Solubility relationships of aluminum and iron minerals associated with acid mine drainage

The ability to properly manage the oxidation of pyritic minerals and associated acid mine drainage is dependent upon understanding the chemistry of the disposal environment. One accepted disposal method is placing pyritic-containing materials in the groundwater environment. The objective of this study was to examine solubility relationships of Al and Fe minerals associated with pyritic waste disposed in a low leaching aerobic saturated environment. Two eastern oil shales were used in this oxidizing equilibration study, a New Albany Shale (unweathered, 4.6 percent pyrite), and a Chattanooga Shale (weathered, 1.5 percent pyrite). Oil shale samples were equilibrated with distilled-deionized water from 1 to 180 d with a 1∶1 solid-to-solution ratio. The suspensions were filtered and the clear filtrates were analyzed for total cations and anions. Ion activities were calculated from total concentrations. Below pH 6.0, depending upon SO ₄ ²⁻ activity, Al³⁺ solubility was controlled by AlOHSO₄ (solid phase) for both shales. Initially, Al³⁺ solubility for the New Albany Shale showed equilibrium with amorphous Al(OH)₃. The pH decreased with time, and Al³⁺ solubility approached equilibrium with AlOHSO_4(s). Below pH 6.0, Fe³⁺ solubility appeared to be regulated by a basic iron sulfate solid phase with the stoichiometric composition of FeOHSO_4(s). The results of this study indicate that below pH 6.0, Al³⁺ solubilities, are limited by basic Al and Fe sulfate solid phases (AlOHSO_4(s) and FeHSO_4(s)). The results from this study further indicate that the acidity in oil shale waters is produced from the hydrolysis of Al³⁺ and Fe³⁺ activities in solution. These results indicate a fundamental change in the stoichiometric equations used to predict acidity from iron sulfide oxidation. The results of this study also indicate that water quality predictions associated with acid mine drainage can be based on fundamental thermodynamic relationships. As a result, waste management decisions can be based on waste-specific/site-specific test methods.     

Iron monosulfide formation and oxidation in drain-bottom sediments of an acid sulfate soil environment

Iron monosulfide formation and oxidation processes were studied in the extensively drained acid sulfate soil environment of the Tweed River floodplain in eastern Australia. Porewater profiles of pH, Eh, SO₄²⁻, Fe²⁺, Fe³⁺, Cl⁻, HCO₃⁻, and metals (Cd, Co, Cr, Cu, Ni, Pb and Zn) were obtained using in situ dialysis membrane samplers (`peepers'). Concentrations of acid volatile S (AVS), pyrite, total S, reactive Fe, total and organic C, simultaneously extracted metals (SEMs) and total elemental composition by X-ray fluorescence, were determined on sediment samples. The oxidation of pyrite in the surrounding landscape provides a source of acidity, Fe, Al, SO₄ and metals, which are exported into the drainage system where they accumulate in the sediments and porewaters. Negative porewater concentration gradients of SO₄²⁻ and Fe²⁺, and large AVS concentrations in the sediments, indicate Fe monosulfides form rapidly under reducing conditions and consume acidity and metals. Oxidation of the sediments during previous drought episodes has resulted in the conversion of monosulfides and pyrite to oxidised Fe minerals and the release of acidity, SO₄²⁻, Fe³⁺, and metals to the surface waters. These formation and oxidation cycles show that Fe monosulfides play an important role in controlling water quality in the drainage system.     

Combinatorial analysis on spatial information statistics for the karst water environment in Guiyang, China

The karst groundwater system is extremely vulnerable and easily contaminated by human activities.To understand the spatial distribution of contaminants in the groundwater of karst urban areas and contributors to the contamination,this paper employs the spatial information statistics analysis theory and method to analyze the karst groundwater environment in Guiyang City.Based on the karst ground water quality data detected in 61 detection points of the research area in the last three years,we made Kriging evaluation isoline map with some ions in the karst groundwater,such as SO4 2-,Fe 3+,Mn 2+and F -,analyzed and evaluated the spatial distribution,extension and variation of four types of ions on the basis of this isoline map.The results of the analysis show that the anomaly areas of SO4 2-,Fe 3+,Mn 2+,Fand other ions are mainly located in Baba’ao,Mawangmiao and Sanqiao in northwestern Gui- yang City as well as in its downtown area by reasons of the original non-point source pollution and the contamination caused by human activities(industrial and domestic pollution).     

