Search

Jezero Crater on Mars is a paleolacustrine environment where Mg-carbonates may host evidence of ancient life. To elucidate the environmental and mineralogical controls on biosignature preservation, we examined samples from five terrestrial analogs: Lake Salda (Turkey), Lake Alchichica (Mexico), Qinghai-Tibetan Plateau (China), Mg-carbonate playas (British Columbia, Canada), and a mine with fine-grained ultramafic tailings (Yukon, Canada). The mineralogical compositions of the samples varied, yet were often dominated by either aragonite (CaCO3) or hydromagnesite [Mg-5(CO3)(4)(OH)(2)center dot 4H(2)O]. Aragonite-rich samples from Alchichica, Mg-carbonate playas, and the ultramafic mine contained an abundance of entombed microbial biomass, including organic structures that resembled cells, whereas hydromagnesite-rich samples were devoid of microfossils. Aragonite often precipitates subaqueously where microbes thrive, thereby increasing the likelihood of biomass entombment, while hydrated Mg-carbonates typically form by evaporation in subaerial settings where biofilms are less prolific. Magnesite (MgCO3), the most stable Mg-carbonate, forms extremely slowly, which may limit the capture of biosignatures. Hydrated Mg-carbonates are prone to transformation via coupled dissolution-precipitation reactions that may expose biosignatures to degradation. Although less abundant, aragonite is commonly found in Mg-carbonate environments and is a better medium for biosignature preservation due to its fast precipitation rates and relative stability, as well as its tendency to form subaqueously and lithify. Consequently, we propose that aragonite be considered a valuable exploration target on Mars.

Tailings dam failures can cause devastation to the environment, loss of human life, and require expensive remediation. A promising approach for de-risking brucite-bearing ultramafic tailings is in situ cementation via carbon dioxide (CO2) mineralization, which also sequesters this greenhouse gas within carbonate minerals. In cylindrical test experiments, brucite [Mg(OH)2] carbonation was accelerated by coupling organic and inorganic carbon cycling. Waste organics generated CO2 concentrations similar to that of flue gas (up to 19%). The abundance of brucite (2-10 wt %) had the greatest influence on tailings cementation as evidenced by the increase in total inorganic carbon (TIC; +0.17-0.84%). Brucite consumption ranged from 64-84% of its initial abundance and was mainly influenced by water availability. Higher moisture contents (e.g., 80% saturation) and finer grain sizes (e.g., clay-silt) that allowed for a better distribution of water resulted in greater brucite carbonation. Furthermore, pore clogging and surface passivation by Mg-carbonates may have slowed brucite carbonation over the 10 weeks. Unconfined compressive strengths ranged from 0.4-6.9 MPa and would be sufficient in most scenarios to adequately stabilize tailings. Our study demonstrates the potential for stabilizing brucite-bearing mine tailings through in situ cementation while sequestering CO2.

Rare earth element (REE) and strontium isotope data (87Sr/86Sr) are presented for hydromagnesite and surface waters that were collected from Dujiali Lake in central Qinghai-Tibet Plateau (QTP), China. The goal of this study is to constrain the solute sources of hydromagnesite deposits in Dujiali Lake. All lake waters from the area exhibit a slight LREE enrichment (average [La/Sm]PAAS = 1.36), clear Eu anomalies (average [Eu/Eu*]PAAS = 1.31), and nearly no Ce anomalies. The recharge waters show a flat pattern (average [La/Sm]PAAS = 1.007), clear Eu anomalies (average [Eu/Eu*] PAAS = 1.83), and nearly no Ce anomalies (average [Ce/Ce*]PAAS = 1.016). The REE+Y data of the surface waters indicate the dissolution of ultramafic rock at depth and change in the hydrogeochemical characteristics through fluid-rock interaction. These data also indicate a significant contribution of paleo-groundwater to the formation of hydromagnesite, which most likely acquired REE and Sr signatures from the interaction with ultramafic rocks. The 87Sr/86Sr data provide additional insight into the geochemical evolution of waters of the Dujiali Lake indicating that the source of Sr in the hydromagnesite does not directly derive from surface water and may have been influenced by both Mg-rich hydrothermal fluids and meteoric water. Additionally, speciation modeling predicts that carbonate complexes are the most abundant dissolved REE species in surface water. This study provides new insights into the origins of hydromagnesite deposits in Dujiali Lake, and contributes to the understanding of hydromagnesite formation in similar modern and ancient environments on Earth.

