Mg-rich Solutions Research Articles

Improving the Performance of a Salt Production Plant by Using Nanofiltration as a Pretreatment.

The Dębieńsko plant in Czerwionka-Leszczyny, Poland, producing evaporated salt from the saline mine water, faces increasing operating costs due to its high energy consumption. To improve the performance of the plant, a two-pass nanofiltration with intermediate crystallization of gypsum was proposed as a pretreatment. Based on the results of pilot-scale research, it was found that the removal of most of the calcium, magnesium, and sulfate allows a substantial reduction in the concentration of these components in the concentrated brine, which is then directed to a sodium chloride crystallization evaporator. This makes it possible to increase salt yield from the current 58.8% to 76.1% and indirectly reduce energy consumption from 1350 kWh/t to 1068 kWh/t. At the same time, the volume of the highly saline post-crystallization lyes is decreased by 66%, and a new stream is obtained: a Mg-rich solution, which could be used for magnesium hydroxide recovery.

Clay minerals affect calcium (magnesium) carbonate precipitation and aging

The effects of pre-existing mineral phases on the nucleation and growth of calcium carbonates from solution are relatively poorly understood, despite the widespread co-occurrence of carbonate minerals with clays and other silicates in rocks, soils, and sediments. Previous studies suggested that sheet silicates template calcite nucleation. Moreover, the presence of certain clay minerals appeared to enhance Mg2+ incorporation, resulting in Mg-bearing calcite or protodolomite. Here, we present the results of titration experiments with an environmentally relevant experimental setup, designed to study the roles of swelling clay minerals in the formation of Mg-bearing CaCO3 phases. We added both Mg-free and Mg-rich calcian solutions to carbonate buffers both in the presence and absence of smectite, and monitored the evolution of the solutions with pH and Ca ion selective electrodes, in order to identify nucleation and phase transition events. Initial products of the titration experiments were aged in their mother solutions for a few months. Both freshly formed and aged materials were studied using a variety of transmission electron microscopy (TEM) techniques. From Mg-free, homogeneous solutions vaterite was the first phase to precipitate. The addition of smectite triggered nucleation at lower supersaturation and generated calcite rather than vaterite. In Mg-rich solutions, aragonite was the first phase to precipitate both without and with clay minerals, and precipitation occurred at similar saturation levels in both samples. In the presence of clays, however, the aragonite nanocrystals were attached to smectite flakes. After the Mg-bearing systems were aged for several months, peculiar assemblages of protodolomite and low-magnesian calcite formed in association with smectite, whereas in the clay-free systems aragonite persisted. These observations suggest that if smectite is present in an environment where carbonates precipitate, the clay mineral has important and complex roles in the formation of Mg-bearing calcium carbonate phases. In addition to enhancing the nucleation of the first carbonate solid, smectite also triggers the formation of calcite-type structures, both at nucleation and in dissolution/reprecipitation reactions during aging.

Microbially Induced Magnesium Carbonate Precipitation and its Potential Application in Combating Desertification

This study investigated the reaction processes of microbially induced magnesium carbonate precipitation (MIMP) with Sporosarcina pasteurii (ATCC 11859) and evaluated its feasibility for controlling desertification in the desert areas in Northwest China. We explored systematically bacterial growth curves, mineralogy of precipitates, and relative chemical conversion efficiencies of the reaction using magnesium carbonate and bacterial urea hydrolysis with Sporosarcina pasteurii. We also compared the results of MIMP with the previously, well-studied microbially induced calcium carbonate precipitation (MICP). Our results indicate that excess Mg2+ motivated bacterial growth slightly. Magnesium carbonate precipitates appeared as nesquehonite, Mg-amorphous calcium carbonate, and Mg-rich calcite. The relative chemical conversion efficiency was higher in Mg medium than in Ca medium. We next evaluated the potential of using MIMP to mitigate desertification. We validated our results using the Mg-rich solution obtained by dissolving abandoned Mg salts that formed from the potassium salt plants nearby salt lakes. MIMP could potentially overcome shortcomings of traditional sand fixing methods, and was particularly suitable for controlling desertification in desert areas in Northwest China where there are abundant Mg resources. If MIMP works at field scales, this approach would further benefit ecosystem reconstruction because MIMP has main products of organic nutrients and ammonia, which would facilitate the development of biomass and soils. Overall, this work provides new insights into MIMP and its geoengineering potential in controlling desertification.

