Iron isotope fractionation and atom exchange during sorption of ferrous iron to mineral surfaces

The application of stable Fe isotopes as a tracer of the biogeochemical Fe cycle necessitates a mechanistic knowledge of natural fractionation processes. We studied the equilibrium Fe isotope fractionation upon sorption of Fe(II) to aluminum oxide (γ-Al₂O₃), goethite (α-FeOOH), quartz (α-SiO₂), and goethite-loaded quartz in batch experiments, and performed continuous-flow column experiments to study the extent of equilibrium and kinetic Fe isotope fractionation during reactive transport of Fe(II) through pure and goethite-loaded quartz sand. In addition, batch and column experiments were used to quantify the coupled electron transfer-atom exchange between dissolved Fe(II) (Fe(II)_aq) and structural Fe(III) of goethite. All experiments were conducted under strictly anoxic conditions at pH 7.2 in 20 mM MOPS (3-(N-morpholino)-propanesulfonic acid) buffer and 23 °C. Iron isotope ratios were measured by high-resolution MC-ICP-MS. Isotope data were analyzed with isotope fractionation models. In batch systems, we observed significant Fe isotope fractionation upon equilibrium sorption of Fe(II) to all sorbents tested, except for aluminum oxide. The equilibrium enrichment factor, , of the Fe(II)_sorb-Fe(II)_aq couple was 0.85 ± 0.10‰ (±2σ) for quartz and 0.85 ± 0.08‰ (±2σ) for goethite-loaded quartz. In the goethite system, the sorption-induced isotope fractionation was superimposed by atom exchange, leading to a δ^56/54Fe shift in solution towards the isotopic composition of the goethite. Without consideration of atom exchange, the equilibrium enrichment factor was 2.01 ± 0.08‰ (±2σ), but decreased to 0.73 ± 0.24‰ (±2σ) when atom exchange was taken into account. The amount of structural Fe in goethite that equilibrated isotopically with Fe(II)_aq via atom exchange was equivalent to one atomic Fe layer of the mineral surface (∼3% of goethite-Fe). Column experiments showed significant Fe isotope fractionation with δ^56/54Fe(II)_aq spanning a range of 1.00‰ and 1.65‰ for pure and goethite-loaded quartz, respectively. Reactive transport of Fe(II) under non-steady state conditions led to complex, non-monotonous Fe isotope trends that could be explained by a combination of kinetic and equilibrium isotope enrichment factors. Our results demonstrate that in abiotic anoxic systems with near-neutral pH, sorption of Fe(II) to mineral surfaces, even to supposedly non-reactive minerals such as quartz, induces significant Fe isotope fractionation. Therefore we expect Fe isotope signatures in natural systems with changing concentration gradients of Fe(II)_aq to be affected by sorption.     

Electron transfer at the mineral/water interface: Selenium reduction by ferrous iron sorbed on clay

The mobility and availability of the toxic metalloid selenium in the environment are largely controlled by sorption and redox reactions, which may proceed at temporal scales similar to that of subsurface water movement under saturated or unsaturated conditions. Since such waters are often anaerobic and rich in Fe²⁺, we investigated the long-term (?1 month) kinetics of selenite sorption to montmorillonite in the presence of Fe²⁺ under anoxic conditions. A synthetic montmorillonite was used to eliminate the influence of structural Fe. In the absence of aqueous Fe²⁺, selenite was sorbed as outer-sphere sorption complex, covering only part of the positive edge sites, as verified by a structure-based MUSIC model and Se K-edge XAS (X-ray absorption spectroscopy). When selenite was added to montmorillonite previously equilibrated with Fe²⁺ solution however, slow reduction of Se and formation of a solid phase was observed with Se K-edge XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine-structure) spectroscopy. Iterative transformation factor analysis of XANES and EXAFS spectra suggested that only one Se reaction product formed, which was identified as nano-particulate Se(0). Even after one month, only 75% of the initially sorbed Se(IV) was reduced to this solid species. Mössbauer spectrometry revealed that before and after addition and reduction of Se, 5% of total sorbed Fe occurred as Fe(III) species on edge sites of montmorillonite (≈2 mmol kg⁻¹). The only change observed after addition of Se was the formation of a new Fe(II) species (15%) attributed to the formation of an outer-sphere Fe(II)-Se sorption complex. The combined Mössbauer and XAS results hence clearly suggest that the Se and Fe redox reactions are not directly coupled. Based on the results of a companion paper, we hypothesize that the electrons produced in the absence of Se by oxidation of sorbed Fe(II) are stored, for example by formation of surface H₂ species, and are then available for the later Se(IV) reduction. The slow reaction rate indicates a diffusion controlled process. Homogeneous precipitation of an iron selenite was thermodynamically predicted and experimentally observed only in the absence of clay. Interestingly, half of Fe was oxidized in this precipitate (Mössbauer). Since DFT calculations predicted the oxidation of Fe at the water-FeSe solid interface only and not in the bulk phase, we derived an average particle size of this precipitate which does not exceed 2 nm. A comparison with the Mössbauer and XAS spectra of the clay samples demonstrates that such homogenous precipitation can be excluded as a mechanism for the observed slow Se reduction, emphasizing the role of abiotic, heterogeneous precipitation and reduction for the removal of Se from subsurface waters.     

