Vanadium,manganese and iron in the carbonate rocks of the Rohtas formation

The environmental conditions that prevailed during the formation of the Rohtas carbonates have been delineated on the basis of the Eh-pH diagrams for V, Mn, Fe⁺² and Fe⁺³ compounds. The high content of vanadium in the insoluble residue is indicative of the prevalence of reducing environment. During early-diagenesis manganese seems to have been mobilised from the soft sediments. Higher manganese content in the carbonates is a result of late-diagenesis. Prior to late diagenesis, ferric iron appears to have been precipitated from the waters while manganese remained in solution, and this process accounts for the low iron content of the carbonates.     

Comparative data on the volatile component composition of magma

Ferrihydrite (2.5 Fe₂O₂-4.5 H₂O) is an unstable colloidal mineral. It dissolves in highly alkaline solutions and is precipitated from them in the form of goethite. Jarosite is stable at very low pH but is decomposed at higher values of pH with separation of iron oxides. Experiments show that in rapid decomposition of jarosite a protohematite substance, ferrihydrite, is formed. This transformation occurs at moderate pH values when solutions percolate through the aggregates of jarosite. Ferrihydrite, an unstable colloidal hydrated oxide of ferric iron, changes spontaneously to stable hematite with time. Very slow decomposition of jarosite results in its replacement by iron hydroxide, goethite. Under laboratory conditions in alkaline solutions lepidocrocite may be obtained from jarosite. The synthesis of this iron hydroxide passes through a stage of intermediate products: ferrihydrite and hydrated ferric oxide - ferriprotolepidocrocite, formed by solution of ferrihydrite in strongly alkaline solutions. The transformation of ferriprotolepidocrocite into lepidocrocite may be regarded as a topotactic reaction. —Authors.     

Discrete element modeling of shear wave propagation in carbonate precipitate–cemented particles

Bioremediation is widely used to improve ground soil by introducing calcium carbonate (CaCO₃). Shear wave velocity (Vs) is usually adopted to evaluate effect but the microscopic mechanism is unclear. The discrete element method (DEM), a promising tool for simulating the behaviors of cohesive and noncohesive materials, was used in this study to simulate Vs evolution and wave propagation path of sand reinforced by calcite precipitates. Two basic calcite precipitate forms are proposed for representing individual calcite precipitation (CaCO₃-P) and calcite aggregation (CaCO₃-C). Contact cementation between adjacent sand grain pairs was the primary association pattern for calcite precipitates at a low calcite content. At a higher calcite content, the preferential shear wave propagation pathway is the clusters cemented by CaCO₃-C. With calcite content increasing from 0 to 9%, the coordination number and average contact force increased. Vs increased from 169.73 to 2132.64 m/s but had high variability due to the spatial distribution. The results suggest that the calibrated DEM model can elucidate the microscopic mechanisms and evaluate the enhancement effect of microorganism-reinforced soil.

     

Aragonite–calcite veins of the ‘Erzberg’ iron ore deposit (Austria): Environmental implications from young fractures