Formation of Mg-carbonates and Mg-hydroxides via calcite replacement controlled by fluid pressure

Reactive fluid flow can control the mineralogical, mechanical and chemical evolution of the Earth’s crust. When rocks are exposed to differential stresses (i.e., vertical stress ≠ horizontal stress ≠ pore-fluid pressure (Pf)) during reactive fluid flow, effective pressure is usually assumed to control the overall reaction process. Here, we show that fluid pressure can play an important role in mineral replacement reactions. We conducted experiments in which calcite (CaCO3) grains (fraction size 53–150 µm) reacted with a Mg-rich solution at ~ 200 °C both in a closed system and under reactive fluid flow conditions with different fluid flow rates and fluid pore pressures, but with similar confining pressure (σn = 10 or 20 MPa) and effective pressure (Pe). Under closed system, vapor-saturated pressures, the magnesite formed with large pores between the magnesite and the calcite. In the open system flow-through experiments, however, brucite (Mg(OH)2) or magnesite (MgCO3) formed, depending on pore-fluid pressure. The main reaction product was brucite at low pore-fluid pressure (0.2 MPa), but magnesite at higher pore-fluid pressures (≥ 1 MPa). Calcite dissolution and precipitation of the product mineral increased concomitantly with flow rate, but the flow rate did not affect the nature of the products. The permeability of the reacting rock was related to the reaction pathway, i.e. the nature of the products. Magnesite replaced the pristine calcite in a pseudomorphic manner, and mantled the pristine calcite with 10–100 µm wide pores. In contrast, tabular and/or platy brucite blocked the porosity and resulted in a decrease in permeability. Our results show that the pore-fluid pressure can be a significant parameter controlling the reaction products and reaction processes in volatile-rich (e.g., CO2, HCl, H2S and SO2) systems at conditions close to phase separation; these conditions occur for example in epithermal and porphyry hydrothermal systems, and in carbonate-replacement and some metamorphic environments.

Fracture dolomite as an archive of continental palaeo-environmental conditions

The origin of Quaternary dolomites in continental environments (e.g. karst and lakes) is barely constrained compared to marine dolomites in sedimentary records. Here we present a study of dolomite and aragonite formations infilling young fractures of the ‘Erzberg’ iron ore deposit, Austria, under continental-meteoric and low temperature conditions. Two dolomite generations formed shortly after the Last Glacial Maximum (~20 kyr BP): dolomite spheroids and matrix dolomite. Clumped isotope measurements and U/Th disequilibrium ages reveal formation temperatures of 0–3 °C (±6 °C) and 3–20 °C (±5 °C) for the both dolomite types, and depositional ages around 19.21 ± 0.10 kyr BP and 13.97 ± 0.08 kyr BP or younger, respectively. Meteoric solution and carbonate isotope compositions (δ18O, δ13C and 87Sr/86Sr) indicate the dolomites formed via aragonite and high-Mg calcite precursors from CO2-degassed, Mg-rich solutions. Our study introduces low temperature dolomite formations and their application as a sedimentary-chemical archive.

Direct Observations of the Coupling between Quartz Dissolution and Mg-Silicate Formation

Although quartz is a stable mineral at Earth surface conditions, field samples have shown its rapid dissolution in combination with the precipitation of Mg-silicate phases. Atomic force microscopy (AFM) experiments were performed to investigate the dissolution of quartz and the precipitation of secondary phases in high-pH, Mg-rich solutions both in situ and ex situ. Experiments were conducted at room temperature with varying MgCl 2 or MgSO 4 concentrations (0.1-100 mM), pH (8.9-12) and ionic strength (<1-530 mM). The results suggest that quartz dissolves by the removal of nanoparticles on the time scale of minutes, and that a nanometer-scale gel-like layer of amorphous silica forms on the quartz surface and is thicker at higher pH. During the in situ experiments, soft and poorly attached precipitates form on the surface when the Mg concentration is high (100 mM). After 20 h in a high-pH, Mg-rich solution, solid Mg-rich precipitates can be observed at places on the surface where the gel-like silica layer is present, predominantly near surface edges where dissolution is enhanced. This suggests a coupling between the dissolution of quartz, that resulted in the gel-like layer, and the formation of secondary phases, indicating an interface-coupled dissolution-precipitation mechanism. The precipitates could not be precisely identified but evidence suggests they are likely to be amorphous Mg-silicate phases. Such a coupled reaction may provide a pathway for Mg-Si phase formation suitable as a new environmentally friendly cement.