The stability and longevity of carbonate minerals make them an ideal sink for surplus atmospheric carbon dioxide. Biogenic magnesium carbonate mineral precipitation from the magnesium-rich tailings generated by many mining operations could offset net mining greenhouse gas emissions, while simultaneously giving value to mine waste products. In this investigation, cyanobacteria in a wetland bioreactor enabled the precipitation of magnesite (MgCO3), hydromagnesite [Mg5(CO3)4(OH)2·4H2O], and dypingite [Mg5(CO3)4(OH)2·5H2O] from a synthetic wastewater comparable in chemistry to what is produced by acid leaching of ultramafic mine tailings. These precipitates occurred as micrometer-scale mineral grains and microcrystalline carbonate coatings that entombed filamentous cyanobacteria. This provides the first laboratory demonstration of low temperature, biogenic magnesite precipitation for carbon sequestration purposes. These findings demonstrate the importance of extracellular polymeric substances in microbially enabled carbonate mineral nucleation. Fluid composition was monitored to determine carbon sequestration rates. The results demonstrate that up to 238 t of CO2 could be stored per hectare of wetland/year if this method of carbon dioxide sequestration was implemented at an ultramafic mine tailing storage facility. The abundance of tailings available for carbonation and the anticipated global implementation of carbon pricing make this method of mineral carbonation worth further investigation.

International audience; The alkaline playas near Atlin, British Columbia, Canada are likely one of the few surface environments on Earth where contemporaneous formation of hydromagnesite and magnesite occurs at temperatures that do not exceed 15 °C. This environment offers a unique opportunity to examine the impact of different formation mechanisms on Mg isotope compositions of Mg-carbonate minerals at low temperature. In this study, we report the Mg isotope composition of ultramafic bedrock, Mg-carbonate sediments, and both surface and ground waters in this geological setting. The composition of hydromagnesite suggests a Rayleigh-type distillation effect on the fluid Mg isotope ratios in unsaturated sediment above the water table. Through this mechanism of formation, hydromagnesite is progressively depleted in 24 Mg obtaining δ 26 Mg values as high as +1.14‰ near the sediment surface. In contrast, magnesite formation is characterized by enrichment of the solid phase in 24 Mg. The apparent Mg isotope fractionation factor during magnesite formation at ~ 10°C ranges between ~ 0.7±0.1‰ and 1.8±0.1‰. The distinct Mg isotope composition of hydromagnesite in comparison to magnesite supports magnesite formation occurring by precipitation from the fluid, or dissolution-reprecipitation, rather than solid-phase transformation from a hydrous Mg-carbonate precursor. Overall, the results provide insights on low temperature Mgcarbonate mineral formation that has implications for long-term storage of CO2.

The salt assemblages precipitated during evaporation of concentrated brine collected from Gasikule Salt Lake (GSL) were studied to better understand the formation of potassium deposits in the Qaidam Basin. The study included isothermal evaporation at 25 °C in the laboratory and solar evaporation in the ponds at GSL field. Brines increased in density and became moderately acidic (pH ≈ 5.30) while major ion geochemistry and precipitate mineralogy all showed broad agreement between both systems. Four salt assemblages were identified in the isothermal evaporation experiment: halite → halite + hexahydrite → halite + bischofite + carnallite → bischofite. Alternately, three salt assemblages were recognized in the solar evaporation: halite → halite + epsomite + carnallite → halite + carnallite + bischofite. The key difference in salt assemblages between the two systems is attributed to differences in relative humidity and temperature conditions. Although the GSL has deep spring inflow recharge, the high abundance of MgSO4 salts demonstrates that the salt assemblages are similar to normal seawater evaporation. Thus, different proportions of deep spring inflow and river water could form both MgSO4-deficient potassium evaporite and normal seawater potassium evaporites. Therefore, nonmarine water may form diverse potassium evaporite deposits in continental basins when the geological structure as well as hydrogeological and climatic conditions is appropriate.

De Beers kimberlite mine operations in South Africa (Venetia and Voorspoed) and Canada (Gahcho Kue, Victor, and Snap Lake) have the potential to sequester carbon dioxide (CO2) through weathering of kimberlite mine tailings, which can store carbon in secondary carbonate minerals (mineral carbonation). Carbonation of ca. 4.7 to 24.0 wt% (average = 13.8 wt%) of annual processed kimberlite production could offset 100% of each mine site’s carbon dioxide equivalent (CO2e) emissions. Minerals of particular interest for reactivity with atmospheric or waste CO2 from energy production include serpentine minerals, olivine (forsterite), brucite, and smectite. The most abundant minerals, such as serpentine polymorphs, provide the bulk of the carbonation potential. However, the detection of minor amounts of highly reactive brucite in tailings from Victor, as well as the likely presence of brucite at Venetia, Gahcho Kue, and Snap Lake, is also important for the mineral carbonation potential of the mine sites.

