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 charge density on silica in alkali and alkaline earth chloride electrolyte solutions

The surface charge density of colloidal SiO₂ (Aerosil 380) was measured in alkali chloride (0.067 and 0.20 M LiCl, NaCl, and KCl) and alkaline earth chloride (0.067 M MgCl₂, CaCl₂, SrCl₂, BaCl₂) solutions. Measurements were conducted at 25°C by potentiometric titrations using the constant ionic medium method in a CO₂-free system. The experimental design measured surface charge for solutions with constant ionic strength as well as constant cation concentration. Alkali chloride solutions promote negative surface charge density in the order LiCl < NaCl < KCl to give the “regular” lyotropic behavior previously reported. In contrast, the alkaline earth chloride solutions exhibit a reversed lyotropic trend with increasing crystallographic radius where increasing negative charge is promoted in the order BaCl₂ < SrCl₂ < CaCl₂ < MgCl₂.The origin of the opposing affinity trends is probed by testing the hypothesis that this reversal is rooted in the differing solvent structuring characteristics of the IA and IIA cations at the silica-water interface. This idea arises from earlier postulations that solvent structuring effects increase entropy through solvent disordering and these gains must be much greater than the small, positive enthalpy associated with electrostatic interactions. By correlating measured charge density with a proxy for the solvent-structuring ability of cations, this study shows that silica surface charge density is maximized by those electrolytes that have the strongest effects on solvent structuring. We suggest that for a given solid material, solvation entropy has a role in determining the ionic specificity of electrostatic interactions and reiterate the idea that the concept of lyotropy is rooted in the solvent-structuring ability of cations at the interface.     

Dissolution of uranophane: An AFM, XPS, SEM and ICP study

Dissolution experiments on single crystals of uranophane and uranophane-β, Ca(H₂O)₅[(UO₂)(SiO₃(OH)]₂, from the Shinkolobwe mine of the Democratic Republic of Congo, were done in an aqueous HCl solution of pH 3.5 for 3 h, in HCl solutions of pH 2 for 5, 10 and 30 min, and in Pb²⁺-, Ba-, Sr-, Ca- and Mg-HCl solutions of pH 2 for 30 min. The basal surfaces of the treated uranophane crystals were examined using atomic-force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Solutions after dissolution experiments on single crystals and synthetic powders were analysed with inductively coupled plasma-optical emission spectroscopy (ICP-OES) and mass spectroscopy (ICP-MS). The morphology of the observed etch pits (measured by AFM) were compared to the morphology, predicted on the basis of the bond-valence deficiency of polyhedron chains along the edges of the basal surface. Etch pits form in HCl solutions of pH 2. Their decrease in depth with the duration of the dissolution experiment is explained with the stepwave dissolution model, which describes the lowering of the surrounding area of an etch pit with continuous waves of steps emanated from the etch pit into the rest of the crystal surface. Hillocks form in an HCl solution of pH 3.5, and the chemical composition of the surface (as indicated by XPS) shows that these hillocks are the result of the precipitation of a uranyl-hydroxy-hydrate phase. Well-orientated hillocks form on the surface of uranophane in a SrCl₂-HCl solution of pH 2. They are part of an aged silica coating of composition Si₂O₂(OH)₄(H₂O)_n. An amorphous layer forms on the surface of uranophane in a MgCl₂-HCl solution of pH 2, which has a composition and structure similar to silicic acid. Small crystallites of uranyl-hydroxy-hydrate phases form on the surface of uranophane after treatment in Pb(NO₃)₂-HCl and BaCl₂-HCl solutions of pH 2. Dissolution experiments on synthetic uranophane powders show that in the early stage of the experiments, the dissolution rate of uranophane increase in the sequence Pb(NO₃)₂-HCl < BaCl₂-HCl < CaCl₂-HCl < HCl < SrCl₂-HCl < MgCl₂-HCl, indicating that the dissolution of uranophane is more enhanced in solutions containing divalent cations of small ionic radii and high Lewis acidity (Mg, MgCl⁺).     

