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Synthesis of magnesite at low temperature
Abstract
Magnesite formed at relatively low temperatures, 40°C, and atmospheric pressure using the experimental duplications is described
in Deelman (1999). The original experiment itself is a variation of Liebermann’s (1967) experiments on dolomite formation. The initial products of the reaction were metastable carbonates, aragonite and dypingite
which will change during the course of the experiment into magnesite. This work reports to have found evidence in support
of an active role of alternations between precipitation and dissolution in bringing about a formation of the thermodynamically
stable phase in the magnesite formation.
KeywordsMagnesite–Low-temperature synthesis
... Previous research showed that dissolution-precipitation cycles could lead to the increasing formation of stable over metastable phases, following Ostwald's rule [16]. Cycling of pH and temperature showed the formation of dolomite at around 40 • C in experiments by Liebermann [17] and Deelman [16], but magnesite was the reaction product in a similar study by dos Anjos et al. [18]. ...
... The acid or dissolution phase in the cycles of our experiments was set to one hour, which is shorter than in similar pH cycling experiments in previous studies [16][17][18]22]. Assuming an average calcite dissolution rate of about 3 × 10 −5 mol m −2 ·s −1 based on a pH of about 6 or more and a temperature of 43 • C [23,24], and a surface area of calcite of 8.72 m 2 ·g −1 [7], it is calculated that 0.288 mol calcite could be dissolved within one hour. ...
... Hence, no ordered dolomite was formed in the experiments. The lack of dolomite formation is consistent with the findings from pH cycling experiments by dos Anjos et al. [18], although those authors showed the formation of magnesite after precursors of aragonite and dypingite. Other dolomite synthesis studies (without pH cycling) at a temperature of less than 100 • C also indicate the formation of metastable phases such as low magnesium calcite, high magnesium calcite, very high magnesium calcite (or protodolomite), and hydrated Ca-Mg carbonate phases [1,27]. ...
The formation of dolomite is very challenging in the laboratory under ambient conditions due to kinetic inhibition. The goal of this study was to test the impact of pH cycling and zinc ions on the formation of magnesium-rich carbonates in saline fluids at a low temperature. Batch reactor experiments were conducted in two series of pH cycling experiments, one without and one with zinc ions, at 43 °C. The results after 36 diel pH cycles indicate a reaction product assemblage of hydromagnesite, aragonite and magnesite in the experiments without zinc ions, and of magnesite and minor aragonite in the experiments with zinc ions. The presence of zinc ions leads to a decrease in the pH in the acid phase of the cycling experiments, which likely plays a role in the reaction product assemblage. Moreover, the hydration enthalpy and other specific ion effects could be additional factors in the formation of magnesium-rich carbonate. The results show a clear evolution towards increasing incorporation of magnesium in the carbonate phase with cycle number, especially in the experiments with zinc ions, reflecting a ripening process that is enhanced by pH cycling. Hence, repeated pH cycling did not lead to more ordered dolomite (from protodolomite), but rather to the formation of magnesite with 92 mol% MgCO3 after 36 cycles, even though geochemical models indicate a higher saturation index for dolomite than for magnesite.
... The idea that physicochemical oscillations are necessary to form dolomite is not new. Since the seminal article by Liebermann in 1967 (1), researchers have tried to crystallize dolomite by inducing dissolution-crystallization cycles at near ambient conditions (1)(2)(3)(4)(5). Nevertheless, no conclusive proof of synthesis of ordered dolomite at low temperature has been provided to date (1)(2)(3)(4)(5). ...
... Since the seminal article by Liebermann in 1967 (1), researchers have tried to crystallize dolomite by inducing dissolution-crystallization cycles at near ambient conditions (1)(2)(3)(4)(5). Nevertheless, no conclusive proof of synthesis of ordered dolomite at low temperature has been provided to date (1)(2)(3)(4)(5). In turn, crystallochemical analyses (6-7) have shown that ordered dolomite can be formed in sedimentary environments by a dissolution-(re)crystallization mechanism operating over a geological timescale (8,9). ...
This eLetter can be found in the following link:
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... At normal temperatures and CO 2 pressures, formation of magnesite is rare: the minimal temperature for magnesite precipitation is generally considered to be 60-100°C (e.g. Hanchen et al., 2008;Hopkinson et al., 2012;Swanson et al., 2014;Qafoku et al., 2014) although it has been synthesised at 40°C under ambient pressures (Deelman, 1999;Alves dos Anjos et al., 2011) or at 35-50°C under water-saturated supercritical CO 2 pressure of 90 atm (Felmy et al., 2012). Magnesite has been found in playa-type settings where it is supposed to be formed at ambient, subaerial conditions with hydrated Mg-carbonate as precursor (Rahimpour-Bonab and Abdi, 2012;Detriche et al., 2013;Power et al., 2014) as well as in marine algae (Nash et al., 2011). ...
