Mg-bearing Minerals Research Articles

Transformation of Mg-bearing minerals and its effect on slagging during the high-alkali coal combustion

To better understand the mineralogical, release-transformation patterns and ash deposition of Mg-bearing minerals, Wucaiwan Coal (WCW), Meihuajing Coal (MHJ), and slag of water wall from a power plant were collected and high-temperature ash transformation experiments were conducted. X-ray fluorescence (XRF), X-ray diffractometer (XRD), scanning electron microscope coupled with energy dispersive spectrometer (SEM-EDS), and simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC) were used to analyze the mineralogy, chemical composition, and characteristics. The results show that the Mg-bearing minerals in high-alkali coal and ash include dolomite, chlorite, forsterite, melilite, akermanite, diopside, bredigite, pyroxene, periclase, spinel, and magnesioferrite. During combustion, the release of Mg is affected by its mode of occurrence. Mg-bearing minerals gradually transform into thermally stable minerals, including spinel, periclase, and pyroxene, with the transformation pathway being dolomite → Mg-bearing silicate (diopside, akermanite, bredigite) → pyroxene/spinel/periclase. Moreover, Ca has a competitive and synergistic effect on the transformation of Mg-bearing minerals, which depends on the relative content of Si and Al in ash. Spinel is the primary Mg-bearing mineral in high-temperature ash, and Fe may substitute for both Mg and Al. In coal combustion, the formation of Mg-Al-Fe and Mg-Al-Fe-Si-Ca eutectic systems from Mg-bearing minerals causes slagging on the water wall and worsens water wall corrosion.

Laboratory experiments of carbon mineralization potential of the main terrestrial basalt reservoirs in China

Against the background of realizing the goal of “carbon peaking and carbon neutrality”, using basaltic rocks for carbon mineralization is one of the most promising approaches to reduce the rise in atmospheric CO2 concentrations. This study conducted a series of experiments to assess carbon mineralization in nine basalt samples from the main terrestrial basalt reservoirs in China within CO2-H2O/brine-rock systems at low temperatures (≤35 °C). The results indicate that the secondary carbonates formed in the CO2-H2O/brine-basalt system are predominantly calcite rather than Mg-carbonate minerals at low temperatures (≤35 °C). Hence, at low temperatures (≤35 °C), basalt rich in Ca-bearing minerals promotes the formation of stable carbonate minerals more effectively than basalt containing Mg-bearing minerals. Furthermore, under conditions of low temperatures (≤35 °C) and pressures of approximately 3 MPa, the results suggest that alkaline olivine basalt, with a higher content of Ca-minerals and typical alkaline minerals (nepheline and Na-sanidine), exhibits the highest pH value and the highest amount of calcite. Hence, the alkaline minerals, nepheline and Na-sanidine, serve as pH buffers to increase the pH and promote the precipitation of calcite within CO2-H2O– basalt systems at low temperatures (≤35 °C). Among the nine evaluated basalts, basalt from the Shandong Linqu-Changle volcanic basin exhibits the highest rate of carbon mineralization at low temperatures (≤35 °C). Hence, Cenozoic alkaline olivine basalt from Shandong Linqu-Changle volcanic basin is one of the most promising basalt reservoirs in China for future in- situ carbonation. As for ex- situ carbonation, compared with olivine, diopside or Ca-plagioclase may be more appropriate for increasing ocean negative emissions.

Magnesium isotope behavior in oceanic magmatic systems: Constraints from mid-ocean ridge lavas from the East Pacific Rise

The magnesium (Mg) isotope composition of oceanic basalts has provided useful constraints on the evolution of the upper mantle and crust-mantle interactions. However, the behavior of Mg isotopes during oceanic magma differentiation is still unclear because of the small range in Mg isotope values in typical normal mid-ocean ridge basalts (N-MORB). Here, we present high-precision Mg isotope data on a well-characterized suite of mid-ocean ridge (MOR) lavas from the 9–10°N segment of the East Pacific Rise. These samples range from relatively primitive basalt to evolved dacite with MgO contents decreasing from 8.62 to 0.80 wt.%, and display a resolvable variation in δ26Mg from -0.27 to -0.17‰. The less-evolved samples (MgO > 7.0 wt.%) that have experienced olivine and plagioclase fractional crystallization have δ26Mg values ranging from -0.23 to -0.17‰ that are negatively correlated with MgO content. Samples containing MgO of 3.5–7.0 wt.%, that have experienced significant crystallization of clinopyroxene, together with plagioclase, show a limited variation of δ26Mg (-0.20 to -0.18‰). For those highly evolved samples (MgO < 3.5 wt.%) saturated with Fe–Ti oxides, the δ26Mg vary from -0.27 to -0.20‰ and are positively correlated with MgO content. The variation of δ26Mg in the MOR lavas is consistent with three stages of magma differentiation, beginning with fractional crystallization of olivine and plagioclase followed by increasing amounts of clinopyroxene and finally joined by Fe-Ti oxides. Quantitative modeling of the Mg isotopic variation in EPR samples, shows that the variation of δ26Mg in MOR lavas is primarily a consequence of fractional crystallization of the Mg-bearing minerals when Δ26MgOl-melt ∼ -0.10‰, Δ26MgCpx-melt ∼ 0.00‰, and Δ26MgTi−Mgt-melt ∼ 0.20‰. This study provides evidence that shallow level crystal fractionation can produce significant and predicable variations in Mg isotopic compositions of MOR lavas and that this process should be carefully considered when using evolved lavas to trace mantle source compositions.