An atomic force microscopy study of calcite dissolution in saline solutions: The role of magnesium ions

In situ Atomic Force Microscopy, AFM, experiments have been carried out using calcite cleavage surfaces in contact with solutions of MgSO₄, MgCl₂, Na₂SO₄ and NaCl in order to attempt to understand the role of Mg²⁺ during calcite dissolution. Although previous work has indicated that magnesium inhibits calcite dissolution, quantitative AFM analyses show that despite the fact that Mg²⁺ inhibits etch pit spreading, it increases the density and depth of etch pits nucleated on calcite surfaces and, subsequently, the overall dissolution rates: i.e., from 10^−11.75 mol cm⁻² s⁻¹ (in deionized water) up to 10^−10.54 mol cm⁻² s⁻¹ (in 2.8 M MgSO₄). Such an effect is concentration-dependent and it is most evident in concentrated solutions ([Mg²⁺] >> 50 mM). These results show that common soluble salts (especially Mg sulfates) may play a critical role in the chemical weathering of carbonate rocks in nature as well as in the decay of carbonate stone in buildings and statuary.     

Carbon isotope effects in the open-system Fischer-Tropsch synthesis

Carbon isotopic composition was measured for products of the Fischer-Tropsch synthesis: catalytic reaction between CO and H₂ to produce CO, CO₂, light hydrocarbons C1-C4 and “oil” fraction. Hydrogen isotopes were also measured in the oil fraction and the produced water. Experimental runs were conducted in the flow-through reactor at 260-310 °C and 30 bar using the synthesis gas composed of 5N₂ + 3H₂ + 2CO, on Fe-catalyst mixed with ZSM-5 synthetic zeolite. In the two of seven runs a Fe + Co-catalyst was used that gives a lower yield of unsaturated hydrocarbons in reaction products. The isotopic effects depended on the conversion of the carbon monoxide. Under steady-state conditions (CO conversion more than 90%) a strong kinetic fractionation was observed between CO and CO₂ (∼−10‰) and CO and hydrocarbons (∼+38‰). At low conversion a clear “inverse” isotopic trend of the depletion in ¹³C of longer hydrocarbon chains was observed. On average, Δ₁₂ = δ¹³C(CH₄) − δ¹³C(C₂H₆) correlates well with the CO conversion: the C₂H₆ is ∼6‰ isotopically lighter than CH₄ at low conversion and ∼2‰ heavier at steady-state regime. Under steady-state conditions there almost no difference was observed in the isotopic composition of methane and ethane and higher hydrocarbons. The chemical composition of light hydrocarbons in the products of flow-through, dynamic FTS is different from that found in the static FTS-type experiments with Fe-catalyst, but isotopic effects are similar. Our results suggest that the isotopic distribution of carbon found in so-called “abiogenic” hydrocarbons from some natural gases (δ¹³C1 > δ¹³C2 > δ¹³C3  >?) is somewhat similar to that at low conversion of CO, but do not resemble the distribution characteristic for the high conversion products, at least, on Fe-catalyst. Other processes (a simple mixing of two or more endmembers) or other P-T conditions of the carbon reduction could be responsible for the “inverse” isotopic trend found in meteorites and some natural gases.     