The well‐known Erzberg site represents the largest siderite (FeCO₃) deposit in the world. It consists of various carbonates accounting for the formation of prominent CaCO₃ (dominantly aragonite) precipitates filling vertical fractures of different width (centimetres to decimetres) and length (tens of metres). These commonly laminated precipitates are known as ‘erzbergite’. This study focuses on the growth dynamics and environmental dependencies of these vein fillings. Samples recovered on‐site and from mineral collections were analyzed, and these analyses were further complemented by modern water analyses from different Erzberg sections. Isotopic signatures support meteoric water infiltration and sulphide oxidation as the principal hydrogeochemical mechanism of (Ca, Mg and Fe) carbonate host rock dissolution, mobilization and vein mineralization. Clumped isotope measurements revealed cool formation temperatures of ca 0 to 10°C for the aragonite, i.e. reflecting the elevated altitude Alpine setting, but unexpectedly low for aragonite nucleation. The ²³⁸U–²³⁴U–²³⁰Th dating yielded ages from 285·1 ± 3·9 to 1·03 ± 0·04 kyr bp and all samples collected on‐site formed after the Last Glacial Maximum. The observed CaCO₃ polymorphism is primarily controlled by the high aqueous Mg/Ca ratios resulting from dissolution of Mg‐rich host rocks, with Mg/Ca further evolving during prior CaCO₃ precipitation and CO₂ outgassing in the fissured aquifer. Aragonite represents the ‘normal’ mode of erzbergite formation and most of the calcite is of diagenetic (replacing aragonite) origin. The characteristic lamination (millimetre‐scale) is an original growth feature and mostly associated with the deposition of stained (Fe‐rich) detrital particle layers. Broader zonations (centimetre‐scale) are commonly of diagenetic origin. Petrographic observations and radiometric dating support an irregular nature for most of the layering. Open fractures resulting from fault tectonics or gravitational mass movements provide water flow routes and fresh chemical reaction surfaces of the host rock carbonates and accessory sulphides. If these prerequisites are considered, including the hydrogeochemical mechanism, modern water compositions, young U‐Th ages and calculated precipitation rates, it seems unlikely that the fractures had stayed open over extended time intervals. Therefore, it is most likely that they are geologically young.     

Grain growth kinetics of dolomite,magnesite and calcite: a comparative study

The rates of grain growth of stoichiometric dolomite [CaMg(CO₃)₂] and magnesite (MgCO₃) have been measured at temperatures T of 700–800°C at a confining pressure P _c of 300 MPa, and compared with growth rates of calcite (CaCO₃). Dry, fine-grained aggregates of the three carbonates were synthesized from high purity powders by hot isostatic pressing (HIP); initial mean grain sizes of HIP-synthesized carbonates were 1.4, 1.1, and 17 μm, respectively, for CaMg(CO₃)₂, MgCO₃, and CaCO₃, with porosities of 2, 28, and 0.04% by volume. Grain sizes of all carbonates coarsened during subsequent isostatic annealing, with mean values reaching 3.9, 5.1, and 27 μm for CaMg(CO₃)₂, MgCO₃, and CaCO₃, respectively, in 1 week. Grain growth of dolomite is much slower than the growth rates of magnesite or calcite; assuming normal grain growth and n = 3 for all three carbonates, the rate constant K for dolomite (≃5 × 10⁻⁵ μm³/s) at T = 800°C is less than that for magnesite by a factor of ~30 and less than that for calcite by three orders of magnitude. Variations in carbonate grain growth may be affected by differences in cation composition and densities of pores at grain boundaries that decrease grain boundary mobility. However, rates of coarsening correlate best with the extent of solid solution; K is the largest for calcite with extensive Mg substitution for Ca, while K is the smallest for dolomite with negligible solid solution. Secondary phases may nucleate at advancing dolomite grain boundaries, with implications for deformation processes, rheology, and reaction kinetics of carbonates.     

Change in carbonate budget and composition during subduction below metal saturation boundary