Thermogravimetric analysis–mass spectrometry, in situ X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy were used to characterize hydromagnesite [Mg5(CO3)4(OH)2·4H2O] from Dujiali Lake in Tibet, China. This study describes the variations in the thermal decomposition mechanisms of hydromagnesite at varying heating rates and under either helium (He) or carbon dioxide (CO2) atmospheres. In a He atmosphere, only two decomposition stages were observed; the loss of the crystalline water followed by the combined dehydroxylation and decarbonation. However, under a CO2 atmosphere, the dehydroxylation and decarbonation occur separately as the inert CO2 gas prevents the decomposition of the MgCO3 component of hydromagnesite. Overall, the thermal decomposition is an endothermic process. A distinctly exothermic process occurs at about 540 °C under conditions of high partial pressure of CO2 or high heating rate and implies the crystallization of magnesite (MgCO3). We propose that the release of H2O and CO2 at different stages likely results from the complicated hydrogen bonds and different carbonate groups in the crystal structure of hydromagnesite.

Mineralogically complex feedstocks, including kimberlite, serpentinite, and wollastonite skarns, have vast capacities to sequester carbon dioxide (CO2) through enhanced rock weathering and CO2 mineralization. However, only a small reactive fraction of these feedstocks will be accessible for carbon dioxide removal at Earth's surface conditions. We have developed a new method to evaluate the reactivity of mineral feedstocks that consists of a batch leach test using CO2 coupled with total inorganic carbon (TIC) analysis to quantify easily extractable Mg and Ca from non-carbonate (desirable) cation sources. Kimberlite residues from the Venetia Diamond Mine (South Africa), serpentinites, wollastonite skarn, and brucite ore were tested and the results were compared to those from commercial ammonium acetate (NH4OAc) leach tests. A strong correlation (R2 = 0.99) between leached Ca and TIC showed that carbonate minerals (e.g., calcite in kimberlite) are a substantial and undesirable source of easily extractable cations that must be excluded in calculating CO2 sequestration potential. Silicate dissolution (e.g., serpentine) was inferred from the strong positive correlation (R2 = 0.94) between Mg and Si concentrations leached from kimberlites and serpentinites. Strong correlations between leached Ca and Si were only detected for wollastonite skarns, whereas Mg leaching from samples with high abundances of brucite showed weak or no relationship to TIC or Si. The ability to distinguish between sources (non-carbonate versus carbonate) of easily extractable cations is necessary to accurately assess CO2 sequestration potential. The maximum CO2 storage capacity of the Venetia kimberlites was 268–342 kg CO2/t, and our leach test estimated an accessible potential in the range of 3–9 kg CO2/t when only accounting for non-carbonate sources. Our CO2 batch leach test is useful to evaluate the reactivity of mineralogically complex feedstocks at Earth's surface conditions for the purpose of carbon dioxide removal.

The genesis of hydromagnesite [Mg5(CO3)4(OH)2·4H2O] has attracted great interest as a pathway for sequestering anthropogenic CO2 and because of its importance to Mg carbonate depositional environments; however, there remain uncertainties regarding the chemical environment for hydromagnesite precipitation in modern and ancient geologic systems. Trace and rare earth element (REE) concentrations in hydromagnesite from Dujiali Lake, central Qinghai–Tibetan Plateau, China identified the formation conditions in the context of the depositional environment. The analyzed hydromagnesite samples had low total REE concentrations, varying from 0.62 to 3.11 ppm, with an average ∑REE value of 1.75 ppm. Comparisons of Ce/Ce* with LaN/SmN, DyN/SmN, and ∑REE showed no correlation indicating preservation of the original redox conditions during hydromagnesite precipitation. Redox-sensitive trace element ratios (U/Th, Ni/Co, V/Cr and V/V + Ni), negative Mn* values, and low authigenic uranium (Ua) values all indicate oxic conditions at the time of hydromagnesite formation. Furthermore, the Post-Archean Australian Shale-normalized REE patterns of the hydromagnesite display slight heavy REE enrichment, a slightly negative Ce anomaly, and a consistently positive Eu anomaly, which are consistent with precipitation in a predominantly oxidizing environment. Data indicate that hydromagnesite precipitated from waters influenced by both Mg-rich hydrothermal fluids and meteoric water with a similar composition to the lake water. This study provides new insights into the conditions of hydromagnesite formation at Dujiali Lake with implications for the understanding of the genesis of modern and ancient Mg carbonate deposits.

Microfluidics experiments were used to examine mineral dissolution-precipitation reactions under evaporative conditions and identify pore-scale processes that control reaction rate. The entrainment of reacting mineral particles by a mobile water-gas interface driven by evaporation dramatically altered the relative abundance of reactive mineral surface area to fluid reservoir volume. This ratio, which directly influences reaction rate and reaction progress, was observed to vary by nearly two orders of magnitude as evaporation progressed in the experiments. Its dynamic evolution may have a correspondingly large impact on mineral-fluid reaction in Earth's shallow subsurface. We predict that the spatial and temporal variability of pore-scale reaction rates will be significant during evaporation, imbibition, or drainage in the vadose zone, with implications for chemical weathering, soil quality, and carbon cycling. Variable reaction rates during particle mobility are likely to be of increased significance as global rainfall patterns and soil moisture contents evolve in response to climate change.