Environmental geochemistry and quality assessment of mine water of Jharia coalfield,India

A long mining history and unscientific exploitation of Jharia coalfield caused many environmental problems including water resource depletion and contamination. A geochemical study of mine water in the Jharia coalfield has been undertaken to assess its quality and suitability for domestic, industrial and irrigation uses. For this purpose, 92 mine water samples collected from different mining areas of Jharia coalfield were analysed for pH, electrical conductivity (EC), major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺), anions (F⁻, Cl⁻, HCO₃ ⁻, SO₄ ²⁻, NO₃ ⁻), dissolved silica (H₄SiO₄) and trace metals. The pH of the analysed mine water samples varied from 6.2 to 8.6, indicating mildly acidic to alkaline nature. Concentration of TDS varied from 437 to 1,593 mg L⁻¹ and spatial differences in TDS values reflect the variation in lithology, surface activities and hydrological regime prevailing in the region. SO₄ ²⁻ and HCO₃ ⁻ are dominant in the anion and Mg²⁺ and Ca²⁺ in the cation chemistry of mine water. High concentrations of SO₄ ²⁻ in the mine water of the area are attributed to the oxidative weathering of pyrites. Ca–Mg–SO₄ and Ca–Mg–HCO₃ are the dominant hydrochemical facies. The drinking water quality assessment indicates that number of mine water samples have high TDS, total hardness and SO₄ ²⁻ concentrations and needs treatment before its utilization. Concentrations of some trace metals (Fe, Mn, Ni, Pb) were also found to be above the desirable levels recommended for drinking water. The mine water is good to permissible quality and suitable for irrigation in most cases. However, higher salinity, residual sodium carbonate and Mg-ratio restrict its suitability for irrigation at some sites.     

Thermodynamics of arsenates,selenites, and sulfates in the oxidation zone of sulfide ores: I. Thermodynamic constants at ambient conditions

Understanding and deciphering processes proceeding near the surface are among the urgent tasks of contemporary mineralogy and geochemistry, which are especially important for resolving ecological challenges and developing principles of rational environmental management. The paper presents systematized data published on thermodynamics of minerals (arsenates, sulfates, selenites, and selenates), which are formed in the weathering zone of sulfide ores, and determines approaches to quantitative physicochemical modeling of their formation conditions. Diagrams of phase and chemical equilibria (Eh-pH, diagrams of solubility) of the subsystems of the model system Fe-Cu-Zn-Pb-Co-Ni-As-Se-S-H₂O (Fe²⁺, Fe³⁺, Cu²⁺, Zn²⁺, Pb²⁺, Ni²⁺, Co²⁺, H⁺//SeO₃²⁻, SeO₄²⁻, AsO₄³⁻, SO₄²⁻, OH⁻-H₂O) are used as a thermodynamic basis for modeling mineral-forming processes in the weathering zone of ore deposits. Seventy-two arsenates, about 70 sulfates, and 7 selenites and selenates have been identified in the framework of this system. The available published values of standard thermodynamic functions of the formation of minerals and chemical compounds are given, as well as the Pitzer equation parameters to describe the sulfate systems, which are substantially specific due to the high solubility of their components.     

