An experimental study of the mechanism of the replacement of magnetite by pyrite up to 300 °C

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 pH_21°C <4 and was the dominant Fe disulfide at pH_21°C 1.11, while pyrite predominated at pH_21°C >2 and formed even under basic conditions (up to pH_21°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; pH_120°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 pH_21°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., pH_155°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.     

A sequential extraction to determine the distribution of apatite in granitoid soil mineral pools with application to weathering at the Hubbard Brook Experimental Forest,NH, USA

Mineral weathering in soils is an important source of many nutrients to forest ecosystems. Apatite, a Ca phosphate mineral, occurs in trace amounts in virtually all igneous and metamorphic rocks and is often found as small mineral inclusions in more weathering-resistant silicate minerals. To better understand the distribution of apatite in soils and its exposure to soil solutions, a sequential extraction method was developed to selectively dissolve minerals from soils so that the amount of apatite in contact with soil solutions versus that armored by silicate minerals could be quantified. The use of three molarities of HNO₃ (0.01, 0.1 and 1 M) at three temperatures (0, 10, or 20 °C) was explored and it was found that apatite congruently dissolved in 1 M HNO₃ at all three temperatures, but did not completely dissolve in weaker HNO₃ solutions. Soil horizons, glacial till (i.e., soil parent material), and individual minerals separated from till collected from the Hubbard Brook Experimental Forest (HBEF), NH, were subjected to a 4-step sequential extraction. Chemical analyses of the extracts indicate that 1 M NH₄Cl (pH 7; 20 °C) removes exchangeable ions, 1 M HNO₃ at 20 °C primarily dissolves apatite in contact with solutions, 1 M HNO₃ at 200 °C primarily dissolves biotite and chlorite (and apatite armored by them), and a mixture of concentrated HNO₃, HCl, and HF at 200 °C dissolves the more refractory minerals including muscovite, alkali feldspar, plagioclase feldspar and quartz (and apatite armored by these minerals). This extraction method was applied to soil profiles from HBEF to demonstrate that it could be used to interpret the abundance of apatite and other minerals as a function of depth. Approximately 70% of the apatite in the HBEF soil parent material is exposed to soil solutions; the remaining 30% is armored in more weathering-resistant micas and feldspars. In the upper soil horizons, the only apatite that has not been weathered from the soil occurs as inclusions in micas and feldspars and thus the rate of dissolution of apatite in weathered soil horizons is controlled by silicate mineral dissolution.     

Detecting the near-surface redox front in crystalline bedrock using fracture mineral distribution,geochemistry and U-series disequilibrium

Oxidizing conditions normally prevail in surface waters and near-surface groundwaters, but there is usually a change to reducing conditions in groundwater at greater depth. Dissolved O₂ originally present is consumed through biogenic and inorganic reactions along the flow paths. Fracture minerals participate in these reactions and the fracture mineralogy and geochemistry can be used to trace the redox front. An important task in the safety assessment of a potential repository for the disposal of nuclear waste in crystalline bedrock, at an approximate depth of 500 m in Sweden, is to demonstrate that reducing conditions can be maintained for a long period of time. Oxygen may damage the Cu canisters that host nuclear waste; additionally, in the event of a canister failure, oxidizing conditions may increase the mobility of some radionuclides. The present study of the near-surface redox front is based on mineralogical (redox-sensitive minerals), geochemical (redox-sensitive elements) and U-series disequilibrium investigations of mineral coatings along open fractures. The fractures have been sampled along drill cores from closely spaced, 100 m deep boreholes, which were drilled during the site investigation work in the Laxemar area, south-eastern Sweden, carried out by the Swedish Nuclear Fuel and Waste Management Co. (SKB). The distribution of the redox-sensitive minerals pyrite and goethite in open fractures shows that the redox front (switch from mainly goethite to mainly pyrite in the fractures) generally occurs at about 15–20 m depth. Calcite leaching by recharging water is indicated in the upper 20–30 m and positive Ce-anomalies suggest oxidation of Ce down to 20 m depth. The U-series radionuclides show disequilibrium in most of the samples, indicating mobility of U during the last 1 Ma. In the upper 20 m, U is mainly removed (due to oxidation) or has experienced complex removal and/or deposition. At depths of 35–55 m, both deposition and removal of U are indicated. Below 55 m, recent deposition of U is generally indicated which suggests removal of U near surface (oxidation) and deposition of U below the redox front. Scattered goethite occurrences below the general redox front (down to ca 80 m) and signs of U removal at 35–55 m mostly correlate with sections of high transmissivity (and/or high fracture frequencies). This shows that highly transmissive fractures are generally required to allow oxygenated groundwaters at depth greater than ca 30 m. Removal of U (oxidation) below 55 m within the last 300 ka is not observed. Although penetration of glacial waters to great depths has been confirmed in the study area, their potential O₂ load seems to have been reduced near the surface.     