Magnesium oxychloride cement (MOC) boards have the potential to offset carbon emissions through carbon mineralization, a process whereby carbon dioxide (CO2) is converted to carbonate minerals. Boards (0–15 years old) contained MOC phase 5 (21–50 wt%), brucite, primary (e.g., magnesite) and secondary (hydromagnesite and chlorartinite) carbonate minerals. Quantitative mineralogy, electron microscopy and carbon abundance data demonstrate that secondary carbonates form through the reactions of MOC and brucite with CO2 within interfacial water layers after board manufacturing. Stable carbon isotopic data confirmed the source of sequestered CO2 as being from the atmosphere. Average carbonation rates were approximately 0.07 kg CO2/m2 board/year or 9.8 kg CO2/t board/year over 15 years, offsetting ∼20–40% of estimated carbon emissions. In experiments using 10% and 100% CO2 gas, carbonation was accelerated by approximately 400 and 1600 times in comparison to the passive rate. Integration of carbonation reactions into MOC board production could provide significant carbon offsets.

The hydromagnesite-magnesite playas near Atlin, British Columba, Canada are unique Mg-carbonate depositional environments that have formed at Earth’s surface since the end of the last deglaciation. This study elucidates the mechanisms, pathways, and rates of magnesite (MgCO3) formation in these near-surface environments, which are challenging to study in short-duration laboratory experiments because magnesite precipitation is extremely slow at low temperature. The Atlin playas, having formed over millennia, contain abundant magnesite as well as a suite of other Mg- and Ca-carbonate minerals. Mineralogical and textural evidence demonstrate that hydromagnesite [Mg5(CO3)4(OH)2·4H2O] forms at least in part through transformation of more hydrated phases, e.g., lansfordite (MgCO3·5H2O). Deposition of these hydrated Mg-carbonate minerals is limited by the evaporative flux, and thus, is effectively transport-controlled at the scale of the playas. Magnesite is a spatially distinct phase from hydromagnesite and its crystal morphology varies with depth indicating variable crystal growth mechanisms and precipitation rates. Particle size distributions and mineral abundance data indicate that magnesite formation is nucleation-limited. Furthermore, mineralogical data as well as stable and radiogenic isotope data support magnesite formation starting after the majority of hydromagnesite had been deposited likely resulting from long induction times and slow precipitation rates. Hydrated Mg-carbonate minerals precipitate relatively rapidly and control pore water chemistry while magnesite remains highly supersaturated, and thus, is reaction-controlled. This difference in controlling regime allows for magnesite abundance to increase over time without the loss of hydromagnesite such as through its transformation, which the data also does not support. We estimate rates of magnesite formation (nucleation + crystal growth) in the range of 10-17 to 10-16 mol/cm2/s over approximately 8000 years. This study helps to elucidate the geochemical conditions needed to form Mg-carbonate minerals in ancient and modern sedimentary environments and provides insights into facilitating long-term storage of anthropogenic CO2 within Mg-carbonate minerals.

Carbonic anhydrase (CA) enzymes have gained considerable attention for their potential use in carbon dioxide (CO2) capture technologies because they are able to catalyze rapidly the interconversion of aqueous CO2 and bicarbonate. However, there are challenges for widespread implementation including the need to develop mineralization process routes for permanent carbon storage. Mineral carbonation of highly reactive feedstocks may be limited by the supply rate of CO2. This rate limitation can be directly addressed by incorporating enzyme-catalyzed CO2 hydration. This study examined the effects of bovine carbonic anhydrase (BCA) and CO2-rich gas streams on the carbonation rate of brucite [Mg(OH)2], a highly reactive mineral. Alkaline brucite slurries were amended with BCA and supplied with 10% CO2 gas while aqueous chemistry and solids were monitored throughout the experiments (hours to days). In comparison to controls, brucite carbonation using BCA was accelerated by up to 240%. Nesquehonite [MgCO3·3H2O] precipitation limited the accumulation of hydrated CO2 species, apparently preventing BCA from catalyzing the dehydration reaction. Geochemical models reproduce observed reaction progress in all experiments, revealing a linear correlation between CO2 uptake and carbonation rate. Data demonstrates that carbonation in BCA-amended reactors remained limited by CO2 supply, implying further acceleration is possible.

This study formulates a comprehensive depositional model for hydromagnesite-magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite-magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metre-scale) and amalgamated mounds composed primarily of hydromagnesite [Mg(CO)(OH)·4HO], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average ≈ 2 m), Ca-Mg-carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post-deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg-HCO groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg-carbonate sediments (hydromagnesite mounds) overlies the Ca-Mg-carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments 'emerged' from the water surface. Capillary action and evaporation draw Mg-HCO water up towards the ground surface, precipitating Mg-carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO·5HO] which partially cements pre-existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca-Mg-carbonate boundary indicates that there has been ca 8000 years of continuous Mg-carbonate deposition at a rate of 0·4 mm yr. The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition.