基于数据科学的矿产资源定量预测的理论与方法探索

矿产资源预测已从定性走向了定量,从数据稀疏型走向了数据密集型,亟须数据科学支撑.本文在前人研究基础上,讨论了基于数据科学的矿产资源定量预测理论与方法,该方法的理论基础为相关性理论与异常理论,前者采用监督的机器学习方法挖掘地质找矿大数据与矿床的相关性为预测未发现矿床提供了理论基础;后者采用非监督的机器学习方法识别地质找矿...     

降低矿产资源定量预测不确定性的双向预测方法

成矿过程是一个复杂的物理化学过程,由于地质自身的不确定性、原始数据采集和处理的不当、预测方法中经验参数的不确定性等多重因素的叠加,造成矿产资源定量预测结果中潜在大量不确定性.在科学认识这些不确定性的基础上,如何降低不确定性是预测评价研究的一个重要方向.以地质异常理论和成矿动力学为指导,双向预测评价方法是降低地质异常分析...     

An integrated method for the quantitative evaluation of mineral resources of cobalt-rich crusts on seamounts

Cobalt-rich crusts on seamounts potentially have the economic value of multiple metals. In the field of exploration, it is important to perform quantitative evaluations of mineral resources and delineate promising areas in survey regions for future mining. Accordingly, this study, based on prior knowledge, develops an integrated method to quantitatively evaluate mineral resources of cobalt-rich crusts on seamounts and gives an application example to demonstrate this method. The method includes four steps: first, defining units with certain areas and shapes on the target seamount (a 20 km² square block in the application example) and estimating characteristic values of the cobalt-rich crust for each unit with known geological survey data using a space interpolation method such as Kriging; second, presenting several model algorithms, i.e. Regional Coverage of Crusts, Suitable Slope Percentage for Mining and Fitting Area on Slopes, to extract the corresponding regional metallogenic factors for each unit by inputting regional surveying data (such as bathymetry data) into these models; third, considering both the features and regional metallogenic factors of cobalt-rich crusts in each unit to estimate their distribution of mineral resources on the entire seamount; and last, according to the distribution of the mineral resources and international social and economic requirements (such as the regulations of the International Seabed Authority), delineating a promising area for future mining.     

中酸性火山岩中造岩矿物含量的定量计算方法

主要造岩矿物的含量是火山岩分类定名的主要依据。对于基质为微晶质、隐晶质的火山岩,采用常规的薄片显微镜统计矿物含量几乎是不可行的。目前已有的利用岩石化学成分计算矿物含量的方法均不适用于中酸性火山岩。因此,本文基于岩石与矿物之间的化学成分质量平衡原理,提出了计算火山岩实际矿物含量的方法。该方法是利用实际矿物成分,采用最优化方法计算岩石中实际矿物的含量,在很大程度上避免了计算中的人为性。对中酸性火山岩实际样品计算表明,该方法使用简单方便,精度较高。     

Biogeochemistry and bacteriology of ferrous iron oxidation in geothermal habitats