Redox potential (Eh) and anion effects of pyrite (FeS2) leaching at pH 1

Pyrite plays the central role in the environmental issue of acid rock drainage. Natural weathering of pyrite results in the release of sulphuric acid which can lead to further leaching of heavy and toxic metals from other associated minerals. Understanding how pyrite reacts in aqueous solution is critical to understanding the natural weathering processes undergone by this mineral. To this end an investigation of the effect of solution redox potential (Eh) and various anions on the rate of pyrite leaching under carefully controlled conditions has been undertaken.Leaching of pyrite has been shown to proceed significantly faster at solution Eh of 900 mV (SHE) than at 700 mV, at pH 1, for the leach media of HCl, H₂SO₄ and HClO₄. The predominant effect of Eh suggests electrochemical control of pyrite leaching with similar mechanism(s) at Eh of 700 and 900 mV albeit with different kinetics. Leach rates at 700 mV were found to decrease according to HClO₄ > HCl > H₂SO₄ while at 900 mV the leach rate order was HCl > HClO₄ > H₂SO₄. Solution Fe³⁺ activity is found to continually increase during all leaches; however, this is not accompanied by an increase in leach rate.Synchrotron based photoemission electron microscopy (PEEM) measurements showed a localised distribution of adsorbed and oxidised surface species highlighting that pyrite oxidation and leaching is a highly site specific process mediated by adsorption of oxidants onto specific surface sites. It appears that rates may be controlled, in part, by the propensity of acidic anions to bind to the surface, which varies according to , thus reducing the reactive or effective surface area. However, anions may also be involved in specific reactions with surface leach products. Stoichiometric dissolution data (Fe/S ratio), XPS and XRD data indicate that the highest leach rates (in HCl media at 900 mV Eh) correlate with relatively lower surface S abundance. Furthermore, there are indications that solution Cl⁻ assists oxidation especially at higher Eh through the prevention of surface S⁰ buildup at reactive surface sites.     

Modeling ferrous-ferric iron chemistry with application to martian surface geochemistry

The Mars Global Surveyor, Mars Exploration Rover, and Mars Express missions have stimulated considerable thinking about the surficial geochemical evolution of Mars. Among the major recent mission findings are the presence of jarosite (a ferric sulfate salt), which requires formation from an acid-sulfate brine, and the occurrence of hematite and goethite on Mars. Recent ferric iron models have largely focused on 25 °C, which is a major limitation for models exploring the geochemical history of cold bodies such as Mars. Until recently, our work on low-temperature iron-bearing brines involved ferrous but not ferric iron, also obviously a limitation. The objectives of this work were to (1) add ferric iron chemistry to an existing ferrous iron model (FREZCHEM), (2) extend this ferrous/ferric iron geochemical model to lower temperatures (<0 °C), and (3) use the reformulated model to explore ferrous/ferric iron chemistries on Mars.The FREZCHEM model is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions using the Pitzer approach for the temperature range from <−70 to 25 °C and the pressure range from 1 to 1000 bars. Ferric chloride and sulfate mineral parameterizations were based, in part, on experimental data. Ferric oxide/hydroxide mineral parameterizations were based exclusively on Gibbs free energy and enthalpy data. New iron parameterizations added 23 new ferrous/ferric minerals to the model for this Na-K-Mg-Ca-Fe(II)-Fe(III)-H-Cl-SO₄-NO₃-OH-HCO₃-CO₃-CO₂-O₂-CH₄-H₂O system.The model was used to develop paragenetic sequences for Rio Tinto waters on Earth and a hypothetical Martian brine derived from acid weathering of basaltic minerals. In general, model simulations were in agreement with field evidence on Earth and Mars in predicting precipitation of stable iron minerals such as jarosites, goethite, and hematite. In addition, paragenetic simulations for Mars suggest that other iron minerals such as lepidocrocite, schwertmannite, ferricopiapite, copiapite, and bilinite may also be present on the surface of Mars. Evaporation or freezing of the Martian brine led to similar mineral precipitates. However, in freezing, compared to evaporation, the following key differences were found: (1) magnesium sulfates had higher hydration states; (2) there was greater total aqueous sulfate (SO_4T = SO₄ + HSO₄) removal; and (3) there was a significantly higher aqueous Cl/SO_4T ratio in the residual Na-Mg-Cl brine. Given the similarities of model results to observations, alternating dry/wet and freeze/thaw cycles and brine migration could have played major roles in vug formation, Cl stratification, and hematite concretion formation on Mars.     

