Stable Mineral Research Articles

Methanogen Productivity and Microbial Community Composition Varies With Iron Oxide Mineralogy.

Quantifying the flux of methane from terrestrial environments remains challenging, owing to considerable spatial and temporal variability in emissions. Amongst a myriad of factors, variation in the composition of electron acceptors, including metal (oxyhydr)oxides, may impart controls on methane emission. The purpose of this research is to understand how iron (oxyhydr)oxide minerals with varied physicochemical properties influence microbial methane production and subsequent microbial community development. Incubation experiments, using lake sediment as an inoculum and acetate as a carbon source, were used to understand the influence of one poorly crystalline iron oxide mineral, ferrihydrite, and two well-crystalline minerals, hematite and goethite, on methane production. Iron speciation, headspace methane, and 16S-rRNA sequencing microbial community data were measured over time. Substantial iron reduction only occurred in the presence of ferrihydrite while hematite and goethite had little effect on methane production throughout the incubations. In ferrihydrite experiments the time taken to reach the maximum methane production rate was slower than under other conditions, but methane production, eventually occurred in the presence of ferrihydrite. We suggest that this is due to ferrihydrite transformation into more stable minerals like magnetite and goethite or surface passivation by Fe(II). While all experimental conditions enriched for Methanosarcina, only the presence of ferrihydrite enriched for iron reducing bacteria Geobacter. Additionally, the presence of ferrihydrite continued to influence microbial community development after the onset of methanogenesis, with the dissimilarity between communities growing in ferrihydrite compared to no-Fe-added controls increasing over time. This work improves our understanding of how the presence of different iron oxides influences microbial community composition and methane production in soils and sediments.

Noble gas constraints on the fate of arsenic in groundwater

Groundwater contamination of geogenic arsenic (As) remains a global health threat, particularly in south-east Asia. The prominent correlation often observed between high As concentrations and methane (CH4) stimulated the analysis of the gas dynamics in an As contaminated aquifer, whereby noble and reactive gases were analysed. Results show a progressive depletion of atmospheric gases (Ar, Kr and N2) alongside highly increasing CH4, implying that a free gas phase comprised mainly of CH4 is formed within the aquifer. In contrast, Helium (He) concentrations are high within the CH4 (gas) producing zone, suggesting longer (groundwater) residence times.We hypothesized that the observed free (CH4) gas phase severely detracts local groundwater (flow) and significantly reduces water renewal within the gas producing zone. Results are in-line with this hypothesis, however, a second hypothesis has been developed, which focuses on the potential transport of He from an adjacent aquitard into the (CH4) gas producing zone. This second hypothesis was formulated as it resolves the particularly high He concentrations observed, and since external solute input from the overlying heterogeneous aquitard cannot be excluded.The proposed feedback between the gas phase and hydraulics provides a plausible explanation of the anti-intuitive correlation between high As and CH4, and the spatially highly patchy distribution of dissolved As concentrations in contaminated aquifers. Furthermore, the increased groundwater residence time would allow for the dissolution of more crystalline As-hosting iron(Fe)-oxide phases in conjunction with the formation of more stable secondary Fe minerals in the hydraulically-slowed (i.e., gas producing) zone; a subject which calls for further investigation.

A clean non-toxic method for leaching of barium tungstate

• A new method for leaching BaWO 4 with Na 2 SO 4 is proposed. • Thermodynamics proves that Na 2 SO 4 is an effective decompose agent. • The leaching rate of BaWO 4 can reach more than 99%. • The product is stable, eco-friendly and non-toxic BaSO 4 . • BaSO 4 can be used as a barium meal, which realizing zero waste. Barium tungstate is one of the important components of complex tungsten minerals and is also an intermediate product of hydrometallurgy. Since it is thermodynamically a very stable mineral, most of the current tungsten ore leaching methods are ineffective for its processing. In addition, due to the high toxicity of barium ions, special attention must be paid during the leaching process. In order to solve these problems, a new method of using sodium sulfate to decompose barium tungstate is proposed in this article. A thermodynamic analysis of the Ba 2+ -SO 4 2– -WO 4 2– -H 2 O system is carried out, in which the changes of various ions and precipitations at different pH values and different amounts of sodium sulfate were explored. Moreover, this method is theoretically proved to be feasible. Also, the leaching of barium tungstate experiment was carried out. Under optimum conditions with liquid/solid ratio 10:1 mL/g, reaction temperature 75 °C, and Na 2 SO 4 dosage 2.5–3.0 M times to BaWO 4 , the leaching rate of barium tungstate was >99%. This method has the advantages of simplicity, and high leaching rate, and it is cost effective with no barium toxicity.

