of precipitates could be discriminated: (1) mineralized filaments at distance from the cells, (2) globules of 100 ± 25 nm in diameter, at the cell surface and (3) a 40-nm thick mineralized layer within the periplasm. All of those phases were shown to be intimately associated with organic molecules. Periplasmic encrustation was accompanied by an accumulation of protein moieties. In the same way, exopolysaccharides were associated with the extracellular mineralized filaments. The evolution of cell encrustation was followed by TEM over the time course of a culture: cell encrustation proceeded progressively, with rapid precipitation in the periplasm (in a few tens of minutes), followed by the formation of surface-bound globules. Moreover, we frequently observed an asymmetric mineral thickening at the cell poles. In parallel, the evolution of iron oxidation was quantified by STXM: iron both contained in the bacteria and in the extracellular precipitates reached complete oxidation within 6 days. While a progressive oxidation of Fe in the bacteria and in the medium could be observed, spatial redox (oxido-reduction state) heterogeneities were detected at the cell poles and in the extracellular precipitates after 1 day. All these findings provide new information to further the understanding of molecular processes involved in iron biomineralization by anaerobic iron-oxidizing bacteria and offer potential signatures of those metabolisms that can be looked for in the geological record.     

Mineralogical characterization of acid weathered phyllosilicates with implications for secondary martian deposits

The detection of phyllosilicates and sulfates on Mars has revealed a complex aqueous history which suggests distinct geochemical environments separated temporally and spatially. Recent observations by MRO CRISM in Mawrth Vallis have shown that phyllosilicate deposits exhibit a specific stratigraphy, which remains incompletely understood. Moreover, MER Spirit has evidenced association between phyllosilicates, amorphous silica and sulfates. We investigated the hypothesis that these parageneses resulted from the acidic weathering of older phyllosilicate deposits. We exposed nontronite (Fe-rich smectite), montmorillonite (Al-rich smectite) and kaolinite to H₂SO₄ solutions at pH 0, 2 and 4, and at a temperature of 60 °C. After the acid treatment, a combination of mineralogical techniques was used to assess the degree of alteration of the three phyllosilicate minerals. XRF, XRD and ESEM measurements show that nontronite was the most unstable when acid leached, followed by montmorillonite and then kaolinite. Progressive acidic leaching of nontronite leads to alteration of the phyllosilicate to amorphous silica, along with Fe-sulfate and anatase, and the formation of an acidic Al,Fe-rich solution. Alteration of montmorillonite resulted in the formation of Fe-, Al-, Ca- and Mg-sulfates, and a Al-rich leaching solution. Comparatively, leaching of kaolinite resulted in the formation of Al-sulfates and a Al-rich solution as well, with only slight alteration of the primary mineralogical features. The effects of acid leaching of the phyllosilicates were also observed in NIR reflectance spectra, allowing a comparison with CRISM spectra from Mawrth Vallis. Based on our results, we propose a new model where acid leaching of mixed phyllosilicate deposits leads to kaolinite overlaying montmorillonite, which in turn caps Fe,Mg-smectites. Leaching of cations and subsequent evaporation leads to sulfate deposits, as supported by geochemical modeling, while amorphous silica remains as a residue. Depending on the intensity (pH) and length of exposure of acidic leaching, our model can explain the stratigraphic distribution of phyllosilicates, and the association of sulfates, silica and smectites.     

An experimental study of the dissolution mechanism and rates of muscovite

Steady-state muscovite dissolution rates have been measured at temperatures from 60 to 201 °C and 1 ? pH ? 10.3 as a function of reactive solution K, Si, and Al concentration. The pegmatitic muscovite used in these experiments has a composition consistent with (Na_0.09, K_0.86)Fe_0.05Al_2.92Si_3.05O₁₀(OH_1.95, F_0.06). All experiments were performed in titanium mixed-flow reactors. All experiments were performed at far-from-equilibrium conditions with respect to muscovite. All reactive solutions were undersaturated with respect to secondary product phases other than for some experiments which were supersaturated with respect to bohemite and diaspore; steady-state dissolution was stoichiometric for all experiments that were undersaturated with respect to these phases.The variation of rates with reactive solution composition depends on the solution pH. At pH ? 7 rates were found to decrease significantly with increasing reactive fluid Al activity but be independent of aqueous SiO₂ activity. pH < 7 rates measured in the present study from 60 to 175 °C are consistent with