Distribution of barium and fulvic acid at the mica-solution interface using in-situ X-ray reflectivity

The interfacial structures of the basal surface of muscovite mica in solutions containing (1) 5 × 10⁻³ m BaCl₂, (2) 500 ppm Elliott Soil Fulvic Acid I (ESFA I), (3) 100 ppm Elliott Soil Fulvic Acid II (ESFA II), (4) 100 ppm Pahokee Peat Fulvic Acid I (PPFA), and (5) 5 × 10⁻³ m BaCl₂ and 100 ppm ESFA II were obtained with high resolution in-situ X-ray reflectivity. The derived electron-density profile in BaCl₂ shows two sharp peaks near the mica surface at 1.98(2) and 3.02(4) Å corresponding to the heights of a mixture of Ba²⁺ ions and water molecules adsorbed in ditrigonal cavities and water molecules coordinated to the Ba²⁺ ions, respectively. This pattern indicates that most Ba²⁺ ions are adsorbed on the mica surface as inner-sphere complexes in a partially hydrated form. The amount of Ba²⁺ ions in the ditrigonal cavities compensates more than 90% of the layer charge of the mica surface. The electron-density profiles of the fulvic acids (FAs) adsorbed on the mica surface, in the absence of Ba²⁺, had overall thicknesses of 4.9-10.8 Å and consisted of one broad taller peak near the surface (likely hydrophobic and positively-charged groups) followed by a broad humped pattern (possibly containing negatively-charged functional groups). The total interfacial electron density and thickness of the FA layer increased as the solution FA concentration increased. The sorbed peat FA which has higher ash content showed a higher average electron density than the sorbed soil FA. When the muscovite reacted with a pre-mixed BaCl₂-ESFA II solution, the positions of the two peaks nearest the surface matched those in the BaCl₂ solution. However, the occupancy of the second peak decreased by about 30% implying that the hydration shell of surface-adsorbed Ba²⁺ was partially substituted by FA. The two surface peaks were followed by a broad less electron-dense layer suggesting a sorption mechanism in which Ba²⁺ acts dominantly as a bridging cation between the mica surface and FA. When the muscovite reacted first with FA and subsequently with BaCl₂, more Ba²⁺ could be adsorbed on the FA-coated mica surface. The peak closest to the mica included Ba²⁺ ions adsorbed directly on the mica in an amount similar to that in the BaCl₂ solution but more broadly distributed. A second peak observed within the FA layer suggests that the FA coating provides additional sites for Ba²⁺ sorption. The results indicate that enhanced uptake of heavy metals can occur when an organic coating already exists on a mineral surface.     

Methyl mercury distributions in relation to the presence of nano- and picophytoplankton in an oceanic water column (Ligurian Sea, North-western Mediterranean)

Recent findings on the distribution of methylated mercury (MeHg_T) in waters have highlighted the importance of organic carbon remineralization on the production of these compounds in the open ocean. Here, we present the first time-series (20 monthly samplings between July 2007 and May 2009) of high-resolution vertical profiles (10-12 depths in a 2350 m water column) of MeHg_T distributions in an open ocean environment, the Ligurian Sea (North-western Mediterranean Sea). Concentrations varied within the sub-picomolar range (general mean: 0.30 ± 0.17 pmol L⁻¹, n = 214) with the lowest values at the surface, increasing with depth up to the oxygen minimum zone, and decreasing slowly at greater depth. Concentrations in the surface waters never exceeded 0.15 pmol L⁻¹, while the highest concentrations (up to 0.82 pmol L⁻¹) were associated to the hypoxycline during the autumn bloom. A detailed vertical MeHg_T profile reveals a “double-peak” pattern, coincidental with the two microbial layers described by Tanaka and Rassoulzadegan (2002), the so-called “microbial food web” in the euphotic zone (<100 m) and the “microbial loop” in the aphotic zone (>100 m). Temporal variations in the MeHg_T abundance and distribution in the water column were linked to seasonality. The highest MeHg_T concentrations were found in the oxygen minimum zone during the period of stratification, and coincide with the greatest abundance of nano- and picophytoplankton (cyanobacteria, nanoflagellates, etc.) in the euphotic layer. None of our deep MeHg_T measurements (∼100 m above the sea bottom) revealed a significant sedimentary source of MeHg_T. We explored the correlation between MeHg_T concentrations and the apparent oxygen utilization, a proxy of organic matter remineralization, over the study period. Results of this study strengthen the hypothesis that net mercury methylation in the open ocean occurs in the water column, is linked to organic matter regeneration, and is promoted by the presence of small-sized nano- and picophytoplankton, that dominate under oligotrophic conditions.     

