Computer thermodynamic modeling of the transport and deposition of Sb and Au during the formation of Au-Sb deposits

Using the Chiller computer program, we performed modeling of the mechanisms of the joint transport and deposition of Au and Sb from various ore-forming solutions during the formation of Au-Sb deposits. Three models are considered by the example of the Uderei Au-Sb deposit in the Yenisei Ridge: (1) simple cooling (cooling only), (2) iso-enthalpy boiling (P = f(T)), and (3) solution–rock interaction (rock titration model). The behavior of Sb(III) and Au(I) in the system Au–Sb–Fe–Cu–Pb–Zn–As–H₂O–Cl–H₂S–CO₂ under hydrothermal conditions was studied. It is shown that both weakly alkaline (near-neutral) and reduced acidic Fe_aq²⁺-enriched low-chloride high-CO₂ and high-chloride hydrothermal solutions play a crucial role in the formation of gold parageneses of Au-Sb ores.     

Zero-valent iron and organic carbon mixtures for remediation of acid mine drainage: Batch experiments

A series of laboratory batch experiments was conducted to evaluate the potential for treatment of acid mine drainage (AMD) using organic C (OC) mixtures amended by zero-valent Fe (Fe⁰). Modest increases in SO₄ reduction rates (SRRs) of up to 15% were achieved by augmenting OC materials with 5 and 10 dry wt% Fe⁰. However, OC was essential for supporting SO₄ reducing bacteria (SRB) and therefore SO₄ reduction. This observation suggests a general absence of autotrophic SRB which can utilize H₂ as an electron donor. Sulfate reduction rates (SRRs), calculated using a mass-based approach, ranged from −12.9 to −14.9 nmol L⁻¹ d⁻¹  g⁻¹ OC. Elevated populations of SRB, iron reducing bacteria (IRB), and acid producing (fermentative) bacteria (APB) were present in all mixtures containing OC. Effective removal of Fe (91.6–97.6%), Zn (>99.9%), Cd (>99.9%), Ni (>99.9%), Co (>99.9%), and Pb (>95%) was observed in all reactive mixtures containing OC. Abiotic metal removal was achieved with Fe⁰ only, however Fe, Co and Mn removal was less effective in the absence of OC. Secondary disordered mackinawite [Fe_1+xS] was observed in field-emission scanning electron microscopy (FE-SEM) backscatter electron micrographs of mixtures that generated SO₄ reduction. Energy dispersive X-ray (EDX) spectroscopy revealed that Fe–S precipitates were Fe-rich for mixtures containing OC and Fe⁰, and S-rich in the absence of Fe⁰ amendment. Sulfur K-edges determined by synchrotron-radiation based bulk X-ray absorption near-edge structure (XANES) spectroscopy indicate solid-phase S was in a reduced form in all mixtures containing OC. Pre-edge peaks on XANES spectra suggest tetragonal S coordination, which is consistent with the presence of an Fe–S phase such as mackinawite. The addition of Fe⁰ enhanced AMD remediation over the duration of these experiments, however long-term evaluation is required to identify optimal Fe⁰ and OC mixtures.     

Chemistry of rainwaters in the south Pacific area of Russia

On the south-eastern edge of Russia, the chemical composition of rainwater is controlled by sea salts, terrestrial material, as well as volcanic (Kuril islands volcanic area) and anthropogenic emissions, mostly in the southern part of the area. The predominant major ions of the Primorye, Sakhalin and the Kuril Islands rainwaters were respectively HCO₃⁻–SO₄²⁻, Ca–Na, and of Cl–Na. Concentration of trace elements changes within 1–2 orders of magnitude but some difference in the distribution of the elements between continental and island rainwater is found. The concentration of the chemical elements in the particulate fraction varies from < 10% to 90% of the total concentration (dissolved + particulate) with the following distribution: Tl, Na, Ca, Sr, Zn, Cd (< 10%)–Be, Th, Bi, Rb, U, K, Sc (10–20%)–Cu, Mn, Mg, Mo, Se, Ba, Ni, As, Ag, Cs, Co, Y, Ga, V (20–50%)–Sb, Pb, Ge, Cr, Fe, Al (50–90%).The concentration of elements of the particulate fraction of the rainwater usually is significantly different from concentrations in the crust, including both higher and lower concentrations. The terrestrial contribution to dissolved elements was evaluated and follows the decreasing order: Fe > K, Mg, Ca > Ba, Sr > Na (65–1%). Close order was found for total (dissolved and solid) concentrations. Sea salt contribution to dissolved element concentration in the rainwater decrease in the following order: Cl, Mg > K, SO₄ > Ca > HCO₃⁻, Ba, Fe (78–0.1%). Calculation of anthropogenic and volcanic inputs for two ions (Cl⁻ and SO₄²⁻) shows that anthropogenic inputs for the Vladivostok and Yuzno-Sakhalinsk cities can be evaluated as 15–20% of Cl⁻ and up to 80–90% of SO₄²⁻. Volcanic components in the Kuril Islands, where anthropogenic inputs are absent, can reach up to 76% of SO₄²⁻ and 36% of Cl⁻.     