Control of temperature and aqueous Mg2+/Ca2+ ratio on the (trans-)formation of ikaite

The calcium carbonate hexahydrate mineral ikaite (CaCO3⋅6 H2O) has been documented in aquatic environments at near-freezing temperatures. An increase of the prevailing temperature in the depositional environment, results in the transformation of natural ikaite into less soluble calcium carbonate phases occasionally leaving calcite pseudomorphs in the sediments, which are considered asan indicator for primary cold water temperatures. Detailed understanding on the physicochemical parameters controlling ikaite (trans-)formation however, such as temperature and reactive solution chemical composition, are still under debate. In order to study the formation of ikaite, we conducted precipitation experiments under controlled physicochemical conditions (pH = 8.3±0.1; T=6, 12, and 18±0.1°C) at defined aqueous molar Mg/Ca ratios. The transformation of ikaite into anhydrous calcium carbonate polymorphs was investigated in solution and at air exposure. The obtained results reveal the formation of ikaite at temperatures up to 12°C, whereas Mg-rich amorphous calcium carbonate precipitated at 18°C. In contact with the reactive solution ikaite transformed into aragonite at aqueous molar Mg2+/Ca2+ ratios of ≥14. In contrast, ikaite separated from the Mg-rich solution and exposed to air transformed in all cases into calcite/vaterite. The herein obtained temperature limit of ≤12 for ikaite formation is significantly higher than formerly expected and most probably caused by (i) the high saturation degree of the solution with respect to ikaite and (ii) the slow dehydration of the aqueous Ca2+ ion at low temperatures. This result questions the suitability of calcite pseudomorphs (i.e. glendonites) as a proxy for near-freezing temperatures. Moreover, our findings show that the CaCO3 polymorph formed from ikaite is strongly controlled by the physicochemical conditions, such as aqueous molar Mg2+/Ca2+ ratio of the reactive fluid and H2O availability throughout the transformation process.

Production and characterization of synthetic aragonite prepared from dolomite by eco-friendly leaching–carbonation process

Dolomite is an alternative material for producing precipitated calcium carbonate (PCC) particles, which have widespread industrial applications depending on their morphology and particle sizes. These properties are readily controlled by the production conditions such as reaction time, temperature, stirring speed, and CO2 flow rate. In this paper, we investigate the influences of these experimental conditions on the production of synthetic aragonite crystals from dolomite using a leaching carbonation process. The proposed process is believed so be more eco-friendly than other methods suggested in the literature because the CO2 released from the dolomite during the leaching stage is stored for use in the carbonation stage. The experimental results indicate that the morphology of the produced PCC is influenced not only by the reaction time and temperature, but also the stirring speed and CO2 flow rate. The required reaction time decreases with an increase in the CO2 flow rates. However, calcite forms along with the aragonite crystals at higher CO2 flow rates. We successfully synthesized pure aragonite crystals in the reaction temperature range of 40–70°C at a fixed CO2 flow rate of 3.00l/min, and at a stirring speed of 750rpm. The d90 values of the aragonite crystals at various temperatures ranged from 18.47 to 25.99μm. We fit the experimental results by a single-term exponential model.Additionally, we obtained a Mg-rich solution and CO2 gas as by-products, which are in high demand.

Changes in mineral assemblages during serpentine carbonation

The carbonation of serpentine (lizardite [Mg3Si2O5(OH)4]) was investigated at a fixed temperature of 290°C under a 30-bar CO2 atmosphere in aqueous solutions of different alkalinities to understand the pH-dependence of the process. To activate the carbonation, the serpentine was heat-treated at 650°C beforehand for 2h. At pH6, the main reaction products were magnesite (MgCO3) and well-crystallized saponite [NaxMg3(Si4Al4−x)O10(OH)2]. The carbonation rate of the Mg ions was limited at acidic pH, and thermogravimetry (TG) analysis of the solid product showed that only 13% carbonation rate of serpentine to magnesite was achieved. The partial dissolution of SiOSi and the resultant Mg-rich solution provided a favorable environment for the formation of saponite. In weakly alkaline and neutral solutions, the rate of carbonation was dramatically enhanced well-faceted rhombohedral magnesite crystals of approximately 1μm in size with a round shape appeared together with a small amount of “curled-flaky type” saponite. The conversion of silicate to carbonate was found to be approximately 45%. As the pH of the solution increased further, the dissociation of carbonic acid accelerated and the free Mg2+ cations then reacted with the bicarbonate ions to form magnesium carbonate. Thus, active conversion to magnesite resulted in poor formation of saponite because of the low concentration of Mg ions in the solution. At pH2, well-crystallized saponite appeared, but magnesite was not observed. Under strongly alkaline conditions, saponite may form by a mechanism wherein the undissolved Mg layer of serpentine is available to act as a substrate for saponite precipitation. The transformation of serpentine to saponite and magnesite was influenced mostly by temperature, alkalinity, and mineral composition.