Experimental investigation of multiple industrial wastes for carbon dioxide removal strategies

Industrial solid waste by-products are being increasingly employed for geochemical carbon dioxide removal (CDR) strategies due to their fine grain size, accessibility, and large annual production tonnages. Here, a range of such by-products has been tested experimentally for their reactivities with CO2 and water. Sample solutions were monitored for 100 h for changes in chemistry, and solid samples were characterised pre- and post-experiment. Samples rich in Ca- and Mg-bearing minerals, such as dunite, kimberlite, and ilmenite mine tailings, as well as marble quarry cuttings, were key cation sources. Ni sulphide, fluorite and borax tailings, coal-fired power plant fly ashes, and red mud samples showed high dissolution rates. The highest reaction rates were often observed during the initial few hours, and compared well to rates determined for rocks typically targeted for CDR purposes, such as basalt and gabbro. Several samples also showed secondary carbonate precipitation, suggesting opportunities for the development of single-step CDR technologies. Overall, the results of this study indicate that several industrial by-products can provide sufficient cations at favourable dissolution rates for geochemical CDR purposes. Any on-site or near-site conditions for reaction acceleration such as heat, concentrated CO2 or microbes, could further increase favourability for geochemical CDR opportunities.

CO2 Mineralization by MgO Nanocubes in Nanometric Water Films.

Water films formed by the adhesion and condensation of air moisture on minerals can trigger the formation of secondary minerals of great importance to nature and technology. Magnesium carbonate growth on Mg-bearing minerals is not only of great interest for CO2 capture under enhanced weathering scenarios but is also a prime system for advancing key ideas on mineral formation under nanoconfinement. To help advance ideas on water film-mediated CO2 capture, we tracked the growth of amorphous magnesium carbonate (AMC) on MgO nanocubes exposed to moist CO2 gas. AMC was identified by its characteristic vibrational spectral signature and by its lack of long-range structure by X-ray diffraction. We find that AMC (MgCO3·2.3-2.5H2O) grew in sub-monolayer (ML) to 4 ML thick water films, with formation rates and yields scaling with humidity. AMC growth was however slowed down as AMC nanocoatings blocked water films access to the reactive MgO core. Films could however be partially dissolved by exposure to thicker water films, driving AMC growth for several more hours until nanocoatings blocked the reactions again. These findings shed new light on a potentially important bottleneck for the efficient mineralization of CO2 using MgO-bearing products. Notably, this study shows how variations in the air humidity affect CO2 capture by controlling water film coverages on reactive minerals. This process is also of great interest in the study of mineral growth in nanometrically thick water films.

Formation of magnesium silicate hydrate (M-S-H) at pH 10 and 50°C in open-flow systems

Magnesium silicate hydrate (M-S-H) is a poorly crystalline Mg-silicate phase formed under alkaline conditions at low temperatures (T < 100 °C). Its formation has been studied in closed systems but not in open-flow systems, which better represent natural surface/subsurface environments. Here, MgO powder was used in reactions at pH ∼10 and 50 °C in flow-through experiments to study the formation of M-S-H as a function of aqueous Si concentration (1.5, 0.15, and 0 mM).Consumption of aqueous Si during precipitation of M-S-H resulted in an increase in the dissolution rate of the primary material (Mg hydroxide). Steady-state Si concentrations were used to calculate the dissolution rate of Mg hydroxide and the precipitation rate of M-S-H. Analyses of retrieved solids by electron microscopy and nuclear magnetic resonance spectroscopy confirmed the formation of M-S-H, although X-ray diffraction patterns provided no clear evidence of the presence of M-S-H because of the small amount precipitated and its nano-crystallinity. The chemical composition (Mg/Si ratio) of the M-S-H varied with the aqueous Si concentration of the injected solution. Mg/Si ratios of 1.00 ± 0.09 and 1.59 ± 0.15 were obtained with Si concentrations of 1.5 and 0.15 mM, respectively. Results indicate that the formation of M-S-H is feasible under Earth surface conditions, with dissolved silica coexisting with Mg-bearing minerals at alkaline pH.