Sideronatrite: A mineral with a {Fe2(SO4)4(OH)2} guildite type chain?

Summary Recently several natural and artificial ferric iron sulphate crystal structures have been solved. Sideronatrite, Na₂Fe³⁺(SO₄)₂(OH)·3H₂O, does not provide good crystals for structural purposes. However if we examine crystallographic, chemical and physical data some useful information about the ...Fe–O–S... structural topology can be inferred. In fact this analysis strengthens the hypothesis that there is a {Fe ₂ ³⁺ (SO₄)₄(OH)₂} chain in sideronatrite like that found in guildite, Cu²⁺Fe³⁺(SO₄)₂(OH)·4H₂O.

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Sideronatrit: Ein Mineral mit einer {Fe₂(SO₄)₄(OH)₂}-Kette vom Typ Guildit?

Zusammenfassung Kürzlich wurden die Kristallstrukturen mehrerer natürlicher und künstlicher Ferrisulfate gelöst. Sideronatrit, Na₂Fe³⁺(SO₄)₂(OH)·3H₂O, liefert keine für die Strukturuntersuchung gut geeigneten Kristalle. Dennoch erhält man aus der Untersuchung der kristallographischen, chemischen und physikalischen Daten nützliche Information über die ...Fe–O–S...-Topologie der Struktur. Eine solche Analyse spricht für die Hypothese, daß der Sideronatrit eine {Fe ₂ ³⁺ (SO₄)₄(OH₂)}-Kette enthält, wie sie im Guildit, Cu²⁺Fe³⁺(SO₄)₂(OH)·4H₂O, gefunden wurde.

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With 1 Figure

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Paper presented at the Sixth European Crystallographic Meeting. Barcelona, Spain 1980.     

Geochemical characteristics of shallow groundwater in Datong basin, northwestern China

Located in semi-arid regions of northwestern China, Datong basin is a Quaternary sedimentary basin, where groundwater is the most important source for water supply. It is very important to study groundwater characteristics and hydrogeochemical processes for better management of the groundwater resource. We have identified five geochemical zones of shallow groundwater (between 5 and 80 m) at Datong: A. Leaching Zone (Zone I); B. Converging Zone (Zone II); C. Enriching Zone (Zone III); D. Reducing Zone (Zone IV); E. Oxidizing Zone (Zone V). In Zones I, II, and V and some parts of Zones III and IV, hydrolysis of albite/K-feldspar/chalcedony system and/or albite/K-feldspar/quartz system enhanced concentrations of Na⁺, K⁺, HCO₃⁻ and silicate. In Zone I, dissolution of carbonate and hydrolysis of feldspar generally controlled the groundwater chemistry. Infiltration of meteoric water promoted the formation of HCO₃⁻ in the water. In Zone II, the main geochemical processes influencing the groundwater chemistry were dissolutions of calcite and dolomite, ion exchange and evaporation. In Zones III and IV, in addition to ion exchange, evaporation and precipitation of calcite and dolomite, leaching of NaHCO₃ in saline–alkaline soils dominated the water quality. Zone IV was under anoxic condition, and reduction reactions led to the decrease of SO₄²⁻, NO₃⁻ and occurrence of H₂S, with the highest arsenic content (mean value of 366 μg/L), far exceeding Maximum Contaminant Level (MCL). Abnormal arsenic in the groundwater resulted in endemic disease of waterborne arsenic poisoning among local people. Zone V overlapped Zone I was intensively affected by coal mining activities. Sulfide minerals, such as pyrite, would have been oxidized when exposed to air due to coal mining, which directly added sulfate to groundwater and thus increased SO₄²⁻ concentration. Oxidization of sulfide minerals also decreased pH and promoted dissolutions of calcite and dolomite.     

