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1.
《Chemical Geology》2006,225(1-2):16-29
Pyrite oxidation rates were examined at various concentrations of dissolved oxygen (DO) in the presence of the sulfur and iron oxidizer Acidithiobacillus ferrooxidans. Five different batch experiments were performed at room temperature for 75 days under various DO levels (273, 129, 64.8, 13.2, and ≤ 0.006 μM), containing pyrite grains (particle size 63–250 μm) and a modified 9K nutrient medium at pH 3. The reactors were inoculated with A. ferrooxidans. In all experiments, pH decreased with time and sulfur and iron were released to the solution, indicating pyrite oxidation at all DO levels. Pyrite oxidation rates (ca. 5 × 10− 10 mol m− 2 s− 1 at 273 μM DO) from all experiments showed positive correlation with DO, Fe(III), and bacterial concentration. These rates were significantly slower than rates presented in other published studies, but this is probably due to the significantly greater Fe(III) concentration at lower pH in these previous studies. The results obtained in this study suggest that ferric iron reduction at the pyrite surface is the primarily mechanism for microbial pyrite oxidation in the presence of DO. The results from our study support the indirect mechanism of sulfide oxidation, where A. ferrooxidans oxidizes ferrous iron in the presence of DO, which then oxidizes pyrite.  相似文献   

2.
In this study we experimentally determine phlogopite/melt partition coefficients of Ra and other trace elements in a lamproitic system. This work was achieved using an analytical technique (LA-ICP-MS) with low detection limits (~ 0.01 fg) permitting the measurement of the very low Ra concentrations feasible in experiments (~ 1 ppb). DRaphlogopite/melt was determined to 2.28 ± 0.44 and 2.84 ± 0.47 in two experiments, the ratio DRa/DBa is around 1.6. The compatibility of Ra in phlogopite results from an ionic radius being close to the apex of the lattice strain parabola for earth alkalis in the large XII-coordinated interlayer site of phlogopite. A re-evaluation of DRa and DRa/DBa for magmatic minerals containing appreciable Ra, yields DRamineral/melt ranging from ~ 2.6 for phlogopite down to 2–3 ? 10? 5 for pyroxenes, and DRa/DBamineral/melt from ~ 4 for leucite to 2 ? 10? 2 for orthopyroxene. The influence of melt composition on DRa/DBa is less than 10%. All investigated minerals have different DRa/DBa, strongly fractionating Ra from Ba. Thus, for magmatic systems, (226Ra)/Ba in the various minerals is not constant, these minerals do not form a straight line in the (226Ra)/Ba–(230Th)/Ba system at the time of crystallization and thus, there is no (226Ra)/Ba–(230Th)/Ba isochron at t0. 226Ra–230Th–Ba mineral dating is thus applicable only to model ages calculated from mineral–glass pairs with known DRa.  相似文献   

3.
Weathering experiments using biotite and phlogopite in the presence of bacteria were conducted to better understand biotic dissolution kinetics and processes (proton- and ligand-promoted dissolution) under aerobic conditions. Miniature batch reactors (300 μl in microplate wells) were used at 24 °C for 3 days with and without bacterial strains. Abiotic experiments were performed with organic and nitric acids in order to calibrate the biotite-phlogopite chemical dissolution. An empirical model was used to fit the pH dependence for iron release rate (rFe) considering the influence of both protons and ligands from acidic to neutral conditions (pH ranging from 3 to 7): rFe=kH(aH+)m+kL(aL)1 where k is the apparent rate constant, aH+ and aL are the activities of protons and ligands, and m and l are the reaction orders. For both minerals in most cases at a given pH, the iron release rates in the presence of bacteria were in good agreement with rates determined by the chemical model and could be explained by a combination of proton- and ligand-promoted processes. Bacteria affect mineral dissolution and iron release rates through the quantities and nature of the organic acids they produce. Three domains were differentiated and proposed as biochemical models of mica dissolution: (1) below pH 3, only proton-promoted dissolution occurred, (2) in weakly acidic solutions both ligand- and proton-promoted mechanisms were involved, and (3) iron immobilization occured, at pH values greater than 4 for biotite and greater than 5 for phlogopite. This model allows us to distinguish the “weathering pattern phenotypes” of strains. Bacteria that are isolated from horizons poor in carbon appear more efficient at weathering micas than bacterial strains isolated from environments rich in carbon. Moreover, our results suggest that the mineral could exert a control on the release of organic acids and the “weathering pattern phenotypes” of bacteria.  相似文献   