     

Isotopic anomalies in organic nanoglobules from Comet 81P/Wild 2: Comparison to Murchison nanoglobules and isotopic anomalies induced in terrestrial organics by electron irradiation

Nanoglobules are a form of organic matter found in interplanetary dust particles and primitive meteorites and are commonly associated with ¹⁵N and D isotopic anomalies that are suggestive of interstellar processes. We report the discovery of two isotopically-anomalous organic globules from the Stardust collection of particles from Comet 81P/Wild 2 and compare them with nanoglobules from the Murchison CM2 meteorite. One globule from Stardust Cometary Track 80 contains highly aromatic organic matter and a large ¹⁵N anomaly (δ¹⁵N = 1120‰). Associated, non-globular, organic matter from this track is less enriched in ¹⁵N and contains a mixture of aromatic and oxidized carbon similar to bulk insoluble organic material (IOM) from primitive meteorites. The second globule, from Cometary Track 2, contains non-aromatic organic matter with abundant nitrile (CN) and carboxyl (COOH) functional groups. It is significantly enriched in D (δD = 1000‰) but has a terrestrial ¹⁵N/¹⁴N ratio. Experiments indicate that similar D enrichments, unaccompanied by ¹⁵N fractionation, can be reproduced in the laboratory by electron irradiation of epoxy or cyanoacrylate. Thus, a terrestrial origin for this globule cannot be ruled out, and, conversely, exposure to high-energy electron irradiation in space may be an important factor in producing D anomalies in organic materials. For comparison, we report two Murchison globules: one with a large ¹⁵N enrichment and highly aromatic chemistry analogous to the Track 80 globule and the other only moderately enriched in ¹⁵N with IOM-like chemistry. The observation of organic globules in Comet 81P/Wild 2 indicates that comets likely sampled the same reservoirs of organic matter as did the chondrite parent bodies. The observed isotopic anomalies in the globules are most likely preserved signatures of low temperature (<10 K) chemistry in the interstellar medium or perhaps the outer regions of the solar nebula. In other extraterrestrial samples, D isotopic anomalies, but not those of ¹⁵N, may be explained in part by exposure to ionizing electron radiation.     

X-ray photoelectron spectroscopic studies of dolomite surfaces exposed to undersaturated and supersaturated aqueous solutions

Cleaved surfaces of dolomite were studied using ex-situ X-ray photoelectron spectroscopy (XPS) following exposure of the surfaces to various experimental conditions. Dolomite samples exposed to air, to a highly undersaturated solution (0.1 M NaCl, pH = 9), and to solution with a supersaturation (−Δμ/kT) of 5.5 (pH = 9) were investigated with semiquantitative methods of analysis to ascertain the degree of non-stoichiometry resulting at the dolomite surface from reactive conditions. It was found that the dolomite cleavage surface in undersaturated solution was not altered significantly from the stoichiometric surface termination. The composition of the cleaved surface after exposure to supersaturated solution, a surface known to have self-limiting growth characteristics under similar conditions, was found to be Ca²⁺ rich (Ca_xMg_2 − x(CO₃)₂, 1.7 > x > 1.3). The observations, while underscoring differences in hydration/dehydration kinetics of the two alkaline earth cations, suggest that achievement of equilibrium at dolomite-water interfaces may be subject to significant barriers from both undersaturated and supersaturated solutions.     

