Precipitation Of Secondary Minerals Research Articles

Seasonal variability of extremely metal rich acid mine drainages from the Tharsis mines (SW Spain)

The Tharsis mine is presently abandoned, but the past intense exploitation has left large dumps and other sulphide-rich mining wastes in the area generating acid mine drainages (AMD). The main goal of this work is to study the effect of hydrogeochemical processes, hydrological regime and the waste typology on the physicochemical parameters and dissolved concentrations of pollutants in a deeply AMD-affected zone. Extreme leachates are produced in the area, reaching even negative pH and concentrations of up to 2.2 g/L of As and 194 g/L of Fe. The results of the comparison of ore grades of sulphide deposits with dissolved concentrations in waters shows that Pb is the least mobile element in dissolution probably due to the precipitation of Pb secondary minerals and/or its coprecipitation on Fe oxyhydroxysulphates. Arsenic, Cr, and V are also coprecipitated with Fe minerals. Seasonal patterns in metal contents were identified: elements coming from the host rocks, such as Al, Mn and Ni, show their maximum values in the dry period, when dilution with freshwater is lower and the interaction of water-rock processes and evaporation is higher. On the other hand, As, Cr, Fe, Pb and V show minimum concentrations in the dry period due to intense Fe oxyhydroxysulphate precipitation. In this sense, large sulphide rich waste heaps would be a temporal sink of these elements (i.e. Pb, As, Cr and V) in the dry period, and a significant source upon intense rainfalls.

Experimental constraints on Mg isotope fractionation during clay formation: Implications for the global biogeochemical cycle of Mg

The direction and magnitude of magnesium (Mg) isotope fractionation attendant to the formation of clay minerals is fundamental to the use of Mg isotopes to decipher the biogeochemical cycling of Mg in the critical zone and for the oceanic Mg budget. This study provides experimental data on the Mg fractionation factor for two smectite-group minerals (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. A series of experiments were performed to asses the impact of temperature and pH on isotope fractionation. Bulk solid samples were treated with ammonium chloride to remove exchangeable Mg in order to distinguish the Mg isotopic fractionation between these sites and octahedral sites.All bulk and residual solids were enriched in 24Mg compared to the initial solution and δ26Mg values of the exchangeable pool were lower than, or within error of, the initial solution. Final solutions were either within error of, or enriched in, 26Mg compared to the initial solution, depending on the fraction of Mg removed from solution (fMg). For experiments with small or negligible fMg, increasing the pH resulted in a higher reaction rate and reduced fractionation from the initial solution. This could point to a kinetic effect, but the composition of the residual solid (Mg/(Li+Mg) ratio) was also dependent on pH. The change in the composition was reflected in the wavenumber of the Mg3-OH stretch in FT-IR data, which is a proxy for bond strength, and suggests an equilibrium control. An equilibrium control is further supported by the observation of reduced fractionation compared to the initial solution with increasing temperature. Rayleigh and batch fractionation models were fitted to the data giving fractionation factors of 0.9991 and 0.9990 respectively.We compare our results with existing field and experimental data and suggest that the apparent contradictions surrounding the direction of Mg isotope fractionation into phyllosilicate minerals could be due to the similarity of Mg bond lengths between clay octahedral sites and dissolved Mg. Thus small changes in mineral structure or initial solution conditions may result in a change in bond length sufficient to alter the direction of fractionation, implying that the magnitude and direction of Mg isotope fractionation into clay minerals could be dependent on local field conditions. Alternatively, if the precipitation of secondary clay minerals in the field preferentially incorporates light Mg, as observed in this experimental study, this implies the contribution of carbonate weathering to dissolved Mg fluxes has been underestimated, with major implications for the global biogeochemical cycle of Mg.

Magnesium isotope fractionation during microbially enhanced forsterite dissolution.