4.
The oolitic ironstones ore deposit of Jebel Ank (central Tunisia), is a simply folded stratiform ore body of about 2.5–8 m thickness located in the upper part of the epicontinental Souar Formation (Late Eocene) and is covered by the continental Segui Formation (Mio-Pliocene). The deposit contains about 20 Mt of ore with an average grade of 50% Fe. Generally, oolitic iron deposition occurs in shallow water lagoonal environments. The Jebel Ank deposit lies between two regional disconformities (Late Eocene and Miocene), and is evidence of a transitional stage at the end of regional regression before renewed transgression. The footwall of the oolitic iron ore-bearing bed consists of a fine-grained sandstone bed (10–20 cm-thick) pinching out laterally westward into green clays. The hanging wall is composed of thin-bedded limestone and clay alternations (2–3.5 m-thick).Iron occurs in the form of cryptocrystalline goethite with limited Al-Fe substitution. The goethite contains around 48% Fe, 5% Al and up to 1.5% P. Jarosite, alunite and manganese minerals (cryptomelane, psilomelane and manjiorite) are supergene secondary minerals, probably related to descending surface fluids. These manganese minerals occur as accessory minerals with the goethite and are most abundant at the lowermost part of the succession showing varied morphologies (local cement, space filling and free centimeter sized nodules). Fe-oolites in the deposit are similar to those documented in many other oolitic ironstone deposits. The dominant Fe-oolite type (>90%) has a concentrically laminated cortex with no nucleus. The nuclei of the oolites that do have a nucleus are most commonly detrital quartz grains.Major elements in high grade samples (Fe2O3 > 65%) vary within a limited range and show higher concentrations of SiO2 (average 7.85%) and Al2O3 (average 5.1%), with minor TiO2, MnO, MgO, Na2O, K2O, and SO3 (less than 1%). PAAS-normalized trace elements of bulk samples and Fe-oolite generally show similar behavior, both are enriched in V, Co, Ni, Mo, As, Zn, and Y and are depleted in Cu, Rb, Zr, Nb, Ba, and Hf. Anomalous V, Cr, Ni, Zn, and REE-Y are correlated with goethite. PAAS-normalized REE-Y patterns of both bulk samples and Fe-oolite show slight HREE enrichment, positive Ce with negative Y anomalies.The mineralogy (goethite and cryptomelane) along with the geochemistry (Si vs. Al; As + Cu + Mo + Pb + V + Zn vs. Ni + Co binary plots; Zn–Ni–Co triangular diagram, REE-Y content and patterns and Ce/Ce1 vs. Nd and Ce/Ce1 vs. YN/HoN binary plots) of the studied oolitic ironstone are congruent with a hydrogenetic type. While two possible sources of iron for Jebel Ank ironstone can be proposed: (i) submarine weathering of glauconite-rich sandstone and (ii) detrital iron from adjacent continental hinterland, the later is the more plausible source of iron, based on paleogeographic setting, the occurrence of fine sandstone underlying the iron level, occurrence of Mn-ores in the lower part of the Fe-ores succession, high phosphorous, zinc, ∑REE-Y concentrations and Y/Ho ratios, and low La/Ce ratios.  相似文献   

5.
Chemical weathering of Mg, Ca-silicates and alumino-silicates contributes significantly to the drawdown of atmospheric CO2 over long time scales. The present work focuses on how this mode of weathering may change in the presence of free-living bacteria in oligotrophic waters, which compose most of the surface freshwaters of the Earth. Forsterite (Fo90) was reacted for 1 week with a stable Escherichia coli population in water maintained at 37 °C and neutral pH in a batch reactor. Control samples with suspensions of pure olivine powders and E. coli cells in pure water were also used for reference. Olivine controls reproduce the Mg, Si and Fe release in solutions predicted from rates published in the literature with pH shifts of less than 0.5 unit. After 1 week, under abiotic conditions, weathered surfaces are enriched in Fe and Fe3+ relative to the initial composition of the mineral. Bacterial controls (without minerals) show decreasing Eh with increasing cell concentrations (−50 mV with 7 × 107 cells/mL and −160 mV with 8 × 108 cells/mL). Magnesium concentrations in bacterial control solutions are in the μg/L range and can be accounted for by the release of Mg from dead cells. More than 80% of the cells were still alive after 1 week. The solutions obtained in the experiments in which olivine reacts in the presence of cells show Mg and Si concentrations a few tens of percent lower than in the mineral control samples, with a prominent depletion of Fe(III) content of the mineral surfaces. Magnesium mass balance discounts both significant bacterial uptake and inhibition of the Mg dissolution rates as a consequence of changing pH and Eh. Coating by bacterial cell layers is also negligible. E. coli reduces the chemical weathering of olivine. This study infers that the presence of free-living Proteobacteria, a prevalent group of subsurface bacteria, should decrease the amount of riverine Mg released by chemical weathering of mafic rocks.  相似文献   