Evolution of Black Talc upon Thermal Treatment

Black talc is a natural silicate clay mineral with a typical 2:1 layered structure, low electrical conductivity, large specific surface area, and high thermal stability. The world’s largest black talc mine, with known reserves of one billion tons, is located in China’s Jiangxi province. Due to the restriction of its color, the application of black talc is only limited to ceramic raw materials, coating filler, waterproof materials, and other low-end application industries. Thermal treatment is a common method of clay mineral modification. It is vital to examine the structural and physical changes of black talc during calcination in order to prepare black-talc-based composites and to broaden their applications. This work discusses the evolution of black talc upon thermal treatment (30–1000 °C) and the corresponding structural changes. The thermal stability of minerals was analyzed via thermogravimetric (TG) analysis and thermogravimetry–mass spectrometry (TG-MS). The decomposition of minerals during calcination consists of four processes: dehydration, organic carbon decomposition, dihydroxylation, and phase transformation. In situ FTIR and in situ XRD were employed to track changes in black talc in real time during thermal treatment. At 800 °C, black talc was found to begin to go through dihydroxylation, and the crystallinity index decreased significantly. The XRD pattern of samples at 950 °C (T950) showed the reflection of the enstatite structure, and the relative crystallinity index was 27.3%, indicating that the mineral had undergone phase transformation. In addition, the Brunauer–Emmet–Teller (BET), laser particle size analyzer, Zeta potential, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques were used to systematically characterize the physicochemical properties of minerals at different temperatures. The results show that black talc’s particle size and specific surface area increase with the calcination temperature. The surface charge changes, and more amorphous SiO2 and MgO appear, indicating that thermal treatment could induce structural changes and activate the surface of black talc.

Performance and mechanisms of microwave-assisted zerovalent iron/pyrite for advance remediation of strongly alkaline high Cr(VI) contaminated soil

Strongly alkaline high Cr(VI) contaminated (SAHCR) soil poses a high risk to the environment and public health, yet lacks rapid and efficient remediation technology. In this study, a novel approach combining microwave irradiation with zerovalent iron/pyrite (FeS2/ZVI) was developed for the remediation of SAHCR soil. The results indicated that fast and efficient remediation of the SAHCR soil was achieved by microwave irradiation-assisted FeS2/ZVI, with 99.9% of removal rate of Cr(VI) within 10 min, and Cr(VI) concentration from 3900.8 plummeted to 2.38 mg kg−1. The data of Cr(VI) reduction kinetics at different temperatures indicated that the activation energies (Ea) for microwave-FeS2/ZVI system was 27.4 kJ mol−1, 28.5% lower than that without microwave irradiation, suggesting that in addition to the heating effect of microwave, the accelerated Cr(VI) reduction also comes from the catalytic effect of “hot spots” on FeS2/ZVI under microwave irradiation. Furthermore, it was demonstrated that microwave irradiation promoted the transformation of reduced Cr(III) into the stable FeCr2O4 mineral and the excellent long-term stability of the remediated SAHCR soil. These findings can provide a perspective for advanced remediation of the difficult-to-treat SAHCR soil by the synergism of microwave irradiation with the iron-sulfur based reducing materials.

