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1.
S. Boursiquot M. Mullet M. Abdelmoula J.-M. Génin J.-J. Ehrhardt 《Physics and Chemistry of Minerals》2001,28(9):600-611
The gradual oxidation of dry mackinawite (tetragonal FeS1? x ) has been studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), transmission Mössbauer spectroscopy (TMS) and X-ray photoelectron spectroscopy (XPS). The initial material and samples exposed to the air (5?min to 6?months) have been analysed. Diffraction patterns showed the slow disappearance of mackinawite with time with concomitant appearance of greigite (Fe3S4) and elemental sulphur (S(0)) as well as iron (oxyhydr)oxides, i.e. magnetite (Fe3O4) and probably goethite (α-FeOOH). After 6 months' air exposure, mackinawite and also greigite were entirely converted into elemental sulphur and iron (oxyhydr)oxide(s), indicating that greigite was an intermediate reaction product. Mössbauer spectra of samples oxidized in air appeared rather complex for interpreting what was easily conceivable in view of the association of several phases, as revealed by the diffraction patterns. The low-temperature Mössbauer spectrum obtained after 6?months air exposure was attributed to magnetite, although a mixture of magnetite and goethite was not completely excluded. XPS iron and oxygen data confirmed the formation of Fe(III) (oxyhydr)oxides at the surface after an induction period. Sulphur spectra demonstrated various oxidation states from S(-II) (monosulphide) to S(VI) (sulphate) for the longest experiments. Mackinawite in these experiments reacted mainly with adsorbed O2 to form elemental sulphur and magnetite. Additionally, sufficient sulphur was generated to react stoichiometrically with mackinawite to produce greigite. Finally, greigite, in the longest experiments, was transformed into elemental sulphur and magnetite. 相似文献
2.
Gujie Qian Joël Brugger William M. Skinner Allan Pring 《Geochimica et cosmochimica acta》2010,74(19):5610-5443
We present the results of an experimental study into the sulfidation of magnetite to form pyrite/marcasite under hydrothermal conditions (90-300 °C, vapor saturated pressures), a process associated with gold deposition in a number of ore deposits. The formation of pyrite/marcasite was studied as a function of reaction time, temperature, pH, sulfide concentration, solid-weight-to-fluid-volume ratio, and geometric surface area of magnetite in polytetrafluoroethylene-lined autoclaves (PTFE) and a titanium and stainless steel flow-through cell. Marcasite was formed only at pH21°C <4 and was the dominant Fe disulfide at pH21°C 1.11, while pyrite predominated at pH21°C >2 and formed even under basic conditions (up to pH21°C 12-13). Marcasite formation was favored at higher temperatures. Fine-grained pyrrhotite formed in the initial stage of the reaction together with pyrite in some experiments with large surface area of magnetite (grain size <125 μm). This pyrrhotite eventually gave way to pyrite. The transformation rate of magnetite to Fe disulfide increased with decreasing pH (at 120 °C; pH120°C 0.96-4.42), and that rate of the transformation increased from 120 to 190 °C.Scanning electron microscope (SEM) imaging revealed that micro-pores (0.1-5 μm scale) existed at the reaction front between the parent magnetite and the product pyrite, and that the pyrite and/or marcasite were euhedral at pH21°C <4 and anhedral at higher pH. The newly formed pyrite was micro-porous (0.1-5 μm); this micro-porosity facilitates fluid transport to the reaction interface between magnetite and pyrite, thus promoting the replacement reaction. The pyrite precipitated onto the parent magnetite was polycrystalline and did not preserve the crystallographic orientation of the magnetite. The pyrite precipitation was also observed on the PTFE liner, which is consistent with pyrite crystallizing from solution. The mechanism of the reaction is that of a dissolution-reprecipitation reaction with the precipitation of pyrite being the rate-limiting step relative to magnetite dissolution under mildly acidic conditions (e.g., pH155°C 4.42).The experimental results are in good agreement with sulfide phase assemblage and textures reported from sulfidized Banded Iron Formations: pyrite, marcasite and pyrrhotite have been found to exist or co-exist in different sulfidized Banded Iron Formations, and the microtextures show no evidence of sub-μm-scale pseudomorphism of magnetite by pyrite. 相似文献
3.