Bacillus subtilis endospore-mediated forsterite dissolution experiments were performed to assess the effects of cell surface reactivity on Mg isotope fractionation during chemical weathering. Endospores present a unique opportunity to study the isolated impact of cell surface reactivity because they exhibit extremely low metabolic activity. In abiotic control assays, 24 Mg was preferentially released into solution during forsterite dissolution, producing an isotopically light liquid phase (δ26 Mg=-0.39±0.06 to -0.26±0.09‰) relative to the initial mineral composition (δ26 Mg=-0.24±0.03‰). The presence of endospores did not have an apparent effect on Mg isotope fractionation associated with the release of Mg from the solid into the aqueous phase. However, the endospore surfaces preferentially adsorbed 24 Mg from the dissolution products, which resulted in relatively heavy aqueous Mg isotope compositions. These aqueous Mg isotope compositions increased proportional to the fraction of dissolved Mg that was adsorbed, with the highest measured δ26 Mg (-0.08±0.07‰) corresponding to the highest degree of adsorption (~76%). The Mg isotope composition of the adsorbed fraction was correspondingly light, at an average δ26 Mg of -0.49‰. Secondary mineral precipitation and Mg adsorption onto secondary minerals had a minimal effect on Mg isotopes at these experimental conditions. Results demonstrate the isolated effects of cell surface reactivity on Mg isotope fractionation separate from other common biological processes, such as metabolism and organic acid production. With further study, Mg isotopes could be used to elucidate the role of the biosphere on Mg cycling in the environment.

CO2 storage in the Paluxy formation at the Kemper County CO2 storage complex: Pore network properties and simulated reactive permeability evolution

The Paluxy formation is being considered as a prospective CO2 reservoir at the Kemper County CO2 Storage Complex. Here, the pore and pore-throat size distributions and connectivity of the Paluxy formation is evaluated through analysis of 3D X-ray Computed Tomography images. In spite of resolution limitations that constrain the pore-throat sizes detectable by imaging, the permeability contributing pore-throats are successfully characterized through 3D imaging analysis. Image-obtained pore and pore-throat size distributions and pore connectivity are then utilized to construct pore network models and simulate permeability. After CO2 is injected, it will dissolve into formation brine and create conditions favorable for dissolution of primary minerals and precipitation of secondary minerals. These reactions will alter the porosity and permeability of the system to varying degrees depending on the spatial location of reactions. Here, the possible porosity-permeability evolution is simulated using pore network models considering mineral reactions occurring uniformly and non-uniformly throughout the network. For a given change in porosity, there is a large range of possible permeability outcomes. Depending on the extent and spatial location of mineral reactions, permeability may decrease by more than one order of magnitude as minerals precipitate. During dissolution, simulated permeability increases as much as 500%.

Characterizing nutrient pathways in Quebec (Canada) vineyards: Insight from stable and radiogenic strontium isotopes

Soils are a crucial interface of Earth's Critical Zone, responsible for the biogeochemical cycling of mineral nutrients. While the sources of essential nutrients such as Ca (and Sr) in plants have been widely studied, the processes by which these nutrients are transferred from the soil to the plants is less clear. We present new evidence for systematic nutrient fractionation between soils, grapes and wines from five vineyards from the Quebec terroir (Canada) using the novel stable Sr (δ88/86Sr) isotope system. We coupled this approach with traditional radiogenic 87Sr/86Sr measurements on the same samples (bulk soils, labile fraction of these soils, whole grapes, and wines) from five vineyards from the Quebec terroir (Canada) to ensure that variations in δ88/86Sr values were provoked by processes involved in Sr transfer instead of mixing between different sources. The δ88/86Sr values generally decrease from the bulk to the labile fractions of the soils to the grapes. A decrease in the difference between the soil fractions (Δ88/86Srbulk-labile) with increasing δ88/86Sr values in the labile fraction is interpreted to reflect increasing pedogenic mineral precipitation and higher soil fertility. The depleted δ88/86Sr values of the grapes are consistent with a preferential uptake of lighter isotopes by plants. This effect is magnified in soils showing higher secondary mineral precipitation. An increase in the δ88/86Sr values from the grapes to the wines reflects the separation of the skin/seeds and the juice (pulp enriched in 88Sr) in the winemaking process. This mixing relationship is supported by a correlation between the Sr concentrations and the δ88/86Sr values of the wines and grapes. These findings suggest promising new applications of stable Sr isotopes values for studying soil fertility and the impact of nutrient availability for plant growth in agricultural ecosystems as well as the impact of the maceration processes in the production of wine.The overall similarity of 87Sr/86Sr ratios of the labile soil fractions, grapes and wines from the same vineyard support the hypothesis of a same Sr source, from the soil solution to its transfer to the grape and wine and confirm the potential of the 87Sr/86Sr ratios as a proxy for geographic provenance of wine.However, slightly lower 87Sr/86Sr ratios in the grape pulps (by 0.00040 on average compared to the skins and seeds) suggest that the sources of nutrients available for plant uptake vary during the vegetative cycle. The grape seeds and skins formed in the early growth cycle are characterized by rapid growth due to enhanced rhizosphere activity (enhanced mineral dissolution) providing higher nutrient levels. The enhanced mineral solubility results in higher 87Sr/86Sr ratios in the skin and seeds derived from enhanced dissolution of 87Sr-enriched silicate minerals. In contrast, the pulp develops later in the growth cycle when rhizosphere activity and mineral solubility is reduced, resulting in a lesser contribution from silicate minerals and thus reflect a higher proportion of dissolved carbonate contributing to the lower 87Sr/86Sr ratios of the pulps.

