Liane G. Benning

Reaction pathways in the Fe–S system below 100°C

Abstract The formation pathways of pyrite are controversial. Time resolved experiments show that in reduced sulphur solutions at low temperature, the iron monosulphide mackinawite is stable for up to 4 months. Below 100°C, the rate of pyrite formation from a precursor mackinawite is insignificant in solutions equilibrated solely with H2S(aq). Mackinawite serves as a precursor to pyrite formation only in more oxidised solutions. Controlled, intentional oxidation experiments below 100°C and over a wide range of pH (3.3–12) confirm that the mackinawite to pyrite transformation occurs in slightly oxidising environments. The conversion to pyrite is a multi-step reaction process involving changes in aqueous sulphur species causing solid state transformation of mackinawite to pyrite via the intermediate monosulphide greigite. Oxidised surfaces of precursors or of pyrite seeds speed up the transformation reaction. Solution compositions from the ageing experiments were used to derive stability constants for mackinawite from 25°C to 95°C for the reaction: FeS(s) +2 H + ⇔ Fe 2+ + H 2 S The values of the equilibrium constant, logKFeS, varied from 3.1 at 25°C to 1.2 at 95°C and fit a linear, temperature-dependent equation: logKFeS=2848.779/T−6.347, with T in Kelvin. From these constants, the thermodynamic functions were derived. These are the first high temperature data for the solubility of mackinawite, where Fe2+ is the dominant aqueous ferrous species in reduced, weakly acidic to acidic solutions.

Hydrosulphide complexing of Au (I) in hydrothermal solutions from 150–400°C and 500–1500 bar

Abstract The solubility of gold has been measured in aqueous sulphide solutions at temperatures between 150°C and 500°C and pressures of 500–1500 bar over a wide range of pH and total dissolved sulphur concentrations. The solubilities ranged from 0.002–1 mg/kg (1 × 10−8 to 5 × 10−6 m) in experiments with low total sulphur and acid pH, and from 2–108 mg/kg (1 × 10−5 to 5 × 10−4 m) in solutions wit)1 high total reduced sulphur concentrations and near neutral pH. The solubilities generally increased with increasing temperature, pH, and total dissolved sulphur. At near neutral pH, an inverse correlation between solubility and pressure was observed, whereas in acid pH solutions, above 150°C, increasing pressure also increased the solubility. In near neutral pH solutions a solubility maximum was observed. This maximum is due to the species Au(HS)2−. However, with increasing temperature, in accordance with the shift of pK1 of H2S towards more alkaline pH, the maximum solubility also shifts to higher pH-values and consequently, at high temperatures the species stable at lower pH will dominate. It has been unambiguously proven that over a wide range of temperatures and pressures in reduced sulphur-containing hydrothermal solutions of low pH, the stoichiometry of the dominant Au (I)-hydrosulphide complex, is AuHS0. High temperature and high pressure: equilibrium constants for the formation of the Au(I)-hydrosulphide complexes, AuHS0, and Au(HS)2−, pertaining to the equilibria Au(s) + H2S = AuHS0 + 1 2 H2(g) (1) and Au(s) + H2S + HS− = Au(HS)2− + 1 2 H2(g), (2) have been calculated. The nonlinear least squares fitted equilibrium constant for reaction (1) varies from log K(1) = −6.81 at 150°C/500 bar to a maximum of −5.90 at 200°C/1500 bar and decreases again at higher temperatures (-7.83 at 400°C/500 bars). For reaction (2), a similar variation occurs: log K(2) = −1.45 at 150°C/500 bar to −1.03 at 250°C/500 bar and −1.75 at 400°C/1500 bar. The thermodynamic functions for the Au(I)-hydrosulphide formation reactions and the cumulative and stepwise formation constants were derived after transforming the above reactions into isocoulombic form. The equilibrium constants were derived after transforming the above reactions into isocoulombic form. role in the transport and deposition of gold in ore depositing environments which are characterised by low pH fluids.