Experimental studies of equilibrium iron isotope fractionation in ferric aquo-chloro complexes

Here we compare new experimental studies with theoretical predictions of equilibrium iron isotopic fractionation among aqueous ferric chloride complexes (Fe(H₂O)₆³⁺, FeCl(H₂O)₅²⁺, FeCl₂(H₂O)₄⁺, FeCl₃ (H₂O)₃, and FeCl₄^-), using the Fe-Cl-H₂O system as a simple, easily-modeled example of the larger variety of iron-ligand compounds, such as chlorides, sulfides, simple organic acids, and siderophores. Isotopic fractionation (⁵⁶Fe/⁵⁴Fe) among naturally occuring iron-bearing species at Earth surface temperatures (up to ∼3‰) is usually attributed to redox effects in the environment. However, theoretical modeling of reduced isotopic partition functions among iron-bearing species in solution also predicts fractionations of similar magnitude due to non-redox changes in speciation (i.e., ligand bond strength and coordination number). In the present study, fractionations are measured in a series of low pH ([H⁺] = 5 M) solutions of ferric chloride (total Fe = 0.0749 mol/L) at chlorinities ranging from 0.5 to 5.0 mol/L. Advantage is taken of the unique solubility of FeCl₄^- in immiscible diethyl ether to create a separate spectator phase, used to monitor changing fractionation in the aqueous solution. Δ⁵⁶Fe_aq-eth = δ⁵⁶Fe (total Fe remaining in aqueous phase)−δ⁵⁶Fe (FeCl₄^- in ether phase) is determined for each solution via MC-ICPMS analysis.Both experiments and theoretical calculations of Δ⁵⁶Fe_aq-eth show a downward trend with increasing chlorinity: Δ⁵⁶Fe_aq-eth is greatest at low chlorinity, where FeCl₂(H₂O)₄⁺ is the dominant species, and smallest at high chlorinity where FeCl₃(H₂O)₃ is dominant. The experimental Δ⁵⁶Fe_aq-eth ranges from 0.8‰ at [Cl^-] = 0.5 M to 0.0‰ at [Cl^-] = 5.0 M, a decrease in aqueous-ether fractionation of 0.8‰. This is very close to the theoretically predicted decreases in Δ⁵⁶Fe_aq-eth, which range from 1.0 to 0.7‰, depending on the ab initio model.The rate of isotopic exchange and attainment of equilibrium are shown using spiked reversal experiments in conjunction with the two-phase aqueous-ether system. Equilibrium under the experimental conditions is established within 30 min.The general agreement between theoretical predictions and experimental results points to substantial equilibrium isotopic fractionation among aqueous ferric chloride complexes and a decrease in ⁵⁶Fe/⁵⁴Fe as the Cl^-/Fe³⁺ ion ratio increases. The effects on isotopic fractionation shown by the modeling of this simple iron-ligand system imply that ligands present in an aqueous environment are potentially important drivers of fractionation, are indicative of possible fractionation effects due to other speciation effects (such as iron-sulfide systems or iron bonding with organic ligands), and must be considered when interpreting iron isotope fractionation in the geological record.     

Hydrogeochemistry and Groundwater Quality Assessment for Drinking and Irrigation Purpose of Raipur City,Chhattisgarh

Groundwater is an important source of drinking and irrigation purpose and the greater part of the total populace relies on groundwater for survival. Present study investigates the hydrogeochemistry and groundwater quality of the study area for drinking and irrigation purpose. In this study, total 100 numbers groundwater samples were collected and analyzed using standard methods (APHA, 1995) during pre-monsoon period (May, 2016). In the study area, there is occurrence of mainly Ca⁺²–Mg⁺²–HCO₃ and Ca⁺²–Mg⁺²–SO ₄ ^–2 water type and the dominant cations and anions are Ca>Mg>Na>K>Fe=HCO₃>Cl>CO₃> SO₄>Fe>F>NH₃. The Gibbs plot shows that, hydrogeochemistry of ground-water is depending upon rock-water interaction. Present study, indicate that groundwater quality in the study area is suitable for irrigation and drinking purpose except some groundwater sample, which are showing high Nitrate, Iron, Sulphate, Ammonia and Calcium concentration.     

Mineral chemistry and chemical zoning in tourmalines, Pampa del Tamboreo, San Luis, Argentina

Tourmalinites that are distally associated with tungsten deposits of the Pampa del Tamboreo area, San Luis, Argentina, contain tourmalines retaining evidence for its origin and evolution. Tourmaline grains uncommonly contain small grains of detrital tourmaline. Analysis of a single detrital tourmaline grain reveals that it is a Ca-rich “oxy-dravite”. Proximal to the detrital cores there are inner domains of asymmetric tourmaline overgrowths that developed during low grade metamorphism. Volumetrically dominant tourmaline overgrowths in the outer domain are concentrically zoned aluminous dravite and “oxy-dravite” with Al/(Al + Fe + Mg) = 0.71–0.74 and Mg/(Mg + Fe) = 0.64–0.71. Variability of Al is primarily controlled by the deprotonation substitution R + OH⁻ = Al + O²⁻ (where R = Fe + Mg), and is a function of the activity of H₂O. A likely evolutionary scenario is one in which volcanogenic material is altered by hydrothermal fluids in the sea floor resulting in an aluminous and magnesian residuum. With further hydrothermal circulation and incipient metamorphism, boron-rich fluids are expelled from metasedimentary and metavolcanic basement rocks and develop Mg-rich tourmalinites in the aluminous, magnesian host rocks. The tourmalinization process occurs over a range of metamorphic conditions and with fluids of variable activity of H₂O.     