The hydrated Mg-carbonate mineral, hydromagnesite [Mg5(CO3)4(OH)2·4H2O], precipitates within mine tailings at the Mount Keith Nickel Mine, Western Australia as a direct result of mining operations. We have used quantitative mineralogical data and δ13C, δ18O and F14C isotopic data to quantify the amount of CO2 fixation and identify carbon sources. Our radiocarbon results indicate that at least 80% of carbon stored in hydromagnesite has been captured from the modern atmosphere. Stable isotopic results indicate that dissolution of atmospheric CO2 into mine tailings water is kinetically limited, which suggests that the current rate of carbon mineralization could be accelerated. Reactive transport modeling is used to describe the observed variation in tailings mineralogy and to estimate rates of CO2 fixation. Based on our assessment, approximately 39,800t/yr of atmospheric CO2 are being trapped and stored in tailings at Mount Keith. This represents an offsetting of approximately 11% of the mine's annual greenhouse gas emissions. Thus, passive sequestration via enhanced weathering of mineral waste can capture and store a significant amount of CO2. Recommendations are made for changes to tailings management and ore processing practices that have potential to accelerate carbonation of tailings and further reduce or completely offset the net greenhouse gas emissions at Mount Keith and many other mines.

Serpentine minerals (e.g., chrysotile) are a potentially important medium for sequestration of CO2 via carbonation reactions. The goals of this study are to report a steady-state, far from equilibrium chrysotile dissolution rate law and to better define what role serpentine dissolution kinetics will have in constraining rates of carbon sequestration via serpentine carbonation. The steady-state dissolution rate of chrysotile in 0.1 m NaCl solutions was measured at 22 °C and pH ranging from 2 to 8. Dissolution experiments were performed in a continuously stirred flow-through reactor with the input solutions pre-equilibrated with atmospheric CO2. Both Mg and Si steady-state fluxes from the chrysotile surface, and the overall chrysotile flux were regressed and the following empirical relationships were obtained: F Mg = - 0.22 pH - 10.02 ; F Si = - 0.19 pH - 10.37 ; F chrysotile = - 0.21 pH - 10.57 where FMg, FSi, and Fchrysotile are the log10 Mg, Si, and molar chrysotile fluxes in mol/m2/s, respectively. Element fluxes were used in reaction-path calculations to constrain the rate of CO2 sequestration in two geological environments that have been proposed as potential sinks for anthropogenic CO2. Carbon sequestration in chrysotile tailings at 10 °C is approximately an order of magnitude faster than carbon sequestration in a serpentinite-hosted aquifer at 60 °C on a per kilogram of water basis. A serpentinite-hosted aquifer, however, provides a larger sequestration capacity. The chrysotile dissolution rate law determined in this study has important implications for constraining potential rates of sequestration in serpentinite-hosted aquifers and under accelerated sequestration scenarios in mine tailings.

Serpentinites offer a highly reactive feedstock for carbonation reactions and the capacity to sequester carbon dioxide (CO2) on a global scale. CO2 can be sequestered in mined serpentinite using high-temperature carbonation reactors, by carbonating alkaline mine wastes, or by subsurface reaction through CO2 injection into serpentinite-hosted aquifers and serpentinized peridotites. Natural analogues to serpentinite carbonation, such as exhumed hydrothermal systems, alkaline travertines, and hydromagnesite–magnesite playas, provide insights into geochemical controls on carbonation rates that can guide industrial CO2 sequestration. The upscaling of existing technologies that accelerate serpentinite carbonation may prove sufficient for offsetting local industrial emissions, but global-scale implementation will require considerable incentives and further research and development.

Carbon mineralization sequesters CO2 by reaction of alkaline earth metal bearing silicate and hydroxide minerals with CO2 to form stable carbonate minerals. Seifritz (1990) proposed harnessing this natural process as a method for sequestration of anthropogenic CO2. It was first studied in detail as an industrial process by Lackner et al. (1995), which is often referred to as “mineral carbonation.” Much of this early research aimed to capitalize on the globally abundant natural deposits of ultramafic and mafic rocks, which are rich in alkaline earth metals, in addition to the long-term stability of the resultant carbonate minerals (Lackner et al. 1995). More recently, other process routes have been investigated that rely on feedstocks other than naturally occurring minerals (e.g., industrial wastes) as a source of cations for carbonate precipitation. Therefore, we use the more general term “carbon mineralization” to refer to any process that sequesters CO2 as a solid carbonate phase. The main advantages of carbon mineralization as a CO2 storage method are that the reactions are thermodynamically favored, the carbonation processes can be readily controlled and manipulated, and the resulting product is benign and stable over geological time. We begin this review with an overview of the fundamental processes that are relevant to carbon mineralization, which provides a basic framework in which to understand CO2 sequestration strategies based on carbon mineralization. We next discuss natural analogues to engineered systems, focusing on (1) exhumed hydrothermal systems in peridotite that have formed listvenite (magnesite + quartz) and soapstone and (2) shallow subsurface peridotite weathering processes and related alkaline springs that form carbonate veins, surficial travertine deposits, and hydromagnesite–magnesite playas. The propensity to form carbonate minerals in these ultramafic terranes reflects the thermodynamic instability of Mg-silicate minerals in the presence of CO2. …