Ca isotope constraints on chemical weathering processes: Evidence from headwater in the Changjiang River, China

This study aims to clarify the relationship between chemical weathering of rocks and the carbon budget of rivers and better understand the weathering mechanisms of plateau watersheds. We chose to study the Jinsha River, which originates from the Tibetan Plateau and also is in the upper reaches of the Changjiang River. Analysis of hydrochemistry, radiogenic strontium isotope and stable calcium isotopes were conducted of the Jinsha River water samples, which were collected along its mainstream and main tributaries in the summer. The results show that the water chemistry of the mainstream waters is dominated by evaporite weathering, which have low 87Sr/86Sr values (0.7098–0.7108) and wide range of Sr contents (2.70–9.35μmol/L). In contrast, tributaries of the Jinsha River have higher 87Sr/86Sr (0.7090–0.7157) and lower Sr contents (∼1μmol/L). Moreover, the Ca isotopic compositions in the mainstream (0.87–1.11‰) are heavier than the tributaries (0.68–0.88‰) and could not be fully explained by the conventional mixing of different sources. We suggest that secondary carbonate precipitation fractionates Ca isotopes in the Jinsha River, and fractionation factors are between 0.99935 and 0.99963. At least 66% of Ca was removed in the mainstream of the Jinsha River through secondary mineral precipitation, and the average value is ∼35% in the tributaries. The results highlight that evaporite weathering results in more carbonate precipitation influencing Ca transportation and cycling in the riverine system constrained by stable Ca isotopic compositions and water chemistry.

Geochemical and mineralogical assessment of reactivity in a full-scale heterogeneous waste-rock pile

Mine waste rock is typically stored on-site in large piles that are heterogeneous in terms of material composition and grain size. This leads to variable reactivity within the pile and makes prediction of drainage quality difficult. Here, we present data from two >100 m continuous boreholes through an operational waste-rock pile at the Antamina mine (Peru). Reactive zones with elevated sulfide and metal content exhibited increased temperatures, reduced oxygen content, as well as pervasive secondary mineral precipitation. Sharp contact between altered and unaltered layers at the meter to centimeter scale indicated strong variability in mineral reactivity. Unaltered waste rock persisted within high-sulfide zones due to low oxygen levels, and surface passivation and galvanic interactions between sulfide couples also limited alteration. Waste-rock layers with gradated alteration that resemble hardpans, as observed in tailings, were in direct contact with non-altered, less-permeable layers. Finally, leaching tests show that sulfate, Cu and Zn were efficiently leached from reactive and mineralogically altered zones, whereas As and Fe were retained through sorption, surface passivation, or secondary mineral formation. This study shows that, over and above bulk geochemical and lithological data, waste-rock reactivity may also be defined by mineralogical data and thereby contribute to improved drainage quality predictions.