Greigite: a true intermediate on the polysulfide pathway to pyrite

The formation of pyrite (FeS2) from iron monosulfide precursors in anoxic sediments has been suggested to proceed via mackinawite (FeS) and greigite (Fe3S4). Despite decades of research, the mechanisms of pyrite formation are not sufficiently understood because solid and dissolved intermediates are oxygen-sensitive and poorly crystalline and therefore notoriously difficult to characterize and quantify.In this study, hydrothermal synchrotron-based energy dispersive X-ray diffraction (ED-XRD) methods were used to investigate in situ and in real-time the transformation of mackinawite to greigite and pyrite via the polysulfide pathway. The rate of formation and disappearance of specific Bragg peaks during the reaction and the changes in morphology of the solid phases as observed with high resolution microscopy were used to derive kinetic parameters and to determine the mechanisms of the reaction from mackinawite to greigite and pyrite.The results clearly show that greigite is formed as an intermediate on the pathway from mackinawite to pyrite. The kinetics of the transformation of mackinawite to greigite and pyrite follow a zero-order rate law indicating a solid-state mechanism. The morphology of greigite and pyrite crystals formed under hydrothermal conditions supports this conclusion and furthermore implies growth of greigite and pyrite by oriented aggregation of nanoparticulate mackinawite and greigite, respectively. The activation enthalpies and entropies of the transformation of mackinawite to greigite, and of greigite to pyrite were determined from the temperature dependence of the rate constants according to the Eyring equation. Although the activation enthalpies are uncharacteristic of a solid-state mechanism, the activation entropies indicate a large increase of order in the transition state, commensurate with a solid-state mechanism.

Bioavailable iron in the Southern Ocean: the significance of the iceberg conveyor belt

Productivity in the Southern Oceans is iron-limited, and the supply of iron dissolved from aeolian dust is believed to be the main source from outside the marine reservoir. Glacial sediment sources of iron have rarely been considered, as the iron has been assumed to be inert and non-bioavailable. This study demonstrates the presence of potentially bioavailable Fe as ferrihydrite and goethite in nanoparticulate clusters, in sediments collected from icebergs in the Southern Ocean and glaciers on the Antarctic landmass. Nanoparticles in ice can be transported by icebergs away from coastal regions in the Southern Ocean, enabling melting to release bioavailable Fe to the open ocean. The abundance of nanoparticulate iron has been measured by an ascorbate extraction. This data indicates that the fluxes of bioavailable iron supplied to the Southern Ocean from aeolian dust (0.01–0.13 Tg yr-1) and icebergs (0.06–0.12 Tg yr-1) are comparable. Increases in iceberg production thus have the capacity to increase productivity and this newly identified negative feedback may help to mitigate fossil fuel emissions.

The Role and Implications of Bassanite as a Stable Precursor Phase to Gypsum Precipitation

Roundabout Gypsum Calcium sulfates are a common but perhaps underappreciated group of minerals used in a number of natural and industrial processes. In many ways, these crystals precipitate from solution in the same way that most other aqueous minerals form; however, mounting evidence suggests that different, unexplored mechanisms may be at work. Van Driessche et al. (p. 69; see the cover) performed high-resolution microscopy of the most common calcium sulfate mineral, gypsum, at various points along time-resolved, fast-quenching growth experiments. The images reveal that gypsum particles actually start out as crystalline nanoparticles of another mineral, bassanite, which then self-assemble into well-ordered nanorods. Finally, the nanorods transform into gypsum following a hydration reaction. The observation that the reaction pathway occurs below the solubility limit of the intermediate phase has wide-ranging implications for biomineralization processes and may provide ways to prevent fouling on the surfaces of desalination membranes. The common mineral gypsum forms when nanoparticles of an undersaturated precursor phase, bassanite, self-assemble into nanorods, followed by ripening. Calcium sulfate minerals such as gypsum play important roles in natural and industrial processes, but their precipitation mechanisms remain largely unexplored. We used time-resolved sample quenching and high-resolution microscopy to demonstrate that gypsum forms via a three-stage process: (i) homogeneous precipitation of nanocrystalline hemihydrate bassanite below its predicted solubility, (ii) self-assembly of bassanite into elongated aggregates co-oriented along their c axis, and (iii) transformation into dihydrate gypsum. These findings indicate that a stable nanocrystalline precursor phase can form below its bulk solubility and that in the CaSO4 system, the self-assembly of nanoparticles plays a crucial role. Understanding why bassanite forms prior to gypsum can lead to more efficient anti-scaling strategies for water desalination and may help to explain the persistence of CaSO4 phases in regions of low water activity on Mars.