The distribution and evolution of mineral–water–gas interfacial areas exert a fundamental yet poorly documented control on mineral–fluid reactions in the unsaturated zone. Here, we explore the impact of changing mineral reactive surface area, water content, and gas distribution on the reaction of brucite [Mg(OH)2] with CO2 gas to form hydrated Mg-carbonate minerals in partially water saturated meter-scale column experiments. Brucite surface area, which is inferred to exert a direct control on mineral dissolution rates, demonstrates a complex evolution including roughening, fracturing and passivation that is inconsistent with conventional models of geometric evolution. Mineral–fluid reaction in the interior of single brucite grains maintains surface area at near-constant values despite the decreasing volumetric brucite content, until solid carbonate precipitates passivate brucite surfaces. The evolution of reactive surface area during passivation also does not follow simple geometric processes. A porous amorphous carbonate phase permits ready access to brucite surfaces for reaction until recrystallization of the amorphous carbonate into bladed, low-porosity nesquehonite [MgCO3·3H2O] abruptly quenches reaction. The varied water content of the experiments illustrates that the extent of mineral–gas reaction is limited by the abundance of water available to facilitate precipitation of hydrated carbonate minerals. Conversely, at high bulk water saturation, the development of preferential gas flow paths limited the exposure of reactive minerals to CO2 and reduced the overall extent of reaction. Thus, bulk mineral–fluid reaction rates were reduced at both high and low bulk water contents.

Atmospheric CO(2) is sequestered within ultramafic mine tailings via carbonation of Mg-bearing minerals. The rate of carbon sequestration at some mine sites appears to be limited by the rate of CO(2) supply. If carbonation of bulk tailings were accelerated, large mines may have the capacity to sequester millions of tonnes of CO(2) annually, offsetting mine emissions. The effect of supplying elevated partial pressures of CO(2) (pCO(2)) at 1 atm total pressure, on the carbonation rate of brucite [Mg(OH)(2)], a tailings mineral, was investigated experimentally with conditions emulating those at Mount Keith Nickel Mine (MKM), Western Australia. Brucite was carbonated to form nesquehonite [MgCO(3) · 3H(2)O] at a rate that increased linearly with pCO(2). Geochemical modeling indicated that HCO(3)(-) promoted dissolution accelerated brucite carbonation. Isotopic and aqueous chemistry data indicated that equilibrium between CO(2) in the gas and aqueous phases was not attained during carbonation, yet nesquehonite precipitation occurred at equilibrium. This implies CO(2) uptake into solution remains rate-limiting for brucite carbonation at elevated pCO(2), providing potential for further acceleration. Accelerated brucite carbonation at MKM offers the potential to offset annual mine emissions by ~22-57%. Recognition of mechanisms for brucite carbonation will guide ongoing work to accelerate Mg-silicate carbonation in tailings.

The nanometer-scaled pore systems of gas shale reservoirs were investigated from the Barnett, Marcellus, Woodford, and Haynesville gas shales in the United States and the Doig Formation of northeastern British Columbia, Canada. The purpose of this article is to provide awareness of the nature and variability in pore structures within gas shales and not to provide a representative evaluation on the previously mentioned North American reservoirs. To understand the pore system of these rocks, the total porosity, pore-size distribution, surface area, organic geochemistry, mineralogy, and image analyses by electron microscopy were performed. Total porosity from helium pycnometry ranges between 2.5 and 6.6%. Total organic carbon content ranges between 0.7 and 6.8 wt. %, and vitrinite reflectance measured between 1.45 and 2.37%. The gas shales in the United States are clay and quartz rich, with the Doig Formation samples being quartz and carbonate rich and clay poor. Higher porosity samples have higher values because of a greater abundance of mesopores compared with lower porosity samples. With decreasing total porosity, micropore volumes relatively increase whereas the sum of mesopores and macropore volumes decrease. Focused ion beam milling, field emission scanning electron microscopy, and transmission electron microscopy provide high-resolution (∼5 nm) images of pore distribution and geometries. Image analysis provides a visual appreciation of pore systems in gas shale reservoirs but is not a statistically valid method to evaluate gas shale reservoirs. Macropores and mesopores are observed as either intergranular porosity or are confined to kerogen-rich aggregates and show no preferred orientation or align parallel with the laminae of the shale. Networks of mesopores are observed to connect with the larger macropores within the kerogen-rich aggregates.