Nickeloan pyrite from Prangenhaus quarry, Niederberg area, Germany—A short communication

The mineral chemistry of two nickeloan pyrite samples from the Prangenhaus quarry, Wülfrath, is presented. Sample PRH 5: main zone=4.32-9.7 wt% Ni (0.089-0.202 atoms per formula unit (a.p.f.u.)), 0.21-1.31 wt% Co (0.004-0.027 a.p.f.u.); rim zone=11.76-22.38 wt% Ni (0.244-0.466 a.p.f.u. ), 1.35-8.16 wt% Co (0.028-0.168 a.p.f.u.). Sample PRH 6: 6.75-11.33 wt% Ni (0.139-0.234 a.p.f.u.), 0.86-2.46 wt% Co (0.018-0.051 a.p.f.u.). Estimated Fe/Ni and Fe/Co ratios of the hydrothermal fluids forming the nickeloan pyrite point to very local fluctuations in the chemistry of the ore forming fluids in the Prangenhaus quarry. The presence of nickeloan pyrite indicates formation temperatures below 137 °C, which is in accordance with earlier findings of formation temperatures for sphalerite (60-130 °C) in the Niederberg area.     

Experimental investigation of main controls to methane adsorption in clay-rich rocks

In this study a series of CH₄ adsorption experiments on clay-rich rocks were conducted at 35 °C, 50 °C and 65 °C and at CH₄ pressure up to 15 MPa under dry conditions. The clay-dominated rock samples used are fresh samples from quarries and mines. Samples are individually dominated by montmorillonite, kaolinite, illite, chlorite, and interstratified illite/smectite. The experimental results show that clay mineral type greatly affects CH₄ sorption capacity under the experimental conditions. In terms of relative CH₄ sorption capacity: montmorillonite ? illite/smectite mixed layer > kaolinite > chlorite > illite. Physisorption is the dominant process for CH₄ absorption on clay minerals, as a result, there is a linear correlation between CH₄ sorption capacity and BET surface area in these clay-mineral dominated rocks. The abundance of micro-mesopores in the size range of a few to a few 10 s of nanometers in montmorillonite clay and illite–smectite interstratified clay results in large BET surface area values for these mineral species.     

Fluids in deeply subducted continental crust: Petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu orogen, eastern China

The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ± omphacite (or jadeite) ± kyanite ± allanite ± zoisite ± rutile ± garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high-field-strength elements (HFSE) and transition metal elements (TME). A kyanite-quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H₂O with solids up to 30-70% of the inclusion volume. The presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing, and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.     

Effect of oxidation potential and zinc sulphate on the separation of chalcopyrite from pyrite

The effects of oxidation potential (Eh) and zinc sulphate on the separation of chalcopyrite from pyrite were investigated at pH 9.0. The flotation recovery of these minerals is Eh dependent with maximum separation obtained at 275 mV SHE. Zinc sulphate addition improved this mineral separation at an Eh value of 275 mV by selectively depressing pyrite flotation. A different result was obtained at lower Eh values where zinc sulphate addition improved chalcopyrite flotation but had no or little effect on pyrite flotation. These opposite effects of zinc sulphate on mineral flotation were reconciled by examining the surface species of these minerals. The selective depression of pyrite flotation by zinc sulphate was also confirmed in the flotation of two copper ores.     