The partitioning of Fe and Mg between olivine and carbonate and the stability of carbonate under mantle conditions

We have investigated the effect of Fe on the stabilities of carbonate (carb) in lherzolite assemblages by determining the partitioning of Fe and Mg between silicate (olivine; ol) and carbonates (magnesite, dolomite, magnesian calcite) at high pressures and temperatures. Fe enters olivine preferentially relative to magnesite and ordered dolomite, but Fe and Mg partition almost equally between disordered calcic carbonate and olivine. Measurement of K _d (X _Fe ^carb X _Mg ^ol /X _Fe ^ol X _Mg ^carb ) as a function of Fe/ Mg ratio indicates that Fe–Mg carbonates deviate only slightly from ideality. Using the regular solution parameter for olivine W _FeMg ^ol of 3.7±0.8 kJ/mol (Wiser and Wood 1991) we obtain for (FeMg)CO₃ a W _FeMg ^carb of 3.05±1.50 kJ/mol. The effect of Ca–Mg–Fe disordering is to raise K _d substantially enabling us to calculate W _CaMg ^carb -W _CaFe ^carb of 5.3±2.2 kJ/mol. The activity-composition relationships and partitioning data have been used to calculate the effect of Fe/Mg ratio on mantle decarbonation and exchange reactions. We find that carbonate (dolomite and magnesian calcite) is stable to slightly lower pressures (by 1 kbar) in mantle lherzolitic assemblages than in the CaO–MgO–SiO₂(CMS)–CO₂ system. The high pressure breakdown of dolomite + orthopyroxene to magnesite + clinopyroxene is displaced to higher pressures (by 2 kbar) in natural compositions relative to CMS. CO₂. We also find a stability field of magnesian calcite in lherzolite at 15–25 kbar and 750–1000°C.     

Thermodynamic properties,low-temperature heat-capacity anomalies,and single-crystal X-ray refinement of hydronium jarosite, (H<Subscript>3</Subscript>O)Fe<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>6</Subscript>

Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO₄ solutions. Single-crystal XRD refinement with R₁=0.0232 for the unique observed reflections (|F_o| > 4_F) and wR₂=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, V^o=160.11(4) cm³, and fractional positions for all atoms except the H in the H₃O groups. The chemical composition of this sample is described by the formula (H₃O)_0.91Fe_2.91(SO₄)₂[(OH)_5.64(H₂O)_0.18]. The enthalpy of formation (H^o_f) is –3694.5 ± 4.6 kJ mol^–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H₂O, and -MgSO₄. The entropy at standard temperature and pressure (S^o) is 438.9±0.7 J mol^–1 K^–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (C_p) data between 273 and 400 K were fitted to a Maier-Kelley polynomial C_p(T in K)=280.6 + 0.6149T–3199700T^–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol^–1. Speciation and activity calculations for Fe(III)–SO₄ solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the C_p data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in C_p, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H₃O)Fe₃(SO₄)₂(OH)₆ was estimated: G^o_f= –3226.4 ± 4.6 kJ mol^–1, H^o_f=–3770.2 ± 4.6 kJ mol^–1, S^o=448.2 ± 0.7 J mol^–1 K^–1, C_p (T in K)=287.2 + 0.6281T–3286000T^–2 (between 273 and 400 K).     