A survey of hot, acid springs in Yellowstone Park has shown that high concentrations of ferrous and ferric iron are often present. Total ionic iron concentrations in different springs ranged from less than 1 ppm to greater than 200 ppm, and up to 50% of the ionic iron was in the ferrous form. Some of these springs also have high concentrations of reduced sulfur species (S^2? and S⁰). Significant populations of the bacterium Sulfolobus, acidocaldarius, an autotrophic organism able to live and oxidize sulfur compounds at low pH and high temperature, were present in most of these springs. The role of this organism in the oxidation of ferrous iron was investigated by incubating natural samples of water and assaying for disappearance of ferrous iron. Controls in which bacterial activity was inhibited by addition of 10% NaCl were also run. Bacterial oxidation of ferrous iron occurred in most but not all of the spring waters. The temperature optimum for oxidation varied from spring to spring, but significant oxidation occurred at temperatures of 80–85°C, but not at 90°C. Thus, 85–90°C is the upper temperature at which bacterial iron oxidation occurs; a similar upper limit has previously been reported for sulfur oxidation in the same kinds of springs. The steady-state concentrations of ferrous and ferric iron are determined by the rate at which these ions move into the spring pools with the ground water (flow rate), by the rate at which ferric iron is reduced to the ferrous state by sulfide, and by the rate of bacterial oxidation. The bacterial oxidation rate is faster than the flow rate, so that the rate of reduction of ferric iron is probably the rate-controlling reaction. In several springs, no decrease in ferrous iron occurred, even though high bacterial populations were present. It was shown that in these springs, ferrous iron oxidation occurred but the ferric iron formed was reduced back to the ferrous state again. These springs were all high in suspended sediment and the reductant was shown to be present in the sediment. X-ray diffraction revealed that the sediment contained three major ingredients, elemental sulfur, natroalunite, and quartz. Chemical analyses showed a small amount of sulfide, too little to reduce the ferric iron. Elemental sulfur itself did not reduce ferric iron but when elemental sulfur was removed from the sediment by CS₂ extraction, the activity of the sediment was abolished. It is hypothesized that the sulfide present in the sediment (possibly bound to natroalunite) reacts with elemental sulfur to form a reductant for ferric iron. The results show that bacteria can have a profound influence on the ferrous/ferric ratios of geothermal systems, but that temperature and mineral composition of the water may significantly influence the overall result.     

Effect of the disproportionation reaction of ferrous iron in impact-evaporation processes

Evidence for the disproportionation of iron was found in model experiments imitating impact melting, evaporation, and condensation. The experiments were carried out using a laser system at a characteristic temperature of ～3000–4000 K and a pulse duration of ～10^?3 s in a He atmosphere (P = 1 atm). Augite and mixtures of peridotite with MnO₂ and WO₃ were used as starting target materials. Experimental products (condensed vapor phase) were analyzed by X-ray photoelectron spectroscopy. The results of condensate analysis provided compelling evidence for the presence of iron in three oxidation states (Fe⁰, Fe²⁺, and Fe³⁺). In an experiment with augite, the proportions of iron species of different valences were similar to the stoichiometry of the disproportionation reaction. Similar evidence for this reaction was first found in a condensate from the samples of the fine fraction of the Luna 16 regolith. In the layers of the lunar condensate, the proportions of the valence states of iron were on average Fe⁰:Fe²⁺:Fe³⁺ = 1.2: 1.9: 0.7.     

Partitioning of ferric and ferrous iron between plagioclase and silicate melt

Using the thermodynamic algorithm of Sugawara (Contributions to Mineralogy and Petrology 141, 2001, p. 659–686), FeO and Fe₂O₃ concentrations in plagioclase were computed for 420 published experiments on tholeiitic, FeTi-tholeiite, calc-alkaline, and alkaline magma compositions. Estimates of the partition coefficient between plagioclase and liquid range from 0.19 to 0.92 for Fe₂O₃ and from 0.008 to 0.050 for FeO, i.e. ca. twenty times greater for Fe₂O₃ than for FeO. Partitioning of Fe₂O₃ and FeO is independent of both oxygen fugacity and plagioclase composition, contradicting the common assumption that partitioning of Fe₂O₃ correlates positively with the amount of aluminium in plagioclase. In contrast, the SiO₂-content of the magma correlates positively with the partition coefficients for Fe₂O₃ and FeO. This is ascribed to increasing activity of iron in polymerised SiO₂-rich magma. Advances of micro-beam Fe-XANES techniques allow the determination of Fe³⁺/Fe in plagioclase. Using such plagioclase data and the partition coefficients for Fe₂O₃ and FeO, the Fe₂O₃/FeO and oxygen fugacity of equilibrium magma may be estimated. As petrological examples, we estimate that the oxygen fugacity of the Palisades sill ranged from the QFM buffer to 0.5 log unit below it (QFM to QFM –0.5), the Lake County basalt from QFM to QFM –2, and Upper Zone a of the Skaergaard intrusion from QFM –1 to QFM –1.5.Editorial responsibility: I. Parsons     