Effects of the Reservoir Environment and Oilfield Operations on the Iron Mineral Surface Charge Development: An Insight into Their Role in Wettability Alteration

Iron minerals are hydrophobic minerals predominately found in sandstone formations with their role in wettability alteration not fully understood. With the reservoir rock being a complex system of multiple minerals, the control of wettability alterations becomes difficult to manage due to the different surface chemistry of constituent minerals. This study presents for the first time the surface charge development of pyrite, magnetite, and hematite minerals in native reservoir environments and the effect of oilfield operations such as acidizing, water injection, polymerization, surfactant flooding, and alkaline flooding on iron minerals, surface charge development. This was achieved using zeta potential measurement, which depicts the type of surface charge and infers the propensity of precipitation. The experimental results show that pyrite, magnetite, and hematite behave similarly under reservoir conditions with all the minerals being positively charged although pyrite and hematite are negatively charged in seawater (SW). Results also show that the surface charge development is controlled by the electrical double-layer effect and ion adsorption on the mineral surfaces. These findings provide key insights into the role of iron minerals in wettability alteration as they provide surfaces that serve as a precursor for polar crude component adsorption. Also, the design of an ion-engineered fluid to control the surface charge of iron minerals was implemented, and the results showed that they behave differently. Furthermore, the effect of low-salinity water on the mineral surface charge was examined. The findings revealed that low-salinity water can produce a negatively charged surface, however, cannot mitigate the iron mineral precipitation challenge as the observed zeta potential values are near zero. Finally, slug and continuous injection of the ethylenediaminetetraacetic acid chelating agent was implemented as an iron mineral precipitation control strategy, with the continuous injection observed to significantly improve the colloidal stability of the iron minerals in the reservoir native state and after SW injection.

Stabilization of clay-rich interburdens using silica nanoparticles

Detachment of clay particles from clay-rich interburdens is the main source of solid production in coal seam gas wells (CSG), costing the Australian operators millions of dollars every year. Several techniques have been proposed to reduce the solid production and consequent pump failure in already constructed open hole/slotted liner wells. Amongst all remedial techniques for these existing problematic wells, chemical stabilization seems to be the most practical method. The high cost of chemical additives, their environmental issues, potential low effectiveness and technical difficulties for their placement downhole have, however, hindered their wide application. In particular, the long-term effect of current chemical additives on interburden stability and their influence on coal seam matrix gas diffusion need further investigation. In this study, the application of colloidal silica nanoparticle solution (silica SOL) as a long-term chemical stabilization method for clay-rich interburden in CSG wells was investigated. A series of comprehensive experiments were performed to investigate the effect of the silica SOL on stability of pure compacted dry smectite clays and clay-rich sandstone samples in stationary and turbulent conditions (resembling the wellbore condition during gas production) using a uniquely designed experimental setup. Additionally the effect of silica SOL on coal and its gas adsorption was analyzed. The results of this study demonstrated that the silica SOL is a suitable candidate for long-term chemical stabilization of clay minerals as a cost-efficient and environmentally friendly additive. Its non-reactivity with coal and its inert response on gas sorption is another significant benefit of the proposed chemical solution.

The Transformation of Green Rusts on the Surface of Portland Cement Hydration Materials

Ordinary Portland cement (OPC) is widely used building material, and its hydration products can be recycling as low-cost absorbents. The loading of iron oxide is helpful to further improve their adsorption performance. In view of the fact that green rusts are frequently occurrence intermediate products in the co-precipitation of iron salt and prone to be oxidized into stable iron minerals. This study simulated co-precipitation to carry out iron-modification on hydration OPC. The results demonstrate that as the carrier materials, hydration OPC behaves excellent affinity with green rusts. Under the conditions of high temperature (70 oC) and high alklinity (pH=11), green rusts mainly transformed into feroxydrate, and a minor part are into magnetite. The enrichment and transformation of green rust can be regarded as an efficient approach for immobilization of iron oxide (oxyhydroxide).