Arsenic release from chlorine-promoted alteration of a sulfide cement horizon: Evidence from batch studies on the St. Peter Sandstone,Wisconsin, USA

Elevated As concentrations have been measured in wells in the St. Peter Sandstone aquifer of eastern Wisconsin, USA. The primary source is As-bearing sulfide minerals (pyrite and marcasite) within the aquifer. There is concern that well disinfection by chlorination may facilitate As release to groundwater by increasing the rate and extent of sulfide oxidation. The objective of this study was to examine the abiotic processes that mobilize As from the aquifer solids during controlled exposure to chlorinated solutions. Thin sections made from sulfidic aquifer material were characterized by quantitative electron probe micro-analysis before and after 24 h exposure to solutions of different Cl₂ concentrations. Batch experiments using crushed aquifer solids were also conducted to examine changes in solution chemistry over 24 h. Results of the combined experiments indicate that Cl₂ addition affects As release and uptake in two ways. First, Cl₂ increases oxidation of sulfide minerals, releasing more As from the mineral structure. Chlorine addition also increases the rate of Fe(II) oxidation and subsequent hydrous ferric oxide (HFO) precipitation, allowing for increased uptake of As onto the mineral surface. Although HFOs can act as sinks for As, they can release As if biogeochemical conditions (e.g. redox, pH) change. These results have implications not only for disinfection of drinking water wells in the study area, but also suggest that introduction of oxidants may adversely affect water quality during aquifer storage and recovery programs in aquifers containing As-bearing minerals.     

Gold deposition on pyrite and the common sulfide minerals: An STM/STS and SR-XPS study of surface reactions and Au nanoparticles

Gold species spontaneously deposited on pyrite and chalcopyrite, pyrrhotite, galena, sphalerite from HAuCl₄ solutions at room temperature, as well as the state of the reacted mineral surfaces have been characterized using synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS), scanning tunneling microscopy and tunneling spectroscopy (STM/STS). The deposition of silver from 10⁻⁴ M AgNO₃ has been examined for comparison. Gold precipitates as metallic nanoparticles (NPs) from about 3 nm to 30 nm in diameter, which tends to aggregate forming larger particles, especially on pyrite. The Au 4f binding energies increase up to 1 eV with decreasing size of individual Au⁰ NPs, probably due to the temporal charging in the final state. Concurrently, a positive correlation between the tunneling current and the particle size was found in STS. Both these size effects were observed for unusually large, up to 20 nm Au particles. In contrast, silver deposited on the minerals as nanoparticles of semiconducting sulfide showed no shifts of photoelectron lines and different tunneling spectra.The quantity of gold deposited on pyrite and other minerals increased with time; it was lower for fracture surfaces and it grew if minerals were moderately pre-oxidized, while the preliminary leaching in Fe(III)-bearing media inhibited the following Au deposition. After the contact of polished minerals with 10⁻⁴ M solution (pH 1.5) for 10 min, the gold uptake changed in the order CuFeS₂ > ZnS > PbS > FeAsS > FeS₂ > Fe₇S₈. It was noticed that the open circuit (mixed) potentials of the minerals varied in approximately the same order, excepting chalcopyrite. We concluded that the potentials of minerals were largely determined by Fe(II)/Fe(III) couple, whereas the reduction of gold complexes had a minor effect. As a result, the deposition of gold, although it proceeded via the electrochemical mechanism, increased with decreasing potential. This suggests, in particular, that the accumulation of “invisible” gold in arsenian pyrites and arsenopyrite under hydrothermal conditions may be explained by the low electrochemical potentials but not structural relationships between As and Au in solids.     

Surface chemistry of kaolinite and Na-montmorillonite in aqueous electrolyte solutions at 25 and 60 °C: Experimental and modeling study

The aqueous interfacial chemistry of kaolinite and Na-montmorillonite samples was investigated by potentiometric measurements using acid/base continuous titrations and batch experiments at 25 and 60 °C. Using the batch experimental method, a continuous drift of pH was observed reflecting the mineral dissolution. Consequently, the continuous titration method appears to be the best way of studying solid surface reactions. For each clay mineral, the net proton surface excess/consumption was calculated as a function of pH and ionic strength (0.025, 0.1 and 0.5 M). At 25 °C, and according to the literature data, the pH corresponding to zero net proton consumption for montmorillonite appears to depend on ionic strength, whereas the value for kaolinite is constant and close to 5. Similar results are obtained at 60 °C, which suggests that the point of zero net proton consumption for clay minerals does not depend on temperature, at least up to 60 °C. On the other hand, the temperature rise induces a slight increase of the net proton surface excess. Finally, the diffuse double layer formalism (DDLM) is used to model the experimental data. The model involves two processes: the protonation/deprotonation of two types of edge sites (aluminol and silanol) and H⁺/Na⁺ exchange reactions on basal surfaces, while a tiny proportion of the negative structural charge remains uncompensated. This last process maintains a negative surface potential whatever the pH of the solution, which is in agreement with electrokinetic data.     