The effect of cyanobacteria on silica precipitation at neutral pH: implications for bacterial silicification in geothermal hot springs

In this study, we performed silica precipitation experiments with the cyanobacteria Calothrix sp. to investigate the mechanisms of silica biomineralization. Batch silica precipitation experiments were conducted at neutral pH as a function of time, Si saturation states, temperature and ferrihydrite concentrations. The experimental results show that in solutions undersaturated with respect to amorphous silica, the interaction between Si and cell surface functional groups is weak and minimal Si sorption onto cyanobacterial surfaces occurs. In solutions at high Si supersaturation states, abiotic Si polymerization is spontaneous, and at the time scales of our experiments (1–50 h) the presence of cyanobacteria had a negligible effect on silica precipitation kinetics. At lower supersaturation states, Si polymerization is slow and the presence of cyanobacteria do not promote Si–solid phase nucleation. In contrast, experiments conducted with ferrihydrite-coated cyanobacteria significantly increase the rate of Si removal, and the extent to which Si is removed increases as a function of ferrihydrite concentration. Experiments conducted with inorganic ferrihydrite colloids (without cyanobacteria) removes similar amounts of Si, suggesting that microbial surfaces play a limited role in the silica precipitation process. Therefore, in supersaturated hydrothermal waters, silica precipitation is largely nonbiogenic and cyanobacterial surfaces have a negligible effect on silica nucleation. D 2003 Elsevier Science B.V. All rights reserved.

Molecular characterization of cyanobacterial silicification using synchrotron infrared micro-spectroscopy

Synchrotron-based Fourier-transform infrared (SR-FTIR) micro-spectroscopy was used to determine the concentration-dependent response of the organic structure of live cyanobacterial cells to silicification. Mid-infrared (4000–600 cm−1) measurements carried out on single filaments and sheaths of the cyanobacteria Calothrix sp. (strain KC97) were used to monitor the interaction between a polymerizing silica solution and the organic functional groups of the cells during progressive silicification. Spectra of whole-cells and sheaths were analyzed and the spectral features were assigned to specific functional groups related to the cell: lipids (-CH2 and -CH3; at 2870–2960 cm−1), fatty acids (>C=O at 1740 cm−1), proteins (amides I and II at 1650 and 1540 cm−1), nucleic acids (>P=O 1240 cm−1), carboxylic acids (C-O at 1392 cm−1), and polysaccharides (C-O between 1165 and 1030 cm−1). These vibrations and the characteristic vibrations for silica (Si-O between 1190 and 1060 cm−1; to some extent overlapping with the C-O frequencies of polysaccharides and Si-O at 800 cm−1) were used to follow the progress of silicification. Relative to unsilicified samples, the intensity of the combined C-O/Si-O vibration band increased considerably over the course of the silicification (whole-cells by > 90% and sheath by ∼75%). This increase is a consequence of (1) extensive growth of the sheath in response to the silicification, and (2) the formation of thin amorphous silica layers on the sheath. The formation of a silica specific band (∼800 cm−1) indicates, however, that the precipitation of amorphous silica is controlled by the dehydroxylation of abiotically formed silanol groups.