Research progress in the application of stable isotope and mineral element analysis in tracing the geographical origin of Chinese medicinal materials

The Geographical Origin of Chinese medicinal materials plays an important role in guaranteeing the authenticity of Chinese medicinal materials, ensuring the quality and efficacy of Chinese medicinal materials, maintaining the market order, and reducing the risk of medical accidents caused by counterfeit Chinese medicinal materials. In recent years, there have been more and more research papers on tracing Chinese medicinal materials by stable isotope and mineral element analysis combined with chemometrics. Using stable isotope and mineral element analysis to trace traditional Chinese medicinal materials has the advantages of high accuracy and strong reliability. Through chemometrics methods, medicinal materials from different origins can be classified, distinguished, and modeled, and finally the purpose of origin traceability can be achieved. Based on the introduction of the basic principles of stable isotopes and mineral elements, this article systematically analyzes the research and application of relevant stable isotope and mineral element analysis techniques in the origin traceability of Chinese medicinal materials in the past 10 years, and proposes the existing problems in origin traceability of Chinese medicinal materials. This article aims at providing reference for the origin traceability, confirmation and supervision of Chinese herbal medicines, and promoting the establishment and improvement of the traceability technical system of Chinese herbal medicines.

An ab-initio study on the thermodynamics of disulfide, sulfide, and bisulfide incorporation into apatite and the development of a more comprehensive temperature, pressure, pH, and composition-dependent model for ionic substitution in minerals

AbstractThe mineral apatite, Ca10(PO4)6(F,OH,Cl)2, incorporates sulfur (S) during crystallization from S-bearing hydrothermal fluids and silicate melts. Our previous studies of natural and experimental apatite demonstrate that the oxidation state of S in apatite varies systematically as a function of oxygen fugacity (fO2). The S oxidation states –1 and –2 were quantitatively identified in apatite crystallized from reduced, S-bearing hydrothermal fluids and silicate melts by using sulfur K-edge X-ray absorption near-edge structure spectroscopy (S-XANES) where S 6+/ΣS in apatite increases from ~0 at FMQ-1 to ~1 at FMQ+2, where FMQ refers to the fayalite-magnetite-quartz fO2 buffer. In this study, we employ quantum-mechanical calculations to investigate the atomistic structure and energetics of S(-I) and S(-II) incorporated into apatite and elucidate incorporation mechanisms.One S(-I) species (disulfide, S22−) and two S(-II) species (bisulfide, HS−, and sulfide, S2−) are investigated as possible forms of reduced S species in apatite. In configuration models for the simulation, these reduced S species are positioned along the c-axis channel, originally occupied by the column anions F, Cl, and OH in the end-member apatites. In the lowest-energy configurations of S-incorporated apatite, disulfide prefers to be positioned halfway between the mirror planes at z = 1/4 and 3/4. In contrast, the energy-optimized bisulfide is located slightly away from the mirror planes by ~0.04 fractional units in the c direction. The energetic stability of these reduced S species as a function of position along the c-axis can be explained by the geometric and electrostatic constraints of the Ca and O planes that constitute the c-axis channel.The thermodynamics of incorporation of disulfide and bisulfide into apatite is evaluated by using solid-state reaction equations where the apatite host and a solid S-bearing source phase (pyrite and Na2S2(s) for disulfide; troilite and Na2S(s) for sulfide) are the reactants, and the S-incorporated apatite and an anion sink phase are the products. The Gibbs free energy (ΔG) is lower for incorporation with Na-bearing phases than with Fe-bearing phases, which is attributed to the higher energetic stability of the iron sulfide minerals as a source phase for S than the sodium sulfide phases. The thermodynamics of incorporation of reduced S is also evaluated by using reaction equations involving dissolved disulfide and sulfide species [HnS(aq)(2−n) and HnS(aq)(2−n); n = 0, 1, and 2] as a source phase. The ΔG of S-incorporation increases for fluorapatite and chlorapatite, and decreases for hydroxylapatite, as these species are protonated (i.e., as n changes from 0 to 2). These thermodynamic results demonstrate that the presence of reduced S in apatite is primarily controlled by the chemistry of magmatic and hydrothermal systems where apatite forms (e.g., an abundance of Fe; solution pH). Ultimately, our methodology developed for evaluating the thermodynamics of S incorporation in apatite as a function of temperature, pH, and composition is highly applicable to predicting the trace and volatile element incorporation in minerals in a variety of geological systems. In addition to solid-solid and solid-liquid equilibria treated here at different temperatures and pH, the methodology can be easily extended to different pressure conditions by just performing the quantum-mechanical calculations at elevated pressures.