6.
An experimental study on the origin of ferric and ferrous carbonate-silicate melts, which can be considered as the potential metasomatic oxidizing agents and diamond forming media, was performed in the (Ca,Mg)CO3-SiO2-Al2O3-(Mg,Fe)(Cr,Fe,Ti)O3 system, at 6.3 GPa and 1350–1650 °C. At 1350–1450 °C and ?O2 of FMQ + 2 log units, carbonate–silicate melt, coexisting with Fe3 +-bearing ilmenite, pyrope-almandine and rutile, contained up to 13 wt.% of Fe2O3. An increase in the degree of partial melting was accompanied by decarbonation and melt enrichment with CO2, up to 21 wt.%. At 1550–1650 °C excess CO2 segregated as a separate fluid phase. The restricted solubility of CO2 in the melt indicated that investigated system did not achieve the second critical point at 6.3 GPa. At 1350–1450 °C and ?O2 close to CCO buffer, Fe2 +-bearing carbonate–silicate melt was formed in association with pyrope-almandine and Fe3 +-bearing rutile. It was experimentally shown that CO2-rich ferrous carbonate-silicate melt can be an effective waterless medium for the diamond crystallization. It provides relatively high diamond growth rates (3–5 μm/h) at P,T-conditions, corresponding to the formation of most natural diamonds.  相似文献   

7.
A peralkaline, ultrapotassic dyke found at ?ebkovice (T?ebí? district, western Moravia) is a mineralogically extreme member of a dyke swarm occurring along the south-eastern border of the Moldanubian Region of the Bohemian Massif. The dyke shows a simple zoning, with a very fine-grained marginal zone grading into a medium-grained central zone. It has a primary mineral assemblage of microcline and potassic amphiboles, with accessory apatite and altered phlogopite. The microcline exhibits an unusual red luminescence colour and pronounced substitution of Fe3+ for Al, with measured contents of Fe2O3 up to 8.5 wt.% (0.31 apfu Fe3+). Amphiboles have very high K (up to 0.99 apfu) and Si contents; their compositions follow an alkaline fractionation trend from potassic-richterite to potassic-magnesio-arfvedsonite, characterized by an increase of Na/K and a decrease of Ca, Mg, Fe2+ and Ti via heterovalent substitutions [B]Ca + [C](Mg,Fe2+) → [B]Na + [C]Fe3+ and Ti + Mg → 2Fe3+. The most evolved apatite is significantly enriched in SrO (up to 9.7 wt.%; 0.49 apfu Sr). The core of the dyke and late veinlets contain unique late- to post-magmatic Ba–Ti–Zr-bearing mineral assemblages of baotite, henrymeyerite, titanite, rutile, benitoite and bazirite. Anhedral baotite fills interstices distributed inhomogeneously in the dyke centre; it is locally replaced by a Ba-bearing titanite + henrymeyerite + rutile + quartz assemblage. Henrymeyerite (the second record in a lamproite) shows variable Fe/Ti ratios and represents a solid solution of the hepta- and hexatitanate components. Euhedral crystals of benitoite and bazirite are enclosed in the late-stage quartz–titanite–apatite veinlets in the fine-grained margin of the intrusion. In terms of a mineralogical–genetic classification, the ?ebkovice dyke can be considered as a new high-silica (~ 57 wt.% SiO2) variety of lamproite (variety ?ebkovice), and represents a unique expression of post-collisional potassic magmatism on the south-eastern border of the Bohemian Massif. The peralkaline dykes from this area show mineralogical and geochemical features similar to those of silica-rich orogenic lamproites emplaced at destructive plate margins. In terms of the modern classification of lamproites, the ?ebkovice dyke is the first lamproite recognised in the Variscan orogenic belt.  相似文献   