D. E. HARLOV R. C. NEWTON E. C. HANSEN & A. S. JANARDHAN 《Journal of Metamorphic Geology》1997,15(6):701-717
The Shevaroy Hills of northern Tamil Nadu, southern India, expose the highest-grade granulites of a prograde amphibolite facies to granulite facies deep-crustal section of Late Archaean age. These highly oxidized quartzofeldspathic garnet charnockites generally show minor high-TiO2 biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements Rb and Th. Peak metamorphic temperatures (garnet–orthopyroxene) and pressures (garnet–orthopyroxene–plagioclase–quartz) are near 750 °C and 8 kbar, respectively. Pervasive veinlets of K-feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene. They are suggested to have been produced by a low H2O activity, migrating fluid phase under granulite facies conditions, most likely a concentrated chloride/carbonate brine with high alkali mobility accompanied by an immiscible CO2-rich fluid. Silicate, oxide and sulphide mineral assemblages record high oxygen fugacity. Pyroxenes in the felsic rocks have high Mg/(Mg+Fe) (0.5–0.7). The major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite. Utilizing three independent oxygen barometers (ferrosilite–magnetite–quartz, ferrosilite–hematite–quartz and magnetite–hematite) in conjunction with garnet–orthopyroxene exchange temperatures, samples with XIlmHm>0.1 yield a consistent oxygen fugacity about two log units above fayalite stability. Less oxidized samples (XIlmHm<0.1) show some scatter with indications of having equilibrated under more reducing conditions. Temperature-f (O2 ) arrays result in self consistent conditions ranging from 660 °C and 10?16 bar to 820 °C and 10?11.5 bar. These trends are confirmed by calculations based on the assemblage clinopyroxene–orthopyroxene–magnetite–ilmenite using the QUIlF program. In the most oxidized granulite samples (XIlmHm>0.4) pyrite is the dominant sulphide and pyrrhotite is absent. Pyrite grains in these samples have marginal alteration to magnetite along the rims, signifying a high-temperature oxidation event. Moderately oxidized samples (0.1no coexisting magnetite. Chalcopyrite is a common accessory mineral of pyrite and pyrrhotite in all the samples. Textures in some samples suggest that it formed as an exsolution product from pyrrhotite. Extensive vein networks of magnetite and pyrite, associated principally with the pyroxene and amphibole, give evidence for a pervasive, highly oxidizing fluid phase. Thermodynamic analysis of the assemblage pyrrhotite, pyrite and magnetite yields consistent high oxidation states at 700–800 °C and 8 kbar. The oxygen fugacity in our most oxidized pyrrhotite-bearing sample is 10?12.65 bar at 770 °C. There are strong indications that the Shevaroy Hills granulites recrystallized in the presence of an alkali-rich, low H2O-activity fluid, probably a concentrated brine. It cannot be demonstrated at present whether the high oxidation states were set by initially oxidized protoliths or effected by the postulated fluids. The high correspondence of maximally Rb-depleted samples with the highest recorded oxidation states suggests that the Rb depletion event coincided with the oxidation event, probably during breakdown of biotite to orthopyroxene+K-feldspar. We speculate that these alterations were effected by exhalations from deep-seated alkali basalts, which provided both heat and high oxygen fugacity, low aH2O fluids. It will be of interest to determine whether greatly Rb-depleted granulites in other Precambrian terranes show similar highly-oxidizing signatures. 相似文献
4.