黄铁矿载金的原因和特征

对102个金矿床载金矿物的统计表明,黄铁矿是最普遍最重要的载金矿物。造成黄铁矿成为主要载金矿物的原因,有三个矿物学方面的因素,即结构因素、成核因素和电化学因素。结构因素表现在黄铁矿晶体结构中存在对硫 ［S2］2－,对硫 形成过程中对金离子具还原效应。成核因素表现在自然金成核常选择原子排布与之最接近的黄铁矿表面为衬底,以降低成核能。电化学因素表现在黄铁矿的热电性导致金离子在其表面发生电化学反应而沉淀结晶。对黄铁矿载金能力的分析表明,细粒、它形、裂隙发育程度高、As和Sb含量高以及P型的黄铁矿载金能力高,自形黄铁矿中的{210}、S面{100}及其聚形晶的载金能力高。     

Iron isotope fractionation between pyrite (FeS2), hematite (Fe2O3) and siderite (FeCO3): A first-principles density functional theory study

In addition to equilibrium isotopic fractionation factors experimentally derived, theoretical predictions are needed for interpreting isotopic compositions measured on natural samples because they allow exploring more easily a broader range of temperature and composition. For iron isotopes, only aqueous species were studied by first-principles methods and the combination of these data with those obtained by different methods for minerals leads to discrepancies between theoretical and experimental isotopic fractionation factors. In this paper, equilibrium iron isotope fractionation factors for the common minerals pyrite, hematite, and siderite were determined as a function of temperature, using first-principles methods based on the density functional theory (DFT). In these minerals belonging to the sulfide, oxide and carbonate class, iron is present under two different oxidation states and is involved in contrasted types of interatomic bonds. Equilibrium fractionation factors calculated between hematite and siderite compare well with the one estimated from experimental data (ln α⁵⁷Fe/⁵⁴Fe = 4.59 ± 0.30‰ and 5.46 ± 0.63‰ at 20 °C for theoretical and experimental data, respectively) while those for Fe(III)_aq-hematite and Fe(II)_aq-siderite are significantly higher that experimental values. This suggests that the absolute values of the reduced partition functions (β-factors) of aqueous species are not accurate enough to be combined with those calculated for minerals. When compared to previous predictions derived from Mössbauer or INRXS data [Polyakov V. B., Clayton R. N., Horita J. and Mineev S. D. (2007) Equilibrium iron isotope fractionation factors of minerals: reevaluation from the data of nuclear inelastic resonant X-ray scattering and Mössbauer spectroscopy. Geochim. Cosmochim. Acta71, 3833-3846], our iron β-factors are in good agreement for siderite and hematite while a discrepancy is observed for pyrite. However, the detailed investigation of the structural, electronic and vibrational properties of pyrite as well as the study of sulfur isotope fractionation between pyrite and two other sulfides (sphalerite and galena) indicate that DFT-derived β-factors of pyrite are as accurate as for hematite and siderite. We thus suggest that experimental vibrational density of states of pyrite should be re-examined.     

Alpha thermochronology of carbonates

Step-heating experiments on 17 calcites from 11 different samples and 6 dolomites from 5 samples suggest a closure temperature of He in carbonates ∼70 ± 10 °C for a cooling rate of 10 °C/m.y. The bulk closure temperature in some samples may tend slightly higher due to the presence of diffusion domains larger (and therefore more retentive) than the sites in which the majority of He resides. The diffusivity of He in calcite is independent of the genesis of the mineral (igneous, sedimentary, metamorphic) or the source of the He (radiogenic, common, or laboratory induced) and in all samples analyzed the effective diffusion dimension for He is smaller than the size of the crystals investigated. Although calcite is a low-U mineral, this shortcoming can be overcome by analyzing large samples (>2 mm diameter) provided samples have a minimum of ∼0.3 ppm U; samples with smaller concentrations of alpha-producers are unlikely to produce enough radiogenic ⁴He sufficient to overwhelm He present in the crystals at the time they passed through their closure temperature.     

Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks

In recent years, serpentinized ultramafic rocks have received considerable attention as a source of H₂ for hydrogen-based microbial communities and as a potential environment for the abiotic synthesis of methane and other hydrocarbons within the Earth’s crust. Both of these processes rely on the development of strongly reducing conditions and the generation of H₂ during serpentinization, which principally results from reaction of water with ferrous iron-rich minerals contained in ultramafic rocks. In this report, numerical models are used to investigate the potential influence of chemical thermodynamics on H₂ production during serpentinization. The results suggest that thermodynamic constraints on mineral stability and on the distribution of Fe among mineral alteration products as a function of temperature are likely to be major factors controlling the extent of H₂ production. At high temperatures (>∼315 °C), rates of serpentinization reactions are fast, but H₂ concentrations may be limited by the attainment of stable thermodynamic equilibrium between olivine and the aqueous fluid. Conversely, at temperatures below ∼150 °C, H₂ generation is severely limited both by slow reaction kinetics and partitioning of Fe(II) into brucite. At 35 MPa, peak temperatures for H₂ production occur at 200-315 °C, indicating that the most strongly reducing conditions will be attained during alteration within this temperature range. Fluids interacting with peridotite in this temperature range are likely to be the most productive sources of H₂ for biology, and should also produce the most favorable environments for abiotic organic synthesis. The results also suggest that thermodynamic constraints on Fe distribution among mineral alteration products have significant implications for the timing of magnetization of the ocean crust, and for the occurrence of native metal alloys and other trace minerals during serpentinization.     

The effect of crystallinity on dissolution rates and CO2 consumption capacity of silicates

Comparison of measured far-from-equilibrium dissolution rates of natural glasses and silicate minerals at 25 °C and pH 4 reveals the systematic effects of crystallinity and elemental composition on these rates. Rates for both minerals and glasses decrease with increasing Si:O ratio, but glass dissolution rates are faster than corresponding mineral rates. The difference between glass and mineral dissolution rates increases with increasing Si:O ratio; ultra-mafic glasses (Si:O ? 0.28) dissolve at similar rates as correspondingly compositioned minerals, but Si-rich glasses such as rhyolite (Si:O ∼ 0.40) dissolve ?1.6 orders of magnitude faster than corresponding minerals. This behaviour is interpreted to stem from the effect of Si-O polymerisation on silicate dissolution rates. The rate controlling step of dissolution for silicate minerals and glasses for which Si:O > 0.28 is the breaking of Si-O bonds. Owing to rapid quenching, natural glasses will exhibit less polymerisation and less ordering of Si-O bonds than minerals, making them less resistant to dissolution. Dissolution rates summarized in this study are used to determine the Ca release rates of natural rocks at far-from-equilibrium conditions, which in turn are used to estimate their CO₂ consumption capacity. Results indicate that Ca release rates for glasses are faster than those of corresponding rocks. This difference is, however, significantly less than the corresponding difference between glass and mineral bulk dissolution rates. This is due to the presence of Ca in relatively reactive minerals. In both cases, Ca release rates increase by ∼two orders of magnitude from high to low Si:O ratios (e.g., from granite to gabbro or from rhyolitic to basaltic glass), illustrating the important role of Si-poor silicates in the long-term global CO₂ cycle.     

Argon diffusion in plagioclase and implications for thermochronometry: A case study from the Bushveld Complex, South Africa

Plagioclase is not only the most abundant mineral in the Earth’s crust, but is present in almost all terrestrial tectonic settings and is widespread in most extraterrestrial material. Applying the K-Ar system to this common mineral would provide a powerful tool for quantifying thermal histories in a wide variety of settings. Nonetheless, plagioclase has rarely been used for thermochronometry, largely due to difficulties in simultaneously acquiring precise geochronologic data and quantifying argon diffusion kinetics from a mineral with low-K concentration. Here we describe an analytical technique that generates high-precision ⁴⁰Ar/³⁹Ar data and quantifies Ar diffusion kinetics of low-K minerals. We present results of five diffusion experiments conducted on single crystals of plagioclase from the Bushveld Complex, South Africa. The observed diffusion kinetics yield internally consistent thermochronological constraints, indicating that plagioclase is a reliable thermochronometer. Individual grains have activation energies of 155-178 kJ/mol and ln(D₀/a²) varies between 3.5 and 6.5. These diffusion parameters correspond to closure temperatures of 225-300 °C, for a 10 °C/Ma cooling rate. Age spectra generally conform to single-domain diffusive loss profiles, suggesting that grain-scale diffusion dominates argon transport in this fairly simple plagioclase. Conjointly examining several single-grain analyses enables us to distinguish episodic reheating from slow cooling and indicates that the Bushveld Complex cooled rapidly and monotonically from magmatic temperature to <300 °C over 3 Ma, followed by protracted cooling to ambient crustal temperatures of 150-200 °C over ∼600 Ma.     