... Its genesis is still enigmatic and it has been suggested that desiccation of hydrated Mgcarbonate is responsible (Renaut and Stead, 1990) or alternations of dissolution and precipitation of hydrated and unhydrated Mg-carbonate due to e.g. changes in pH (Deelman, 1999(Deelman, , 2012Alves dos Anjos et al., 2011). Nesquehonite, a hydrated Mg carbonate with the formula MgCO 3 ·3H 2 O, is able to precipitate at a temperature of 25°C and when the CO 2 partial pressures are not too high (Hanchen et al., 2008). ...
Enhanced weathering of olivine has been suggested as a measure to lower the atmospheric CO2 level and it might also mitigate ocean acidification. This study aimed to characterise how olivine can weather in seawater, to elucidate the role of secondary precipitation and to ascertain the efficiency in terms of molar CO2 removal per mole of olivine dissolution. Geochemical thermodynamic equilibrium modelling was used, which considered both the variable mineralogical composition of olivine and the kinds of secondary precipitates that may be formed. The advantage is that such an approach is independent from local or regional factors as temperature, related kinetics, mineralogy, etc. The results show that the efficiency falls when secondary precipitates are formed. When Fe-bearing olivine undergoes weathering in an oxic environment, Fe(III) hydroxides will inevitably be formed, and as a result of this acidifying process, CO2 could be released to the atmosphere. This might also enhance ocean acidification when Fe-rich olivine becomes used. Ocean alkalinisation only happens when more than 1 mol/kgH2O Mg-rich olivine weathers. Maintenance of supersaturation for calcite or aragonite as holds in seawater reduces the efficiency by about a factor of two compared to the efficiency without secondary precipitation. Precipitation of sepiolite as Mg silicate reduces the efficiency even more. Magnesite precipitation has a similar effect to Ca carbonate precipitation, but according to the literature magnesite precipitation is improbable at ambient conditions and relatively low supersaturation. When less than 0.05 mmol olivine/kg(seawater) weathers the efficiency is slightly different than at higher intensities, due to strong buffering by seawater alkalinity.
... Dolomite ripening under oscillating conditions, following Ostwald's step rule, was originally derived from experimental synthesis results (Deelman 1999;Liebermann 1967). Still, similar experiments with pH cycling led to the formation of magnesite in other studies (dos Anjos et al. 2011;Vandeginste 2021). In addition, the kinetic barrier that is specific to the formation of dolomite, thus the cation ordening, could be of entropic nature, and be related to the mineral surface, in particular the interfacial energy of a growing particle, which can be affected by oscillating environmental conditions (Meister et al. 2023). ...
Predicting the type and rate of reactions between minerals and fluids is of utmost importance in many applications. Due to the presence of background ions, natural environments are often much more complex than laboratory experimental conditions that are used to derive mineral dissolution or precipitation rates. Dolomitisation is one of the most important diagenetic processes affecting carbonate rocks. Still, its underlying mechanisms are not yet completely unraveled. Here, we test the impact of background ions in saline solutions on the dolomitisation rate. Using batch reactor experiments at 200 °C and mineralogical characterisation, we demonstrate that the presence of background ions influences the fluid starting pH and specific ion effect, both impacting the dolomitisation rate. The results indicate that ions with a stronger hydration enthalpy correlate with a shorter dolomitisation induction time, and that Lewis acid AlCl3 is more effective than Brønsted acid HCl. Importantly, dolomitisation occurred at a slightly acidic pH, and carbon speciation modelling showed that carbonate ions did not dominate in any of the experiments. Hence, dolomitisation in our experiments is faster in saline, slightly acidic rather than alkaline solutions and the rate is influenced by the solution composition, with specific ion effects influencing dolomite surface charge, interfacial tension and the structure of water. These new insights have implications for interpretations on natural environments, such as deep reservoirs with saline, slightly acidic formation water, and predictions related to geological carbon dioxide storage.
... This was done because magnesite is kinetically inhibited from nucleating and is rarely observed to form at temperatures below 100°C (Hänchen et al., 2008). However, precipitation of MgCO 3 has occasionally been achieved at low temperatures, and may be enhanced by cyclic changes in the environment (Deelman, 1999;Dos Anjos et al., 2011), such as diurnal changes in temperature and RH. A second Mg-carbonate phase that has been suppressed is huntite (Mg 3 Ca(CO 3 ) 4 ). ...
Thermodynamic modeling has been used to predict chemical compositions of brines formed by the deliquescence of sea salt aerosols. Representative brines have been mixed, and physical and chemical properties have been measured over a range of temperatures. Brine properties are discussed in terms of atmospheric corrosion of austenitic stainless steel, using spent nuclear fuel dry storage canisters as an example. After initial loading with spent fuel, during dry storage, the canisters cool over time, leading to increased surface relative humidities and evolving brine chemistries and properties. These parameters affect corrosion kinetics and damage distributions, and may offer important constraints on the expected timing, rate, and long-term impacts of canister corrosion.
... This was done because magnesite is kinetically inhibited from nucleating and is rarely observed to form at temperatures below 100°C (Hänchen et al., 2008). However, precipitation of MgCO 3 has occasionally been achieved at low temperatures, and may be enhanced by cyclic changes in the environment (Deelman, 1999;Dos Anjos et al., 2011), such as diurnal changes in temperature and RH. A second Mg-carbonate phase that has been suppressed is huntite (Mg 3 Ca(CO 3 ) 4 ). ...