The use of super-critical carbon dioxide as the working fluid in enhanced geothermal systems (EGSs): A review study

Naturally-occurring high-temperature geothermal energy resources contribute significantly to the world’s emerging energy demand by allowing renewable and clean power generation, and the technology of enhanced geothermal systems (EGSs) continues to progress at deeper levels, providing the ability to extract heat stored in deep underground at 2–5 km depths, which would have previously been considered impractical. Water-based EGSs are fairly well established, and a number of research studies have been carried out on the performance of water as the working fluid in geothermal reservoirs. Besides, the applicability of super-critical carbon dioxide (ScCO2) as the working fluid in EGSs by replacing water was recently investigated by research studies due to a number of advantages accompanied in CO2 as the working fluid in EGSs. The aim of this review study is to comprehensively investigate the applicability of ScCO2 as a working fluid in EGSs by summarising the recent research studies conducted on the operation of CO2-based geothermal systems. The study is further intended to highlight the advantages of ScCO2 in the EGS environment compared to conventional water-based methods. Of the advantages, a reduction of the external power requirement for the pumping of ScCO2 due to enhanced injectivity and flowability is the most attractive. In terms of heat extraction, higher mass flowrates can be achieved using ScCO2 as a working fluid, depending on the density-to-viscosity ratio, and the ratio is doubled with ScCO2 compared to water in the temperature range of 200–250 °C. Moreover, an accelerated energy recovery can be achieved through the higher thermal extraction rates of ScCO2, which are 50% greater than for water. Importantly, ScCO2 has the ability of inducing an interconnected fracture network with multiple secondary branches under comparatively low breakdown pressures compared to water. ScCO2 being a linear molecule with lower viscosity has the percolation ability into microcracks of the rock matrix, and thereby, the pressurisation of these microcracks induce multiple flow paths long the rock matrix by interconnecting the microcracks. In addition, the use of ScCO2 eliminates the problems associated with silica dissolution followed by precipitation, takes place in surface piping, heat exchangers and other surface equipment, because dry ScCO2 is less reactive in hot-dry reservoirs. However, in water-based EGSs, enhanced reactivity of water with the increase of pH accelerates the dissolution and precipitation of minerals, and is a critical issue in water-based EGSs. The enhanced reactivity of ScCO2 in the presence of pore fluid may also affect the long-term integrity of EGSs, because it alters the internal pore structure, due to the dissolution and precipitation of primary and secondary rock minerals, resulting in altered porosity and permeability, which ultimately affect the energy extraction process.

Release of technology critical metals during sulfide oxidation processes: the case of the Poderosa sulfide mine (south-west Spain)

Environmental contextNatural weathering of rocks may release technology critical elements (TCEs) to the environment, and anthropogenic activities can noticeably increase TCE release rates. We investigated acid mine drainage outflows from an underground sulfide mine in south-west Spain, reporting TCE concentrations orders of magnitude higher than those observed in natural waters. The findings improve our knowledge on mobility of TCEs in different geological settings. AbstractExtensive extraction of technology critical elements (TCEs) from the lithosphere and their use results in a growing dispersion and remobilisation of these elements within the environmental compartments. We investigated the concentration and mobility of different TCEs (rare earth elements (REEs), Sc, Y, Ga and Tl) in acid mine drainage (AMD) outflows from a massive sulfide underground mine in south-west Spain for around 2 years. High levels of TCEs were observed; average concentrations of 8.2mgL−1 of REEs, 1.5mgL−1 of Y, 80µgL−1 of Ga, 53µgL−1 of Sc and 42µgL−1 of Tl were reported, several orders of magnitude higher than those observed in natural waters. The TCEs source in the study site is primarily accessory minerals in the host rocks, although the contribution of Ga and Tl by sulfides cannot be discarded. A seasonal variability in TCEs is observed in AMD waters, although their maximum concentrations do not coincide with those of sulfide-related elements. TCEs seem not to be controlled by the precipitation of secondary minerals, but by the intensity of chemical weathering inside the mined zone. A positive correlation between REEs and the Si/Na+K ratio seems to indicate that these elements are linked to resistant minerals to weathering.