Preservation of ancestral Cretaceous microflora recovered from a hypersaline oil reservoir.

Microbiology of a hypersaline oil reservoir located in Central Africa was investigated with molecular and culture methods applied to preserved core samples. Here we show that the community structure was partially acquired during sedimentation, as many prokaryotic 16S rRNA gene sequences retrieved from the extracted DNA are phylogenetically related to actual Archaea inhabiting surface evaporitic environments, similar to the Cretaceous sediment paleoenvironment. Results are discussed in term of microorganisms and/or DNA preservation in such hypersaline and Mg-rich solutions. High salt concentrations together with anaerobic conditions could have preserved microbial/molecular diversity originating from the ancient sediment basin wherein organic matter was deposited.

Evidence of montmorillonite/Fe-rich smectite transformation in the Morrón de Mateo bentonite deposit (Spain): Implications for the clayey barrier behaviour

The Morrón de Mateo bentonite deposit (Cabo de Gata region, Spain) has been studied as a natural analogue of the thermal and geochemical effects on the clayey barrier in a deep geological repository of high level radioactive wastes. This deposit was intruded by a volcanic dome inducing an increase of temperature and supplying Fe–Mg-rich solutions, which were responsible for the alteration of the previously bentonitised pyroclastic materials (white tuff formation). The textural, mineralogical and crystallochemical features of the smectites located within the vicinity and away from the dome have been studied in order to elucidate the intrusion effects on these smectites. The results obtained show that distal smectites are Al-montmorillonites, similar to those from other bentonite deposits in the Cabo de Gata region; whereas proximal smectites are a mixture of Al-montmorillonites, Fe-rich montmorillonites and beidellites, and intermediate smectites between beidellite and Fe-rich-saponite. The textural relationships between these smectites indicate that smectites with intermediate composition come from the transformation of Al-montmorillonite, through gradual steps, consisting of increasingly Mg and Fe-rich smectites. This transformation process is confirmed when the structural formulae of proximal smectites are plotted on the MgVIvs (Al+Fe3+)VI diagram. Thus, a gradual transition from dioctahedral smectites to smectites with an intermediate chemical composition is observed, which tend toward trioctahedral smectites. These observations suggest that the transformation of dioctahedral smectites into an intermediate term between di- and trioctahedral Fe-rich smectites could be originated under natural conditions. This transformation can be considered as an analogue process to that expected in the bentonite barrier from a deep geological repository of radioactive wastes whether the two following conditions would occur: i) an increase of temperature due to the radioactive decay of the fission products, and ii) a supply of Fe as result of the canister corrosion under intermediate redox conditions. Furthermore, similar results were obtained in laboratory experiments focused on the Fe/Al-smectite interaction processes, related to the durability of the clayey engineered barrier.

Preparation of magnesium hydroxide from serpentinite by sulfuric acid leaching for CO2 mineral carbonation

Carbon capture and storage (CCS) by mineral carbonation is a promising way for CO2 emissions mitigation that has been under studied for decades. In this work, the preparation of magnesium hydroxide from Finnish serpentinite using sulfuric acid leaching as the first step of a CO2 mineral carbonation process was studied. Some details of leaching behavior of the ore were revealed and a valuable metal was recovered in this study. It was found that leaching yield of magnesium increased with sulfuric acid dosage, limited by a product layer formed on the ore particles, resulting in incomplete serpentinite decomposition. Agitation and ultrasonication were demonstrated to be effective in controlling the thickness of product layer. About 95% of iron was recovered from the leachate and leaching residues and valuable Fe-rich substances were obtained as by-products. After the iron extraction, a fine Mg(OH)2-rich powder could be prepared from the Mg-rich solution by precipitation using sodium hydroxide solution.