Kinetics of diffusion-controlled growth of fayalite 总被引：1，自引：0，他引：1
The rate of growth of fayalite (Fe2SiO4) has been measured at one atmosphere total pressure, temperatures from 1000° to 1120° C, and oxygen fugacities controlled by CO/CO2 gas-mixing from 10-9.9 to 10-13.0atm, chosen to span the fayalite stability field. The fine-grained polycrystalline fayalite layer was formed by reacting the oxides FeO or Fe3O4 with a thin slice of single-crystal quartz. The rate of growth of the fayalite increases with increasing temperature and decreasing oxygen fugacity, and is consistent with a parabolic rate law, indicating that the growth rate is controlled by diffusion through the fayalite. Microstructural observations and platinum marker experiments suggest that the reaction phase is formed at the quartz-fayalite interface, and is therefore controlled by the diffusion of iron and oxygen. The parabolic rate constant was analyzed in terms of the oxide activity gradient to yield mean chemical diffusivities for the rate-limiting ionic species, assuming bulk transport through the fayalite layer. Given that iron diffusion in olivine polycrystals occurs either by lattice diffusion, which shows a positive dependence on oxygen activity, or by grain boundary diffusion, which would result in growth rates significantly faster than we observe, we conclude that the diffusivities derived in this study represent oxygen diffusion. However, since oxygen lattice diffusion in fayalite has been established to be much slower than our measurements, it is likely that the transport path for oxygen is along the grain boundaries. Thus, the mean grain boundary diffusivity of oxygen in fayalite $\bar D$ O gb (m2 s-1), using the measured grain size of 0.25 μm, is then given by $$\bar D_O^{gb} {\mathbf{ }}\delta = 1.28 \times 10^{ - 3} f_{O_2 }^{ - 0.17} {\mathbf{ }}e^{ - 540/RT} $$ , where δ is the grain boundary width (in m), and the activation energy is in kJ/mol. Assuming δ=10-9 m (Ricoult and Kohlstedt 1983), the oxygen grain boundary diffusivities are about a factor of 30 × slower than those reported by Watson (1986) for Fo90 olivine. 相似文献
5.
Air entrainment in fragmented magmas controls the dynamics of volcanic eruptions. Pyroclast oxidation kinetics may be applied to quantify the degree of magma–air interaction. Pyrrhotite (Po) in volcanic rocks is often oxidized to form magnetite (Mt) and hematite (Hm), and its reaction mechanisms are well constrained. To test utilizing Po oxidation as a marker for magma–air interactions, we compared the occurrence of Po oxidation products from three different eruption styles during the Sakurajima 1914–1915 eruption. Pumices from the Plinian eruption include columnar-type Fe oxides (Mt with subordinate width of Hm) often accompanied by relict Po. This columnar type is also found in clastogenic lava, where it is almost completely oxidized to Hm. The effusive lava contains framboidal aggregates of subhedral to anhedral Mt crystals without Hm. The formation mechanisms of columnar and framboidal Fe oxides were estimated. The columnar type Fe oxides were formed syn-eruptively through gaseous reactions, as opposed to the melt in a magma chamber, as demonstrated by the Ti-free nature of the columnar Mt and its synchronous oxidation to Hm. By contrast, the framboidal type was formed in a melt with decreasing fS2. The calculation of Hm growth in a conductively cooling pumice clast constrains the surface temperature of pumice in the eruption column. The paragenesis and oxidation degree of Po and Fe oxides are consistent with the eruption processes in terms of magma fragmentation, air entrainment, and welding, and can, therefore, be a responsive marker for the magma–air interaction. 相似文献
6.
The development of orthopyroxene-Fe/Mg ferrite symplectites associated with olivine is discussed with respect to the chemical reactions by which they form. Previously proposed reactions are presented graphically and the differences between them are reviewed. With the exception of exsolution, these are all discontinuous reactions in the sense that olivine is replaced by the two-phase symplectite assemblage.Olivine-hosted symplectites developed in the margins of lherzolite xenoliths from Kauai, Hawaii, demonstrate a reaction mechanism which has not been previously documented from natural samples. Original Fo90 olivine in these samples oxidized to a new assemblage consisting of orthopyroxene (En92–95)-Fe/Mg ferrite (Mf35–50) symplectites developed within more magnesian olivine (Fo92–96) hosts. Thus, by this mechanism, olivine of a different composition persists as part of a final three-phase assemblage. As oxidation advanced, the compositions of all three product phases became continuously more magnesian and the stoichiometric coefficients of the orthopyroxene and Fe/Mg ferrite continuously increased, whereas those of the product olivine decreased in the mass-balance equations. These characteristics demonstrate that the reaction was controlled by oxygen diffusion into the xenoliths from the highly oxidized alkali picrite melt in which they were entrained. Thermodynamic calculations suggest that a gradient in oxygen fugacity of 100.9 bars existed across the xenolith rims and resulted in compositional gradients of 4 mol% fayalite and ferrosilite and 15 mol% magnetite. 相似文献
7.