The iron-coating role on the oxidation kinetics of a pyritic sludge doped with fly ash

The present study examines the processes that control the oxidation attenuation of a pyrite-rich sludge (72 wt% pyrite) from the Iberian Pyrite Belt by the buffer capacity of a fly ash from Los Barrios power station (S Spain), using saturated column experiments. In addition, in order to understand the behaviour of both materials inside these experiments, a fly-ash leaching test and flow-through experiments with pyritic sludge were carried out. The fly-ash leaching test showed that after leaching this material with a slightly acid solution (Millipore MQ water; pH 5.6) the pH raised up to 10.2 and that the metals released by the fly-ash dissolution did not increase significantly the metal concentrations in the output solutions. The flow-through experiments with the pyritic sludge were performed at pH 9, 22 °C and O₂ partial pressure of 0.21 atm, to calculate the dissolution rate of this residue simulating the fly-ash addition. In the experiments Fe bearing oxyhydroxides precipitated as the sludge dissolved. In two non-stirred experiments the iron precipitates formed Fe-coatings on the pyrite surfaces preventing the interaction between the oxidizing agents and the pyrite grains, halting pyrite oxidation (this process is known as pyrite microencapsulation), whereas in two stirred experiments, stirring hindered the iron precipitates to coat the pyrite grains. Thus, based on the release of S (aqueous sulphate) the steady-state pyritic sludge dissolution rate obtained was 9.0 ± 0.2 × ⁻¹¹ mol m⁻² s⁻¹.In the saturated column experiments, the sludge dissolution was examined at acidic and basic pH at 22 °C and oxygen-saturated atmosphere. In a saturated column experiment filled with the pyritic sludge, pyrite oxidation occurred favourably at pH approx. 3.7. As the leachates of the fly ash yielded high basic pH, in another saturated column, consisting of an initial thick layer of fly-ash material and a layer of pyritic sludge, the pyrite dissolution took place at pH approx. 10.45. In this experiment, iron was depleted completely from the solution and attenuation of the sludge oxidation was produced in this conditions. The attenuation was likely promoted by precipitation of iron-bearing phases upon the pyritic surface forming Fe-coatings (of ferrihydrite and/or Fe(III) amorphous phases) that halted the pyrite oxidation (as in non-stirred flow-through experiments). Results suggest that buffering capacity of fly ash can be used to attenuate the pyrite-rich sludge oxidation.     

Effects of pH and temperature on dimerization rate of glycine: Evaluation of favorable environmental conditions for chemical evolution of life

To evaluate favorable environmental conditions for the chemical evolution of life, we studied the effects of pH and temperature on the dimerization rate of glycine (Gly: NH₂-CH₂-COOH), one of the simplest amino acids. Gly dimerizes to form glycylglycine (GlyGly), and GlyGly further reacts to form diketopiperazine (DKP). Gly solutions with pH ranging from 3.1 to 10.9 were heated for 1-14 days at 140 °C, and changes in concentrations of Gly, GlyGly, and DKP were evaluated. At pH 9.8, the experiments were conducted at 120, 140, 160, and 180 °C. The dimerization rate of Gly was nearly constant at pH 3-7 and increased with increasing pH from 7 to 9.8 and then decreased with further increases in pH. To elucidate the reason for this pH dependency, we evaluated the role of the three dissociation states of Gly (cationic state: Gly⁺, zwitterionic state: Gly^±, and anionic state: Gly⁻). For pH >6, the dominant forms are Gly^± and Gly⁻, and the molar fraction of Gly^± decreases and that of Gly⁻ increases with increasing pH. The dimerization rate was determined for each dissociation state. The reaction between Gly^± and Gly⁻ was found to be the fastest; the rate constant of the reaction between Gly^± and Gly⁻ was 10 times the size of that between Gly⁻ and Gly⁻ and 98 times that between Gly^± and Gly^±. The dimerization rate became greatest at pH 9.8 because the molar fractions of Gly^± and Gly⁻ are approximately equal at this pH. The dimerization rate increased with temperature, and an activation energy of 88 kJ mol⁻¹ was obtained. Based on these results and previous reports on the stability of amino acids under hydrothermal conditions, we determined that Gly dimerizes most efficiently under alkaline pH (∼9.8) at about 150 °C.