Formation and transformations of Fe-rich serpentines by asteroidal aqueous alteration processes: A nanoscale study of the Murray chondrite

Fe-rich serpentines are an abundant product of the early aqueous alteration events that affected the parent bodies of CM carbonaceous chondrites. Alteration assemblages in these meteorites show a large chemical variability and although water–rock interactions occurred under anoxic conditions, serpentines contain high amounts of ferric iron. To date very few studies have documented Fe valence variations in alteration assemblages of carbonaceous chondrites, limiting the understanding of the oxidation mechanisms. Here, we report results from a nanoscale study of a calcium–aluminum-rich inclusion (CAI) from the Murray chondrite, in which alteration resulted in Fe import and Ca export by the fluid phase and in massive Fe-rich serpentines formation. We combined scanning and transmission electron microscopies and scanning transmission X-ray microscopy for characterizing the crystal chemistry of Fe-serpentines. We used reference minerals with known crystallographic orientations to quantify the Fe valence state in Fe-rich serpentines using X-ray absorption spectroscopy at the Fe L2,3-edges, yielding a robust methodology that would prove valuable for studying oxidation processes in other terrestrial or extra-terrestrial cases of serpentinization. We suggest that aqueous Fe2+ was transported to the initially Fe-depleted CAI, where local changes in pH conditions, and possibly mineral catalysis by spinel promoted the partial oxidation of Fe2+ into Fe3+ by water and the formation of Fe-rich serpentines close to the cronstedtite endmember. Such mechanisms produce H2, which opens interesting perspectives as hydrogen may have reacted with carbon species, or escaped and yield increasingly oxidizing conditions in the parent asteroid. From the results of this nanoscale study, we also propose transformations of the initial cronstedtite, destabilized by later input of Al- and Mg-rich solutions, leading to Fe2+ leaching from serpentines, as well as to random serpentine-chlorite interstratifications. Such transformations towards polysomatic assemblages that are un-equilibrated from the structural, chemical and redox point of views are probably controlled by the various rates of alteration of primary minerals, but also by porosity gradients, as in terrestrial hydrothermal systems. We suggest that the proposed mechanisms may have played a role in the early formation of (Fe2+,Fe3+)-rich serpentines documented in CM chondrites, as well as in their transformation with on-going alteration towards Fe-poorer compositions inferred from previous petrologic, mineralogical and magnetic studies of CM chondrites.

Production of High-purity Magnetite Nanoparticles from a Low-grade Iron Ore via Solvent Extraction

We produced magnetite nanoparticles (MNPs) and a Mg-rich solution as a nano-adsorbent and a coagulant for water treatment, respectively, using a low-grade iron ore. The ore was leached with aqueous hydrochloric acid and its impurities were removed by solvent extraction of the leachate using tri-n-butyl phosphate as an extractant. The content of Si and Mg, which inhibit the formation of MNPs, was reduced from 10.3 wt% and 15.5 wt% to 28.1 mg/L and < 1.4 mg/L, respectively. Consequently, the Fe content increased from 68.6 wt% to 99.8 wt%. The high-purity <TEX>$Fe^{3+}$</TEX> solution recovered was used to prepare 5-15-nm MNPs by coprecipitation. The wastewater produced contained a large amount of <TEX>$Mg^{2+}$</TEX> and can be used to precipitate struvite in sewage treatment. This process helps reduce the cost of both sewage and iron-orewastewater treatments, as well as in the economic production of the nano-adsorbent.

Replacement of olivine by serpentine in the Queen Alexandra Range 93005 carbonaceous chondrite (CM2): Reactant–product compositional relations, and isovolumetric constraints on reaction stoichiometry and elemental mobility during aqueous alteration