金在黄铁矿表面沉淀机理的实验研究

为了研究金在黄铁矿表面沉淀的机理,于室温、常压,在氯化物溶液中进行了黄铁矿粉末吸附金的实验。在不同pH的溶液中,黄铁矿均可吸附金,而且pH值明显地影响吸附速率。扫描电镜观察表明,反应后黄铁矿粒表面有金晶体形成。XPS研究得知,黄铁矿光片与含金氯化物溶液反应后表面有A⁰存在;硫在反应初期为S⁰、S₂O₃^2-,随后转变为SO₄^2-,而铁成为Fe³⁺.黄铁矿中的Fe²⁺和S₂^2-是溶液中金的还原剂。金在黄铁矿表面沉淀可能涉及吸附、还原和晶体生长等过程。     

Contamination risk assessment of fresh groundwater using the distribution and chemical speciation of some potentially toxic elements in Calabar (southern Nigeria)

The computer program PHREEQC was used to determined the distribution, chemical speciation and mineral saturation indices in a fresh groundwater environment with limited mining activities in the adjoining areas. The aim was mainly to determine the potential risk of a coastal plain aquifer contamination by some potentially toxic elements. The results show that the elements Ba, Cd, Cu, Fe, Mn, Ni, Rb, Sr, and Zn are distributed as free metal ions. Arsenic is in the neutral form of H₃AsO₃ ^o, while three species of aluminium [Al³⁺, AlOH₂, Al(OH)₂ ⁺] dominate. The major species of uranium include UO₂CO₃, UO₂²⁺+, UO₂⁺, and UO₂OH⁺, respectively, in order of abundance. The groundwater is saturated with respect to alunite [KAl₃ (SO₄)₂ (OH)₆], basaluminite [Al₄ (OH)₁₀ SO₄], boehmite [Al(OH)], Cu metal (Cu), cuprous ferrite (CuFeO₂), diaspore [AlO(OH)], gibbsite [Al(OH)₃], goethite (FeOOH), hematite (Fe₂O₃), magnetite (Fe₃O₄) and uraninite (UO₂). Most of the species are not mobile under the prevailing pH (3.3 to 5.9) and Eh (7 to 158 mV) conditions. The mobile ones are very low in concentration and will be immobilized by precipitation of mineral phases. The study concludes that presently these species do not pose any risk to the aquifer.     

Activity variations attending tungsten skarn formation,Pine Creek,California

An integrated geochemical analysis of the well-exposed Pine Creek, California tungsten skarn deposit has been undertaken to evaluate changes in chemical gradients across various lithologies. Thermodynamic calculations using available experimental and thermodynamic data allow limits to be assigned to the activities of important chemical components in the metasomatic environment. Quantifiable changes in “non-volatile” component activites (CaO, MgO, Al₂O₃, Fe₂O₃, WO₃) and in fugacities (O₂, F₂) have been traced across the system. The activities of Al₂O₃, Fe₂O₃ and WO₃ generally increase from the marble (<10², <10⁻⁶, <10⁻⁵ respectively), through the outer skarn zone and into the massive garnet skarn (10^−1.7±0.3, 10^−3.4±0.4, 10^−4.8±0.1) While CaO and MgO activities decrease for the same traverse from 10⁻⁵ and 10^−2.1±1 respectively, to <10^−5.7 and <10⁻³. Calculated oxygen fugacities are 10^−23.5+1.0 at T=800 K (527° C), about one log unit below QFM, and more reducing than that required by Mt-Py-Po. The high variance of the garnet-pyroxene-quartz assemblages adds sufficient uncertainty to the calculated activities for individual specimens that only the large-scale trends survive the small-scale scatter. None of the chemical variables emerge as major independent or controlling factors for the mineralogy or phase compositions. Changes in the activity of one component may be offset by compensatory changes in another resulting in an environment that, while different from Pine Creek, could still host scheelite mineralization. Mass balance calculations indicate that the exposed endoskarn cannot have supplied the necessary chemical components to convert the country rock to skarn.     