Frieder Klein Wolfgang Bach Tom McCollom Thelma Berquó 《Geochimica et cosmochimica acta》2009,73(22):6868-6893
Aqueous dihydrogen (H2,aq) is produced in copious amounts when seawater interacts with peridotite and H2O oxidizes ferrous iron in olivine to ferric iron in secondary magnetite and serpentine. Poorly understood in this process is the partitioning of iron and its oxidation state in serpentine, although both impose an important control on dihydrogen production. We present results of detailed petrographic, mineral chemical, magnetic and Mößbauer analyses of partially to fully serpentinized peridotites from the Ocean Drilling Program (ODP) Leg 209, Mid-Atlantic Ridge (MAR) 15°N area. These results are used to constrain the fate of iron during serpentinization and are compared with phase equilibria considerations and peridotite-seawater reaction path models. In samples from Hole 1274A, mesh-rims reveal a distinct in-to-out zoning from brucite at the interface with primary olivine, followed by a zone of serpentine + brucite ± magnetite and finally serpentine + magnetite in the outermost mesh-rim. The compositions of coexisting serpentine (Mg# 95) and brucite (Mg# 80) vary little throughout the core. About 30-50% of the iron in serpentine/brucite mesh-rims is trivalent, irrespective of subbasement depth and protolith (harzburgite versus dunite). Model calculations suggest that both partitioning and oxidation state of iron are very sensitive to temperature and water-to-rock ratio during serpentinization. At temperatures above 330 °C the dissolution of olivine and coeval formation of serpentine, magnetite and dihydrogen depends on the availability of an external silica source. At these temperatures the extent of olivine serpentinization is insufficient to produce much hydrogen, hence conditions are not reducing enough to form awaruite. At T < 330 °C, hydrogen generation is facilitated by the formation of brucite, as dissolution of olivine to form serpentine, magnetite and brucite requires no addition of silica. The model calculations suggest that the iron distribution observed in serpentine and brucite is consistent with formation temperatures ranging from <150 to 250 °C and bulk water-to-rock ratios between 0.1 and 5. These conditions coincide with peak hydrogen fugacities during serpentinization and are conducive to awaruite formation during main stage serpentinization. The development of the common brucite rims around olivine is either due to an arrested reaction olivine → brucite → serpentine + brucite, or reflects metastable olivine-brucite equilibria developing in the strong gradient in silica activity between orthopyroxene (talc-serpentine) and olivine (serpentine-brucite). 相似文献
8.
The iron-rich olivine end-member, fayalite, occurs in the matrix, chondrules, Ca-Al-rich inclusions (CAIs), silicate aggregates, and dark inclusions in the Kaba and Mokoia oxidized CV3 chondrites. In most occurrences, fayalite is associated with magnetite and troilite. To help constrain the origin of the fayalite (Fa98-100), we measured oxygen and silicon isotopic compositions and Mn-Cr systematics in fayalite from two petrographic settings of the Kaba meteorite. One setting consists of big fayalite laths embedded in the matrix and radiating from a core of fine-grained magnetite and sulfide, while the other setting consists of small fayalite-magnetite-sulfide assemblages within or at the surface of Type I barred or porphyritic olivine chondrules. Oxygen in the big fayalite laths and small chondrule fayalites falls on the terrestrial fractionation line, and is distinct from that in chondrule forsterites, which are enriched in 16O (Δ17O = ∼−4‰). Oxygen in the big fayalite laths may be isotopically heavier than that in chondrule fayalites. Silicon isotopes suggest that forsterite is ∼1‰/amu heavier than adjacent fayalite within Kaba chondrules. However, we were unable to confirm large silicon isotopic differences among fayalites reported previously. The Mn-Cr data for big Kaba fayalites give an initial 53Mn/55Mn ratio of (2.07 ± 0.17) × 10−6, consistent with literature results on Mokoia chondrule fayalites. The combined data suggest that fayalites in both petrographic settings formed at about the same time, ∼9.7 Ma after the formation of CAIs. Our data indicate that those fayalite-magnetite-troilite assemblages replacing metal inside and around chondrules formed by aqueous alteration on the meteorite parent body. The formation site and mechanism for the big fayalite laths is less clear, but the petrographic setting indicates that they did not form in situ. None of the models that have been suggested for formation of these fayalites is entirely satisfactory. 相似文献
9.