8.
Pure-iron end-member hibbingite, Fe2(OH)3Cl(s), may be important to geological repositories in salt formations, as it may be a dominant corrosion product of steel waste canisters in an anoxic environment in Na–Cl- and Na–Mg–Cl-dominated brines. In this study, the solubility of Fe2(OH)3Cl(s), the pure-iron end-member of hibbingite (FeII, Mg)2(OH)3Cl(s), and Fe(OH)2(s) in 0.04 m to 6 m NaCl brines has been determined. For the reactionFe2(OH)3Cl(s) + 3H+ ? 3 H2O + 2 Fe2+ + Cl?,the solubility constant of Fe2(OH)3Cl(s) at infinite dilution and 25 °C has been found to be log10 K = 17.12 ± 0.15 (95% confidence interval using F statistics for 36 data points and 3 parameters). For the reactionFe(OH)2(s) + 2H+ ? 2 H2O + Fe2+,the solubility constant of Fe(OH)2 at infinite dilution and 25 °C has been found to be log10 K = 12.95 ± 0.13 (95 % confidence interval using F statistics for 36 data points and 3 parameters). For the combined set of solubility data for Fe2(OH)3Cl(s) and Fe(OH)2(s), the Na+–Fe2+ pair Pitzer interaction parameter θNa+/Fe2+ has been found to be 0.08 ± 0.03 (95% confidence interval using F statistics for 36 data points and 3 parameters). In nearly saturated NaCl brine we observed evidence for the conversion of Fe(OH)2(s) to Fe2(OH)3Cl(s). Additionally, when Fe2(OH)3Cl(s) was added to sodium sulfate brines, the formation of green rust(II) sulfate was observed, along with the generation of hydrogen gas. The results presented here provide insight into understanding and modeling the geochemistry and performance assessment of nuclear waste repositories in salt formations.  相似文献   

9.
The magnetic fractions of ilmenite from the beach placer deposit of Chavara, southwest India have been studied for mineralogical and chemical composition to assess the range of their physical and chemical variations with weathering. Chavara deposit represents a highly weathered and relatively homogenous concentration. Significant variation in composition has been documented with alteration. The most magnetic of the fractions of ilmenite, separated at 0.15 Å, and with a susceptibility of 3.2 × 10?6 m3 kg?1, indicates the presence of haematite–ilmenite intergrowth. An iron-poor, titanium-rich component of the ilmenite ore has been identified from among the magnetic fractions of the Chavara ilmenite albeit with an undesirably high Nb2O5 (0.28%), Cr2O3 (0.23%) and Th (149 ppm) contents. The ilmenite from Chavara is compared with that from the nearby Manavalakurichi deposit of similar geological setting and provenance. The lower ferrous iron oxide (2.32–14.22%) and higher TiO2 (56.31–66.45%) contents highlight the advanced state of alteration of Chavara. This is also evidenced by the relatively higher Fe3+/Fe2+ ratio compared to Manavalakurichi ilmenite. In fact, the ilmenite has significantly been converted to pseudorutile/leucoxene.  相似文献   

10.
This study focused on the ferric sulfate precipitates formed during the culture of Acidithiobacillus ferrooxidans (A. ferrooxidans) in a modified 9K medium by applying a potential control on the electrode. X-ray diffraction (XRD), environmental scanning electron microscope (ESEM), Raman spectroscopy (Raman) and Fourier Transform Infrared spectroscopy (FTIR) were carried out to characterize and identify the precipitates which were formed, respectively, in the electrochemical cultivation with a fixed cathode potential (bias-experiment) and in the conventional batch cultivation without cathode potential control (no-bias-experiment). The results indicated that K-jarosite presented in both experiments while NH4-jarosite and schwertmannite were only found in the no-bias-experiment. The formation of different precipitates could be attributed to the different growth statuses and rates of A. ferrooxidans and the different concentrations of Fe3+. In the bias-experiment, external electrons reproduced Fe2+ and promoted the growth of A. ferrooxidans, thus resulting in the low Fe3+ concentration and the rapid depletion of NH4 + as the nitrogen source, in which K-jarosite was preferentially formed. In the no-bias-experiment, the lower concentration of A. ferrooxidans was observed, which was due to the continuous consumption of Fe2+ by bacteria, thus resulting in the relatively higher Fe3+ and the NH4 + concentration in culture. The high concentration of Fe3+ favored the precipitation of the solid solution of K-NH4-H3O jarosite, and led to the formation of schwertmannite after K+ and NH4 + were depleted.  相似文献   