Geochemical characteristics of waters in mineralised area of Peloritani Mountains (Sicily,Italy)

This paper presents the results of a study on the geochemistry of waters circulating in the mineralised area of the south-eastern sector of Mt. Peloritani (north-eastern Sicily, Italy), aimed at basic understanding of the geochemical processes influencing their chemical composition. Chemico-physical parameters and data on 26 major and minor chemical elements are reported for 103 water samples. Water chemistry is mainly dominated by dissolution of carbonates and hydrolysis of aluminosilicate minerals. Total dissolved salts (TDS) range from 80 to 1398 mg/L. All the waters exhibit E_H characteristic of an oxygenated environment. Excluding two samples, which show very high H⁺ activity (pH = 3.0 and 2.7), all the waters have pH values in the range 6.2–8.6. Cluster analysis based on major ion contents defined three main chemical water types, reflecting different hydrochemical processes. The first, group I, has low salinity (average TDS = 118 ± 30 mg/L) and abundance orders (meq/L) Na > Ca ≈ Mg > K and Cl ≈ HCO₃ > SO₄. With increased water–rock interaction, waters in groups II and III become more saline, changing composition towards SO₄–Cl-alkaline earth and HCO₃-alkaline earth types. Weathering of carbonate minerals causes waters to become saturated with respect to calcite and dolomite, whereas the incongruent dissolution of aluminosilicate minerals causes the solution to reach equilibrium with kaolinite and to form smectites. Trace element geochemistry in the analysed waters reflects interactions between waters and existing mineralisation, with elemental concentrations showing highly variable values, and higher concentrations of As, Pb, Sb and Zn near known mineralisation. Lead–Zn and As–Sb statistical associations, probably distinguishing interactions with different mineralogical phase paragenesis, were revealed by factor analysis. The main aqueous chemical forms of trace elements predicted by chemical speciation calculations are also reported. As most of the analysed spring waters provide the main source of freshwater for domestic purposes, attention should be given to As and Sb, whose concentrations exceed the recommended limits.     

Decomposition reactions of magnesium sulfate hydrates and phase equilibria in the MgSO4-H2O and Na-Mg-Cl-SO4-H2O systems with implications for Mars

We report new measurements of equilibrium relative humidities for stable and metastable hydration-dehydration equilibria involving several magnesium sulfates in the MgSO₄·nH₂O series. We also report a comprehensive thermodynamic treatment of the system including solution properties and experimental data from the published literature, i.e. solubilities, heat capacities and additional decomposition humidities. While for some magnesium sulfate hydrates solubility data in the binary system MgSO₄-H₂O are sparse, there is a reasonable database of solubility measurements of these hydrates in the ternary MgCl₂-MgSO₄-H₂O and the quaternary reciprocal Na⁺-Mg²⁺-Cl⁻-SO₄²⁻-H₂O systems. To make these data suitable for the determination of solubility products, we parameterized a Pitzer ion interaction model for the calculation of activity coefficients and water activities in mixed solutions of these systems and report the ion interaction parameters for the Na⁺-Mg²⁺-Cl⁻-SO₄²⁻-H₂O system. The model predicted solubilities in the reciprocal system are in very good agreement with experimental data. Using all available experimental data and the solution model an updated phase diagram of the MgSO₄-H₂O system covering the whole temperature range from about 170 to 473 K is established. This treatment includes MgSO₄·H₂O (kieserite), MgSO₄·4H₂O (starkeyite), MgSO₄·5H₂O (pentahydrite), MgSO₄·6H₂O (hexahydrite), MgSO₄·7H₂O (epsomite) and MgSO₄·11H₂O (meridianiite). It is shown that only kieserite, hexahydrite, epsomite and meridianiite show fields of stable existence while starkeyite and pentahydrite are always metastable. Due to sluggish kinetics of kieserite formation, however, there is a rather extended field of metastable existence of starkeyite which makes this solid a major product in dehydration reactions. The model predicted behavior of the magnesium sulfates is in excellent agreement with observations reported in the literature under terrestrial temperature and relative humidity conditions. We also discuss the implications of the new phase diagram for sulfates on Mars.     