It is generally accepted that carbonates can be subducted to the mantle depths, where they are reduced with iron metal to produce a diamond. In this work, we found that this is not always the case. The mantle carbonates from inclusions in diamonds show a wide range of cation compositions (Mg, Fe, Ca, Na, and K). Here we studied the reaction kinetics of these carbonates with iron metal at 6–6.5 GPa and 1000–1500 °C. We found that the reduction of carbonate with Fe produces C-bearing species (Fe, Fe-C melt, Fe₃C, Fe₇C₃, C) and wüstite containing Na₂O, CaO, and MgO. The reaction rate constants (k = Δx²/2t) are log-linear relative to 1/T and their temperature dependences are determined to bek_MgCO3 (m²/s) = 4.37 × 10^?3 exp [?251 (kJ/mol)/RT]k_CaMg(CO3)2 (m²/s) = 1.48 × 10^?3 exp [?264 (kJ/mol)/RT]k_CaCO3 (m²/s) = 3.06 × 10^?5 exp [?245 (kJ/mol)/RT] andk_Na2CO3 (m²/s) = 1.88 × 10^?10 exp [?155 (kJ/mol)/RT].According to obtained results at least, 45–70 vol% of carbonates preserve during subduction down to the 660-km discontinuity if no melting occurs. The slab stagnation and warming, subsequent carbonate melting, and infiltration into the mantle saturated with iron metal are accompanied by a reduction of carbonate melt with Fe. The established sequence of reactivity of carbonates: FeCO₃ ≥ MgCO₃ > CaMg(CO₃)₂ > CaCO₃ ? Na₂CO₃, where K₂CO₃ does not react at all with iron metal, implies that during reduction carbonate melt with Fe evolves toward alkali-rich. The above conclusions are consistent with the findings of carbonates in inclusions in diamonds from the lower mantle and high concentrations of alkalis, particularly K, in mantle carbonatite melts entrapped by diamonds from kimberlites and placers worldwide.     

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

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

Cycling of calcite and hydrous metal oxides and chemical changes of major element and REE chemistry in monomictic hardwater lake: Impact on sedimentation

The variation of major and rare earth elements and yttrium (REY) in the monomictic hardwater Lake Tiberias during the wet and dry seasons of the hydrological year was studied in two profiles. The average volume and Cl concentration of the known and unknown saline inflows of 1.6 × 10⁷ m³ and 1.2 × 10⁹ mol are derived by closing both balances. This brine corresponds to a mixture of 83% of groundwater from Cretaceous aquifers and 17% of very saline deep brine. Taking cycling of calcite in the hypolimnion into account, the settling rate of authigenic calcite is estimated to be 3.3 mol m⁻² a⁻¹.In the stratified lake of the dry season dissolved inorganic carbon increases by 490 μM at the thermo-/chemocline due to microbial reduction of SO₄²⁻, NO₃⁻, chemical reduction of Fe(III) and MnO₂ colloids, and cycling of calcite in the hypolimnion. REY distribution in the stratified water column is dominantly controlled by coprecipitation with calcite, hydrous ferric oxides and MnO₂ in the epilimnion and cycling of these compounds in the hypolimnion. The positive Ce anomaly in the hypolimnetic water is produced by cycling of MnO₂. The simulation of the increase of REY in the hypolimnion reveals that hydrous ferric and manganese oxides only play a negligible role except Ce. Only about 10% of REY from cycled matter enhance REY in solution. Most of the released REY are adsorbed by particular matter and thus settling on the floor of the lake.Different from Na, U, SO₄²⁻ and SiO₂, the other elements, in particular REY, increase in the mixed water column from the top to the lower third and mostly decrease thereafter toward the bottom in the mixed lake during the wet season. The behavior of REY is caused by some cycling of calcite and pH-dependent re-equilibration of REY bound to hydrous ferric and manganese oxides adsorbed by particular matter.     

Reactions of metal ions at surfaces of hydrous iron oxide

Cu, Ag and Cr concentrations in natural water may be lowered by mild chemical reduction involving ferric hydroxide-ferrous ion redox processes. V and Mo solubilities may be controlled by precipitation of ferrous vanadate or molybdate. Concentrations as low as 10^?8.00 or 10^?9.00 M are readily attainable for all these metals in oxygen-depleted systems that are relatively rich in Fe. Deposition of manganese oxides such as Mn₃O₄ can be catalyzed in oxygenated water by coupling to ferrous-ferric redox reactions. Once formed, these oxides may disproportionate, giving Mn⁴⁺ oxides. This reaction produces strongly oxidizing conditions at manganese oxide surfaces. The solubility of As is significantly influenced by ferric iron only at low pH. Spinel structures such as chromite or ferrites of Cu, Ni, and Zn, are very stable and if locally developed on ferric hydroxide surfaces could bring about solubilities much below 10^?9.00 M for divalent metals near neutral pH. Solubilities calculated from thermodynamic data are shown graphically and compared with observed concentrations in some natural systems.     