Major ion chemistry of groundwaters in the Bahar area, Hamadan, western Iran

Hydrochemical investigations were carried out in Bahar area, Hamadan, western Iran, to assess the chemical composition of groundwater. The area falls in a semi-arid type of climate. In this area, groundwater has been exploited over the past century mainly for irrigation and water supply. A total of 135 representative groundwater samples were collected from different wells to monitor the water chemistry of various ions. Chemical analysis of the groundwater shows that the mean concentration of the cations is of the order Ca²⁺>Mg ²⁺>Na⁺>K⁺, while that for anions is SO₄^2–>HCO₃^–>Cl^–>NO₃^–. Statistical analyses indicate positive correlation between the following pairs of parameters Cl^– and Mg ²⁺ (r=0.71), Cl^– and Na⁺ (r=0.76), HCO₃^– and Na⁺ (r=0.56), SO₄^2– and Mg²⁺ (r=0.76), SO₄^2– and Na⁺ (r=0.69). Water presents a large spatial variability of the chemical facies (Ca-HCO₃, Ca-SO₄, Mg-HCO₃, Mg-SO₄, Na-HCO₃) which is in relation to their interaction with the geological formations of the basin (carbonates, dolomite and various silicates) and evaporation. The hydrochemical types Ca-HCO₃ and Ca-SO₄ dominate the largest part of the studied area. The dissolution of halite, calcite, dolomite, and gypsum explains part of the contained Na ⁺, Ca²⁺, Mg²⁺, Cl^–, SO₄^2– and HCO₃^–, but other processes, such as cation exchange and weathering of aluminosilicates also contribute to the water composition.     

Modelling ion composition in seepage water from a column experiment with an open cut coal mine sediment

Secondary reactions occurring in pyrite-containing sediments from open cut coal mines are complex and not fully understood. In this study, the changes in seepage water composition in a column experiment with a sediment containing pyrite (5.6 g kg⁻¹) were evaluated using a chemical equilibrium model. A column experiment with artificial irrigation (730 mm water yr⁻¹) was carried out for 2 yr with a sediment from the open pit mine Garzweiler, Germany, at the Institute of Applied Geology. Tracer (LiCl) was added to the sediment. Seepage water composition at 52 cm depth was sampled weekly. Redox potential and the water potential were also recorded weekly. Sulphate and Fe(II) were the dominant ions in the seepage water with concentration maxima of 500 and 350 mmol l⁻¹ after 50 days (0.7 pore volumes (PV)). Minimum pH values were around 0.8 after 100 days (1.4 PV), but increased subsequently and reached 2.4 after 700 days (9.5 PV). Ion activity product calculations indicated the intermediate formation of gypsum (19th–480th day of the experiment). Solutions were undersaturated with respect to alunite, jarosite, jurbanite, schwertmannite, melanterite, gibbsite and goethite during the whole experiment. The model of coupled equilibria which included inorganic complexation, precipitation/dissolution of gypsum and multiple cation exchange was tested. Pyrite oxidation and pH-dependent silicate weathering were considered using simple input functions. Transport was modelled using a field capacity cascade submodel. Model results showed satisfactory agreement with measured values for pH and concentrations of SO₄, Fe, Mg, Ca and Al. Correlation coefficients lay between 0.7 and 0.9 and linear regression coefficients (modelled against measured) were 1.5 (Ca), 1.0 (Fe, SO₄), 0.8 (Mg), 0.7 (pH) and 0.6 (Al). The results showed that the protons produced during pyrite oxidation (94 mmol_c H⁺ kg⁻¹) were mainly released into seepage water (as HSO₄⁻ and H⁺). Cation exchange reactions buffered 20 mmol_c of H⁺ kg⁻¹ sediment, and Al released by silicate weathering accounted for 3.6 mmol_c H⁺ kg⁻¹. Modelling was useful to further understand the significance of different pH buffer reactions.     

Calcite solubility and speciation in supercritical NaCl-HCl aqueous fluids

The solubility of calcite in NaCl-H₂O and in HCl-H₂O fluids was measured using an extraction-quench hydrothermal apparatus. Experiments were conducted at 2 kbar, between 400° C and 600° C. Measurements in NaCl-H₂O were conducted in two ways: 1) at constant pressure and NaCl concentration, as a function of temperature; and 2) at constant pressure and temperature, as a function of NaCl concentration. In both the NaCl-H₂O and the HCl-H₂O systems, the solubility of calcite increases with increasing chlorine concentrations. For example, the log calcium molality in equilibrium with calcite increases from –3.75 at 2 kbar and 500° C, in pure H₂O to –3.10 at 2 kbar and 500° C at log NaCl molality=–1.67. At fixed pressure and NaCl molality, the solubility of calcite is almost constant from 400° C to 550° C, but increases somewhat at higher temperatures. The results can be used to determine the dominant calcium species in the experimental solutions as a function of NaCl concentration and to obtain values for the second dissociation constant of CaCl_2(aq). At 2 kbar, 400° C, 500° C, and 600° C, we calculate values for the log of the dissociation constant of CaCl⁺ of –2.1, –3.2, and –4.3, respectively. The 400° C and 500° C values are consistent with those obtained by Frantz and Marshall (1982) using electrical conductance techniques. However, our 600° C value is 0.8 log units higher than that reported by Frantz and Marshall. The calcite solubilities in the NaCl-H₂O and HCl-H₂O systems are inconsistent with the solubilities of calcite in pure H₂O reported by Walther and Long (1986). They are, however, consistent with the measurements of calcite solubilities in pure H₂O presented in this study. These results allow for the calculation of the solubilities of calcium silicates and carbonates in fluids that contain CO₂ and NaCl.     