Effects of Fe and Al incorporations on the bridgmanite–postperovskite coexistence domain

The postperovskite phase transition of Fe and Al-bearing MgSiO₃ bridgmanite, the most aboundant mineral in the Earth's lower mantle, is believed to be a key to understanding seismological observations in the D″ layer, e.g., the discontinuous changes in seismic wave velocities. Experimentally reported phase transition boundaries of Fe and Al-bearing bridgmanite are currently largely controversial and generally suggest wide two-phase coexistence domains. Theoretical simulations ignoring temperature effects cannot evaluate correctly two-phase coexistence domains under high-temperature. We show high-pressure and high-temperature phase transition boundaries for various compositions with geophysically relevant impurities of Fe²⁺SiO₃, Fe³⁺Fe³⁺O₃, Fe³⁺Al³⁺O₃, and Al³⁺Al³⁺O₃ derived from the ab initio finite-temperature free energies calculated combining the internally consistent LSDA + U method and a lattice dynamics approach. We found that at ～ 2500 K, incorporations accompanied by Fe³⁺ expand the two-phase coexistence domains distinctly, implying that D″ seismic discontinuities likely arise from the phase transition of Fe²⁺-bearing bridgmanite.     

O and S isotopic composition of dissolved and attached oxidation products of pyrite by Acidithiobacillus ferrooxidans: Comparison with abiotic oxidations

The acidophilic iron-oxidizing bacterium, Acidithiobacillus ferrooxidans, plays a part in the pyrite oxidation process and has been widely studied in order to determine the kinetics of the reactions and the isotopic composition of dissolved product sulphates, but the details of the oxidation processes at the surface of pyrite are still poorly known. In this study, oxygen and sulphur isotopic compositions (δ¹⁸O and δ³⁴S) were analyzed for dissolved sulphates and water from experimental aerobic acidic (pH < 2) pyrite oxidation by A. ferrooxidans. The oxidation products attached to the pyrite surfaces were studied for their morphology (SEM), their chemistry (Raman spectroscopy) and for their δ¹⁸O (ion microprobe). They were compared to abiotically (Fe³⁺, H₂O₂, O₂) oxidized pyrite surface compounds in order to constrain the oxidation pathways and to look for the existence of potential biosignatures for this system.The pyrite dissolution evolved from non-stoichiometric (during the first days) to stoichiometric (with increasing time) resulting in dissolved sulphates having distinct δ¹⁸O (e.g. +11.0‰ and −2.0‰, respectively) and δ³⁴S (+4.5‰ and +2.8‰, respectively) values. The “oxidation layer” at the surface of pyrite is complex and made of iron oxides, sulphate, polysulphide, elemental sulphur and polythionates. Bio- and Fe³⁺-oxidation favour the development of monophased micrometric bumps made of hematite or sulphate while other abiotic oxidation processes result in more variable oxidation products. The δ¹⁸O of these oxidation products at the surface of oxidized pyrites are strongly variable (from ≈−40‰ to ≈+30‰) for all experiments.Isotopic fractionation between sulphates and pyrite, Δ³⁴S_SO4-pyrite, is equal to −1.3‰ and +0.4‰ for sulphates formed by stoichiometric and non-stoichiometric processes, respectively. These two values likely reflect either a S-S or a Fe-S bond breaking process. The Δ¹⁸O_SO4-H2O and Δ¹⁸O_SO4-O2 are estimated to be ≈+16‰ and ≈−25‰, respectively. These values are higher than previously published data and may reflect biological effects. The large δ¹⁸O heterogeneity measured at the surfaces of oxidized pyrites, whatever the oxidant, may be related (i) to the existence of local surface environments isolated from the solution in which the oxidation processes are different and (ii) to the stabilization at the pyrite surface of reaction intermediates that are not in isotopic equilibrium with the solution. Though the oxygen isotopic composition of surface oxidation products cannot be taken as a direct biosignature, the combined morphological, chemical and isotopic characterization of the surfaces of oxidized pyrites may furnish clues about a biological activity on a mineral surface.     