Phosphorus Speciation in Stream Bed Sediments from an Agricultural Watershed: Solid-Phase Associations and Sorption Behavior

The sorption behavior and solid-phase associations of phosphorus (P) in fine-grained sediments (<63 μm) from two upstream tributaries and one downstream main stem site of the Spoon River in west-central Illinois were characterized to better understand phosphorus bioavailability in this agriculturally dominated watershed. The P sorption affinities, as indicated by linear distribution coefficients (K _d), of all sediments were 330–5,150 L/kg, and negatively correlated with equilibrium phosphorus concentration (EPC_o) values, which ranged between 0.2 and 2.2 μM. pH values measured at the conclusion of the sorption experiments varied only slightly (7.45–8.10) but were nonetheless strongly positively correlated to EPC_o values, and negatively correlated to K _d values, suggesting the importance of pH to the observed sorption behavior. K _d values were generally lower and EPC_o values higher at the main stem site than at the upstream tributary sites, suggesting dissolved reactive P (DRP) bioavailability (specifically orthophosphate) increased downstream. The solid phase associations of P were operationally assessed with the streamlined SEDEX (sedimentary extraction) procedure, and most sediment P (≥50%) was released during the step designed to determine iron oxide–associated P. On average, 70–90% of the total sediment P pool was potentially bioavailable, as estimated by the sum of the iron oxide-, authigenic carbonate-, and organic-associated P fractions. Considerable calcium was also extracted from some sediments during the step designed to specifically remove iron oxide–associated P. It is hypothesized that the severe drought conditions that persisted between April and October, 2005 allowed authigenic carbonates (perhaps partly amorphous) to accumulate, and that these carbonates dissolved during the iron oxide extraction step. The extensive benthic algal populations also present may have aided carbonate precipitation, which under more normal hydrologic conditions would be periodically flushed downstream and replaced by fresh sediment. This suggests antecedent hydrologic conditions played a dominant role in the P sorption and solid phase associations identified.     

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.     

Geochemical reactions and dynamics during titration of a contaminated groundwater with high uranium, aluminum, and calcium

This study investigated possible geochemical reactions during titration of a contaminated groundwater with a low pH but high concentrations of aluminum, calcium, magnesium, manganese, and trace contaminant metals/radionuclides such as uranium, technetium, nickel, and cobalt. Both Na-carbonate and hydroxide were used as titrants, and a geochemical equilibrium reaction path model was employed to predict aqueous species and mineral precipitation during titration. Although the model appeared to be adequate to describe the concentration profiles of some metal cations, solution pH, and mineral precipitates, it failed to describe the concentrations of U during titration and its precipitation. Most U (as uranyl, UO₂²⁺) as well as Tc (as pertechnetate, TcO₄⁻) were found to be sorbed and coprecipitated with amorphous Al and Fe oxyhydroxides at pH below ∼5.5, but slow desorption or dissolution of U and Tc occurred at higher pH values when Na₂CO₃ was used as the titrant. In general, the precipitation of major cationic species followed the order of Fe(OH)₃ and/or FeCo_0.1(OH)_3.2, Al₄(OH)₁₀SO₄, MnCO₃, CaCO₃, conversion of Al₄(OH)₁₀SO₄ to Al(OH)_3,am, Mn(OH)₂, Mg(OH)₂, MgCO₃, and Ca(OH)₂. The formation of mixed or double hydroxide phases of Ni and Co with Al and Fe oxyhydroxides was thought to be responsible for the removal of Ni and Co in solution. Results of this study indicate that, although the hydrolysis and precipitation of a single cation are known, complex reactions such as sorption/desorption, coprecipitation of mixed mineral phases, and their dissolution could occur simultaneously. These processes as well as the kinetic constraints must be considered in the design of the remediation strategies and modeling to better predict the activities of various metal species and solid precipitates during pre- and post-groundwater treatment practices.     