... At the same time, the ancient sedimentary strata are composed of magnesite, which is the most stable Mg-carbonate mineral phase [11,12]. Therefore, the question about the possible mechanism of magnesite formation is still debatable: Is it formed by the decomposition/aging of the hydrated phase of metastable compounds such as hydromagnesite [13], or is it the primary precipitation product [14]? Another interesting aspect of this problem is related to the fact that most of the examples of Mg-carbonate formation under modern conditions are associated with algal mats [12,15,16], which in turn suggests that biomineralization is the main factor in their formation. ...
The formation of Mg-rich carbonates in continental lakes throughout the world is highly relevant to irreversible CO2 sequestration and the reconstruction of paleo-sedimentary environments. Here, preliminary results on Mg-rich carbonate formation at the coastal zone of Lake Vtoroe Zasechnoe, representing the Setovskiye group of water bodies located in the forest-steppe zone of Southwest Western Siberia, are reported. The Setovskiye lakes are Cl−–Na+–(SO42−) type, alkaline, and medium or highly saline. The results of microscopic and mineralogical studies of microbialites from shallow coastal waters of Lake Vtoroe Zasechnoe demonstrated that Mg in the studied lake was precipitated in the form of hydrous Mg carbonates, which occur as radially divergent crystals that form clusters in a dumbbell or star shape. It is possible that hydrous Mg carbonate forms due to the mineralization of exopolymeric substances (EPS) around bacterial cells within the algal mats. Therefore, the Vtoroe Zasechnoe Lake represents a rare case of Mg-carbonates formation under contemporary lacustrine conditions. Further research on this, as well as other lakes of Setovskiye group, is needed for a better understanding of the possible role of biomineralization and abiotic mechanisms, such as winter freezing and solute concentration, in the formation of authigenic Mg carbonate in modern aquatic environments.
... The transformation between these carbonates is tightly related to the fluctuation of some parameters, e.g. pH ¢ Mg 2+ concentration, temperature, etc. [215]. The processes of precipitation are quite complex, since the family of magnesium carbonates consists of a variety of compounds and they can be transformed into each other under certain conditions [214,216,217]. ...
Magnesium (Mg) and its alloys have been widely investigated as biomaterials due to their remarkable biodegradability and bioresorbability. The discrepancy of degradation results between in vitro and in vivo observations demands a much better understanding for the mechanism of the degradation processes. However, the roles of organic molecules in Mg degradation remain unclear. In this thesis, several typical organic components, L-ascorbic acid, L-glutamine, L-alanyl-L-glutamine, bovine serum albumin, fibrinogen and fetal bovine serum were chosen to elucidate the effects of organic components on the degradation of pure Mg under cell culture conditions. The results reveal that the influence of organic components on the degradation of pure Mg is time- and medium-dependent. Small organic molecules increase the degradation rate of pure Mg after relatively long-term immersion, while proteins generally reduce the degradation of Mg. On the other hand, they play an important role in the formation of the degradation products. The addition of organic components favours the precipitation of crystalline nesquehonite rather than hydromagnesite in the ‘outer’ layer in HBSS. Whereas, in HBSSCa and DMEM organic components accelerate the formation of Ca/P-rich products in the top of degradation layer, presenting an in vivo-like degradation layer. Moreover, proteins seem to stabilize the top of Ca/P-rich layer and protect the integrity of degradation layer, which are of importance to Mg degradation. The formation of Ca/P-rich products reduces the surface roughness and changes the surface chemistry and charge, eventually weakening the adsorption of proteins. A promising result is that the addition of organic molecules, especially FBS, can weaken the difference of Mg degradation caused by the different conditions used, such as the composition of media, the ratio of medium volume to sample and the static or semi-static conditions, which enables the results more comparable.
... Aunque interesantes, los experimentos realizados a altas temperaturas están fueran de los objetivos de esta tesis. 5. Experimentos de reacción de cristales de minerales de carbonato cálcico (calcita y aragonito) con soluciones que contienen magnesio, calcio y/o carbonato (Usdowski, 1989(Usdowski, , 1994Land, 1998). En algunos experimentos se indujeron variaciones del pH mediante el burbujeo intermitente de CO 2 con el fin de promover reacciones de disolución y cristalización Deelman, 1999Deelman, , 2011Dos-Anjos et al., 2011). ...
Dolomite, CaMg(CO3)2, is a calcium magnesium carbonate mineral ubiquitous in the earth crust. Dolomite is the second most abundant carbonate mineral after calcite, CaCO3. Since its first description in 1791, a number of different unsuccessful methodologies has been used to tried to synthesize dolomite in the laboratory at ambient pressure and temperature. In mineralogy, this is known as the “dolomite problem”.