Plant-driven fungal weathering: Early stages of mineral alteration at the nanometer scale

Plant-driven fungal weathering is a major pathway of soil formation, yet the precise mechanism by which mycorrhiza alter minerals is poorly understood. Here we report the first direct in situ observations of the effects of a soil fungus on the surface of a mineral over which it grew in a controlled experiment. An ectomycorrhizal fungus was grown in symbiosis with a tree seedling so that individual hyphae expanded across the surface of a biotite flake over a period of three months. Ultramicroscopic and spectroscopic analysis of the fungus-biotite interfaces revealed intimate fungal-mineral attachment, biomechanical forcing, altered interlayer spacings, substantial depletion of potassium (~50 nm depth), oxidation of the biotite Fe(II), and the formation of vermiculite and clusters of Fe(III) oxides. Our study demonstrates the biomechanical-chemical alteration interplay at the fungus-biotite interface at the nanometer scale. Specifically, the weathering process is initiated by physical distortion of the lattice structure of biotite within 1 μm of the attached fungal hypha. Only subsequently does the distorted volume become chemically altered through dissolution and oxidation reactions that lead to mineral neoformation.

Ice sheets as a significant source of highly reactive nanoparticulate iron to the oceans

The Greenland and Antarctic Ice Sheets cover ~\n10% of global land surface, but are rarely considered as active components of the global iron cycle. The ocean waters around both ice sheets harbour highly productive coastal ecosystems, many of which are iron limited. Measurements of iron concentrations in subglacial runoff from a large Greenland Ice Sheet catchment reveal the potential for globally significant export of labile iron fractions to the near-coastal euphotic zone. We estimate that the flux of bioavailable iron associated with glacial runoff is 0.40–2.54 Tg per year in Greenland and 0.06–0.17 Tg per year in Antarctica. Iron fluxes are dominated by a highly reactive and potentially bioavailable nanoparticulate suspended sediment fraction, similar to that identified in Antarctic icebergs. Estimates of labile iron fluxes in meltwater are comparable with aeolian dust fluxes to the oceans surrounding Greenland and Antarctica, and are similarly expected to increase in a warming climate with enhanced melting.

The dynamics of cyanobacterial silicification: an infrared micro-spectroscopic investigation

Abstract The dynamics of cyanobacterial silicification was investigated using synchrotron-based Fourier transform infrared micro-spectroscopy. The changes in exo-polymeric polysaccharide and silica vibrational characteristics of individual Calothrix sp. filaments was determined over time in a series of microcosms in which the microbially sorbed silica or silica and iron load was increased sequentially. The changes in intensity and integrated area of specific infrared spectral features were used to develop an empirical quantitative dynamic model and to derive silica load-dependent parameters for each quasi-equilibrium stage in the biomineralization process. The degree of change in spectral features was derived from the increase in integrated area of the combined silica/polysaccharide region (Si-O/C-O, at 1150–950 cm−1) and the Si-O band at 800 cm−1, the latter representing specific silica bonds corresponding to hydrated amorphous SiO4 tetrahedra. From the degree of change, a two-phase model with concurrent change in process was derived. In the first phase, a biologically controlled increase in thickness of the exo-polymeric polysaccharide sheath around the cell was observed. In phase two, a transition to an inorganically controlled accumulation of silica on the surface of the cyanobacterial cells was derived from the change in integrated area for the mixed Si-O/C-O spectral region. This second process is further corroborated by the synchronous formation of non-microbially associated inorganic SiO4 units indicated by the growth of the singular Si-O band at 800 cm−1. During silicification, silica accumulates (1) independently of the growth of the sheath polysaccharides and (2) via an increase in chain lengths of the silica polymers by expelling water from the siloxane bonds. IR evidence suggest that an inorganic, apparently surface catalyzed process, which leads to the accumulation of silica nanospheres on the cyanobacterial surfaces governs this second stage. In experiments where iron was present, the silicification followed similar pathways, but at low silica loads, the iron bound to the cell surfaces slightly enhanced the reaction dynamics.