The threshold value of epikarst runoff in forest karst mountain area

Hydrochemical investigations were carried out in Damagh area, Hamadan, western Iran, to assess chemical composition of groundwater. Forty representative groundwater samples were collected from different wells to monitor the water chemistry of various ions. Chemical analysis of the groundwater showed that the mean concentration of the cations is in the order Na⁺ > Ca²⁺ > Mg²⁺ > K⁺, while that for anions was HCO₃⁻ > Cl⁻ > SO₄^{2 −} > NO₃⁻. All of the investigated groundwaters present two different chemical facies (Ca–HCO₃ and Na–HCO₃) which is in relation with their interaction with the geological formations of the basin, cation exchange between groundwater and clay minerals and anthropogenic activities. The principal component analysis (PCA) performed on groundwater identified three principal components controlling their variability in groundwater. Electrical conductivity, Mg²⁺, Na⁺, SO₄²⁻, and Cl⁻ content were associated in the same component (PC1) (salinity), determined principally by anthropogenic activities. The pH, CO₃^{2 −}, HCO₃⁻, and Ca²⁺ (PC2) content were related to the geogenic factor. Finally, the NO₃⁻, Cl⁻ and K⁺ (PC3) were controlled by anthropogenic activity as a consequence of inorganic fertilizers.     

Influence of pH on mineral speciation in a bioreactor simulating acid mine drainage

Mineral precipitates formed under conditions representative of acid mine drainage were prepared by oxidizing 0.1 M FeS0₄ · 7H₂0 solutions at 24°C and pH 2.3, 2.6, 3.0, 3.3 and 3.6 using a bioreactor and a strain ofThiobacillus ferrooxidans. The oxidation of dissolved Fe²⁺ was monitored colorimetrically and was completed within 90 to 120 h at all pHs. Schwertmannite, Fe₈O₈(OH)₆SO₄, was a major component of the precipitates and was the only phase formed at pH 3.0. Jarosite, (H,Na,K)Fe₃(OH)₆(SO₄)₂, increased in abundance with decreasing pH whereas goethite, α-FeOOH, appeared at pH 3.3 and 3.6. A similar relationship between pH and mineralogy has been reported in natural specimens of mine drainage ochres.     

Geochemical Styudy of Gold and Arsenic Mineralization of the Carlin-Type Gold Deposits,Qinling Region,China

Element geochemistry of gold arsenic and mineralogical features of their sulfides in the Carlin-type gold depostis of the Qinling region are discussed in this paper.The initial contents of ore-forming elements such as glod and arsenic are high the ore-bearing rock series in the Qinling region.Furthermore,both the metals are concentrated mainly in the diagenetic pyrite.Study on the mineralogy of arsenic-bearing sulfide minerals in the ores demonstrated that there is a poistive correlation between gold and arsenic in the sulfide minerals.Available evidence suggests that gold in the As-bearing sulfide minerals in likely to be presented as a charge species(Au ),and it is most possible for it to replace the exxcess arsenic at the site of iron and war probably deposited together with arsenic as solid in the sulfide minerals. Pyrite is composed of(Aux^3 ,Fe1-2^2 )([AsS]x^3-[S2]1-x^2-),and arenopyrite of (Aux^3 ,Fe1-x^3 )([AsS]x^3-[AsS2]1-x^3-).The occurrence of glod in the As-sulfied minerals from the Carlin-type gold depostis in the Qinling region has been confirmed by electron probe and transmission electron microscopic studies.The results show that gold was probably depostied together with arsenicas coupled solid solutions in sulfide minerals in the early stage of mineralization.Metallogenic chemical reactions concerning gold deposition in the Carlin-type As-rich gold deposits would involve oxidation of glod and concurrent reduction of arsenic.Later,the deposited gold as solid was remobilized and redistributed as exsolutions,as a result of increasing hydrothermal alteration and crystallization,and decreasing resistance to refractoriness of the host minerals.Gold occurs as sub-microscopic grains(ranging from 0.04tp 0.16μm in diameter)of native gold along micro factures in and crystalline grains of the sulfiedes.     