Isovolumetric replacement of euhedral and anhedral olivine by serpentine produced both centripetal and meshwork textures in the CM2 chondrites ALH 81002 and Nogoya. The compositions of these textural varieties of serpentine are uniform within narrow limits within each previously studied meteorite, independent of the composition of olivine being replaced, and different between the two meteorites. In QUE 93005 (CM2), coarse olivines of widely varying compositions (Fo<76–99) are replaced in a texturally similar manner by compositionally uniform serpentine (Mg0.73±0.05Fe0.27±0.05)3Si2O5(OH)4. The narrow compositional range of serpentine replacing coarse olivine indicates that the aqueous solution from which the serpentine formed was compositionally uniform on scales at least as large as the meteorite (∼2.5cm in longest dimension).Isovolumetric textures and compositional observations constrain elemental redistribution from coarse olivine to serpentine and to surrounding phases during serpentinization. Regardless of olivine’s composition, isovolumetric replacement of coarse olivines by serpentine of the observed composition released more Mg and Si from olivine than was required to form the serpentine. Excess Mg and Si released by olivine destruction and not retained in serpentine were exported from the replaced volume.Olivines with different Fa/Fo proportions contributed different amounts of Fe and Mg to the serpentine. Ferroan olivines released more Fe than required to form the serpentines replacing them, so some of the Fe released from ferroan olivine was exported from the replaced volumes. Forsteritic olivines released less Fe than required to form the serpentines replacing them, so some Fe was imported into the replaced volumes augmenting the small amount of Fe released from forsteritic olivine. In QUE 93005 Fo83.8 is the threshold composition between Fe-exporting and Fe-importing behavior in individual olivine–serpentine pairs, which released exactly the amount of Fe required to form serpentine of the observed uniform composition. Compositions of serpentines isovolumetrically replacing olivines, and threshold olivine compositions, in QUE 93005 differ from the corresponding values in Nogoya.Solvent and solute species diffused through the serpentine between the olivine–serpentine interface and the aqueous solution outside the isovolumetrically replaced volume. In QUE 93005, some of the Fe released from ferroan olivine in excess of the amount required to form serpentine reacted with S sourced from outside the pseudomorphs to form Fe-sulfide decorating the margins of the pseudomorphs of serpentine after fayalitic olivine. Such Fe-sulfide-decorated outlines after fayalitic olivine do not occur in ALH 81002 or Nogoya, indicating different Fe and S mass transfer regimes in different CM2 chondrites. Mg, Fe, Si, and S in the aqueous solution, including the excess Mg and Si exported from all serpentine pseudomorphs after olivine of any composition, were available to be incorporated into other phases spatially separate from the pseudomorphs after olivine, including regularly interstratified serpentine–tochilinite. Serpentines that replaced coarse olivines in QUE 93005 and ALH 81002 are less magnesian than those in Nogoya, indicating that the Nogoya aqueous-alteration environment was more evolved toward Mg-rich solutions. This easily located and characterized phase assemblage may be potentially useful for characterizing clasts of varying degrees of alteration in brecciated and heterogeneous CM chondrites, and future returned samples from mineralogically similar asteroids.

Carbon sequestration via carbonic anhydrase facilitated magnesium carbonate precipitation

Strategies for sequestering CO2 directly from the atmosphere are likely required to achieve the desired reduction in CO2 concentration. This study examined the effect of bovine carbonic anhydrase (BCA) on the rate of atmospheric CO2 uptake into solution and Mg–carbonate precipitation as a means of sequestering CO2. Alkaline Mg-rich solutions, prepared using either NaOH or natural brucite [Mg(OH)2], were amended with BCA and supplied with laboratory air (~470ppm CO2). BCA catalyzed the hydration of aqueous CO2, thereby minimizing the rate-limiting step for Mg–carbonate precipitation. Geochemical modelling of solution chemistry and the δ13C values of the dissolved inorganic carbon indicated that solutions amended with BCA approached equilibrium more rapidly. Rates of CO2 uptake were accelerated by up to 600% and 150% in systems using NaOH and brucite in comparison to controls, respectively. CO2 sequestration was enhanced by up to 360% with CO2 stored in a Mg–carbonate mineral that is mineralogically similar to dypingite [Mg5(CO3)4(OH)2·5H2O]. CO2 uptake and sequestration rates increased non-linearly with BCA concentration, suggesting that BCA concentration could be better optimized to achieve maximum efficiency. Given the global abundance of available cation sources (e.g., saline waste waters), carbonic anhydrase facilitated Mg–carbonate precipitation is a promising means of sequestering CO2.