Decomposed and weighted characteristic analysis for the quantitative estimation of mineral resources

Characteristic analysis has been employed as a means of selecting favorable targets for mineral exploration. This paper describes a new version of characteristic analysis that is designed to estimate mineral resources as well as delineating exploration targets. The new version, referred to as decomposed and weighted characteristic analysis, employs a weighting scheme for both samples and variables involved in the model to extract the useful information on tonnages and the order of importance of variables. To construct a decomposed model, an optimum cutting technique for the ordered quantity is developed. The model is demonstrated on a case study of pegmatitic Nb-Ta deposits in China.     

The kinetics of the oxidation of ferrous iron in synthetic and natural waters

The rate of oxidation of ferrous iron in a seasonally anoxic lake was measured on 39 occasions with respect to both depth and time. Sample disturbance was minimal as only oxygen had to be introduced to initiate the reaction. The data were consistent with the simple rate law for homogeneous chemical kinetics previously established for synthetic solutions. The rate constant for the oxidation reaction in lake water was indistinguishable from that measured in synthetic samples. It did not appear to be influenced by changes in the microbial populations or by changes in any particulate or soluble components in the water, including iron and manganese. Analysis of the errors inherent in the kinetic measurements showed that the estimation of pH was the major source of inaccuracy and that values of the rate constant determined by different workers could easily differ by a factor of six.The present data, together with a comprehensive survey of the literature, are used to suggest a ‘universal’ rate constant of ca. 2 × 10¹³ M^?2 atm^?1 min^?1 (range 1.5–3 × 10¹³) in the rate law $\text{?d[Fe II]}\text{dt}\text{= k[Fe II]pO}{}_2\text{(OH?)}{}^2$ for natural freshwaters in the pH range 6.5–7.4. Discrepancies in the effects of ionic strength and interfering substances reported in the literature are highlighted. Generally substances have only been found to interfere at concentrations which far exceed those in most natural waters.     

A model for the quantitative estimation of mineral liberation by grinding

An exact expression is derived for the fraction of particles of mesh size D that contain less than a prescribed fraction of any particular mineral. The expression is obtained entirely in terms of the distributions of linear intercept lengths of the minerals in the ore. These distributions can be obtained by line traverses across a section of the ore. No other statistical information regarding the mineral grain sizes is required. The theory is completely free of empirical constants or other parameters and in particular no assumptions are made regarding the shape of the mineral grains in the ore or of the particles.The theory predicts that the fractional liberation of mineral at mesh size D is given by:

$\text{L(D)=1?}\text{1}\text{μ}\text{∫}\text{0}\text{D}{}_{\mathrm{u}}\text{{1?N(}\text{l}\text{D}\text{)} {2?F(l)}dl}$

where F(l) is the distribution of linear intercept lengths for the mineral and μ is the mean linear intercept length for the mineral. N(l/D) is the linear intercept distribution function for particles of mesh size D and Du is the largest intercept length across any particle of mesh size D.The theory was confirmed experimentally for the liberation of pyrite from Witwatersrand quartzite.     

碱熔重铬酸钾滴定岩石中全铁的不确定度评定

通过碱熔重铬酸钾滴定岩石中全铁的分析,找出了影响滴定结果的各个分量。影响滴定结果的分量有:配制重铬酸钾标准溶液的浓度、称取试样的质量、滴定过程中所消耗重铬酸钾标准溶液的体积以及铁的摩尔质量等。在此基础上对不确定度的分量进行了评定与合成,给出了扩展不确定度,并按照计量技术规范,结出了相应的表示形式。     

AUSTRALIS: A new tool for the study of isotopic systems and geochronology in mineral systems

AUSTRALIS (AMS for Ultra Sensitive TRAce eLement and Isotopic Studies) is a microbeam accelerator mass spectrometry (AMS) system designed for in situ microanalysis of geological samples for trace elements and radiogenic and stable isotope data. The AMS method eliminates molecular and isobaric interferences in in situ mass spectrometric measurements, opening up new opportunities in geochronology and tracer applications. Tests have been carried out for measurements of Pb, S and Os isotopes, conducted mainly at 1.5 MV accelerating voltage. In Pb and S tests, precision as high as 0.3‰ has been obtained, made possible by a fast isotope switching system to counter the effect of instabilities in the ion source and beam transport system. In trace‐element analysis, a detection limit for Au at the sub‐ppb level was obtained.     