Geochemical models of the impact of acidic groundwater and evaporative sulfate salts on Boulder Creek at Iron Mountain,California

During dry season baseflow conditions approximately 20% of the flow in Boulder Creek is comprised of acidic metals-bearing groundwater. Significant amounts of efflorescent salts accumulate around intermittent seeps and surface streams as a result of evaporation of acid rock drainage. Those salts include the Fe-sulfates — rhomboclase ((H₃O)Fe³⁺(SO₄)₂·3H₂O), ferricopiapite (Fe³⁺₅(SO₄)₆O(OH)·20H₂O), and bilinite (Fe²⁺Fe₂³⁺(SO₄)₄·22H₂O); Al-sulfates — alunogen (Al₂(SO₄)₃·17H₂O) and kalinite (KAl(SO₄)₂·11H₂O); and Ca- and Mg-sulfates — gypsum (CaSO₄·2H₂O), and hexahydrite (MgSO₄·6H₂O). The dissolution of evaporative sulfate salt accumulations during the first major storm of the wet season at Iron Mountain produces a characteristic hydrogeochemical response (so-called “rinse-out”) in surface waters that is subdued in later storms. Geochemical modeling shows that the solutes from relatively minor amounts of dissolved sulfate salts will maintain the pH of surface streams near 3.0 during a rainstorm. On a weight basis, Fe-sulfate salts are capable of producing more acidity than Al- or Mg-sulfate salts. The primary mechanism for the production of acidity from salts involves the hydrolysis of the dissolved dissolved metals, especially Fe³⁺. In addition to the lowering of pH values and providing dissolved Fe and Al to surface streams, the soluble salts appear to be a significant source of dissolved Cu, Zn, and other metals during the first significant storm of the season.     

Surface oxidation of pyrite under ambient atmospheric and aqueous (pH = 2 to 10) conditions: electronic structure and mineralogy from X-ray absorption spectroscopy

The nature of the surface oxidation phase on pyrite, FeS₂, reacted in aqueous electrolytes at pH = 2 to 10 and with air under ambient atmospheric conditions was studied using synchrotron-based oxygen K edge, sulfur L_III edge, and iron L_II,III edge X-ray absorption spectroscopy. We demonstrate that O K edge X-ray absorption spectra provide a sensitive probe of sulfide surface oxidation that is complementary to X-ray photoelectron spectroscopy. Using total electron yield detection, the top 20 to 50 Å of the pyrite surface is characterized. In air, pyrite oxidizes to form predominantly ferric sulfate. In aqueous air-saturated solutions, the surface oxidation products of pyrite vary with pH, with a marked transition occurring around pH 4. Below pH = 4, a ferric (hydroxy)sulfate is the main oxidation product on the pyrite surface. At higher pH, we find iron(III) oxyhydroxide in addition to ferric (hydroxy)sulfate on the surface. Under the most alkaline conditions, the O K edge spectrum closely resembles that of goethite, FeOOH, and the surface is oxidized to the extent that no FeS₂ can be detected in the X-ray absorption spectra. In a 1.667 × 10⁻³ mol/L Fe³⁺ solution with ferric iron present as FeCl₃ in NaCl, the oxidation of pyrite is autocatalyzed, and formation of the surface iron(III) oxyhydroxide phase is promoted at low pH.     

Geochemistry characterization of groundwater in an agricultural area of Razan,Hamadan, Iran

This study was conducted to evaluate factors regulating groundwater quality in an area with agriculture as main use. Thirty groundwater samples have been collected from Razan area (Hamadan, Iran) for hydrochemical investigations to understand the sources of dissolved ions and assess the chemical quality of the groundwater. The chemical compositions of the groundwater are dominated by Na⁺, Ca²⁺, HCO₃ ⁻, Cl⁻ and SO₄ ²⁻, which have been derived largely from natural chemical weathering of carbonate, gypsum and anthropogenic activities of fertilizer’s source. The production of SO₄ ²⁻ has multiple origins, mainly from dissolution of sulphate minerals, oxidation of sulphide minerals and anthropogenic sources. The major anthropogenic components in the groundwater include Na⁺, Cl⁻, SO₄ ²⁻ and NO₃ ⁻, with Cl⁻ and NO₃ ⁻ being the main contributors to groundwater pollution in Razan area.     