A new technique for extracting zirconium form Egyptian zircon concentrate

Zircon is the most important commercial source of zirconium, its compounds and alloys. Several methods are used for industrial processing of zircon for production of zirconium dioxide and tetrachloride. These methods include sintering of zircon with sodium carbonate or sodium hydroxide or calcium oxide or calcium carbonate and with potassium fluorosilicate, chlorination of zircon mixture with coal in blast furnace and carbidization of zircon in a mixture of coal in electric arc furnace. All these methods are carried out at high temperatures and have many disadvantages.The present work illustrates a study of a new technique for extracting zirconium from Egyptian zircon concentrate by its simultaneous ball-milling and pressure alkaline leaching, to improve the recovery of zirconium from zircon. Experiments were carried out in stainless steel ball-mills of cylindrical shape under different conditions of temperature, pressure and time. The ball-mills were heated and mechanically rotated in an electric furnace by means of roll mechanism.The filtrate after leaching of zircon, containing excess of sodium hydroxide was regenerated by its treatment with calcium hydroxide for purification from silicon impurity. Then, the solution was evaporated to the desired concentration (500 g/l Na₂O) and recycled to the reactor of leaching.The results obtained show that complete recovery of zirconium from zircon (99.7%) by simultaneous ball-milling and alkaline leaching was attained at 250 °C within 3 h, using amount of sodium hydroxide 150% of theoretical requirement (satisfying favourable conditions of zirconate cake for subsequent acid leaching).The standard free energy (ΔF^o) and equilibrium constant (K) of the reaction of zircon with sodium hydroxide were calculated as—19.58 k cal mol^− 1 and 2.29 · 10¹⁴, respectively. The kinetics of the technological alkaline processing of zircon in ball-mall autoclaves was studied over the temperature range 150–275 °C.     

Gold losses by cementation and thermal reduction in the gold recovery circuits

In this study, gold losses in a carbon-in-pulp (CIP) cyanidation gold recovery process and potential sources of these losses were investigated. Gold was found in samples from different streams through the CIP-cyanidation process, pointing to incidental losses. Mineralogical studies showed that gold losses occurred in two main forms, either as attached to larger entities or in the form of dendritic precipitates. SEM and EDS studies revealed that iron bearing minerals acted as the major media in cases when gold associations were observed as losses. The highly alkaline pH (≈ 13), elevated process temperature (≈ 145 °C), and high cyanide concentration (≥ 250 ppm) in the elution column along with a fine iron bearing material implied that gold attachment occurred through an electrochemical cementation mechanism. It was anticipated that the presence of iron in the process, which facilitated gold cementation, relied on the oxidative breakdown of the iron bearing minerals in the ore and/or due to the formation of porous iron oxides due to the roasting of iron sulfides in the regeneration kiln. In the elution column some part of the auro–cyanide complexes would remain non-eluted and be discharged into the carbon generation kiln and the carbon generation kiln was another section promoting gold losses. The high temperature condition in the carbon regeneration kiln (> 500 °C) caused thermal reduction of the non-eluted auro–cyanide complexes to metallic gold, leading to the formation of dendritic gold precipitates and their eventual loss.     

Ternary Ca–Fe–Mg carbonates: subsolidus phase relations at 3.5 GPa and a thermodynamic solid solution model including order/disorder