Ultramafic mine tailings from the Diavik Diamond Mine, Canada and the Mount Keith Nickel Mine, Western Australia are valuable feedstocks for sequestering CO₂ via mineral carbonation. In microcosm experiments, tailings were leached using various dilute acids to produce subsaline solutions at circumneutral pH that were inoculated with a phototrophic consortium that is able to induce carbonate precipitation. Geochemical modeling of the experimental solutions indicates that up to 2.5% and 16.7% of the annual emissions for Diavik and Mount Keith mines, respectively, could be sequestered as carbonate minerals and phototrophic biomass. CO₂ sequestration rates are mainly limited by cation availability and the uptake of CO₂. Abundant carbonate mineral precipitation occurred when heterotrophic oxidation of acetate acted as an alternative pathway for CO₂ delivery. These experiments highlight the importance of heterotrophy in producing sufficient DIC concentrations while phototrophy causes alkalinization of waters and produces biomass (fatty acids = 7.6 wt.%), a potential feedstock for biofuel production. Tailings storage facilities could be redesigned to promote CO₂ sequestration by directing leachate waters from tailings piles into specially designed ponds where carbonate precipitation would be mediated by both chemical and biological processes, thereby storing carbon in stable carbonate minerals and potentially valuable biomass.

The mineral waste from some mines has the capacity to trap and store CO(2) within secondary carbonate minerals via the process of silicate weathering. Nesquehonite [MgCO(3)·3H(2)O] forms by weathering of Mg-silicate minerals in kimberlitic mine tailings at the Diavik Diamond Mine, Northwest Territories, Canada. Less abundant Na- and Ca-carbonate minerals precipitate from sewage treatment effluent deposited in the tailings storage facility. Radiocarbon and stable carbon and oxygen isotopes are used to assess the ability of mine tailings to trap and store modern CO(2) within these minerals in the arid, subarctic climate at Diavik. Stable isotopic data cannot always uniquely identify the source of carbon stored within minerals in this setting; however, radiocarbon isotopic data provide a reliable quantitative estimate for sequestration of modern carbon. At least 89% of the carbon trapped within secondary carbonate minerals at Diavik is derived from a modern source, either by direct uptake of atmospheric CO(2) or indirect uptake though the biosphere. Silicate weathering at Diavik is trapping 102-114 g C/m(2)/y within nesquehonite, which corresponds to a 2 orders of magnitude increase over the background rate of CO(2) uptake predicted from arctic and subarctic river catchment data.

Microbialites were discovered in an open pit pond at an abandoned asbestos mine near Clinton Creek, Yukon, Canada. These microbialites are extremely young and presumably began forming soon after the mine closed in 1978. Detailed characterization of the periphyton and microbialites using light and scanning electron microscopy was coupled with mineralogical and isotopic analyses to investigate the mechanisms by which these microbialites formed. The microbialites are columnar in form (cm scale), have an internal spherulitic fabric (mm scale), and are mostly made of aragonite, which is supersaturated in the subsaline pond water. Initial precipitation is seen as acicular aragonite crystals nucleating onto microbial biomass and detrital particles. Continued precipitation entombs benthic diatoms (e.g. Brachysira vitrea), filamentous algae (e.g. Oedogonium sp.), dinoflagellates, and cyanobacteria. The presence of phototrophs at spherulite centers strongly suggests that these microbes play an important initial role in aragonite precipitation. Substantial growth of individual spherulites occurs abiotically through periodic precipitation of aragonite that forms concentric laminations around spherulite centers while pauses in spherulite growth allow for colonization by microbes. Aragonite associated with biomass (δ(13)C = -4.6‰ VPDB) showed a (13)C-enrichment of 0.8‰ relative to aragonite exhibiting no biomass (δ(13)C = -5.4‰ VPDB), which suggests a modest removal of isotopically light dissolved inorganic carbon by phototrophs. The combination of a low sedimentation rate, high calcification rate, and low microbial growth rate appears to result in the formation of these microbialites. The formation of microbialites at an historic mine site demonstrates that an anthropogenically constructed environment can foster microbial carbonate formation.