This Ph.D. thesis provides a new contribution to the future resolution of the dolomite problem. The thesis comprises: (I) a study of the formation process of dolomite and dolomite analogue phases at ambient temperature and pressure and (II) a study of the reactivity of (10.4) dolomite and kutnohorite (CaMn(CO3)2) surfaces in contact with supersaturated aqueous solution with respect to various monocationic carbonates (i. e. calcite [CaCO3], otavite [CdCO3], sphaerocobaltite [CoCO3] and zabuyelite [Li2CO3]).
The study of the processes leading to the formation of dolomite-like phases and dolomite analogue phases was carried out using the following methodologies: (I) mixing aqueous solutions, (II) ageing of suspended precursor phases in aqueous solutions, (III) ageing of precursor phases in solutions with acidification – basification cycles and (IV) precipitating phases using seawater and acidification – basification cycles in presence of additives.
By the ageing of the precipitates previously obtained by mixing aqueous solutions the synthesis of norsethite (BaMg(CO3)2) and PbMg(CO3)2 with cationic ordering was achieved in less than 10 days. Using the same method, the phases CaMg(CO3)2, BaCa(CO3)2 and CdMg(CO3)2 without cationic ordering were also synthesized. Alternatively, norsethite was synthesized by the ageing of precursor phases suspended in aqueous solutions. However, the time required in these experiments of synthesis of norsethite was longer than that in the experiments by mixing solutions.
The synthesis of dolomite was not achieved by using any of the four methodologies mentioned above.
The reactivity experiments on the (10.4) dolomite and kutnohorite surfaces was conducted by promoting the growth of different phases (i. e. calcite, otavite, sphaerocobaltite and zabuyelite) on that surfaces. These experiments were carried out using an atomic force microscopy (AFM). A nanotribological study was also performed on the substrates and overgrowths surfaces.
The calcite, otavite, sphaerocobaltite and zabuyelite overgrowths showed a different formation behaviour. In the case of calcite, otavite and sphaerocobaltite, the growth was epitaxial. The nanotribological study of the overgrowths consisted in both the quantification of the adhesion between the overgrowth and the substrate (calcite on dolomite and on kutnohorite) and the quantification of the frictional forces between the AFM tip and the overgrowths or the substrates. The different frictional response observed in the overgrowths and the substrates allowed us to distinguish them quickly.
The main conclusions drawn from this thesis are:
(I) The formation and cationic ordering processes that lead to the crystallization of the dolomite like phases and the dolomite analogues have different kinetics, mainly depending on the ratios of cationic radii (cation2+:Mg2+): the larger the ratios, the fastest is the process.
(II) The experimental results suggest that the hydration of Mg2+ is not the main factor that inhibits the formation of dolomite at ambient temperature in the laboratory. Since norsethite and PbMg(CO3)2 were synthesized under ambient conditions, and the formation of both phases required (as for dolomite) an equal incorporation of Mg2+ and the other cations (i. e. Ba2+ and Pb2+) in their structures, we can conclude that dehydration of Mg2+ is not a significant rate-limiting factor.
(III) Different reaction pathways towards the formation of norsethite and PbMg(CO3)2 were identified. Moreover, the precursor phases of such dolomite analogue phases, as well as their evolution, were described.
(IV) The growth of monocationic carbonates on the (10.4) dolomite and kutnohorite surfaces is mainly controlled by the lattice misfits between the overgrowths and the substrates. Calcite, otavite and sphaerocobaltite grew epitaxially on the substrates. This was verified by the analysis of the high resolution AFM images of the overgrowth and substrate surfaces, in which the crystalline structures of the monocationic carbonate of the overgrowths resulted to be parallel to the structure of the substrates. When zabuyelite highly supersaturated solutions were used, the growth of zabuyelite on these substrates is epitaxial. However, when zabuyelite slightly supersaturated or subsaturated solutions were used, the growth mechanism could not be identified, because high resolution AFM images were not obtained on those overgrowths, due to (I) there is no zabuyelite surfaces parallel to the substrate surfaces or (II) the growing phase is an amorphous phase.
(V) On the (10.4) dolomite and kutnohorite surfaces, the scanning with the AFM tip affects the nucleation of the phases (i. e. calcite and zabuyelite). While the nucleation of calcite islands was inhibited by the scanning, the nucleation of zabuyalite was favored by it.
(VI) The nanotribological study of the overgrowths indicates that the different overgrowths show different frictional responses. While otavite and sphaerocobaltite have a higher friction coefficient than the substrates on which they grow, zabuyelite has a lower friction coefficient than such substrates. Through nanomanipulation experiments conducted on the (10.4) dolomite and kutnohorite surfaces, the minima shear strength required for detaching calcite islands from such surfaces were estimated. The shear strength between calcite and kutnohorite is larger than that between calcite and dolomite.
(VII) In summary, the processes leading to the formation of a number of dolomite analogue phases at ambient temperature were determined and monitorized. Moreover, the crystal growth of some monocationic carbonates on the surfaces of two minerals with dolomite structure were investigated. The results presented in thesis can be considered as a starting point for future works on the crystallization of dolomite and dolomite analogue phases at ambient conditions, and for investigating crystal growth phenomena on the surfaces of double carbonates.