Olivine dissolution and hydrous Mg carbonate and silicate precipitation in the presence of microbial consortium of photo-autotrophic and heterotrophic bacteria

Olivine is an important mineral that controls the sequestration of atmospheric CO2 in the form of secondary carbonate minerals during chemical and biological weathering of mafic rocks on Earth. Despite significant efforts in characterization of olivine reactivity and coupled secondary mineral precipitation both in abiotic and biotic systems, little is known on olivine behavior in the presence of bacterial consortia, which are the dominant forms of microbial life impacting mineral reactivity in natural settings. To address this gap, we studied the interaction of olivine with a bacterial consortium composed of typical freshwater cyanobacterium Synechococcus sp. and heterotrophic aerobic Pseudomonas reactans, isolated from a CO2 storage site in Icelandic basalts. We quantified the impact of this consortium on the dissolution rate of olivine and we characterized the precipitation of secondary mineral phases while monitoring various biological (number of cells, bacterial biomass) and physicochemical (pH, Si, Mg, Ca, alkalinity, dissolved organic and inorganic carbon) parameters of the medium over a period of ∼21 days. Heterotrophic bacteria and their organic exometabolites enhanced the release of Mg and Si and produced leaching features (etch pits) at the olivine surface whereas cyanobacterial photosynthesis raised the pH and favored precipitation of hydrous Mg carbonates and silicates in the vicinity of the cells. During microbially-induced transformation of aquatic carbon, the latter was sequestered in the form of cyanobacterial biomass (about 66%) and their soluble organic exometabolites (11%), and stored as secondary Mg carbonates (23%). Overall, the impact of bacterial consortium is higher than that of individual species and may represent important and understudied biotically-controlled mechanism of CO2 sequestration in natural waters.

Assessment of origin and fate of contaminants along mining-affected Rio Montevecchio (SW Sardinia, Italy): A hydrologic-tracer and environmental mineralogy study

Hydrologic tracer techniques were applied to Rio Montevecchio (SW Sardinia, Italy), a stream affected by mine drainage, allowing the calculation of discharge and contaminant loads. Discharge along the stream showed a constant increase throughout the 2.7 km-long study reach, up to 13.6 l/s at the last synoptic point. Calculated loads of mine-related constituents were large, reaching values of 1780 kg/day for SO42−, 340 kg/day for Zn, 47 kg/day for Fe, and 50 kg/day for Mn. The difference of the cumulative instream metal loads between the first and the last synoptic sampling points indicated gains of 421 kg/day for Zn, 2080 kg/day for SO42−, 56 kg/day for Mn, and 50 kg/day for Fe. The source areas critical for contaminants loading were almost all concentrated in the first 800 m of the stream, with the exception of Pb, whose loading occurs evenly along the whole study reach. Precipitation of secondary minerals along the streambed was responsible for a very high attenuation of Al and Fe loads (66% and 77%) and affected also SO42− and Zn loads, though less effectively.Rio Montevecchio has the second highest metal load among the rivers investigated with tracer techniques in SW Sardinia. In comparison with Rio Irvi, which has one order of magnitude higher metal loads, natural attenuation processes limit the loads in Rio Montevecchio. Results are useful to clarify the hydrogeochemical paths involved in the release and attenuation of pollutants, improving our understanding of stream responses to contamination and aiding development of site-specific remediation actions.

Silicon isotopic re-equilibration during amorphous silica precipitation and implications for isotopic signatures in geochemical proxies

Abstract Water-rock interactions influence the isotopic signatures archived during secondary mineral formation and thus the fidelity with which these geochemical proxies retain pristine geochemical records over geologic timescales. The extent to which these isotopic signatures are “re-equilibrated” or “reset” has been suggested to be dependent on surface area and the depth into mineral surface that the surrounding fluid can access, but this relationship has yet to be incorporated in a generalized modeling framework. In this study, we evaluate the timing and extent of silicon isotopic re-equilibration during amorphous silica formation, a common secondary mineral in weathering environments, through a combination of both laboratory experiments and multi-component numerical modeling. A series of amorphous silica precipitation experiments were conducted over a range of surface areas and solid to fluid ratios at 20 °C and near neutral pH for a period of 30 days. These experiments show evidence that surface area exerts a first order control on the extent of isotopic re-equilibration following secondary mineral precipitation. New mineral growth onto higher surface areas seeds resulted in rapid isotopic re-equilibration (over a period of ∼2 weeks) whereas precipitation onto lower surface area grains retained a kinetic signature due to overall slower kinetics. Experimental results were further supported by a modified numerical reaction network model that has the novel capacity to explicitly treat the depth into the mineral that the fluid can chemically and isotopically exchange with through time. These numerical simulations demonstrate that the fluid-mineral surface interaction depth is larger for low surface area seed crystals where both a larger mass of amorphous silica was formed, and slower precipitations rates kept the reaction far from equilibrium, thereby impeding the influence of the back reaction and corresponding isotopic re-equilibration. These results highlight the significant role that water-rock interaction and, specifically, the mineral surface plays in the preservation of isotopic signatures within secondary silicate phases. Further, we suggest that a transient model approach is necessary to improve quantitative interpretations of observed isotope behavior in natural systems.