Replacement of olivine by serpentine in the carbonaceous chondrite Nogoya (CM2)

Coarse (chondrule and isolated) olivine in some CM chondrites is replaced by serpentine in both centripetal and meshwork replacement textures. Locally preserved textures formed by partial replacement of coarse olivine by serpentine in the carbonaceous chondrite Nogoya (CM2) establish unique associations between each individual mass of serpentine and the specific olivine from which that serpentine formed. Electron probe microanalyses show that the composition of serpentine replacing coarse olivine is uniform throughout all analyzed volumes of Nogoya, and is independent of the composition of the olivine being replaced. If, as previously proposed, late-stage alteration fluids were Mg-rich because Fe-source minerals were depleted in earlier stages, then the uniform Mg-rich composition of the serpentine replacing large silicate grains during advanced stages of alteration may indicate diffusional homogenization of the aqueous solutions over progressively larger spatial scales, enabled by long timescales and previously proposed stagnant or slow-moving fluids.The range of olivine compositions replaced in Nogoya is even larger than previously reported from ALH 81002 (CM2). This militates against hypotheses of strong primary-mineral control on the compositions of alteration products, at least at advanced stages of alteration. The serpentine formed by olivine replacement in Nogoya is more magnesian than the counterpart serpentine replacing all anhydrous primary silicates in ALH 81002. This intermeteorite heterogeneity of replacement-serpentine composition between ALH 81002 and Nogoya indicates that the aqueous solutions in which the olivine-serpentine replacement reactions occurred were of different compositions in the two different CM parent-body volumes sampled by ALH 81002 and Nogoya. The more magnesian character of serpentines in Nogoya than in ALH 81002 indicates that the Nogoya aqueous-alteration environment was even more highly evolved toward Mg-rich solutions than the environment indicated by the composition of the serpentine in ALH 81002.Persistence of primary-silicate remnants within centripetal and meshwork serpentine indicates that either the aqueous alteration episodes in the parent-body volumes represented by individual meteorites were too short to allow complete replacement of olivine by serpentine, or one or more reactants (most likely water) were completely consumed before the coarse primary silicate was completely replaced. Seemingly incompatible arguments for and against primary-mineral control of serpentine composition during CM chondrite alteration may be reconciled by considering the different grain sizes and reaction timescales that likely existed in different textural settings.

Dissolution and carbonation of a serpentinite: Inferences from acid attack and high P–T experiments performed in aqueous solutions at variable salinity

Abstract Dissolution experiments on a serpentinite were performed at 70 °C, 0.1 MPa, in H 2 SO 4 solution, in open and closed systems, in order to evaluate the overall dissolution rate of mineral components over different times (4, 9 and 24 h). In addition, the serpentinite powder was reacted with a NaCl-bearing aqueous solution and supercritical CO 2 for 24 h at higher pressures (9–30 MPa) and temperatures (250–300 °C) either in a stirred reactor or in an externally-heated pressure vessel to assess both the dissolution rate of serpentinite minerals and the progress of the carbonation reaction. Results show that, at 0.1 MPa, MgO extraction from serpentinite ranges from 82% to 98% and dissolution rate varies from 8.5 × 10 −10 mole m −2 s −1 to 4.2 × 10 −9 mole m −2 s −1 . Attempts to obtain carbonates from the Mg-rich solutions by increasing their pH failed since Mg- and NH 4 - bearing sulfates promptly precipitated. On the other hand, at higher pressures, significant crystallization (5.0–10.4 wt%) of Ca- and Fe-bearing magnesite was accomplished at 30 MPa and 300 °C using 100 g L −1 NaCl aqueous solutions. The corresponding amount of CO 2 sequestered by crystallization of carbonates is 9.4–15.9 mole%. Dissolution rate (from 6.3 × 10 −11 mole m −2 s −1 to 1.3 × 10 −10 mole m −2 s −1 ) is lower than that obtained at 0.1 MPa and 70 °C but it is related to pH values much higher (3.3–4.4) than that (−0.65) calculated for the H 2 SO 4 solution. Through a thorough review of previous experimental investigations on the dissolution kinetics of serpentine minerals the authors propose adopting: (i) the log rate [mole m −2 s −1 ] value of −12.08 ± 0.16 (1σ), as representative of the neutral dissolution mechanism at 25 °C and (ii) the following relationship for the acidic dissolution mechanism at 25 °C: log rate = - 0.45 ( ± 0.09 ) × pH - 10.01 ( ± 0.30 ) . The initial dissolution rate (for 25 °C) by acid attack obtained in this work is consistent with this relationship. In contrast, the average dissolution rate (for 25 °C) determined in this study through the pressure-vessel experiments is ∼4.5 orders of magnitude lower than that computed through this equation, suggesting that silica armoring of serpentinite grains played a significant role in these experiments. Overall, the obtained data may improve both the planning of ex-situ mineral carbonation using the CO 2 separated from biogas and the modeling of in-situ mineral carbonation.