Calculated phase equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O for silica-undersaturated sapphirine-bearing mineral assemblages

Silica‐undersaturated, sapphirine‐bearing granulites occur in a large number of localities worldwide. Such rocks have historically been under‐utilized for estimating P–T evolution histories because of limited experimental work, and a consequent poor understanding of the topology and P–T location of silica‐undersaturated mineral equilibria. Here, a calculated P–T projection for sapphirine‐bearing, silica‐undersaturated metapelitic rock compositions is constructed using THERMOCALC for the FeO‐MgO‐Al₂O₃‐SiO₂ (FMAS) and KFMASH (+K₂O + H₂O) chemical systems, allowing quantitative analysis of silica‐undersaturated mineral assemblages. This study builds on that for KFMASH sapphirine + quartz equilibria [Kelsey et al. (2004) Journal of Metamorphic Geology, vol. 22, pp. 559–578]. FMAS equilibria are significantly displaced in P–T space from silicate melt‐bearing KFMASH equilibria. The large number of univariant silica‐undersaturated KFMASH equilibria result in a P–T projection that is topologically more complex than could be established on the basis of experiments and/or natural assemblages. Coexisting sapphirine and silicate melt (with or without corundum) occur down to c. 900 °C in KFMASH, some 100 °C lower than in silica‐saturated compositions, and from pressures of c.≤1 to ≥12 kbar. Mineral compositions and composition ranges for the calculated phases are consistent with natural examples. Bulk silica has a significant effect on the stability of sapphirine‐bearing assemblages at a given P–T, resulting in a wide variety of possible granulite facies assemblages in silica‐undersaturated metapelites. Calculated pseudosections are able to reproduce many naturally occurring silica‐undersaturated assemblages, either within a single assemblage field or as the product of a P–T trajectory crossing several fields. With an understanding of the importance of bulk composition on sapphirine stability and textural development, silica‐undersaturated assemblages may be utilized in a quantitative manner in the detailed metamorphic investigation of high‐grade terranes.     

Experimental analysis of arsenic precipitation during microbial sulfate and iron reduction in model aquifer sediment reactors

Microbial SO₄²⁻ reduction limits accumulation of aqueous As in reducing aquifers where the sulfide that is produced forms minerals that sequester As. We examined the potential for As partitioning into As- and Fe-sulfide minerals in anaerobic, semi-continuous flow bioreactors inoculated with 0.5% (g mL⁻¹) fine-grained alluvial aquifer sediment. A fluid residence time of three weeks was maintained over a ca. 300-d incubation period by replacing one-third of the aqueous phase volume of the reactors with fresh medium every seven days. The medium had a composition comparable to natural As-contaminated groundwater with slightly basic pH (7.3) and 7.5 μM aqueous As(V) and also contained 0.8 mM acetate to stimulate microbial activity. Medium was delivered to a reactor system with and without 10 mmol L⁻¹ synthetic goethite (α-FeOOH). In both reactors, influent As(V) was almost completely reduced to As(III). Pure As-sulfide minerals did not form in the Fe-limited reactor. Realgar (As₄S₄) and As₂S₃(am) were undersaturated throughout the experiment. Orpiment (As₂S₃) was saturated while sulfide content was low (∼50 to 150 μM), but precipitation was likely limited by slow kinetics. Reaction-path modeling suggests that, even if these minerals had formed, the dissolved As content of the reactor would have remained at hazardous levels. Mackinawite (Fe_1 + xS; x ? 0.07) formed readily in the Fe-bearing reactor and held dissolved sulfide at levels below saturation for orpiment and realgar. The mackinawite sequestered little As (<0.1 wt.%), however, and aqueous As accumulated to levels above the influent concentration as microbial Fe(III) reduction consumed goethite and mobilized adsorbed As. A relatively small amount of pyrite (FeS₂) and greigite (Fe₃S₄) formed in the Fe-bearing reactor when we injected a polysulfide solution (Na₂S₄) to a final concentration of 0.5 mM after 216, 230, 279, and 286 days. The pyrite, and to a lesser extent the greigite, that formed did sequester As from solution, containing 0.84 and 0.23 wt.% As on average, respectively. Our results suggest that As precipitation during Fe-sulfide formation in nature occurs mainly in conjunction with pyrite formation. Our findings imply that the effectiveness of stimulating microbial SO₄²⁻ reduction to remediate As contamination may be limited by the rate and extent of pyrite formation and the solubility of As-sulfides.