Catalytic peptide hydrolysis by mineral surface: Implications for prebiotic chemistry

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

Internally consistent thermodynamic data for magnesium sulfate hydrates

Experimental studies on the stability of several Mg-sulfate hydrates including epsomite (MgSO₄·7H₂O), hexahydrite (MgSO₄·6H₂O), starkeyite (MgSO₄·4H₂O), and kieserite (MgSO₄·H₂O) as a function of temperature and relative humidity are in poor agreement with calculations based on thermodynamic properties of these substances taken from the literature. Therefore, we synthesized four different MgSO₄ hydrates and measured their enthalpies of formation by solution calorimetry at T = 298.15 K. The resulting enthalpies of formation from the elements are:

•

Δ_fH⁰₂₉₈ (epsomite) = −3387.7 ± 1.3 kJmol⁻¹

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Δ_fH⁰₂₉₈ (hexahydrite) = −3088.1 ± 1.1 kJmol⁻¹

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Δ_fH⁰₂₉₈ (sanderite, MgSO₄·2H₂O) = −1894.9 ± 1.3 kJmol⁻¹

•

Δ_fH⁰₂₉₈ (kieserite) = −1612.4 ± 1.3 kJmol⁻¹

Using mathematical programming (MAP) techniques, standard thermodynamic values consistent both with our calorimetric data and previously published humidity brackets could be derived:

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Epsomite: Δ_fH⁰₂₉₈ = −3388.7 kJmol⁻¹, S⁰₂₉₈ = 371.3 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −2871.0 kJmol⁻¹

•

Hexahydrite: Δ_fH⁰₂₉₈ = −3087.3 kJmol⁻¹, S⁰₂₉₈ = 348.5 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −2632.3 kJmol⁻¹

•

Starkeyite: Δ_fH⁰₂₉₈ = −2496.1 kJmol⁻¹, S⁰₂₉₈ = 259.9 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −2153.8 kJmol⁻¹

•

Kieserite: Δ_fH⁰₂₉₈ = −1611.5 kJmol⁻¹, S⁰₂₉₈ = 126.0 Jmol⁻¹ K⁻¹, Δ_fG⁰₂₉₈ = −1437.9 kJmol⁻¹

Additionally, heat capacity measurements and standard entropy determinations of several magnesium sulfate hydrate minerals from the literature are analyzed and judged against estimates obtained from a linear combination of the heat capacities of MgSO₄ and hexagonal ice. The results of the MAP analysis are compared to these estimates to conclude that heat capacity and entropy correlate well with the number of waters of hydration. However, even the good correlation is not good enough to capture the fine variations in these properties. Consequently, their experimental measurement is inevitable if reliable thermodynamic data are sought. Our MAP thermodynamic data show that epsomite, hexahydrite, and kieserite have stability fields in the T-%RH space. Starkeyite is metastable. Although no MAP data could have been derived for pentahydrite (MgSO₄·5H₂O) and sanderite, their transient existence suggest that both of them are metastable as well.     

Geochemistry and mineralogy of arsenic in (natural) anaerobic groundwaters

Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO₄ to stimulate metabolism of indigenous SO₄-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO₄, and contained <10 μg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained ∼10 mg/L Fe and ∼100 μg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 μg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO₄ reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS₂. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO₄ reduction commenced but increased to pre-experiment level once SO₄ reduction ended. In the ongoing experiment, both SO₄ and Fe(II) were amended to groundwater to evaluate if FeS/FeS₂ formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field completely dominates in reducing Eh–pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions.     