Occurrence of Fe–Mg-rich smectites and corrensite in the Morrón de Mateo bentonite deposit (Cabo de Gata region, Spain): A natural analogue of the bentonite barrier in a radwaste repository

► A hydrothermal process transformed Fe–Mg smectites into corrensite. ► This transformation was favoured by the intrusion of the Morrón de Mateo dome. ► The intrusion caused a temperature increased and a supply of Fe–Mg rich solutions. ► The system can be a good natural analogue of bentonite barrier in a radwaste disposal. ► Experimental studies of stability of bentonite are in agreement with the results. The Morrón de Mateo bentonite deposit is being studied as a natural analogue of the thermal and geochemical effects on a bentonite barrier in a deep geological repository of high level radioactive wastes. This bentonite deposit and its host rocks were intruded by a rhyodacitic volcanic dome that induced a hydrothermal metasomatic process affecting the biocalcarenite beds close to the dome. In this work, the mineralogical and chemical features of the clay minerals of the hydrothermally altered pyroclastic (white tuffs) and epiclastic rocks (mass flow), located in the NE sector of the Morrón de Mateo deposit are described. White tuffs have a high content of phyllosilicates, mainly composed of dioctahedral smectites, while mass flow have a higher proportion of inherited minerals, the neoformed phyllosilicates are dioctahedral smectites and an interlayer chlorite/smectite mineral of corrensite type. The chemical composition of smectites reflects the different nature of the parent rocks, in such a way that smectites from white tuffs have a quite homogeneous chemical composition and their structural formulae correspond to montmorillonite type, while smectites from mass flow show more chemical variability, higher Fe and Mg contents and a mean structural formulae corresponding to Fe–Mg-rich beidellite and/or to an intermediate smectite member between beidellite and saponite. In addition, chemical composition and textural features of corrensite-like clay minerals in relation to Fe–Mg-rich smectites in the samples have also been studied, suggesting that the former seems to be formed from Fe–Mg-rich smectites. The presence of corrensite in the epiclastic rocks suggests that in the Morrón de Mateo area a hydrothermal alteration process occurred after bentonite formation, which transformed Fe–Mg-rich smectites into corrensite. This transformation was probably favoured by the intrusion of the Morrón de Mateo volcanic dome, which produced a temperature increase in the geological media and a supply of Fe–Mg-rich solutions. These physicochemical conditions were also responsible for the metasomatic transformations observed in the biocalcarenite beds located on the top of the bentonite deposit. All these data suggest that the Morrón de Mateo natural system could be a good natural analogue of both thermal and chemical effects on a bentonite barrier related to the radioactive decay of fission products and the interaction between the corrosion products of steel over-packs and the bentonite. These circumstances would favour the transformation of the candidate Al-rich smectites into Fe–Mg-rich smectites and corrensite, as steps prior to formation of chlorite. In this case, all the physicochemical and mechanical properties of Al-rich smectites would disappear and the clayey barrier would fail.

Dolomite in caves: Recent dolomite formation in oxic, non-sulfate environments. Castañar Cave, Spain

Dolomite is a common mineral in the rock record but rare in recent superficial environments. Where it does occur, it is related to anoxic, sulfate-rich environments and microbial activity. The occurrence of some dolomite deposits in caves, however, indicates that dolomite formation is also possible in oxic, non-sulfate settings. Dolomite is forming at 17 °C and in oxic-vadose conditions on the host rocks and aragonite speleothems of the Castañar Cave, Cáceres, Spain. It appears as spheroids and dumbbells 50–300 μm in diameter that internally consist of micron-sized rhombic to rounded crystals. Initially this dolomite is Ca-rich, non-stoichiometric and poorly ordered. Mg-rich solutions allow the precipitation of metastable Mg-rich carbonates, such as huntite. This soon transforms into this Ca-rich dolomite, which later “ages” to form a more stoichiometric dolomite. These dolomites show similarities to those grown under anoxic, sulfate-rich conditions and their presence in caves provides a different setting that may contribute to the understanding of the “dolomite problem”, including their initial formation and later recrystallization processes.