The reaction between dissolved sulfide and synthetic iron (oxyhydr)oxide minerals was studied in artificial seawater and 0.1 M NaCl at pH 7.5 and 25°C. Electron transfer between surface-complexed sulfide and solid-phase Fe(III) results in the oxidation of dissolved sulfide to elemental sulfur, and the subsequent dissolution of the surface-reduced Fe. Sulfide oxidation and Fe(II) dissolution kinetics were evaluated for freshly precipitated hydrous ferric oxide (HFO), lepidocrocite, goethite, magnetite, hematite, and Al-substituted lepidocrocite. Reaction kinetics were expressed in terms of an empirical rate equation of the form:
10.
The mechanism of thermally induced oxidation of Fe2+ from natural pyrope has been studied at 1000 and 1100 °C using 57Fe Mössbauer spectroscopy in conjunction with XRD, XRF, AFM, QELS, TG, DTA and electron microprobe analyses. At 1000 °C, the non-destructive oxidation of Fe2+ in air includes the partial stabilization of Fe3+ in the dodecahedral 24c position of the garnet structure and the simultaneous formation of hematite particles (15–20 nm). The incorporation of the magnesium ions to the hematite structure results in the suppression of the Morin transition temperature to below 20 K. The general garnet structure is preserved during the redox process at 1000 °C, in accordance with XRD and DTA data. At 1100 °C, however, oxidative conversion of pyrope to the mixed magnesium aluminium iron oxide, Fe-orthoenstatite and cristoballite was observed. During this destructive decomposition, Fe2+ is predominantly oxidized and incorporated into the spinel structure of Mg(Al,Fe)2O4 and partially stabilized in the structure of orthoenstatite, (Mg,Fe)SiO3. The combination of XRD and Mössbauer data suggest the definite reaction mechanism prevailing, including the refinement of the chemical composition and quantification of the reaction products. The reaction mechanism indicates that the respective distribution of Fe2+and Fe3+ to the enstatite and spinel structures is determined by the total content of Fe2+ in pyrope. 相似文献
11.
X-ray absorption and emission spectra were used to characterize the surface of chalcopyrite after oxidation both in air and in air-saturated aqueous solution (pH = 2-10). For chalcopyrite oxidized in aqueous solution, the Cu and Fe L-edge spectra show that the surface oxidation layer is copper deficient. As the pH increases, O K-edge spectra reveal a change in the nature of the oxidation layer. An iron (hydroxy)sulfate is dominant at low pH, whereas FeOOH is the major surface phase under alkaline conditions. Fe2O3 may be present at intermediate pH. The surfaces of chalcopyrite samples oxidized in air consist of a mixture of copper oxides, FeOOH, and sulfate phases. Sulfate is much more abundant on the surface of air-oxidized chalcopyrite because of its high solubility in aqueous solution. Likewise, copper oxidation products can be observed in the O K-edge spectra of air-oxidized chalcopyrite in contrast to the aqueous samples. 相似文献
12.