The separate production of H2S from the thermal reaction of hydrocarbons with magnesium sulfate and sulfur: Implications for thermal sulfate reduction

The yields and stable C and H isotopic composition of gaseous products from the reactions of pure n-C₂₄ with (1) MgSO₄; and (2) elemental S in sealed Au-tubes at a series of temperatures over the range 220–600 °C were monitored to better resolve the reaction mechanisms. Hydrogen sulfide formation from thermochemical sulfate reduction (TSR) of n-C₂₄ with MgSO₄ was initiated at 431 °C, coincident with the evolution of C₂–C₅ hydrocarbons. Whereas the yields of H₂S increased progressively with pyrolysis temperature, the hydrocarbon yields decreased sharply above 490 °C due to subsequent S consumption. Ethane and propane were initially very ¹³C depleted, but became progressively heavier with pyrolysis temperature and were more ¹³C enriched than the values of a control treatment conducted on just n-C₂₄ above 475 °C. TSR of MgSO₄ also led to progressively higher concentrations of CO₂ showing relatively low δ¹³C values, possibly due to input of isotopically light CO₂ derived from gaseous hydrocarbon oxidation (e.g., more depleted CH₄).     

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.     

Role of lithosphere-asthenosphere interaction in the genesis of Quaternary alkali and tholeiitic basalts from Datong, western North China Craton

The geochemistry of Quaternary volcanic rocks from Datong provides important constraints on the petrogenesis of continental alkali and tholeiitic basalts and lithospheric evolution in the western North China Craton. Alkali basalts in north Datong have trace element compositions similar to oceanic island basalts (OIB). They show nearly homogenous isotopic compositions (?_Nd = 5.4-6.8 and ⁸⁷Sr / ⁸⁶Sr = 0.7035-0.7037) that resemble the nearby Hannuoba Miocene basalts, indicating that the two lava suites share a similar asthenospheric source. However, Datong basalts have conspicuously lower Al₂O₃ and CaO, higher SiO₂ and HREE contents and Na / Ti ratios, compared to Hannuoba lavas at comparable MgO. This compositional difference is attributable to the combined effect of source difference and temporal decrease in melting depth. The latter reflects Cenozoic lithospheric thinning of the western North China Craton.Tholeiitic basalts in southeast Datong have incompatible element ratios that differ from OIB; they have lower ?_Nd (1.3-3.7) and higher ⁸⁷Sr / ⁸⁶Sr (0.7039-0.7046) compared to alkali basalts. These moderately evolved rocks (MgO < 7%) display unusually high Cr concentrations (> 200 ppm), a nearly flat LREE pattern and a fractionated HREE with the “kink” occurring at Gd. A shallow melting depth (< 60 km), suggested by their Q-normative composition, is in conflict with the residual garnet in the source (> 75 km) as required by REE modeling. This paradox, which is reminiscent of that for Hawaiian tholeiites, can be reconciled if garnet lherzolite melts react with refractory peridotites during which orthopyroxene is dissolved and olivine precipitates. The diagnostic consequence of this melt-rock reaction includes increases in SiO₂ and Cr, decreases in Al₂O₃ and CaO, and formation of “kinked” REE patterns. Involvement of lithospheric mantle in the genesis of Datong tholeiites may be related to the Cenozoic lithospheric thinning/erosion in the western North China Craton. The spatial distribution of Datong alkali and tholeiitic basalts may be related to enhanced extension along the lithospheric boundary between the Western Block of the North China Craton and the Trans-North China Orogen.     