Fibroferrite: A mineral with a {Fe(OH) (H2O)2SO4} spiral chain and its relationship to Fe(OH)SO4, butlerite and parabutlerite

Summary The mineral fibroferrite has the chemical formula Fe(OH)SO₄·xH₂O; the value forx has not been definitely settled, but as a rule it is found to be near five. Several symmetries are given in the literature.A sample from Saint Felix de Paillères, France, proved to be rhombohedral with space group R3; lattice constants for the hexagonal cell area=24.176,c=7.656 Å. As calculated from the experimental density (=1.95 g·cm^–3)Z=18 for this cell. Intensities were collected on an automated X-ray diffractometer from a thin fiber extended along [00.1]. The structure was determined by Patterson and Fourier methods. Least squares refinement with 818 observed reflections resulted inR=0.076.The structure contains hydroxo-bridged {Fe(OH)(H₂O)₂SO₄} spiral chains built of [Fe(OH)₂(H₂O)₂O₂] octahedra and SO₄ tetrahedra. Hydrogen bonds provide connections between these chains. The spiral chains are a stereoisomer variant of the hydroxo-bridged linear chains of Fe(OH)SO₄, butlerite and parabutlerite. A comparison of these compounds is givenm to understand the relationship between the structure and their water content.

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Fibroferrit: Ein Mineral mit einer {Fe(OH)(H₂O)₂SO₄} Spiralkette und seine Beziehung zu Fe(OH)SO₄, Butlerit und Parabutlerit

Zusammenfassung Das Mineral Fibroferrit hat die chemische Formel Fe(OH)SO₄·xH₂O; der Wert furx scheint nicht endgültig geklärt zu sein, liegt aber meist nahe 5. Verschiedene Symmetrien werden in der Literatur angegeben.Eine Probe von Saint Felix de Paillères, Frankreich, erwies sich als rhomboedrisch mit der Raumgruppe R3; die Gitterkonstanten der hexagonalen Zelle sinda=24,176,c=7,656 Å. Die experimentelle Bestimmung der Dichte (=1,95 g·cm^–3) führt für diese Zelle zuZ=18. Von einer nach [00.1] gestreckten dünnen Faser wurden die Intensitäten auf einem automatischen Röntgendiffraktometer gesammelt. Die Struktur wurde mit Patterson-und Fouriersynthesen gelöst. Eine Verfeinerung nach der Methode der kleinsten Quadrate führte für 818 beobachtete Reflexe aufR=0,076.Die Struktur enthält durch Hydroxil-Gruppen verknüpfte {Fe(OH)(H₂O)₂SO₄}-Spiralketten, die aus [Fe(OH)₂(H₂O)₂O₂]-Oktaedern und SO₄-Tetraedern aufgebaut sind. Die Spiralketten von Fibroferrit sind eine stereoisomere Variante der annähernd linearen Fe–O–S-Ketten von Fe(OH)SO₄, Butlerit und Parabutlerit. Diese Verbindungen werden mit Fibroferrit verglichen, um Beziehungen zwischen Struktur und Wassergehalt zu verstehen.

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Paper presented at the Fifth European Crystallography Meeting, Copenhagen, Denmark 1979.     

Internally consistent gahnitic spinel-cordierite-garnet equilibria in the FMASHZn system: geothermobarometry and applications

The equilibrium (Mg, Fe, Zn)₃Al₂Si₃O₁₂+2Al₂SiO₅=3(Mg, Fe, Zn)Al₂O₄+5SiO₂ garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100^*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO –MgO–Al₂O₃–SiO₂–ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe–Mg–Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinel's stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths.     

Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal lowlands

We examined the transformations of Fe and S associated with schwertmannite (Fe₈O₈(OH)₆SO₄) reduction in acidified coastal lowlands. This was achieved by conducting a 91 day diffusive-flux column experiment, which involved waterlogging of natural schwertmannite- and organic-rich soil material. This experiment was complemented by short-term batch experiments utilizing synthetic schwertmannite. Waterlogging readily induced bacterial reduction of schwertmannite-derived Fe(III), producing abundant pore-water Fe^II, SO₄ and alkalinity. Production of alkalinity increased pH from pH 3.4 to pH ∼6.5 within the initial 14 days, facilitating the precipitation of siderite (FeCO₃). Interactions between schwertmannite and Fe^II at pH ∼6.5 were found, for the first time, to catalyse the transformation of schwertmannite to goethite (αFeOOH). Thermodynamic calculations indicate that this Fe^II-catalysed transformation shifted the biogeochemical regime from an initial dominance of Fe(III)-reduction to a subsequent co-occurrence of both Fe(III)- and SO₄-reduction. This lead firstly to the formation of elemental S via H₂S oxidation by goethite, and later also to formation of nanoparticulate mackinawite (FeS) via H₂S precipitation with Fe^II. Pyrite (FeS₂) was a quantitatively insignificant product of reductive Fe and S mineralization. This study provides important new insights into Fe and S geochemistry in settings where schwertmannite is subjected to reducing conditions.     

A new view on gold speciation in sulfur-bearing hydrothermal fluids from in situ X-ray absorption spectroscopy and quantum-chemical modeling

Despite the common belief that Au^I complexes with hydrogen sulfide ligands (H₂S/HS⁻) are the major carriers of gold in natural hydrothermal fluids, their identity, structure and stability are still subjects of debate. Here we present the first in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structure of aqueous Au^I–S complexes at temperatures and pressures (T–P) typical of natural sulfur-rich ore-forming fluids. The solubility of native gold and the local atomic structure around the dissolved metal in S–NaOH–Na₂SO₄–H₂SO₄ aqueous solutions were characterized at temperatures 200–450 °C and pressures 300–600 bar using an X-ray cell that allows simultaneous measurement of the absolute concentration of the absorbing atom (Au) and its local atomic environment in the fluid phase. Structural and solubility data obtained from XAFS spectra, combined with quantum-chemical calculations of species geometries, show that gold bis(hydrogensulfide) Au(HS)₂⁻ is the dominant Au species in neutral-to-basic solutions (5.5  pH  8.5; H₂O–S–NaOH) over a wide range of sulfur concentrations (0.2 < ΣS < 3.6 mol/kg), in agreement with previous solubility studies. Our results provide the first direct determination of this species structure, in which two sulfur atoms are in a linear geometry around Au^I at an average distance of 2.29 ± 0.01 Å. At acidic conditions (1.5  pH  5.0; H₂O–S–Na₂SO₄–H₂SO₄), the Au atomic environment determined by XAFS is similar to that in neutral solutions. These findings, together with measured high Au solubilities, are inconsistent with the predominance of the gold hydrogensulfide Au(HS)⁰ complex suggested by recent solubility studies. Our spectroscopic data and quantum-chemical calculations imply the formation of species composed of linear S–Au–S moieties, like the neutral [H₂S–Au–SH] complex. This species may account for the elevated Au solubilities in acidic fluids and vapors with H₂S concentrations higher than 0.1–0.2 mol/kg. However, because of the complex sulfur speciation in acidic solutions that involves sulfite, thiosulfate and polysulfide species, the formation of Au^I complexes with these ligands (e.g., AuHS(SO₂)⁰, Au(HS₂O₃)₂⁻, Au(HS_n)₂⁻) cannot be ruled out. The existence of such species may significantly enhance Au transport by high T–P acidic ore-forming fluids and vapors, responsible for the formation of a major part of the gold resources on Earth.     

Chemistry and sulfur isotopic composition of precipitation at Bologna, Italy

Individual and monthly precipitation samples from the polluted atmosphere of Bologna (Emilia-Romagna province) were collected during March 1996 to May 1997 and analyzed for major ions in solution and S isotopes in dissolved SO₄.Weighted mean enrichment factors relative to seawater are found to be 1.0 for Na, 15.2 for K, 105 for Ca, 3.3 for Mg, 17.3 for SO₄ and 663 for HCO⁻₃. Very good positive correlations are observed for the Ca²⁺–Mg²⁺–HCO⁻₃–SO²⁻₄–NO⁻₃ system, indicating that dissolution of Ca (±Mg)-carbonate particles by H₂SO₄ and HNO₃ from combustion of oil and gas is a major process controlling the chemical composition of rain and snow. Na⁺ and Cl⁻ in monthly precipitation derive essentially from sea spray, but the contribution of Na⁺ from continental sources is appreciable in a number of individual rains. NH⁺₄ appears to be on average more abundant in spring and summer precipitation, its main sources being microbial activity in soils and application of fertilizers. K⁺ is probably of continental origin from soil dust.The S isotopic composition of SO₄ is systematically positive, with mean δ³⁴S values of +3.2±1.6‰ (n=40) in individual precipitation and +2.8±1.4‰ (n=12) in monthly precipitation. These isotopic compositions are interpreted in terms of a dominant contribution of S from anthropogenic emissions and subordinate contributions from biogenic and marine sources. Pollutant SO₄ is estimated to have a δ³⁴S value in the range +2.5 to +4.5‰, whereas a distinctive δ³⁴S of −4.5‰ or lower indicates SO₄ from oxidation of biogenic gases.The isotopic and chemical compositions of SO₄ do not depend on wind direction, thus testifying to a mostly local source for pollutant S in the Bologna atmosphere.     