I. Keesmann S. Matthes W. Schreyer F. Seifert 《Contributions to Mineralogy and Petrology》1971,31(2):132-144
Almandine, although decomposing in the presence of metallic iron into the anhydrous subsolidus assemblage fayalite + ferrocordierite + hercynite solid solution at low pressures, melts incongruently to hercynitess + quartz + liquid at 10 kb. At pressures between about 12 and 20 kb the products of incongruent melting are hercynitess + liquid only, and at still higher pressures almandine melts congruently. For the intermediate pressures between 2 and 10 kb not investigated a sequence of probable breakdown and melting relations involving the phases ferrocordierite, fayalite, hercynitess, quartz, and liquid is derived through Schreinemakers' analyses.The lower temperature stability limit of almandine in the presence of water at low oxygen fugacities and pressures of 15 to 20 kb lies between 550° and 600° C as at low pressures. It is marked, however, by the breakdown to a hydrous assemblage involving chloritoid and the new phase aluminous deerite. Since the anhydrous melting at these pressures occurs between 1300° and 1400° C, the thermal stability range of almandine increases drastically with pressure. Its upper breakdown limit shows in principle a similar behavior as those of other garnet end members. 相似文献
13.
W.C Shanks James L Bischoff Robert J Rosenbauer 《Geochimica et cosmochimica acta》1981,45(11):1977-1995
Sulfate reduction during seawater reaction with fayalite and with magnetite was rapid at 350°C, producing equilibrium assemblages of talc-pyrite-hematite-magnetite at low water/rock ratios and talc-pyrite-hematite-anhydrite at higher water/rock ratios. At 250°C, seawater reacting with fayalite produced detectable amounts of dissolved H2S, but extent of reaction of solid phases was minor after 150 days. At 200°C, dissolved H2S was not detected, even after 219 days, but mass balance calculations suggest a small amount of pyrite may have formed. Reaction stoichiometry indicates that sulfate reduction requires large amounts of H+, which, in subseafloor hydrothermal systems is provided by Mg metasomatism. Seawater contains sufficient Mg to supply all the H+ necessary for quantitative reduction of seawater sulfate.Systematics of sulfur isotopes in the 250 and 350°C experiments indicate that isotopic equilibrium is reached, and can be modeled as a Rayleigh distillation process. Isotopic composition of hydrothermally produced H2S in natural systems is strongly dependent upon the seawater/basalt ratio in the geothermal system, which controls the relative sulfide contributions from the two important sulfur sources, seawater sulfate and sulfide phases in basalt. Anhydrite precipitation during geothermal heating severely limits sulfate ingress into high temperature interaction zones. Quantitative sulfate reduction can thus be accomplished without producing strongly oxidized rocks and resultant sulfide sulfur isotope values represent a mixture of seawater and basaltic sulfur. 相似文献
14.
The replacement of magnetite by hematite was studied through a series of experiments under mild hydrothermal conditions(140 -220℃, vapour saturated pressures) to quantify the kinetics of the transformation and the relative effects of redox and non-redox processes on the transformation. The results indicate that oxygen is not an essential factor in the replacement reaction of magnetite by hematite, but the addition of excess oxidant does trigger the oxidation reaction, and increases the kinetics of the transformation. However, even under high O_2(aq) environments, some of the replacement still occurred via Fe2+ leaching from magnetite. The kinetics of the replacement reaction depends upon temperature and solution parameters such as pH and the concentrations of ligands, all of which are factors that control the solubility of magnetite and affect the transport of Fe2+ (and the oxidant) to and from the reaction front. Reaction rates are fast at ~200℃, and in nature transport properties of Fe and,in the case of the redox-controlled replacement, the oxidant will be the rate-limiting control on the reaction progress. Using an Avrami treatment of the kinetic data and the Arrhenius equation, the activation energy for the transformation under non-redox conditions was calculated to be 26 ± 6 kJ mol-1.This value is in agreement with the reported activation energy for the dissolution of magnetite, which is the rate-limiting process for the transformation under non-redox conditions. 相似文献
15.