The effects of a gas well blow out on groundwater chemistry

Water wells were sampled near North Madison, Ohio, following a gas well blow out that injected large amounts of CH₄ into near-surface groundwater Chemical analyses showed elevated levels of Fe⁺², Mn⁺², Ca⁺², sulfide, alkalinity, and pH, and low levels of dissolved oxygen, SO₄ ⁻², and NO₃ ⁻ in CH₄-affected wells compared to unaffected wells. Sulfate reduction is quantitatively the most important vehicle for CH₄ oxidation Equilibrium thermodynamic computer models were used to simulate groundwaters from the North Madison area Model results showed that CH₄ is oxidized to HCO₃ ⁻, SO₄ ⁻² is reduced, iron and manganese oxides are reduced and dissolved, and pH increases These simulations are in excellent agreement with trends observed in the field data A laboratory experiment was designed to simulate CH₄ ⁻ perturbed groundwater in the methane-perturbed system, sulfide increased significantly, providing direct evidence for methane oxidation by sulfate reduction Although suitable anaerobic methane-oxidizing bacteria have not been isolated from groundwater aquifers, the combination of field data, laboratory experiment, and computer simulation form a convincing argument that CH₄ perturbation of aquifers can and does affect groundwater chemistry     

Hydrogen bonding in coquimbite, nominally Fe2(SO4)3·9H2O, and the relationship between coquimbite and paracoquimbite

Using single-crystal X-ray diffraction at 293, 200 and 100 K, and neutron diffraction at 50 K, we have refined the positions of all atoms, including hydrogen atoms (previously undetermined), in the structure of coquimbite ( $ P {\bar 3}1c $ , a?=?10.924(2)/10.882(2) Å, c?=?17.086(3) / 17.154(3) Å, V?=?1765.8(3)/1759.2(5) ų, at 293 / 50 K, respectively). The use of neutron diffraction allowed us to determine precise and accurate hydrogen positions. The O–H distances in coquimbite at 50 K vary between 0.98 and 1.01 Å. In addition to H₂O molecules coordinated to the Al³⁺ and Fe³⁺ ions, there are rings of six “free” H₂O molecules in the coquimbite structure. These rings can be visualized as flattened octahedra with the distance between oxygen and the geometric center of the polyhedron of 2.46 Å. The hydrogen-bonding scheme undergoes no changes with decreasing temperature and the unit cell shrinks linearly from 293 to 100 K. A review of the available data on coquimbite and its “dimorph” paracoquimbite indicates that paracoquimbite may form in phases closer to the nominal composition of Fe₂(SO₄)₃·9H₂O. Coquimbite, on the other hand, has a composition approximating Fe_1.5Al_0.5(SO₄)₃·9H₂O. Hence, even a “simple” sulfate Fe_2-x Al _x (SO₄)₃·9H₂O may be structurally rather complex.     