Integrated hydrochemical assessment of the Quaternary alluvial aquifer of the Guadalquivir River, southern Spain

The alluvial aquifer of the Guadalquivir River comprises shallow Quaternary deposits located in the central-eastern part of the Province of Jaén in southern Spain, where groundwater resources are used mainly for crop irrigation in an important agricultural area. In order to establish the baseline hydrochemical conditions and processes determining the groundwater quality, groundwater and river water samples were collected as part of an integrated investigation that coupled multivariate statistical analysis with hydrochemical methods to identify and interpret the groundwater chemistry of the aquifer system. Three main hydrochemical types (Mg–Ca–HCO₃, Ca–Mg–SO₄–HCO₃–Cl and Na–Ca–Mg–Cl–SO₄) were identified. Further interpretation, using R-mode principal components analysis (PCA) conducted with 13 hydrochemical variables, identified two principal components which explain ⅔ of the variance in the original data. In combination with the hydrochemical interpretation, mineralogical analyses of the aquifer sediment together with inverse geochemical modelling using NETPATH showed that dedolomitization (calcite precipitation and dolomite dissolution driven by gypsum dissolution) is the principal hydrochemical process controlling the regional groundwater chemistry. Other processes such as silicate weathering, ion exchange, mixing between river water and groundwater, and agricultural practices also affect the groundwater chemistry.     

Solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions

An experimental study of the solubility of Pt and Pd sulfides and Au metal in aqueous bisulfide solutions was conducted at temperatures from 200° to 350 °C and at saturated vapor pressure. A 500-mL Bridgemantype pressure vessel constructed of titanium, and equipped with a motor-driven magnetic stirrer was employed. The pH and the oxidation state were buffered by the coexistence of H₂S/HS^–/SO _inf4 ^sup2– . The pH at temperature was calculated to be in the range 5.91–9.43, and S was 0.3–2.2 m. Under the experimental conditions, the measured solubility of gold is about two to three orders of magnitude greater than that of either platinum and palladium, and the measured solubility of platinum is, in general, approximately equal to that of palladium, in molal units. The solubilities are found to be in the range: platinum 4–800 ppb, palladium 1–400 ppb, and gold 2–300 ppm. The solubility data can be modeled adequately using the following reactions: Au+H₂S+HH^–=Au(HS) ₂ ^– +1/2H₂ (K₁₄); PtS+HS^–+H⁺=Pt (HS) ₂ ⁰ (K₁₅); PdS+HS^–+H⁺=Pd (HS) ₂ ⁰ (K₁₆); PtS₂+H₂=Pt (HS) ₂ ⁰ (K₂₁).With equilibrium constants determined as follows (errors represent two standard deviations): Preliminary measurements of the solubilities of metallic Pt, Pd and Au as hydroxide complexes were also conducted using a second titanium pressure vessel, at temperatures of 200° to 350 °C and vapor saturation pressure, with pH and the oxidation state controlled or buffered by adding known amounts of NaOH and H₂ gas. The concentration of NaOH was in the range 0.01–1.3 m, and the partial pressure of H₂ at 200 °C was 62–275 bars, initially. Under the temperature and pressure conditions of these experiments, the solubility of platinum in 1 m NaOH solution is less than 100 ppb, that of palladium is less than 10 ppb and that of gold is less than 0.2 ppm; and in 0.01 m NaOH solutions, both Pt and Pd solubilities are less than 1 ppb. These data indicate that the contributions of hydroxide complexes to the total solubilities in the bisulfide runs, where the pH was in the range of 5.9–9.4, are negligible. The concentrations of both Pt and Pd as bisulfide complexes in the Salton Sea geothermal system predicted using the stability constants determined in this work agree very well with those values measured by McKibben et al. (1990). This calculation strongly suggests that the PGE are transported in moderately reducing, near neutral hydrothermal fluids as bisulfide complexes, as is gold. However, the much lower maximum solubility of the PGE relative to gold severely constrains models of re genesis, and may explain the relative rarity of hydrothermal PGE deposits compared to the relative abundance of hydrothermal Au deposits.