Experimental partitioning of fluorine and barium in lamproites

Abstract The dynamic properties and melting behavior of the Earth’s mantle are strongly influenced by the presence of volatile species, including water, carbon dioxide, and halogens. The role that halogens play in the mantle has not yet been fully quantified: their presence in only small quantities has dramatic effects on the stability of mantle minerals, melting temperatures, and in generating halogen-rich melts such as lamproites. Lamproites are volumetrically small volcanic deposits but are found on every continent on the planet: they are thought to be melts generated from volatile-rich mantle sources rich in fluorine and water. To clarify the mantle sources of lamproites, we present experimentally determined mineral/melt partition coefficients for fluorine and barium between phlogopite and lamproite melts. Both fluorine and barium are compatible in phlogopite [DF(Phl/Melt)0.96 ± 0.02 – 3.44 ± 0.33, DBa(Phl/Melt)0.52 ± 0.05 – 3.68 ± 0.43] at a range of pressures (5–30 kbar), temperatures (1000–1200 °C), and fluid compositions (C-O-H mixtures). Using our partition coefficients, we model the melt compositions produced by potential lamproite sources, including phlogopite garnet lherzolite, phlogopite harzburgite, and hydrous pyroxenite. The results demonstrate that hydrous pyroxenites and phlogopite garnet lherzolite can produce melts with F and Ba contents similar to lamproites, but only hydrous pyroxenites fully reproduce other geochemical characteristics of lamproites including high K2O, low CaO contents, and high F/H2O ratios.

Climate-driven drainage reorganization of small mountainous rivers in Taiwan (East Asia) since the last glaciation: The Zhuoshui River example

As an important component in East Asia sediment source-to-sink systems, small mountainous rivers in Taiwan deliver disproportionately large amounts of sediments to oceans. Although the modern fluvial sediment transport processes, discharge fluxes and sediment compositions have been well investigated, the drainage evolution of these mountainous rivers remains understudied and sediment fluxes are expected to vary greatly in glacial-interglacial cycles due to the tremendous climatic fluctuations. To define how the drainage of Taiwan mountainous rivers has evolved since the last glaciation, we target a sediment core (98 m in length) from the Zhuoshui River delta, western Taiwan and conduct sediment petrography, heavy mineral analysis, detrital zircon U Pb geochronology and clay mineralogy to investigate provenance variations and river basin evolution since 60 ka. Sediments of the last glaciation show comparatively high illite crystallinity index values, low metasedimentary lithic fragment and stable heavy mineral contents and similar detrital zircon U Pb ages with downstream signatures, indicating prominent sediment contributions from the Coastal Plain and Western Foothills regions (elevation <1 km) during the glaciation. However, characteristics of the deglacial and Holocene sediments indicate high contributions from the higher Hsueshan Range and Central Range regions (elevation >1 km). We suggest that headward extension and drainage capture since the deglaciation, which was most likely due to the increasing monsoon rainfall, account for the provenance variations in the discharged sediments. This implies a climate-driven drainage reorganization of the small mountainous rivers in Taiwan since the last glaciation. Our findings highlight the previously-overlooked, variable provenance information from Taiwan in glacial-interglacial cycles, and the dynamic source signatures are important to East Asia sediment source-to-sink studies. • The provenance of Zhuoshui River sediments has changed since the last glaciation. • The provenance variation was caused by climate-driven drainage reorganization. • The river system of Zhuoshui River was not developed during the last glaciation. • The Zhuoshui River headward extended and captured other basins in the Holocene.

Soil-chronosequence and Quaternary landscape evolution at the marine terraces of Harmony Point, Nelson Island, Maritime Antarctica.