... The transformation between these carbonates is tightly related to the fluctuation of some parameters, eg. pH, Mg 2+ concentration, temperature, etc. [70]. The processes of precipitation are quite complex, since the family of magnesium carbonates consists of a variety of compounds and they can be transformed into each other under certain conditions [69,71,72]. ...
Several typical organic components, l-ascorbic acid (l-AA), l-glutamine (l-Gln), l-alanyl-l-glutamine (l-Ala-l-Gln) and fetal bovine serum (FBS) were chosen to elucidate the effects of organic components on the degradation of pure Mg under cell culture conditions. The results revealed that the influence of organic components on the degradation of pure Mg is time-dependent and they play an important role in the formation of the degradation layer. The addition of organic components favors the precipitation of nesquehonite rather than hydromagnesite in the "outer" layer, while in the "inner" layer the organic components accelerates the formation of phosphate (Mg-PO4, Ca-P salts) during immersion.
... between dissolution and precipitation are those ofDeelman ( , 2010) and recently, Alves dosAnjos et al., (2011). It is possible, that intermittent conditions of wetting and drying as well as of photosynthesis and respiration, typical for tidal and super-tidal microbial photosynthetic mats, can provide such fluctuating environments. ...
Dolomite is a common carbonate mineral CaCO3∙MgCO3, which is widely used in metallurgy, glass manufacture, chemical industry, and also as a sorbent for fluorine, boron and heavy metals in contaminated waters. The present describes the use of dolomite as a substrate for manganese and copper oxide catalysts for purification of groundwater from Fe (II) compounds. The actuality of developing such materials is due to the fact that groundwater is often characterized by a high iron content and may not be used for drinking purposes. For example, in Belarus, more than 80% of the explored groundwater sources do not meet sanitary standards for iron content. Supported catalysts are obtained by impregnation of a thermally activated dolomite with manganese (II) and copper (II) salt solutions, followed by drying and heat treatment in air. It is obvious that the catalytic activity of obtained materials depends on a number of factors: such as thermal activation of dolomite, the concentration of the impregnating solutions, metal oxide precursors, calcination temperature of the catalyst.
In this paper we present the results of systematic investigations of the main factors which influence the physical-chemical properties of manganese and copper oxide catalysts deposited on dolomite substrate, and demonstrate their effectiveness for groundwater purification from compounds of divalent iron. This allowed to obtain efficient catalysts for the oxidation of Fe (II) in aqueous media, to find out the optimal conditions of their production, and to establish the relationship between the mineral phase and chemical compositions, the parameters of porous structure and catalytic activity of developed catalysts.
... It is possible that magnesite precipitation took place in the form of a hydrated Mg-carbonate, such as hydromagnesite, which later dehydrated to form magnesite, as is seen in higher temperature lab experiments (Hänchen et al., 2008). Alternatively, the magnesite may have formed as a result of multiple cycles of alternating dissolution and precipitation of carbonates, in which each cycle forms progressively more stable phases like magnesite and dolomite and less metastable phases like dypingite and aragonite (Deelman, 1999;dos Anjos et al., 2011). Dolomite precipitation kinetics at low temperature are also poorly understood, despite years of study devoted to the "dolomite problem." ...
The Samail ophiolite in Oman is undergoing modern hydration and carbonation of peridotite and may host a deep subsurface biosphere. Previous investigations of hyperalkaline fluids in Oman have focused on fluids released at surface seeps, which quickly lose their reducing character and precipitate carbonates upon contact with the O2/CO2-rich atmosphere. In this work, geochemical analysis of rocks and fluids from the subsurface provides new insights into the operative reactions in serpentinizing aquifers. Serpentinite rock and hyperalkaline fluids (pH > 10), which exhibit millimolar concentrations of Ca2+, H2 and CH4, as well as variable sulfate and nitrate, were accessed from wells situated in mantle peridotite near Ibra and studied to investigate their aqueous geochemistry, gas concentrations, isotopic signatures, mineralogy, Fe speciation and microbial community composition.
The bulk mineralogy of drill cuttings is dominated by olivine, pyroxene, brucite, serpentine and magnetite. At depth, Fe-bearing brucite is commonly intermixed with serpentine, whereas near the surface, olivine and brucite are lost and increased magnetite and serpentine is detected. Micro-Raman spectroscopy reveals at least two distinct generations of serpentine present in drill cuttings recovered from several depths from two wells. Fe K-edge X-ray absorption near-edge spectroscopy (XANES) analysis of the lizardite shows a strong tetrahedral Fe coordination, suggesting a mixture of both Fe(II) and Fe(III) in the serpentine. Magnetite veins are also closely associated with this second generation serpentine, and 2–10 ?m magnetite grains overprint all minerals in the drill cuttings. Thus we propose that the dissolved H2 that accumulates in the subsurface hyperalkaline fluids was evolved through low temperature oxidation and hydration of relict olivine, as well as destabilization of pre-existing brucite present in the partially serpentinized dunites and harzburgites. In particular, we hypothesize that Fe-bearing brucite is currently reacting with dissolved silica in the aquifer fluids to generate late-stage magnetite, additional serpentine and dissolved H2.