Geochemical effects of cement mineral variations on water–rock–CO2 interactions in a sandstone reservoir as an experiment and modeling study

AbstractThe complexity of water–rock–CO2 interactions associated with CO2 geological storage requires more comprehensive understanding with regard to the geochemical characteristics of reservoirs. In this paper, five groups of batch reactions and coupled simulations were conducted, in which rocks of different cementation types were reacted with purified water and CO2 at 180°C and 18 Mpa for 15 days, to reveal the possible geochemical effects of cement mineral variations on water–rock–CO2 interactions. With water chemistry and mineral alteration being monitored, the thermodynamics and kinetic characteristics of each CO2–water–rock system were fully analyzed in PHREEQC by the method of mineral saturation index, mineral phases diagram, and kinetics modeling to reveal the possible reaction paths and to compare their geochemical differences, which are caused by cement mineral variations. The experiment identified quite different dissolution characteristics and rates for cement minerals, and as a result, favored a diverse water chemistry and precipitation of different secondary minerals. Generally, the sensitive orders of cement mineral variations due to water–rock–CO2 interactions are carbonates, argillaceous, and siliceous minerals. The modeling showed good consistency with experiment results especially in cation evolution but underestimated the dissolution rate of alkali feldspar and carbonate mineral solubility. In addition, the modeling also predicted the carbonate reprecipitation of dolomite and calcite as cement minerals themselves, as well as dolomite precipitation at the expense of chlorite and calcite dissolution, which we failed to observe in the experiment. Thus, special attention should be paid to cement mineral variations when conducting CO2 injection in sandstone reservoirs. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

Mobilization of Metal(oid) Oxyanions through Circumneutral Mine Waste-Rock Drainage.

Most studies on the weathering of mine waste rock focus on the generation of acidic drainage with high metal concentrations, whereas metal(loid) release under neutral-rock drainage (NRD) conditions has received limited attention. Here, we present geochemical and mineralogical data from a long-term (>10 years) kinetic testing program with 50 waste-rock field barrels at the polymetallic Antamina mine in Peru. The weathering of most rock lithologies in the field experiments generated circumneutral to alkaline drainage (6 < pH < 9) but with concentrations of the oxyanion-forming metal(loid)s As, Mo, Se, and Sb in the mg/L range. The mobilization of As and Sb was particularly efficient from intrusive, marble and hornfels rocks that contained labile As- and Sb-sulfides, irrespective of bulk elemental content or waste-rock reactivity. High-alkalinity drainage from these materials sustained neutral-pH conditions that are unfavorable to oxyanion adsorption onto Fe-(oxyhydr)oxides and, therefore, enhanced As and Sb leaching. The release of Mo and Se from sulfidic skarn and intrusive waste rock was more proportional to elemental content but equally enhanced by pH-inhibited adsorption and negligible secondary mineral precipitation under NRD conditions. Our results demonstrate that oxyanion concentrations of environmental concern may be conveyed by neutral- to alkaline-pH waste-rock drainage and should be a focus of mine wastewater monitoring programs.

PORE SYSTEM QUANTIFICATION AND CHARACTERIZATION IN VOLCANIC ROCKS: A CASE STUDY FROM THE LOWER CRETACEOUS SERRA GERAL GROUP, PARANÁ BASIN, SOUTHERN BRAZIL