11.
The trace element compositions of melts and minerals from high-pressure experiments on hydrous pyroxenites containing K-richterite are presented. The experiments used mixtures of a third each of the natural minerals clinopyroxene, phlogopite and K-richterite, some with the addition of 5% of an accessory phase ilmenite, rutile or apatite. Although the major element compositions of melts resemble natural lamproites, the trace element contents of most trace elements from the three-mineral mixture are much lower than in lamproites. Apatite is required in the source to provide high abundances of the rare earth elements, and either rutile and/or ilmenite is required to provide the high field strength elements Ti, Nb, Ta, Zr and Hf. Phlogopite controls the high levels of Rb, Cs and Ba.Since abundances of trace elements in the various starting mixtures vary strongly because of the use of natural minerals, we calculated mineral/melt partition coefficients (DMin/melt) using mineral modes and melting reactions and present trace element patterns for different degrees of partial melting of hydrous pyroxenites. Rb, Cs and Ba are compatible in phlogopite and the partition coefficient ratio phlogopite/K-richterite is high for Ba (1 3 6) and Rb (12). All melts have low contents of most of the first row transition elements, particularly Ni and Cu ((0.1–0.01) × primitive mantle). Nickel has high DMin/melt for all the major minerals (12 for K-richterite, 9.2 for phlogopite and 5.6 for Cpx) and so behaves at least as compatibly as in melting of peridotites. Fluorine/chlorine ratios in melts are high and DMin/melt for fluorine decreases in the order apatite (2.2) > phlogopite (1.5) > K-richterite (0.87). The requirement for apatite and at least one Ti-oxide in the source of natural lamproites holds for mica pyroxenites that lack K-richterite. The results are used to model isotopic ageing in hydrous pyroxenite source rocks: phlogopite controls Sr isotopes, so that lamproites with relatively low 87Sr/86Sr must come from phlogopite-poor source rocks, probably dominated by Cpx and K-richterite. At high pressures (>4 GPa), peritectic Cpx holds back Na, explaining the high K2O/Na2O of lamproites.  相似文献   

12.
Physical, physicochemical, and mineralogical-petrographic methods have been applied to samples of ophiolite-hosted chromite ore from different deposits and occurrences in the Urals. Temperature dependences of dielectric loss obtained for nine chromite ore samples consisting of 95–98% Cr spinel show prominent peaks indicating a relaxation origin of the loss. The analyzed samples have the loss peaks at different temperatures depending mainly on H = (FeO/Fe2O3)? : (FeO/Fe2O3)??, where (FeO/Fe2O3)? and (FeO/Fe2O3)?? are, respectively, the ferrous/ferric oxide ratios in the samples before and after heating to 800 °C, and H is thus the heating-induced relative change in the FeO/Fe2O3 ratio. These peak temperatures vary from 550 °C (sample 1, high-Cr chromium spinel with more than 52% Cr2O3) to 750 °C (sample 2, aluminous and magnesian spinel with less than 30% Cr2O3), and H ranges correspondingly from 1.61 to 5.49. The temperature of the loss peaks is related with H as H = 34.30 ? 11.52N + 1.20N2, with an error of σ = 0.19 (N = T · 10?2, T is temperature in °C).  相似文献   

13.
《Applied Geochemistry》2006,21(4):580-613
To quantify and explain the contributions by pollution and chemical weathering to their composition, we studied the chemistries of springs and surface waters in the mountainous part of the Vouga River basin. Water samples were collected during a number of consecutive summer campaigns. Recharge rates were derived from monitored discharge rates within the basin. Very large contributions by meteoric, agricultural and domestic sources to the water chemistries were found, identified by the chloride, sulfate and nitrate concentrations: on average only 1/4 to 1/3 of the solutes could be attributed to chemical weathering. Two petrologic units characterize the river basin: granites and metasediments. The waters collected within metasediment units are distinct from those in granite terrain by a higher magnesium concentration. On that basis, it could be estimated that the Rio Vouga, when leaving the mountainous part of the basin, has for some 2/5 a signature determined by chemical weathering in the metasediments. The dominant primary minerals subject to chemical weathering are plagioclase (Pl) and biotite (in granite) or Pl and chlorite (in metasediment). Kaolinite, gibbsite and vermiculite are the major weathering products where annual precipitation (P) > 1000 mm y−1, and kaolinite, vermiculite and smectite where P was lower. Using an algorithm based on the ratio of dissolved silica to bicarbonate, the contributions of chemical weathering of primary minerals could be unraveled. The results show that in granite the export rate (as mol ha−1 y−1 wt%mineral−1) of oligoclase (Pl with An10–30) was 5.0 ± 2.6 and of biotite 3.2 ± 2.6, while in metasediment these rates for albite (Pl with An0–10) are 16.5 ± 8.9 and for chlorite are 0.5 ± 0.5. The observed decrease of dissolved silica in surface waters relative to springs was ascribed to (summer) uptake by aquatic biota.  相似文献   