Dissolved CH4 in the fluids exhibits the most isotopically heavy carbon in CH4 reported in the literature thus far. The CH4 may have formed through abiotic reduction of dissolved CO2 or through biogenic pathways under extreme carbon limitation. The methane isotopic composition may have also been modified by significant methane oxidation. 16S rRNA sequencing of DNA recovered from filtered hyperalkaline well fluids reveals an abundance of Meiothermus, Thermodesulfovibrionaceae (sulfate-reducers) and Clostridia (fermenters). The fluids also contain candidate phyla OP1 and OD1, as well as Methanobacterium (methanogen) and Methylococcus sp. (methanotroph). The composition of these microbial communities suggests that low-temperature hydrogen and methane generation, coupled with the presence of electron acceptors such as nitrate and sulfate, sustains subsurface microbial life within the Oman ophiolite.
... Lansfordite (MgCO 3 ·5H 2 O, not shown) and nesquehonite (MgCO 3 ·3H 2 O) were predicted to be undersaturated. Inorganic synthesis of dolomite, magnesite and huntite at atmospheric pressure and temperature below 40ºC has never been reported (Deelman, 1999;Saldi et al., 2009;Deelman, 2011;dos Anjos et al., 2011) so they are not expected to precipitate in the Oman spring waters. Hydrous magnesium carbonate phases, such as artinite, hydromagnesite and dypingite (Mg 5 (CO 3 ) 4 (OH) 2 ·5H 2 O), are far more likely to form (Hsu, 1967;Botha and Strydom, 2001;Hopkinson et al., 2008;Hänchen et al., 2008). ...
Ultramafic rocks, such as the Semail Ophiolite in the Sultanate of Oman, are considered to be a potential storage site for CO2. This type of rock is rich in divalent cations that can react with dissolved CO2 and form carbonate minerals, which remain stable over
geological periods of time. Dissolution of the ophiolite mobilizes heavy metals, which can threaten the safety of surface and groundwater supplies but secondary phases, such as iron oxides, clays and carbonate minerals, can take up significant quantities of trace elements both in their structure
and adsorbed on their surfaces. Hyperalkaline spring waters issuing from the Semail Ophiolites can have pH as high as 12. This water absorbs CO2 from air, forming carbonate mineral precipitates either as thin crusts on the surface of placid water pools or bottom precipitates
in turbulent waters. We investigated the composition of the spring water and the precipitates to determine the extent of trace element uptake. We collected water and travertine samples from two alkaline springs of the Semail Ophiolite. Twenty seven elements were detected in the spring waters.
The bulk of the precipitate was CaCO3 in aragonite, as needles, and rhombohedral calcite crystals. Traces of dypingite (Mg5(CO3)4(OH)2·5H2O) and antigorite ((Mg,Fe)3Si2O5(OH)4)
were also detected. The bulk precipitate contained rare earth elements and toxic metals, such as As, Ba, Cd, Sr and Pb, which indicated scavenging by the carbonate minerals. Boron and mercury were detected in the spring water but not in the carbonate phases. The results provide confidence
that many of the toxic metals released by ophiolite dissolution in an engineered CO2 injection project would be taken up by secondary phases, minimizing risk to water quality.
... It is possible that magnesite precipitation took place in the form of a hydrated Mg-carbonate, such as hydromagnesite, which later dehydrated to form magnesite, as is seen in higher temperature lab experiments (Hänchen et al., 2008). Alternatively, the magnesite may have formed as a result of multiple cycles of alternating dissolution and precipitation of carbonates, in which each cycle forms progressively more stable phases like magnesite and dolomite and less metastable phases like dypingite and aragonite (Deelman, 1999;dos Anjos et al., 2011). Dolomite precipitation kinetics at low temperature are also poorly understood, despite years of study devoted to the "dolomite problem." ...
The peridotite section of the Samail Ophiolite in the Sultanate of Oman offers insight into the feasibility of mineral carbonation for engineered, in situ geological CO2 storage in mantle peridotites. Naturally occurring CO2 sequestration via mineral carbonation is well-developed in the peridotite; however, the natural process captures and sequesters CO2 too slowly to significantly impact the concentration of CO2 in the atmosphere. A reaction path model was developed to simulate in situ CO2 mineralization through carbonation of fresh peridotite, with its composition based on that of mantle peridotite in the Samail Ophiolite and including dissolution kinetics for primary minerals. The model employs a two-stage technique, beginning with an open system and progressing to three different closed system scenarios- a natural system at 30 °C, an engineered CO2 injection scenario at 30 °C, and an engineered CO2 injection scenario at 90 °C. The natural system model reproduces measured aqueous solute concentrations in the target water, signifying the model is a close approximation of the natural process. Natural system model results suggest that the open system achieves steady state within a few decades, while the closed system may take up to 6,500 years to reach observed fluid compositions. The model also identifies the supply of dissolved inorganic carbon as the limiting factor for natural CO2 mineralization in the deep subsurface. Engineered system models indicate that injecting CO2 at depth could enhance the rate of CO2 mineralization by a factor of over 16,000. CO2 injection could also increase mineralization efficiency – kilograms of CO2 sequestered per kilogram of peridotite – by a factor of over 350. These model estimates do not include the effects of precipitation kinetics or changes in permeability and reactive surface area due to secondary mineral precipitation. Nonetheless, the faster rate of mineralization in the CO2 injection models implies that enhanced in situ peridotite carbonation could be a significant sink for atmospheric CO2.