This study characterized the physical, chemical, macro- and micromorphological soil properties from three successive marine terrace levels from Harmony Point (Nelson Island, Maritime Antarctica) in order to understand the pedological signatures of Quaternary coastal landscape evolution of Maritime Antarctica. Soils were sampled on the Late Holocene beach (current beach) and Mid Holocene marine terraces higher up, at 3, 8, and 12 m a.s.l. At the lower levels, the predominant soils were Gelorthents, whereas Haplogelepts dominate the higher terraces. Soil properties are mostly influenced by parent material and faunal activity, in which cryoclastic (thermal weathering) and phosphatization are the main soil-forming processes. Soils from the upper levels are more developed, deeper with reddish colors, granular structures and incipient formation B horizon. These horizonation features highlight that soils vary according with age of glacier-isostatic terrace uplift, representing a Quaternary soil chronosequence. All marine terrace levels are Ornithogenic soils, at varying degrees. However, the presence of old bird nesting sites for long periods led to formation of phosphatic horizons, stable Fe-phosphate minerals and abundant vegetation in the highest terraces of this part of Maritime Antarctica.

Status of CO2 mineralization and its utilization prospects

Action is currently being taken globally to mitigate global warming.The objective of reducing CO2 emissions is not a burden for society but is a significant opportunity for evolution in various industries for the sustainable production of energy and the essential minerals, metals, and materials required for modern society. CO2 mineralization is one of the most promising methods to effectively reduce CO2 emissions via the formation of stable mineral carbonates. Accelerated mineral carbonation requires high capital costs for implementation. Accordingly, it has thus far not been economically feasible to carry out accelerated CO2 mineralization alone. Accelerated CO2 mineralization must be combined with other associated technologies to produce high-value products. The technical developments in enhanced metal recovery, nanomaterials, enhanced flotation, H2 production and applications in the cement industry may be suitable options. The utilization and generation of valuable byproducts may determine the economic feasibility of CO2 mineralization processes. The need for CO2 reduction and utilization can contribute to driving the development of many innovative and sustainable technologies for the future benefit of society. The implementation of carbon taxation may also significantly motivate the development of these technologies and their potential application.

Oxalic Acid Modification Improves the Removal Effect of Cr(VI) by Biochar through Three Important Means: Adsorption, Reduction and Mineralization

Oxalic acid, one of the common organic acids secreted by microorganisms, was used to modify the biochar of sludge (SB) and rice husk (RB), and the adsorption function of biochar was combined with the reductive property of oxalic acid to repair Cr(VI). The biochar materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform attenuated total reflection infrared spectroscopy (ATR-IR), and the mechanism of chromium hexavalent removal was studied. Adsorption experiments showed that the removal rate of Cr(VI) with the modified SB and RB was 1.68 and 2.19 mg/g within 4h, which was 7.4 and 33 times of that before modification. Oxalic acid modification can enhance the repair of Cr(VI) by biochar in three aspects: 1. increases the BET surface area (109.34-174.13%) and decrease the average pore width (86.3-93.7%) of biochar, which increases the adsorption sites. 2. the addition of reducing oxygen-containing functional groups improves biocahr reduction performance. 3. improve reduced and unreduced Cr ion forms stable minerals on the surface of biochar. Therefore, oxalic acid can not only improve the adsorption and reduction of Cr(VI) by biochar, but also further improve the fixation/passivation of Cr ion after reduction and reduce the migration of Cr ion

Thermodynamic analysis of primary and secondary minerals stability in melilite-nephelinite tuff with Australopithecus Afarensis footprints, Laetoli, Tanzania