空白试验对铬铁矿中亚铁测定准确度的影响初探

本文初步探讨了空白试验对铬铁矿中亚铁测定准确度的影响,提出了最佳分析操作条件,应用于分析测试实践中取得了较为满意的效果。     

Evaluation of mineral substrates for in situ iron removal from groundwater

Calcium carbonate-based materials (CCBM) have been found to remove Fe(II) and other divalent metal cations from aqueous solution and thus have the potential for incorporation into remediation systems to remove Fe(II) from groundwater at landfills. Research was conducted to examine the ability of a range of CCBM to remove Fe(II) and assess the mechanism of removal. Different CCBM (limestone, concrete, dolomite, marble), as well as gypsum, witherite, and quartz sand, were tested for their ability to remove Fe(II) from water using batch tests conducted under anaerobic conditions. Limestone (specific surface area of approximately 0.46 m²/g) was found to have the best removal effectiveness, and the final Fe(II) concentration was reduced from 50 to <0.01 mg/L. Kinetics experiments conducted over a 72 h period indicated that the removal process of Fe(II) by CCBM was a two-step process. The first step is rapid sorption of Fe(II) onto the CCBM surfaces within the first hour, and the second step is relatively slow co-precipitation of iron-containing solids formed through various chemical reactions. The two best performing CCBM (limestone and concrete) were evaluated for their removal ability based on media particle size (diameters of 3–5, 7–10, 15–25, and 40–50 mm) and revealed statistically significant (p < 0.01) increases in Fe(II) removal for each particle size class examined. SEM analysis of reacted materials revealed visible precipitates on the reactive material surface; XRD analysis was not able to detect crystalline Fe minerals on limestone surface.     

Influence of ferric iron on the stability of mineral assemblages

Ferric iron is present in all metamorphic rocks and has the ability to significantly affect their phase relations. However, the influence of ferric iron has commonly been ignored, or at least not been considered quantitatively, mainly because its abundance in rocks and minerals is not determined by routine analytical techniques. Mineral equilibria calculations that explicitly account for ferric iron can be used to examine its effect on the phase relations in rocks and, in principle, allow the estimation of the oxidation state of rocks. This is illustrated with calculated pseudosections in NCKFMASHTO for mafic and pelitic rock compositions. In addition, it is shown that ferric iron has the capacity to significantly increase the stability of the corundum + quartz assemblage, making it possible for this assemblage to exist at crustal P–T conditions in oxidized rocks of appropriate composition.     

大数据时代数字找矿与定量评价

这是一个科技飞速发展的时代,也是一个数据爆炸的时代。不同学科领域及不同类型工作都需要针对其特点开发和建立自己的数据科学,培养各自领域的科学人才。通过对大数据时代概念和形成的深入理解,将其引入到地学领域的研究工作中,从而提出数字找矿的概念,实现数学地质到数字地质的飞跃,弥补传统定性找矿的缺陷,结合四大找矿系统理论,对矿产预测做出科学的定量评价与分析,是未来的必然发展趋势。     

大数据时代数字找矿与定量评价

这是一个科技飞速发展的时代,也是一个数据爆炸的时代。不同学科领域及不同类型工作都需要针对其特点开发和建立自己的数据科学,培养各自领域的科学人才。通过对大数据时代概念和形成的深入理解,将其引入到地学领域的研究工作中,从而提出数字找矿的概念,实现数学地质到数字地质的飞跃,弥补传统定性找矿的缺陷,结合四大找矿系统理论,对矿产预测做出科学的定量评价与分析,是未来的必然发展趋势。     

Limitations on the applicability of the crystallographic method for determining the topology of the universe

The regular crystallographic method for determining the topology of the spatial section of the Universe from observational data is of limited applicability if the mean density of the Universe has its critical value.