Application of the Pitzer ion interaction model to isopiestic data for the Fe2(SO4)3-H2SO4-H2O system at 298.15 and 323.15 K

Recent isopiestic studies of the Fe₂(SO₄)₃-H₂SO₄-H₂O system at 298.15 K are represented with an extended version of Pitzer’s ion interaction model. The model represents osmotic coefficients for aqueous {(1 − y)Fe₂(SO₄)₃ + yH₂SO₄} mixtures from 0.45 to 3.0 m at 298.15 K and 0.0435 ? y ? 0.9370. In addition, a slightly less accurate representation of a more extended molality range to 5.47 m extends over the same y values, translating to a maximum ionic strength of 45 m. Recent isopiestic data for the system at 323.15 K are represented with the extended Pitzer model over a limited range in molality and solute fraction. These datasets are also represented with the usual “3-parameter” version of Pitzer’s model so that it may be incorporated in geochemical modeling software, but is a slightly less accurate representation of thermodynamic properties for this system. Comparisons made between our ion interaction model and available solubility data display partial agreement for rhomboclase and significant discrepancy for ferricopiapite. The comparisons highlight uncertainty remaining for solubility predictions in this system as well as the need for additional solubility measurements for Fe³⁺-bearing sulfate minerals. The resulting Pitzer ion interaction models provide an important step toward an accurate and comprehensive representation of thermodynamic properties in this geochemically important system.     

Crystallization behavior of Na2SO4–MgSO4 salt mixtures in sandstone and comparison to single salt behavior

We report on the crystallization behavior and the salt weathering potential of Na₂SO₄, MgSO₄ and an equimolar mixture of these salts in natural rock and porous stone. Geochemical modeling of the phase diagram of the ternary Na₂SO₄–MgSO₄–H₂O system was used to determine the equilibrium pathways during wetting (or deliquescence) of incongruently soluble minerals and evaporation of mixed electrolyte solutions. Model calculations include stable and metastable solubilities of the various hydrated states of the single salts and the double salts Na₂Mg(SO₄)₂·4H₂O (bloedite), Na₂Mg(SO₄)₂·5H₂O (konyaite), Na₁₂Mg₇(SO₄)₁₃·15H₂O (loeweite) and Na₆Mg(SO₄)₄ (vanthoffite). In situ Raman spectroscopy was used to study the phase transformations during wetting of pure MgSO₄·H₂O (kieserite) and of the incongruently soluble salts bloedite and konyaite. Dissolution of kieserite leads to high supersaturation resulting in crystallization of higher hydrated phases, i.e. MgSO₄·7H₂O (epsomite) and MgSO₄·6H₂O (hexahydrite). This confirms the high damage potential of magnesium sulfate in salt damage of building materials. The dissolution of the incongruently soluble double salts leads to supersaturation with respect to Na₂SO₄·10H₂O (mirabilite). However, the supersaturation was insufficient for mirabilite nucleation. The damage potential of the two single salts and an equimolar salt mixture was tested in wetting–drying experiments with porous sandstone. While the high damage potential of the single salts is confirmed, it appears that the supersaturation achieved during wetting of the double salts at room temperature is not sufficient to generate high crystallization pressures. In contrast, very high damage potentials of the double salts were found in experiments at low temperature under high salt load.1     

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.     

The role of metal sulfates in thermochemical sulfate reduction (TSR) of hydrocarbons: Insight from the yields and stable carbon isotopes of gas products