A multi-analytical study of the crystal structure of unusual Ti–Zr–Cr-rich Andradite from the Maronia skarn, Rhodope massif, western Thrace, Greece

Unusual Ti–Cr–Zr-rich garnet crystals from high-temperature melilitic skarn of the Maronia area, western Thrace, Greece, were investigated by electron-microprobe analysis, powder and single-crystal X-ray diffraction, IR, Raman and Mössbauer spectroscopy. Chemical data showed that the garnets contain up to 8 wt.% TiO₂, 8 wt.% Cr₂O₃ and 4 wt.% ZrO₂, representing a solid solution of andradite (Ca₃Fe³⁺ ₂Si₃O₁₂ ≈46 mol%), uvarovite (Ca₃Cr₂Si₃O₁₂ ≈23 mol%), grossular (Ca₃Al₂Si₃O₁₂ ≈10 mol%), schorlomite (Ca₃Ti₂[Si,(Fe³⁺,Al³⁺)₂]O₁₂ ≈15 mol%), and kimzeyite (Ca₃Zr₂[Si,Al₂]₃O₁₂ ≈6 mol%). The Mössbauer analysis showed that the total Fe is ferric, preferentially located at the octahedral site and to a smaller extent at the tetrahedral site. Single-crystal XRD analysis, Raman and IR spectroscopy verified substitution of Si mainly by Al³⁺, Fe³⁺ and Ti⁴⁺. Cr³⁺ and Zr⁴⁺ are found at the octahedral site along with Fe³⁺, Al³⁺ and Ti⁴⁺. The measured H₂O content is 0.20 wt.%. The analytical data suggest that the structural formula of the Maronia garnet can be given as: (Ca_2.99Mg_0.03)_Σ=3.02(Fe³⁺ _0.67Cr_0.54Al_0.33Ti_0.29Zr_0.15)_Σ=1.98(Si_2.42Ti_0.24Fe_0.18Al_0.14)_Σ=2.98O₁₂OH_0.11. Ti-rich garnets are not common and their crystal chemistry is still under investigation. The present work presents new evidence that will enable the elucidation of the structural chemistry of Ti- and Cr-rich garnets.     

Chemical and isotopic variations in the Wiśniówka Mała mine pit water,Holy Cross Mountains (south-central Poland)

In 2005 and 2006, hydrogeochemical study was carried out in the bipartite Wiśniówka Mała pit lake of the Holy Cross Mountains (south-central Poland). This is the largest acidic water body in Poland. This report presents the element concentrations in the water and sediment, stable sulfur and oxygen isotope ratios in the soluble sulfates, and stable oxygen isotope ratio in the water. The scope of the investigation also encompassed mineralogical examinations (scanning electron microscope, X-ray diffraction) of the sediment. The results of this study show that there is a spatial and temporal variability in concentrations of most elements and sulfur isotope ratios in the examined pit lake. The water of the western pond displayed a lower pH with a mean of 3.73 and higher conductivity (390 μS cm⁻¹) as well as higher concentrations of sulfates (156 mg L⁻¹) and most of the cations and anions. The concentrations of Fe²⁺ and Fe³⁺ averaged 0.8 and 0.4 mg·L⁻¹. In contrast, the eastern pond water revealed a higher pH (mean of 4.36), lower conductivity (293 μS cm⁻¹) and lower sulfate (90 mg L⁻¹) and trace metal levels. Similar variations were recorded in the stable sulfur isotope ratios. The δ³⁴S_V-CDT(SO₄ ²⁻) values in the water of the western pit pond were in the range of −6.7 to −4.6‰ (mean of −5.6‰), whereas that in the eastern pit pond ranged from −2.2 to −0.9‰ (−1.6‰). The alkalinity of the entire lake water was below 0.1 mg·L⁻¹ CaCO₃. No distinct difference in the δ¹⁸O_V-SMOW(SO₄ ²⁻) was noted between the western and eastern pit ponds. Compared to the Purple Pond in the Sudetes (Poland) and similar sites throughout the world, the examined pit lake is highlighted by distinctly low concentrations of sulfates, iron and other trace metals. Based on this and other studies performed in the Holy Cross Mountains, a conclusion can be drawn that the SO₄ ²⁻ in the Wiśniówka Mała pit lake water is a mixture of SO₄ ²⁻ derived from the following sources: (1) pyrite oxidation (especially in the western pond water), (2) leaching of soluble sulfates from soils and waste material, as well as (3) subordinate deposition of airborne sulfate precipitation.