Anthropogenic greenhouse gas emissions may be offset by sequestering carbon dioxide (CO2) through the carbonation of magnesium silicate minerals to form magnesium carbonate minerals. The hydromagnesite [Mg5(CO3)4(OH)2·4H2O] playas of Atlin, British Columbia, Canada provide a natural model to examine mineral carbonation on a watershed scale. At near surface conditions, CO2 is biogeochemically sequestered by microorganisms that are involved in weathering of bedrock and precipitation of carbonate minerals. The purpose of this study was to characterize the weathering regime in a groundwater recharge zone and the depositional environments in the playas in the context of a biogeochemical model for CO2 sequestration with emphasis on microbial processes that accelerate mineral carbonation. Regions with ultramafic bedrock, such as Atlin, represent the best potential sources of feedstocks for mineral carbonation. Elemental compositions of a soil profile show significant depletion of MgO and enrichment of SiO2 in comparison to underlying ultramafic parent material. Polished serpentinite cubes were placed in the organic horizon of a coniferous forest soil in a groundwater recharge zone for three years. Upon retrieval, the cube surfaces, as seen using scanning electron microscopy, had been colonized by bacteria that were associated with surface pitting. Degradation of organic matter in the soil produced chelating agents and acids that contributed to the chemical weathering of the serpentinite and would be expected to have a similar effect on the magnesium-rich bedrock at Atlin. Stable carbon isotopes of groundwater from a well, situated near a wetland in the southeastern playa, indicate that ∼ 12% of the dissolved inorganic carbon has a modern origin from soil CO2. The mineralogy and isotope geochemistry of the hydromagnesite playas suggest that there are three distinct depositional environments: (1) the wetland, characterized by biologically-aided precipitation of carbonate minerals from waters concentrated by evaporation, (2) isolated wetland sections that lead to the formation of consolidated aragonite sediments, and (3) the emerged grassland environment where evaporation produces mounds of hydromagnesite. Examination of sediments within the southeastern playa–wetland suggests that cyanobacteria, sulphate reducing bacteria, and diatoms aid in producing favourable geochemical conditions for precipitation of carbonate minerals. The Atlin site, as a biogeochemical model, has implications for creating carbon sinks that utilize passive microbial, geochemical and physical processes that aid in mineral carbonation of magnesium silicates. These processes could be exploited for the purposes of CO2 sequestration by creating conditions similar to those of the Atlin site in environments, artificial or natural, where the precipitation of magnesium carbonates would be suitable. Given the vast quantities of Mg-rich bedrock that exist throughout the world, this study has significant implications for reducing atmospheric CO2 concentrations and combating global climate change.

Within the subarctic climate of Clinton Creek, Yukon, Canada, lies an abandoned and flooded open-pit asbestos mine that harbors rapidly growing microbialites. To understand their formation we completed a metagenomic community profile of the microbialites and their surrounding sediments. Assembled metagenomic data revealed that bacteria within the phylum Proteobacteria numerically dominated this system, although the relative abundances of taxa within the phylum varied among environments. Bacteria belonging to Alphaproteobacteria and Gammaproteobacteria were dominant in the microbialites and sediments, respectively. The microbialites were also home to many other groups associated with microbialite formation including filamentous cyanobacteria and dissimilatory sulfate-reducing Deltaproteobacteria, consistent with the idea of a shared global microbialite microbiome. Other members were present that are typically not associated with microbialites including Gemmatimonadetes and iron-oxidizing Betaproteobacteria, which participate in carbon metabolism and iron cycling. Compared to the sediments, the microbialite microbiome has significantly more genes associated with photosynthetic processes (e.g., photosystem II reaction centers, carotenoid and chlorophyll biosynthesis) and carbon fixation (e.g., CO dehydrogenase). The Clinton Creek microbialite communities had strikingly similar functional potentials to non-lithifying microbial mats from the Canadian High Arctic and Antarctica, but are functionally distinct, from non-lithifying mats or biofilms from Yellowstone. Clinton Creek microbialites also share metabolic genes (R2 < 0.750) with freshwater microbial mats from Cuatro Ciénegas, Mexico, but are more similar to polar Arctic mats (R2 > 0.900). These metagenomic profiles from an anthropogenic microbialite-forming ecosystem provide context to microbialite formation on a human-relevant timescale.

The evolution of mineral reactive surface area is an important control on the progress of carbon mineralization reactions that sequester anthropogenic CO2. Dry conditions in unsaturated porous media and the passivation of reactive surface area by secondary phase precipitation complicate predictions of reactive surface during carbon mineralization reactions. Unsaturated brucite [Mg(OH)2] bearing column experiments were used to evaluate the effects of water saturation and hydrous Mg-carbonate precipitation on reaction of brucite with 10% CO2 gas streams at ambient conditions. We demonstrate that a lack of available water severely limits reaction progress largely due to the requirement of water as a reactant to form hydrated Mg-carbonates. The precipitation of a poorly crystalline carbonate phase in the early stages of the reaction does not significantly hinder brucite dissolution, as the carbonate coating remains sufficiently permeable. It is postulated that the conversion of this phase to substantially less porous, crystalline nesquehonite [MgCO3·3H2O] results in passivation of the brucite surface. Although a mechanistic model describing the passivating effect of nesquehonite remains elusive, reactive transport modeling using MIN3P-DUSTY confirms that conventional geometric surface area update models do not adequately reproduce observed reaction progress during brucite carbonation, while an empirically based model accounting for surface passivation is able to capture the transient evolution of CO2 uptake. Both water limits and surface passivation effects may limit the efficiency of CO2 sequestration efforts that rely on the conversion of mafic and ultramafic rock to carbonate minerals.