The mechanism of thermochemical sulfate reduction (TSR) was investigated by separately heating n-C₂₄ with three different sulfates (CaSO₄, Na₂SO₄, MgSO₄) in sealed gold tubes at 420 °C and measuring the stable carbon isotope values of hydrocarbon (C₁-C₅) and non-hydrocarbon (CO₂) products. Extensive TSR was observed with the MgSO₄ reactant as reflected by increasing concentrations of H₂S, ¹³C depleted CO₂ and relatively low concentrations of H₂ (compared to the control). H₂S yields were already very high at the first monitoring time (12 h) when the temperature had just reached 420 °C, suggesting that TSR had commenced well prior to this temperature. Only trace amounts of n-C₂₄ and secondary C₃-C₅ alkanes were detected at 12 h, reflecting the efficient TSR utilization of the reactant and lower molecular weight alkane products. Ethane levels were still relatively high at 12 h, but declined thereafter as it was subject to TSR in the absence of higher molecular weight alkanes which had already been utilized. Methane yields were consistently high throughout the 48 h MgSO₄ treatment. The temporal decrease in the concentrations of alkanes available for TSR may also contribute to the sharp enhancement of CO₂ after 36 h. Absence or dampening of the molecular and isotopic trends of MgSO₄ TSR was observed with Na₂SO₄ and CaSO₄ respectively, directly reflecting the levels of TSR reached using these sulfate treatments.For all treatments, the δ¹³C values of C_1-5n-alkanes showed an increase with both molecular weight and treatment time. MgSO₄ TSR led to a 5-10‰ increase in the δ¹³C values of the C₁-C₅ hydrocarbons and a 20‰ decrease in the δ¹³C value of CO₂. The significant ¹³C depletion of the CO₂ may be due to co-production of ¹³C enriched MgCO₃, although this remains unproven as the δ¹³C of MgCO₃ was not measured. The difference in the δ¹³C values of ethane and propane (Δδ¹³C_EP) increased in magnitude with the degree of TSR, and this trend could be used to help evaluate the occurrence and extent of TSR in subsurface gas reservoirs.     

Initial responses of carbonate-rich shelf sediments to rising atmospheric pCO2 and “ocean acidification”: Role of high Mg-calcites

Carbonate-rich sediments at shoal to shelf depths (<200 m) represent a major CaCO₃ reservoir that can rapidly react to the decreasing saturation state of seawater with respect to carbonate minerals, produced by the increasing partial pressure of atmospheric carbon dioxide (pCO₂) and “acidification” of ocean waters. Aragonite is usually the most abundant carbonate mineral in these sediments. However, the second most abundant (typically ∼24 wt%) carbonate mineral is high Mg-calcite (Mg-calcite) whose solubility can exceed that of aragonite making it the “first responder” to the decreasing saturation state of seawater. For the naturally occurring biogenic Mg-calcites, dissolution experiments have been used to predict their “stoichiometric solubilities” as a function of mol% MgCO₃. The only valid relationship that one can provisionally use for the metastable stabilities for Mg-calcite based on composition is that for the synthetically produced phases where metastable equilibrium has been achieved from both under- and over-saturation. Biogenic Mg-calcites exhibit a large offset in solubility from that of abiotic Mg-calcite and can also exhibit a wide range of solubilities for biogenic Mg-calcites of similar Mg content. This indicates that factors other than the Mg content can influence the solubility of these mineral phases. Thus, it is necessary to turn to observations of natural sediments where changes in the saturation state of surrounding waters occur in order to determine their likely responses to the changing saturation state in upper oceanic waters brought on by increasing pCO₂. In the present study, we investigate the responses of Mg-calcites to rising pCO₂ and “ocean acidification” by means of a simple numerical model based on the experimental range of biogenic Mg-calcite solubilities as a function of Mg content in order to bracket the behavior of the most abundant Mg-calcite phases in the natural environment. In addition, observational data from Bermuda and the Great Bahama Bank are also presented in order to project future responses of these minerals. The numerical simulations suggest that Mg-calcite minerals will respond to rising pCO₂ by sequential dissolution according to mineral stability, progressively leading to removal of the more soluble phases until the least soluble phases remain. These results are confirmed by laboratory experiments and observations from Bermuda. As a consequence of continuous increases in atmospheric CO₂ from burning of fossil fuels, the average composition of contemporary carbonate sediments could change, i.e., the average Mg content in the sediments may slowly decrease. Furthermore, evidence from the Great Bahama Bank indicates that the amount of abiotic carbonate production is likely to decline as pCO₂ continues to rise.