Studies of pore systems in volcanic rocks are of increasing importance due to these rocks' potential as reservoirs for hydrocarbons. For this paper, samples of basaltic pahoehoe, rubbly pahoehoe and acidic lava from the Lower Cretaceous Serra Geral Group (Paraná Basin, southern Brazil) were analysed in order to quantify and characterize the constituent pore systems. The Serra Geral Group volcanics were erupted as part of the Paraná‐Etendeka Igneous Province in the Early Cretaceous (Valanginian – Hauterivian). Analyses included experimental measurements by permo‐porosimeter integrated with X‐ray microtomography (μ‐CT) image analysis of vertical and horizontal sample plugs. In addition petrographic analyses were carried out to characterize pore types in thin section. The experimental porosity values ranged from 0.11 to 13.08% and most permeability values were generally lower than 0.0004 mD. Values varied as much within flow zones as they did between them.Porosity and permeability values were not sufficient for the Serra Geral Group volcanics to be considered as a potential reservoir analogue. However the wide range in values was attributed to the processes which controlled the origin and development of the pore system. Primary pores observed included intracrystalline sieve, vesicular and interflow laminar types; secondary porosity, such as spongy, interclast and intra‐matrix pore types, was related to dissolution and the precipitation of secondary minerals. The porosity values obtained by μ‐CT (between &lt;0.01 and 3.37%) were lower than those experimentally measured by permo‐porosimeter. This was attributed to the presence of multi‐scale pores in the volcanic rocks sampled, and also to limitations with image resolution. Even so, the use of μ‐CT allowed the visualization of porosity variations and was useful in characterizing the pore system.The results presented in this paper demonstrate that the volcanic rocks in the Serra Geral Group have a heterogeneous pore system, similar to that in carbonate rocks.

Saline lakes of Northern Kazakhstan: Geochemical correlations of elements and controls on their accumulation in water and bottom sediments

This paper is a review of the saline endorheic lakes of the Northern Kazakhstan, with the provided data on chemical composition of their waters and sediments, diversity/uniformity, mineral precipitation sequences and brine evolution. Study saline lakes are located in the Ishim steppe geographical unit of the southern Western Siberia and are ancient closed basins. The lakes are of Cl–Na and Cl–Na–Mg chemical types (the most saline Zhalauly lake (428 g/L) is Cl–Mg type), their pH values vary from 6.0 (Kalibek lake) to 9.1 (Shureksor Lake). The common trend for the lakes chemical state is a low calcium concentration with high values of sodium, i.e. calcium < magnesium < sodium (with three exceptions). The Na/Cl, Cl/SO4 and Mg/Ca ratios were calculated for the solutions from the lakes in order to reveal their primary trends of geochemical evolution. The geochemical indexes of enrichment and pollution (EF and Igeo) of the bottom sediments show that enrichment is low or absent in comparison with the geochemical background for the lakes of Western Siberia. We imply four main stages of lake evolution indicated by precipitation of different secondary minerals: calcite and Mg-carbonates, gypsum, halite, thenardite and bischofite. We suggest the onset of crystallization of each mineral as a tipping point of the water evolution that resulted in the complete removal of a certain chemical element consistent with the solubility constant of corresponding mineral.

A coupled microscopy approach to assess the nano-landscape of weathering

Mineral weathering is a balanced interplay among physical, chemical, and biological processes. Fundamental knowledge gaps exist in characterizing the biogeochemical mechanisms that transform microbe-mineral interfaces at submicron scales, particularly in complex field systems. Our objective was to develop methods targeting the nanoscale by using high-resolution microscopy to assess biological and geochemical drivers of weathering in natural settings. Basalt, granite, and quartz (53–250 µm) were deployed in surface soils (10 cm) of three ecosystems (semiarid, subhumid, humid) for one year. We successfully developed a reference grid method to analyze individual grains using: (1) helium ion microscopy to capture micron to sub-nanometer imagery of mineral-organic interactions; and (2) scanning electron microscopy to quantify elemental distribution on the same surfaces via element mapping and point analyses. We detected locations of biomechanical weathering, secondary mineral precipitation, biofilm formation, and grain coatings across the three contrasting climates. To our knowledge, this is the first time these coupled microscopy techniques were applied in the earth and ecosystem sciences to assess microbe-mineral interfaces and in situ biological contributors to incipient weathering.