14.
《Chemical Geology》2007,236(3-4):199-216
The chemical characteristics of freshwaters draining the silicate rocks in the northern part of Okinawa Island were studied to understand solute generation processes, and to determine rates of chemical weathering and CO2 consumption. It was observed that the water chemistry is highly influenced by marine aerosols, contributing more than 60% of total solute. Significant positive correlations observed for chloride versus dissolved silica and chloride versus bicarbonate suggest a strong influence of evapotranspiration on the seasonality of solute concentration. It was also found that chemical weathering has been highly advanced in which the dominant kaolinite minerals are being gibbsitized. Carbonic acid was found to be the major chemical weathering agent, releasing greater than 80% of weathering-derived dissolved cations and silica while the remaining portion was attributed to weathering by sulfuric acid generated via oxidation of pyrite contained in the rocks. The flux of basic cations, weathering-derived silica and CO2 consumption were relatively high due to favourable climatic condition, topography and high rate of mechanical erosion. Silicate weathering rates for basic cations were estimated to be 6.7–9.7 ton km− 2 y− 1. Carbon dioxide consumed by silicate weathering was 334–471 kmol km− 2 y− 1 which was slightly higher than that consumed by carbonate weathering. In general, divalent cations (Mg and Ca) and bicarbonate alkalinity derived from carbonate dissolution were higher than those from silicate weathering. As a consequence, the evolution of chemical species in the freshwaters of northern area of Okinawa Island to a large extent could be explained by mixing of two components, characterized by waters with Na+ and Cl− as predominant species and waters enriched with Ca2+ and HCO3−.  相似文献   

15.
Various iron-bearing primary phases and rocks have been weathered experimentally to simulate possible present and past weathering processes occurring on Mars. We used magnetite, monoclinic and hexagonal pyrrhotites, and metallic iron as it is suggested that meteoritic input to the martian surface may account for an important source of reduced iron. The phases were weathered in two different atmospheres: one composed of CO2 + H2O, to model the present and primary martian atmosphere, and a CO2 + H2O + H2O2 atmosphere to simulate the effect of strong oxidizing agents. Experiments were conducted at room temperature and a pressure of 0.75 atm. Magnetite is the only stable phase in the experiments and is thus likely to be released on the surface of Mars from primary rocks during weathering processes. Siderite, elemental sulfur, ferrous sulfates and ferric (oxy)hydroxides (goethite and lepidocrocite) are the main products in a water-bearing atmosphere, depending on the substrate. In the peroxide atmosphere, weathering products are dominated by ferric sulfates and goethite. A kinetic model was then developed for iron weathering in a water atmosphere, using the shrinking core model (SCM). This model includes competition between chemical reaction and diffusion of reactants through porous layers of secondary products. The results indicate that for short time scales, the mechanism is dominated by a chemical reaction with second order kinetics (k = 7.75 × 10−5 g−1/h), whereas for longer time scales, the mechanism is diffusion-controlled (DeA = 2.71 × 10−10 m2/h). The results indicate that a primary CO2- and H2O-rich atmosphere should favour sulfur, ferrous phases such as siderite or Fe2+-sulfates, associated with ferric (oxy)hydroxides (goethite and lepidocrocite). Further evolution to more oxidizing conditions may have forced these precursors to evolve into ferric sulfates and goethite/hematite.  相似文献   