The Laetoli area in northern Tanzania is an important palaeo-anthropological site, where the oldest footprints of Australopithecus afarensis reside. Aeolian tuffs are the major rock type at Laetoli and they are divided into Lower and Upper Laetolil Beds that were deposited at an interval of 4.36 and 3.63 million years. The Upper Laetolil Beds contain eight layers of air-fall tuffs known as marker tuffs. The Australopithecus afarensis footprints are observed on the surface of the white tuff, which is a part of the Upper Laetolil marker tuff 7, also known as the “Footprint Tuff.” The interpolated age of the marker tuff 7 is 3.66 million years. Two mineral assemblages are distinguished in the Upper Laetolil marker tuffs. The first assemblage consists of primary tuff minerals and includes clinopyroxene (diopside, augite, aegirine-augite), nepheline, melilite (åkermanite and alumoåkermanite), garnet (andradite and schorlomite), magnetite, and others. The second mineral assemblage consists of secondary minerals, montmorillonite, calcite, and phillipsite. They were formed during replacement of the primary minerals, volcanic glass, and ash cementation. Thermodynamic calculations show that the major primary tuff minerals (melilite and nepheline) are stable at variable sodium activity and pH values. Replacement of melilite and nepheline by montmorillonite is caused by a decrease of sodium activity in slightly alkaline, neutral and acidic conditions (рН &lt; 10). Montmorillonite is not present in the altered nephelinitic tuff of the Sadiman volcano (which is considered as a source of the Laetolil Beds) where kaolinite is the major secondary mineral. This is explained by the difference in H2O fugacity with higher lgfH2O values in Sadiman and lower values in Laetoli. Relationships between primary and secondary tuffs minerals on the lgaHCO3 vs pH plot suggest mineral transformation within the Laetolil Beds in slightly acid and neutral con- ditions (рН = 5–7) compared with more alkaline conditions at Sadiman (рН &gt; 10).

Partial dehydration of brucite and its implications for water distribution in the subducting oceanic slab

Hydrous minerals within the subducting oceanic slab are important hosts for water. Clarification of the stability field of hydrous minerals helps to understand transport and distribution of water from the surface to the Earth’s interior. We investigated the stability of brucite, a prototype of hydrous minerals, by means of electrical conductivity measurements in both open and closed systems at 3 GPa and temperatures up to 1300 K. Dramatic increase of conductivity in association with characteristic impedance spectra suggests that partial dehydration of single-crystal brucite in the open system with a low water fugacity occurs at 950 K, which is about 300 K lower than those previously defined by phase equilibrium experiments in the closed system. By contrast, brucite completely dehydrates at 1300 K in the closed system, consistent with previous studies. Partial dehydration may generate a highly defective structure but does not lead to the breakdown of brucite to periclase and water immediately. Water activity plays a key role in the stability of hydrous minerals. Low water activity (aH2O) caused by the high wetting behavior of the subducted oceanic slab at the transition zone depth may cause the partial dehydration of the dense hydrous magnesium silicates (DHMSs), which significantly reduces the temperature stability of DHMS (this mechanism has been confirmed by previous study on super hydrous phase B). As a result, the transition zone may serve as a ‘dead zone’ for DHMSs, and most water will be stored in wadsleyite and ringwoodite in the transition zone.

The Main Features of Phosphorus Transport in World Rivers

Data on the geochemistry of phosphorus in the continental runoff of dissolved and solid substances were systematized and generalized, with a separate consideration of the processes of runoff transformation in river mouth areas. It has been established that atmospheric deposition, which many authors consider to be an important source of phosphorus in river runoff and not associated with mobilization processes in catchments, actually contains phosphorus from soil-plant recycling. This is confirmed by the fact that the input of phosphorus from the atmosphere into catchments exceeds its removal via water runoff. An analysis of the mass ratio of phosphorus in the adsorbed form and in the form of its own minerals was carried out. It was shown that the maximum mass of adsorbed phosphorus is limited by the solubility of its most stable minerals. The minimum concentrations of dissolved mineral and total phosphorus were observed in the rivers of the Arctic and subarctic belts; the maximum concentrations were confined to the most densely populated temperate zone and the zone of dry tropics and subtropics. In the waters of the primary hydrographic network, the phosphorus concentration exhibited direct relationships with the population density in the catchments and the mineralization of the river water and was closely correlated with the nitrogen content. This strongly suggests that economic activity is one of the main factors in the formation of river phosphorus runoff. The generalization of the authors’ and the literature’s data on the behavior of phosphorus at the river–sea mixing zone made it possible to draw a conclusion about the nonconservative distribution of phosphorus, in most cases associated with biological production and destruction processes. The conservative behavior of phosphorus was observed only in heavily polluted river mouths with abnormally high concentrations of this element.