Subduction carries atmospheric and crustal carbon hosted in the altered oceanic crystalline basement and in pelagic sediments back into the mantle. Reactions involving complex carbonate solid solutions(s) lead to the transfer of carbon into the mantle, where it may be stored as graphite/diamond, in fluids or melts, or in carbonates. To constrain the thermodynamics and thus reactions of the ternary Ca–Mg–Fe carbonate solid solution, piston cylinder experiments have been performed in the system CaCO₃–MgCO₃–FeCO₃ at a pressure of 3.5 GPa and temperatures of 900–1,100°C. At 900°C, the system has two miscibility gaps: the solvus dolomite–calcite, which closes at X _MgCO3 ~0.7, and the solvus dolomite–magnesite, which ranges from the Mg to the Fe side of the ternary. With increasing temperature, the two miscibility gaps become narrower until complete solid solutions between CaCO₃–Ca_0.5Mg_0.5CO₃ is reached at 1,100°C and between CaCO₃–FeCO₃ at 1,000°C. The solvi are characterized by strong compositional asymmetry and by an order–disorder mechanism. To deal with these features, a solid solution model based on the van Laar macroscopic formalism has been calculated for ternary carbonates. This thermodynamic solid solution model is able to reproduce the experimentally constrained phase relations in the system CaCO₃–MgCO₃–FeCO₃ in a broad P–T range. To test our model, calculated phase equilibria were compared with experiments performed in carbonated mafic protolithes, demonstrating the reliability of our solid solution model at pressures up to 6 GPa in complex systems.     

Advances of ferrous and ferric Mössbauer recoilless fractions in minerals and glasses

Mössbauer spectroscopy has been used widely to characterize the ferric (Fe³⁺) and ferrous (Fe²⁺) proportions and coordination of solid materials. To obtain these accurately, the recoilless fraction is indispensible. The recoilless fractions (f) of iron-bearing minerals, including oxides, oxyhydroxides, silicates, carbonates, phosphates and dichalcogenides, and silicate glasses were evaluated from the temperature dependence of their center shifts or absorption area with the Debye model approximation. Generally, the resolved Debye temperature (θ_D) of ferric iron in minerals, except dichalcogenides, through their center shifts ranging from 400 to 550 K, is significantly larger than ferrous iron ranging from 300 to 400 K, which is consistent with the conclusion from previous work. The resolved f (Fe³⁺)_RT with the center shift model (CSM) ranges from 0.825 to 0.925, which is larger than that obtained for f(Fe²⁺)_RT, which ranges from 0.675 to 0.750. Meanwhile, the θ_D and f resolved from temperature-dependence of absorption are generally lower than from center shifts, especially for ferric iron. The significant difference between f(Fe³⁺) and f(Fe²⁺) indicates the necessity of recoilless fraction correction on the Fe³⁺/(Fe³⁺+Fe²⁺) resolved from Mössbauer spectra.     

Production of mud‐grade carbonates by marine fish: Crystalline products and their sedimentary significance

Recent work has established that marine teleost (bony) fish represent a prolific source of mud grade, mainly high‐Mg calcite, carbonate sediment by means of primary precipitation within the intestine. Previously documented crystalline products display a diverse array of morphologies, many unique in shallow tropical marine settings, and have a wide range of magnesium contents (from 18 to 39 mol% MgCO₃). This study utilizes scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, X‐ray diffraction and liquid ion chromatography to provide a more extensive and expansive morphological, mineralogical and chemical characterization of the crystalline forms produced by a wider range of piscine functional groups (covering 21 different fish species common in The Bahamas). Several crystal morphologies not previously described in fish‐derived carbonates are documented, and chemical composition is found to be more variable than previously reported: in addition to high‐Mg calcites with >18 mol% MgCO₃, high‐Mg calcites with lower MgCO₃ contents and low‐Mg calcites are identified. From the expanded species range, MgCO₃ content in fish‐derived carbonates ranges from ca 0˙5 to > 40 mol%, and particle length is in the range 0˙1 to >50 μm (typically <2 μm for individual crystals). Amorphous Mg‐carbonates (with detectable CaCO₃ of <2 mol%) are also found to varying extent in the precipitates of many species. Dominant mineralogy and MgCO₃ content varies with producing species and crystal morphology (itself a species‐dependent variable). Given the very small grain size and often high MgCO₃ contents of these carbonates, interesting questions arise about their preservation potential. Thus, the extent to which carbonates produced by different species may follow different post‐excretion preservation pathways is considered.     