16.
Laboratory experiments were conducted to simulate chalcopyrite oxidation under anaerobic and aerobic conditions in the absence or presence of the bacterium Acidithiobacillus ferrooxidans. Experiments were carried out with 3 different oxygen isotope values of water (δ18OH2O) so that approach to equilibrium or steady-state isotope fractionation for different starting conditions could be evaluated. The contribution of dissolved O2 and water-derived oxygen to dissolved sulfate formed by chalcopyrite oxidation was unambiguously resolved during the aerobic experiments. Aerobic oxidation of chalcopyrite showed 93 ± 1% incorporation of water oxygen into the resulting sulfate during the biological experiments. Anaerobic experiments showed similar percentages of water oxygen incorporation into sulfate, but were more variable. The experiments also allowed determination of sulfate–water oxygen isotope fractionation, ε18OSO4–H2O, of ~ 3.8‰ for the anaerobic experiments. Aerobic oxidation produced apparent εSO4–H2O values (6.4‰) higher than the anaerobic experiments, possibly due to additional incorporation of dissolved O2 into sulfate. δ34SSO4 values are ~ 4‰ lower than the parent sulfide mineral during anaerobic oxidation of chalcopyrite, with no significant difference between abiotic and biological processes. For the aerobic experiments, a small depletion in δ34SSO4 of ~? 1.5 ± 0.2‰ was observed for the biological experiments. Fewer solids precipitated during oxidation under aerobic conditions than under anaerobic conditions, which may account for the observed differences in sulfur isotope fractionation under these contrasting conditions.  相似文献   

17.
Tooeleite, nominally Fe63+(As3+O3)4(SO4)(OH)4·4H2O, is a relatively uncommon mineral of some acid-mine drainage systems. Yet, if it does occur, it does so in large quantities, indicating that some specific conditions favor the formation of this mineral in the system Fe-As-S-O-H. In this contribution, we report the thermodynamic properties of synthetic tooeleite. The sample was characterized by powder X-ray diffraction, scanning electron microscopy, extended X-ray absorption fine-structure spectroscopy, and Mössbauer spectroscopy. These methods confirmed that the sample is pure, devoid of amorphous impurities of iron oxides, and that the oxidation state of arsenic is 3+. Using acid-solution calorimetry, the enthalpy of formation of this mineral from the elements at the standard conditions was determined as −6196.6 ± 8.6 kJ mol−1. The entropy of tooeleite, calculated from low-temperature heat capacity data measured by relaxation calorimetry, is 899.0 ± 10.8 J mol−1 K−1. The calculated standard Gibbs free energy of formation is −5396.3 ± 9.3 kJ mol−1. The log Ksp value, calculated for the reaction Fe6(AsO3)4(SO4)(OH)4·4H2O + 16H+ = 6Fe3+ + 4H3AsO3 + SO42− + 8H2O, is −17.25 ± 1.80. Tooeleite has stability field only at very high activities of aqueous sulfate and arsenate. As such, it does not appear to be a good candidate for arsenic immobilization at polluted sites. An inspection of speciation diagrams shows that the predominance field of Fe3+ and As3+ overlap only at strongly basic conditions. The formation of tooeleite, therefore, requires strictly selective oxidation of Fe2+ to Fe3+ and, at the same time, firm conservation of the trivalent oxidation state of arsenic. Such conditions can be realized only by biological systems (microorganisms) which can selectively oxidize one redox-active element but leave the other ones untouched. Hence, tooeleite is the first example of an “obligatory” biomineral under the conditions prevailing at or near the Earth's surface because its formation under these conditions necessitates the action of microorganisms.  相似文献   

18.
The Sangan iron skarn deposit is located on the eastern edge of the Sabzevar-Doruneh Magmatic Belt, northeastern Iran. Mineralization occurs at the contact between Eocene igneous rocks and Cretaceous carbonates. The silicate-dominant prograde skarn stage consists of garnet and clinopyroxene, whereas the retrograde stage is dominated by magnetite associated with minor hematite, phlogopite, pyrite, and chalcopyrite. Phase equilibria and mineral chemistry studies reveal that the skarn formed within a temperature range of ∼375° to 580 °C and that the mineralizing fluid evolved from a hot, low oxygen fugacity, alkaline fluid during the silicate-dominant stage to a fluid of relatively lower temperature and higher oxygen fugacity at the magnetite-dominant stage. The δ18O values of magnetite and garnet vary from +3.1 to +7.5‰ and +7.7 to +11.6‰, respectively. The calculated δ18OH2O values of fluid in equilibrium with magnetite and garnet range from +9.8 to +11.1‰ and +10.1 to +14.8‰, respectively. These elevated δ18OH2O values suggest interaction of magmatic water with 18O-enriched carbonates. The high δ34S values (+10.6 to +17.0‰) of pyrite separates from the Sangan iron ore indicate that evaporites had an important role in the evolution of the hydrothermal fluid. Phlogopite separates from the massive ores yield 40Ar/39Ar plateau ages of 41.97 ± 0.2 and 42.47 ± 0.2 Ma, indicating that the skarn formation and associated iron mineralization was related to the oldest episode of magmatism in Sangan at ∼42 Ma. Eocene time marked a peak of magmatic activity and associated skarn in the post-collisional setting in northeastern Iran, whereas Oligo-Miocene magmatic activity and associated skarn in the Urumieh-Dokhtar Magmatic Belt are related to subduction. In addition, skarn mineralization in northeastern and eastern Iran is iron type, but skarn mineralization in the Urumieh-Dokhtar magmatic belt is copper – iron and copper type.  相似文献   