Incorporating magnesium into the carbonate structure is difficult at low temperatures. However, Veitsch-type anhydrous magnesites (MgCO3) are found in evaporative environments without the typical hydrothermal conditions associated with magnesite formation. To address the disconnect, this study refined a temperature and pH cycling method to achieve up to 91 mol% MgCO3 of (Mg,Ca)CO3 carbonates without exceeding 40 °C. Individual parameters were tested to develop a novel simultaneous growth and replacement mechanism for magnesite formation. The experimental conditions ideal for magnesite growth match very well with an evaporative/lagoonal/playa geologic setting: small thermal mass for daily temperature swings, algal mass for pH control, and a solution highly concentrated in Mg²⁺ with high Mg:Ca ratios. Additionally, this study suggests the inclusion of dissolved silica in the system may play an important, but previously unacknowledged, catalytic role in the development of magnesium bearing carbonates. Finally, a high mol% MgCO3 carbonate precursor, such as the one produced in this study, is needed for the diagenetic alteration towards pure magnesite.
As magnesium and many of its alloys are a promising class of degradable implant materials, a thorough understanding of their degradation under physiological conditions is a key challenge in the field of biomaterial science. In order to increase the predictive power of in vitro studies, it is necessary to imitate the in vivo conditions, track the decomposition process and identify the products that form during the degradation pathway. In this in vitro study, slices of pure magnesium were exposed to Hank’s Balanced Salt Solution (HBSS), Dulbecco’s Modified Eagle Medium (DMEM) and simulated body fluid (SBF), respectively, under cell culture conditions, which included CO
Laboratory experiments were conducted to investigate the possible (bio-) chemical pathways involved in the low-temperature nucleation of magnesite, MgCO 3. Two compounds known for their power to prevent hydrolysis (amorphous silica and magnesium trisilicate) were tested in contact with a slowly desiccating solution of magnesium bicarbonate. Amorphous silica gave rise to precipitates of nesquehonite instead of the more usual magnesium hydroxide carbonate, thereby illustrating its influence on hydrolysis. Magnesium tri-silicate led to Xray amorphous precipitates. The presence of reducing compounds, in particular the strongly reducing hydrogen gas, does not lead to the low-temperature nucleation of magnesite.
Till date, the peculiar compound called oxymagnesite, MgO∙2MgCO₃: an intermediate formed during thermal decomposition of hydrated magnesium carbonates, has only been described a handful of times without a distinct description of its formation or morphology. In the current work we present the first scanning and transmission electron microscopy images of an oxymagnesite crystal together with crystallographic data. The oxymagnesite was synthesized in a controlled manner via decomposition of amorphous magnesium carbonates (AMCs) subjected to varying relative humidity. We show that oxymagnesite only is formed when AMC is hydrated above a certain level, which we attribute to a structural inequivalence between CO3 groups induced by water in AMC subjected to high humidity resulting in a weakening of some of the Mg-CO3 bonds. The study provides an understanding of the conditions needed for oxymagnesite formation and shows how hydrated AMCs can be used as precursors for different types of magnesium carbonates.
Most Holocene dolomite occurrences in the near coastal lakes of the Coorong region are associated with Mg-calcite. Only in the more arid settings found at the northwestern end of the Coorong region, near Salt Creek, is dolomite commonly found in association with magnesite and occasionally with hydromagnesite and aragonite. Textures found in vertical sequences from lakes in the Coorong coastal plain are largely independent of mineralogy; similar vertical transitions occur whether the lake is filling with carbonate, magnesium carbonates, or gypsum. -from Author
Wilhelm Ostald has claimed as a universa, principle, that if a metastable form of a particular compound exists, this metastable form will precipitate before the stable phase. In nature, a number of exceptions to Ostwald's Rule have been found. In some instances, the metastable modification of a compound occurs together with the stable form, or the metastable equivalent is completely lacking and only the stable phase is found. In a thermodynamic sense, minerals such as kaolinite, anatase, magnesite, and dolomite have to be considered as stable phases. However, their metastable equivalents usually precipitate in laboratory experiments conducted at room temperature and under atmospheric pressure. Low-temperature syntheses of the stable phase ( for example of the stable magnesite instead of the metastable magnesium hydroxy carbonate) is possible under the influence of fluctuations in physico-chemical conditions. Such fluctuations are virtually omnipresent in nature, but are seldom applied on purpose in the laboratory. The active role of fluctuations consists of favouring the prolonged growth of the stable phase by way of periodically dissolving part of the metastable phase. By definition the metastable phase precipitates faster than the stable phase, and therefore the metastable phase will dissolve faster than the stable phase. Fluctuations involving cycles of precipitation alternating with dissolution lead to more and more of the stable phase at the expense of the metastable phase. The process known as Ostwald Ripening can be influenced by fluctuations as well: in that case surface diffusion along the surfaces of a nucleus will be enhanced.