Natural arsenic contamination in waters from the Pesariis village,NE Italy

High arsenic (As) concentrations, >900 μg/L, were measured in Ca–Mg–SO₄ waters from springs and drainages in the village of Pesariis in the Carnic Alps (NE Italy). Oxidation of the outcropping arsenian marcasite ore deposits of the area is proposed as the mechanism for As release into oxygenated waters during runoff. Nevertheless, the limited extension of the ore deposit and the relatively low As content of the mineralization suggest that sulfide weathering might not be the only process responsible for the highest As concentration in groundwaters. An additional mechanism involves As adsorption onto ferric iron particulate during oxidation, the drawdown in reducing environment at depth during water infiltration, and the release of ferrous iron and sorbed arsenic to the water columns by reductive dissolution of hydrous ferric oxides (HFO). This yields the observed Fe–As correlation. Newly formed HFO precipitates when groundwaters discharge to aerated conditions, leading to the removal of As, which strongly partitions into the iron-rich sediments, adsorbed onto the surface of amorphous Fe₂O₃·xH₂O. The calculated and measured As concentration in sediments exceeds 10% by weight. Furthermore, geochemical and isotopic data indicate that the As-rich reservoir partly mixes with shallower aquifers, commonly tapped for drinking supply, representing a natural hazard for inhabitants.     

Rezente marine Eisenerze auf Santorin,Griechenland

Recent iron sediments forming at present in a bay of the volcanic island Palaea Kameni within the caldera of Santorini, Aegean Sea, have been investigated for their mineralogy and geochemistry. For the first time siderite has been found in a marine environment to be major constituent of a recent sediment. Further main constituents are opal, ferric hydroxide, vivianite, ferrous hydroxide, and possibly ferrous silicate. The chemical composition both of the solid material of the sediment cores and of their pore solution indicate that the ore forming solutions have originated from the leaching of volcanic kalk-alcaline rocks by hot acid solutions. This is in agreement with experimental leaching of these rock types. No enrichment of lead, copper, zinc etc. was found in the sediments. Iron oxidizing bacteria in the reddish-brown ferric hydroxide sediments now forming in bays of the Kameni Islands have been studied by light and electron microscopic investigations. Samples from the uppermost parts of the sediment consist mainly of the ferric hydroxide stalks of the iron bacteriumGallionella ferruginea. The stalks showing their morphological characteristics occur in such masses that there is no doubt concerning the presence, activity and share of these bacteria in the process of iron sedimentation. Phases of sedimentation process and kinetics of ferric hydroxide stalk formation have been determined qualitatively and quantitatively by in-situ-experiments using artificial growing surfaces (underwater “Aufwuchs” on glass slides). The results obtained are compared to similar iron sedimentation in fresh water habitats and iron rich carbonate springs discussed in literature in connection with the problem of submarine exhalative sedimentary iron ore formation.     

Carbonate precipitation in artificial soils as a sink for atmospheric carbon dioxide

Turnover of C in soils is the dominant flux in the global C cycle and is responsible for transporting 20 times the quantity of anthropogenic emissions each year. This paper investigates the potential for soils to be modified with Ca-rich materials (e.g. demolition waste or basic slag) to capture some of the transferred C as geologically stable CaCO₃. To test this principal, artificial soil known to contain Ca-rich minerals (Ca silicates and portlandite) was analysed from two sites across NE England, UK. The results demonstrate an average C content of 30 ± 15.3 Kg C m⁻² stored as CaCO₃, which is three times the expected organic C content and that it has accumulated at a rate of 25 ± 12.8 t C ha⁻¹ a⁻¹ since 1996. Isotopic analysis of the carbonates gave values between −6.4‰ and −27.5‰ for δ¹³C and −3.92‰ and −20.89‰ for δ¹⁸O, respectively (against V-PDB), which suggests that a combination of carbonate formation mechanisms are operating including the hydroxylation of gaseous CO₂ in solution, and the sequestration of degraded organic C with minor remobilisation/precipitation of lithogenic carbonates. This study implies that construction/development sites may be designed with a C capture function to sequester atmospheric C into the soil matrix with a maximum global potential of 290 Mt C a⁻¹.