I. M. Varentsov N. V. Bakova Yu. P. Dikov T. S. Gendler R. Giovanoli 《Mineralium Deposita》1979,14(3):281-296
Experiments on the sorption of dissolved Ni, Co, Mn, Fe from seawater by Mn3O4 reveal a sequence of reactions taking place: Ion exchange, hydrolysis, then autocatalytic oxidation and layer formation on the interface. The composition of the new compounds depends on the kinetics of i) sorption, and ii) interface oxidation. The highest oxidized Me ions accumulate at low sorption rates, i. e. when sorption does not inhibit interface oxidation: 60% Mn4+, 30% Ni3+ & 30% Co3+ are a representative example for that layer type. Iron is present in this layer as amorphous FeOOH·xH2O according to Mössbauer spectra. Specific for the Me sorption by Mn3O4 is the interaction of Ni & Co with Mn2+ and Mn3+ of the sorbent lattice. Mn is found in the solute phase equivalent to 16, 14% of the adsorbed Co or 17, 96% of the adsorbed Ni. These results confirm the earlier presented model on the transition metal accumulation in recent basins as taking place in distinct stages with interface autocatalysis for the Me oxidation playing the main role. 相似文献
16.
Damien Faivre Pierre Agrinier Pierpaolo Zuddas Katavut Pachana Jean-Yves Laval 《Geochimica et cosmochimica acta》2004,68(21):4395-4403
Inorganic magnetite nanocrystals were synthesized in an aqueous medium at 25°C, atmospheric pressure, ionic strength of 0.1 M, oxygen fugacity close to 0, and under controlled chemical affinity, which was maintained constant during an experiment and varied between different experiments. The total concentration of iron in the initial solutions, with Fe(III)/Fe(II) ratios of 2, was varied in order to measure the role of this parameter on the reaction rate, particle morphology, and oxygen isotopic composition. The reaction rates were followed by a pHstat apparatus. The nature and morphology of particles were studied by transmission electron microscopy and electron energy loss spectroscopy. Fractionation factors of oxygen isotopes were determined by mass spectrometry after oxygen extraction from the solid on BrF5 lines. At low total iron concentrations, goethite and poorly crystalline iron oxides were observed coexisting with magnetite. At higher concentrations, euhedral single crystals of pure magnetite with an average characteristic size of 10 nm were formed, based on a first-order rate law with respect to total iron concentration. These results confirm that, under high supersaturation conditions, low-temperature inorganic processes can lead to the formation of well-crystallized nanometric magnetite crystals with narrow size distribution. The observed oxygen isotope fractionation factor between magnetite crystals and water was of 0-1‰, similar to the fractionation factor associated with bacterially produced magnetite. We suggest that the solution chemistry used in this study for inorganic precipitation is relevant to better understanding of magnetite precipitation in bacterial magnetosomes, which might thus be characterized by high saturation states and pH. 相似文献
17.
Claudia L. Caldeira Virginia S.T. Ciminelli Kwadwo Osseo-Asare 《Geochimica et cosmochimica acta》2010,74(6):1777-1789
The mechanism of pyrite oxidation in carbonate-containing alkaline solutions at 80 °C was investigated with the help of rate experiments, thermodynamic modeling and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrite oxidation rate increased with pH and was enhanced by addition of bicarbonate/carbonate ions. The carbonate effect was found to be limited to moderately alkaline conditions (pH 8-11). Metastable Eh-pH diagrams, at 25 °C, indicate that soluble iron-carbonate complexes (FeHCO3−, FeCO30, Fe(CO3)(OH)− and FeCO32−) may coexist with pyrite in the pH range of 6-12.5. Above pH 11 and 13, the Fe(II) and Fe(III) hydroxocomplexes, respectively, become stable, even in the presence of carbonate/bicarbonate ions. Surface-bound carbonate complexes on iron were also identified with DRIFTS as products of pyrite oxidation in addition to iron oxyhydroxides and soluble sulfate species. The conditions under which thermodynamic and DRIFTS analyses indicate the presence of carbonate compounds also correspond to those in which the fastest rate of pyrite oxidation in carbonate solutions was observed. Following the Singer-Stumm model for pyrite oxidation in acidic solutions, it is assumed that Fe(III) is the preferred pyrite oxidant under alkaline conditions. We propose that carbonate ions facilitate the electron transfer from soluble iron(II)-carbonate to O2, increase the iron solubility, and provide buffered, favorable alkaline conditions at the reaction front, which in turn favors the overall kinetics of pyrite oxidation. Therefore, the electron transfer from sulfur atoms to O2 is facilitated by the formation of the cycle of Fe(II)-pyrite/Fe(III)-carbonate redox couple at the pyrite surface. 相似文献
18.