By way of duplicating an experiment described by LIEBERMANN (1967), nucleation of magnesite, huntite and/or dolomite has been attained at temperatures between 313 K and 333 K and under atmospheric pressure. Essential to these experiments are fluctuations in pH value. After interrupting an experiment after 1, 3, 5, or 8 of such fluctuations, the change from one or more metastable phases into the stable phase (magnesite or dolomite) could be followed. A theoretical explanation for these low-temperature syntheses can be found in stability relations. OSTWALD's Rule stipulates the nucleation of a metastable phase before that of the stable phase. However, fluctuations of sufficient amplitude and duration are capable of crossing the border between the metastable and the stable fields. As a result, the stable phase will nucleate together with the metastable phase. Conditions opposing the subsequent growth of the metastable phase (such as the slightly acidic conditions resulting from periodically introducing CO2 into the solution) will favour the continued growth of the stable phase.
It has been suggested that the highly hydrated character of the Mg 2+ ion in aqueous solution is responsible for the often encountered difficulty of precipitating stable, anhydrous phases of magnesium carbonate and calcium-magnesium carbonate. In an effort to investigate this, a study of magnesite crystallization kinetics was undertaken, utilizing the reaction of hydromagnesite plus CO 2 to yield magnesite at 126°C. The reactions were characterized by prolonged initial quiescent periods prior to the onset of detectable crystallization. The length of the initial period was found to vary with Mg concentration, pCO 2 and ionic strength. Contrary to classical kinetics, the reaction studied was inhibited by increased Mg concentration. Ionic strength and pCO 2 acted as positive catalysts.
When the solubilities of calcium and magnesium carbonates approach one another, dolomite will precipitate directly from solution. In sea water this happens at a minimum salinity of four to six times that of normal sea water and when the pH after effective abstraction of carbon dioxide is 8-9
A variety of fine-grained carbonate minerals, including dolomite, magnesite, hydro-magnesite, magnesian calcite and aragonite are found in association with the Coorong Lagoon, in southeast South Australia. These minerals occur in definite associations on lake and lagoon floors, as well as along stranded marginal flats. A relationship is shown to exist between the environment and carbonate mineral assemblage in each situation. Radiocarbon age determinations on dolomite from two lakes have verified the fact that this mineral is in the process of forming at the present day. From a series of lake water analyses over a two-year period, it is possible to broadly outline the conditions necessary for formation of dolomite in such an environment. It is concluded that this mineral is currently forming by extremely slow nucleation and crystal growth in the shallow ephemeral lakes marginal to the Coorong Lagoon. Better crystallized and partially ordered dolomites are associated with lake waters of significantly higher pH and Mg/Ca ratio than poorly crystallized protodolomite-magnesian calcite assemblages in the area.
Calcium and magnesium adsorb in near-stoichiometric proportions to dolomite over wide ranges in [Ca2+]/[Mg2+], ionic strength, and solution composition pointing to minimal mixing of metal cations between the CaCO3 and MgCO3 layer edges exposed at the dolomite surface. Near-neutral pH Mg and Ca adsorb as hydrated ions, or, in sulfate-rich solutions, as metal sulfate complexes. Near-stoichiometric adsorption of Ca and Mg points to dehydration and subsequent carbonation of adsorbed Mg as the likely rate-limiting step for dolomite growth at near-Earth surface conditions. We propose that one path for dolomite growth from low-temperature natural waters is through the initial adsorption of Mg-sulfate complexes onto either (1) growing dolomite crystals or (2) rate-limiting dolomite nucleii. Field relations, as well as homogeneous syntheses at low temperatures (25 °C < T < 100 °C) support this hypothesis and provide a mechanistic explanation for dolomite growth from sulfate-rich natural waters.
The crystallization of magnesite from aqueous solution The distribution and preliminary geochem-istry of modern carbonate sediments of the Coorong area, South Australia Sedimentology and mineralogy of dolomitic Coorong lakes, South Australia
- Fl Sayles
- Fyfe
Geology, mineralogy, geochemistry, formation of Mg-carbonates: Berlin u. Stuttgart, Borntraeger, p 300 Sayles FL, Fyfe WS (1973) The crystallization of magnesite from aqueous solution. Geochim Cosmochim Acta 37:87–99 von der Borch CC (1965) The distribution and preliminary geochem-istry of modern carbonate sediments of the Coorong area, South Australia. Geochim Cosmochim Acta 29:781–799 Warren JK (1990) Sedimentology and mineralogy of dolomitic Coorong lakes, South Australia. J Sed Petrol 60:843–858 Carbonates Evaporites (2011) 26:213–215
Nucleation processes of magnesite
- P Möller