19.
The relationship among subducted oxidized oceanic crust and oxidation state of the subarc mantle, and arc magmas is one of the important aspects to evaluate convergent margin tectonics. However details of the oxidized mass transferred from buried oceanic crust to the overlying subarc mantle wedge remain obscure. Here we investigate the Songduo eclogites from south Tibet formed by the subduction of the paleo-Tethyan oceanic crust, and identify an abrupt decrease in pyrope and increase in almandine contents from the mantle to rim of garnet grains. This is coupled with a decrease in the Fe3 + content of epidote and Fe3 +/(Fe2 ++ Fe3 +) ratios from garnet core to rim domains, as well as speciation of calcite, a new mineral phase, in the rock matrix. Minor sulfates occur only as inclusions in garnet core domains, whereas sulfides are confined to the matrix as an accessory mineral phase. Aegirine augite occurs as relics or inclusions in garnet and omphacite. These features clearly suggest that oxidized components, Fe3 + and S6 +, were reduced as Fe2 + and S2 −, respectively, at the subduction zone. Thermodynamic modeling in the P–T-log10fO2 space using updated Perplex_X programs further revealed that the Songduo eclogites experienced oxygen fugacity variation of up to 8 log10 units, with decreasing pressure. Petrological observations further suggest that the strong redox processes took place, after breaking of garnet, during the initial exhumation of the eclogites. CO2 and minor sulfur are subsequently transferred from the cold oceanic subduction zone to the overlying mantle wedge, partially released by arc volcanoes to atmosphere. Our study presents a case of C and S recycling between the Earth's exterior and interior.  相似文献   

20.
Toxic metalliferous mine-tailings pose a significant health risk to ecosystems and neighboring communities from wind and water dispersion of particulates containing high concentrations of toxic metal(loid)s (e.g., Pb, As, Zn). Tailings are particularly vulnerable to erosion before vegetative cover can be reestablished, i.e., decades or longer in semi-arid environments without intervention. Metal(loid) speciation, linked directly to bioaccessibility and lability, is controlled by mineral weathering and is a key consideration when assessing human and environmental health risks associated with mine sites. At the semi-arid Iron King Mine and Humboldt Smelter Superfund site in central Arizona, the mineral assemblage of the top 2 m of tailings has been previously characterized. A distinct redox gradient was observed in the top 0.5 m of the tailings and the mineral assemblage indicates progressive transformation of ferrous iron sulfides to ferrihydrite and gypsum, which, in turn weather to form schwertmannite and then jarosite accompanied by a progressive decrease in pH (7.3–2.3).Within the geochemical context of this reaction front, we examined enriched toxic metal(loid)s As, Pb, and Zn with surficial concentrations 41.1, 10.7, 39.3 mmol kg−1 (3080, 2200, and 2570 mg kg−1), respectively. The highest bulk concentrations of As and Zn occur at the redox boundary representing a 1.7 and 4.2-fold enrichment relative to surficial concentrations, respectively, indicating the translocation of toxic elements from the gossan zone to either the underlying redox boundary or the surface crust. Metal speciation was also examined as a function of depth using X-ray absorption spectroscopy (XAS). The deepest sample (180 cm) contains sulfides (e.g., pyrite, arsenopyrite, galena, and sphalerite). Samples from the redox transition zone (25–54 cm) contain a mixture of sulfides, carbonates (siderite, ankerite, cerrusite, and smithsonite) and metal(loid)s sorbed to neoformed secondary Fe phases, principally ferrihydrite. In surface samples (0–35 cm), metal(loid)s are found as sorbed species or incorporated into secondary Fe hydroxysulfate phases, such as schwertmannite and jarosites. Metal-bearing efflorescent salts (e.g., ZnSO4·nH2O) were detected in the surficial sample. Taken together, these data suggest the bioaccessibility and lability of metal(loid)s are altered by mineral weathering, which results in both the downward migration of metal(loid)s to the redox boundary, as well as the precipitation of metal salts at the surface.  相似文献   

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