James Wilson Gordon Cressey Javier Cuadros David Savage 《Geochimica et cosmochimica acta》2006,70(2):323-336
Several designs proposed for high-level nuclear waste (HLW) repositories include steel waste canisters surrounded by montmorillonite clay. This work investigates montmorillonite stability in the presence of native Fe, magnetite and aqueous solutions under hydrothermal conditions. Two series of experiments were conducted. In the first, mixtures of Na-montmorillonite, magnetite, native Fe, calcite, and NaCl solutions were reacted at 250 °C, Psat for between 93 and 114 days. In the second series, the starting mixtures included Na-montmorillonite, native Fe and solutions of FeCl2 which were reacted at temperatures of 80, 150, and 250 °C, Psat, for 90-92 days. Experiments were analysed using XRD, FT-IR, TEM, ICP-AES, and ICP-MS. In the first series of experiments, native Fe oxidised to produce magnetite and the starting montmorillonite material was transformed to Fe-rich smectite only when the Fe was added predominantly as Fe metal rather than Fe oxide (magnetite). The Fe-rich smectite was initially Fe(II)-rich, which oxidised to produce an Fe(III)-rich form on exposure to air. The expansion of this material on ethylene glycol solvation was much reduced compared to the montmorillonite starting material. TEM imaging shows that partial loss of tetrahedral sheets occurred during transformation of the montmorillonite, resulting in adjacent layers becoming H-bonded with a 7 Å repeat. The reduced swelling property of the Fe-smectite product may be due predominantly to the structural disruption of smectite layers and the formation of H-bonds. Solute activities corresponded to the approximate stability field calculated for hypothetical Fe(II)-saponite. In the second series of experiments, significant smectite alteration was only observed at 250 °C and the product contained a small proportion of a 7 Å repeat structure, observable by XRD. In these experiments, solute activities coincide with berthierine. The experiments indicate that although bentonite is still a desirable choice of backfill material for HLW repositories, some loss of expandability may result if montmorillonite is altered to Fe-rich smectite at the interface between steel canisters and bentonite. 相似文献
19.
The oxidation of carbonate green rust, GR(CO32−), in NaHCO3 solutions at T = 25°C has been investigated through electrochemical techniques, FTIR, XRD, TEM and SEM. The used GR(CO32−) samples were made of either suspended solid in solution or a thin electrochemically formed layer on the surface of an iron disc. Depending on experimental conditions, oxidation occurs, with or without major modifications of the GR(CO32−) structure, suggesting the existence of two pathways: solid-state oxidation (SSO) leading to a ferric oxyhydroxycarbonate as the end product, and a dissolution-oxidation-precipitation (DOP) mechanism leading to ferric oxihydroxides such as lepidocrocite, goethite, or ferrihydrite. A formula was proposed for this ferric oxyhydroxycarbonate, Fe6IIIO(2+x)(OH)(12-2x)(H2O)x(CO3), assuming that the solid-state oxidation reaction is associated to a deprotonation of the water molecules within the interlayers, or of the hydroxyl groups in the Fe(O,H) octahedra layers. The DOP mechanism involves transformation via solution with the occurrence of soluble ferrous-ferric intermediate species. A discussion about factors influencing the oxidation of carbonate green rust is provided hereafter. The ferric oxyhydroxycarbonate can be reduced back to GR(CO32−) by a reverse solid-state reduction reaction. The potentiality for a solid-state redox cycling of iron to occur may be considered. The stability of the ferric oxyhydroxycarbonate towards thermodynamically stable ferric phases, such as goethite and hematite, was also studied. 相似文献