A halogen budget of the bulk silicate Earth points to a history of early halogen degassing followed by net regassing

Halogens are important tracers of various planetary formation and evolution processes, and an accurate understanding of their abundances in the Earth's silicate reservoirs can help us reconstruct the history of interactions among mantle, atmosphere, and oceans. The previous studies of halogen abundances in the bulk silicate Earth (BSE) are based on the assumption of constant ratios of element abundances, which is shown to result in a gross underestimation of the BSE halogen budget. Here we present a more robust approach using a log-log linear model. Using this method, we provide an internally consistent estimate of halogen abundances in the depleted mid-ocean ridge basalts (MORB)-source mantle, the enriched ocean island basalts (OIB)-source mantle, the depleted mantle, and BSE. Unlike previous studies, our results suggest that halogens in BSE are not more depleted compared to elements with similar volatility, thereby indicating sufficient halogen retention during planetary accretion. According to halogen abundances in the depleted mantle and BSE, we estimate that ∼87% of all stable halogens reside in the present-day mantle. Given our understanding of the history of mantle degassing and the evolution of crustal recycling, the revised halogen budget suggests that deep halogen cycle is characterized by efficient degassing in the early Earth and subsequent net regassing in the rest of Earth history. Such an evolution of deep halogen cycle presents a major step toward a more comprehensive understanding of ancient ocean alkalinity, which affects carbon partitioning within the hydrosphere, the stability of crustal and authigenic minerals, and the development of early life.

Interlayer water structure of phyllomanganates: Insights from MD simulations of chalcophanite-group oxide dehydration

Phyllomanganates transform and concentrate a variety of metals in marine and terrestrial environments through sorption and redox reactions. Water molecules present in the interlayer region of phyllomanganates are assumed to play an important role in the stability, cation exchange, and redox reactions of manganese oxide minerals. The molecular structure of interlayer water remains elusive due to insufficient analytical and computational capabilities for investigating these confined water molecules. This study explores the interlayer water structure of phyllomanganates based on classical molecular dynamics (MD) simulations performed for crystalline chalcophanite group oxides (Me2+Mn3O7·3H2O, Me2+ = Zn2+, Ni2+, Mg2+, Mn2+, or Ca2+) as a function of the interlayer water content. As the oxides were dehydrated, MD simulations revealed collapse of the layer spacing in Zn- and Ni-chalcophanite and gradual decrease of the layer spacing in Mg-, Mn-, and Ca-chalcophanite. MD simulations also revealed that the cation-specific reduction in layer spacing with dehydration is controlled by the reorganization of interlayer water molecules, the extent of which strongly depends on the interlayer cations. Water molecules in chalcophanite group oxides occupied the midplane of the interlayer prior to dehydration, with the exception of Ca-chalcophanite which contained two split atomic planes of water molecules near the oxide surface. When Zn- and Ni-chalcophanite were monohydrated per formula unit, water molecules moved directly above or below the interlayer cations, leading to two split atomic planes of water molecules and a minor increase in the c-axis. In monohydrated Mg-, Mn-, and Ca-chalcophanite, water molecules occupied the midplane in the interlayer, with the c-axis reduced. Dehydration of Zn- and Ni-chalcophanite induced the dipole-moment vectors of water molecules to rotate simultaneously and become arranged perpendicular to the (001) basal plane, forming H-bonds between a water molecule and a single Mn octahedral sheet surface. However, in dehydrated Mg-, Mn-, and Ca-chalcophanite, water molecules were induced to rotate in the opposite direction; in this case, the dipole-moment vectors were nearly parallel to the basal plane, and H-bonds formed between a water molecule and two adjacent sheet surfaces. MD simulations with and without Mn4+-vacancy site disorder demonstrated that vacancy site-disorder causes significant disorder in interlayer cation and water structures. The current study provides atomistic insight into the effects on interlayer water structure of chemical and structural disorders in birnessite-like phyllomanganates.