An experimental study on dynamic coupling process of alkaline feldspar dissolution and secondary mineral precipitation

In order to clarify the dynamic process of feldspar dissolution–precipitation and explore the formation mechanism of secondary porosity, six batch reactor experiments were conducted at 200 °C and pH = 7 measured at room temperature. Temporal evolution of fluid chemistry was analyzed with an inductively coupled plasma optical emission spectrometer (ICP-OES). Solid reaction products were retrieved from six batch experiments terminated after 36, 180, 276, 415, 766 and 1008 h. Scanning electron microscopy (SEM) revealed dissolution features and significant secondary mineral adhered on the feldspar surface. The process of feldspar dissolution–precipitation proceeded slowly and full equilibrium was not achieved after 1008 h. Saturation indices suggested that the albite and K-feldspar dissolution occurred throughout the experiments. The average dissolution rates for albite and K-feldspar were 2.28 × 10−10 and 8.51 × 10−11 mol m−2 s−1, respectively. Based on the experimental data, the reaction process of alkaline feldspar was simulated and the secondary porosity had increased 0.3% after the experiment.

Effect of biochar on fraction and species of antimony in contaminated soil

Antimony (Sb) is a highly toxic heavy metal, and its amount in soil is increasing due to anthropogenic activities. Excessive Sb intake could ultimately threat human health. Recently, biochar (BC) has been accepted for remediation of Sb-contaminated soil. Understanding the interaction between BC and Sb and the effect of BC-induced changes in soil properties on immobilization/mobilization of Sb will help, therefore, to elucidate the mechanism of BC in immobilization/mobilization of Sb in contaminated soils. Wheat straw-derived BC (SBC) and fruit (apple) tree-derived BC (FBC) were obtained at the pyrolysis temperatures of 500 °C. Sb-contaminated soil was incubated with/without 0.5, 5, or 10 wt% of SBC and FBC for 130 days. Change of soil properties induced by BC was explored during the incubation. Dynamic change of Sb fraction and speciation were assessed by the sequential chemical extraction and citric acid extraction, respectively. The X-ray photoelectron spectroscopy (XPS) was employed to check elemental change of soil with 10% SBC at different incubation times. The correlation analysis and principal component analysis (PCA) were used to analyze relationship between Sb immobilization/mobilization and change of soil properties induced by BC. The obvious change of soil properties can be observed when the soil was treated with 10% BC instead of 5 and 0.5% BC. During the first 20 days with SBC incubation and 50 days with FBC incubation, Sb mobilization increased may be because of the electrostatic repulsion of functional groups in BC; OM and functional group of BC govern the reduction reactions, anionic competition, electrostatic repulsion, and biological reduction in soil induced by BC. By contrast, after 20 days with SBC incubation and 50 days with FBC incubation, the mobilization of Sb decreased, which may be attributed to the formation of complexes between Sb and OM of BC, secondary mineral precipitation, and organic complex between Sb and humus acid in soil induced by BC. It is noteworthy that the application of BC has a potential mobilizing risk for Sb and the final effect depends on BC characteristic and the change of soil properties induced by BC. The possible risks induced by BC should be considered before applying the BC to Sb-contaminated soil.

The kinetics of siderophore-mediated olivine dissolution.

Silicate minerals represent an important reservoir of nutrients at Earth's surface and a source of alkalinity that modulates long-term geochemical cycles. Due to the slow kinetics of primary silicate mineral dissolution and the potential for nutrient immobilization by secondary mineral precipitation, the bioavailability of many silicate-bound nutrients may be limited by the ability of micro-organisms to actively scavenge these nutrients via redox alteration and/or organic ligand production. In this study, we use targeted laboratory experiments with olivine and the siderophore deferoxamine B to explore how microbial ligands affect nutrient (Fe) release and the overall rate of mineral dissolution. Our results show that olivine dissolution rates are accelerated in the presence of micromolar concentrations of deferoxamine B. Based on the non-linear decrease in rates with time and formation of a Fe3+ -ligand complex, we attribute this acceleration in dissolution rates to the removal of an oxidized surface coating that forms during the dissolution of olivine at circum-neutral pH in the presence of O2 and the absence of organic ligands. While increases in dissolution rates are observed with micromolar concentrations of siderophores, it remains unclear whether such conditions could be realized in natural environments due to the strong physiological control on microbial siderophore production. So, to contextualize our experimental results, we also developed a feedback model, which considers how microbial physiology and ligand-promoted mineral dissolution kinetics interact to control the extent of biotic enhancement of dissolution rates expected for different environments. The model predicts that physiological feedbacks severely limit the extent to which dissolution rates may be enhanced by microbial activity, though the rate of physical transport modulates this limitation.