Electrochemically Enhanced Deposition of Scale from Chosen Formation Waters from the Norwegian Continental Shelf

Reservoir formation waters typically contain scaling ions which can precipitate and form mineral deposits. Such mineral deposition can be accelerated electrochemically, whereby the application of potential between two electrodes results in oxygen reduction and water electrolysis. Both processes change the local pH near the electrodes and affect the surface deposition of pH-sensitive minerals. In the context of the plugging and abandonment of wells, electrochemically enhanced deposition could offer a cost-effective alternative to the established methods that rely on setting cement plugs. In this paper, we tested the scale electro-deposition ability of six different formation waters from selected reservoirs along the Norwegian continental shelf using two experimental setups, one containing CO2 and one without CO2. As the electrochemical deposition of scaling minerals relies on local pH changes near the cathode, geochemical modelling was performed to predict oversaturation with respect to the different mineral phases at different pH values. In a CO2-free environment, the formation waters are mainly oversaturated with portlandite at pH &gt; 12. When CO2 was introduced to the system, the formation waters were oversaturated with calcite. The presence of mineral phases was confirmed by powder X-ray diffraction (XRD) analyses of the mineral deposits obtained in the laboratory experiments. The geochemical-modelling results indicate several oversaturated Mg-bearing minerals (e.g., brucite, dolomite, aragonite) in the formation waters but these, according to XRD results, were absent in the deposits, which is likely due to the significant domination of calcium-scaling ions in the solution. The amount of deposit was found to be proportional to the concentration of calcium present in the formation waters. Formation waters with a high concentration of Ca ions and a high conductivity yielded more precipitate.

Phase Transition of Ca- and Mg-Bearing Minerals of Steel Slag in Acidic Solution for CO2 Sequestration

Phase Transition of Ca- and Mg-Bearing Minerals of Steel Slag in Acidic Solution for CO2 Sequestration

Multistep concentration of lizardite/antigorite from chrysotile mine tailings – case of the Carey Mine site in East-Broughton (Québec)

Abstract Revalorization of mining residues is of central concerns to the mining industry and the environment. Specifically, environmental management of residual products from the exploitation of chrysotile in the Thetford Mines region is one of the government concerns in Quebec and Canada. This work uses mining wastes in a second resource generation for production of magnesium from cheap and health-friendly mineral sources; the goal being to produce chrysotile-depleted pre-concentrates for a use as precursors in the leach off extraction of magnesium. The concentration of lizardite/antigorite from chrysotile containing serpentine rock mine tailings originating from the Carey Mine site in East-Broughton (Québec) was carried out using a suite of hydrocyclone, settling/decantation and magnetic separations. Four size classes of the mining residue, namely (−3150,+1580), (−1580, +600), (−600, +300) and (−300, +150) μm, were tested with an aim to reduce the level of objectionable asbestos fibers to allow access to the safer Mg-bearing minerals contained in the mine waste sources. The asbestos fibers clean-up consisted of subjecting the sieved fractions to two hydrocyclone steps, six settling/decantation steps and two magnetic separation steps. The best results were achieved when the hydrocyclone separators led to Mg recovery of 85% (±4) for the coarsest size fraction size. Both hydrocyclone underflow streams underwent settling/decantation separations. The settling tests lasted 30 min and led to Mg recoveries of 82.5% (±1.8) of Mg in the ultimate concentrate. SEM characterizations revealed that it was possible to reduce substantially the amount of chrysotile fibers to render the coarse-sized fraction in the mining waste usable while significantly lowering the health risk of the fibers. A two-step magnetic separation was applied to the final settling/decantation underflow to remove magnetic minerals such as magnetite from the lizardite/antigorite concentrate. The final quasi-non-magnetic chrysotile-depleted lizardite/antigorite concentrate allowed sample recovery of 62.5% (±0.9) wt. of Mg. These preliminary results are intended as a first compulsory step in support of viable restoration and sustainable development scenarios for the Thetford Mines mining sites as second-breath sources for valuable magnesium.

Validation study of water weakening research from outcrop chalks performed on Eldfisk reservoir cores

Seawater injection for Enhanced Oil Recovery (EOR) purposes can increase the hydrocarbon recovery factor in carbonate reservoirs but are also responsible for weakening their mechanical strength. This study investigates the geomechanical behavior of oil-bearing reservoir chalk subject to reactive flow at reservoir conditions. The obtained results are compared with previous results from experiments on water-saturated outcrop chalks.Two unwashed oil-bearing reservoir chalk cores from the Eldfisk Field in the North Sea are mechanically tested in a triaxial cell. The cores are saturated with NaCl brine prior to testing, in addition to residual oil. The cores’ axial and radial deformations were monitored during hydrostatic stress loading and creep under constant 50 MPa stress, at 130 °C. During the test, the cores were flooded with 0.657 M NaCl, 0.219 M MgCl2, 0.219 M MgCl2 + 0.130 M CaCl2 and synthetic seawater (SSW). The average creep strain increased from 0.03% to 0.04% per day radially and from 0.03% to 0.06% axially after changing from the NaCl brine to SSW brine. Flooding MgCl2 brine after NaCl increased the average compaction rate from 0.05% to 0.09% per day radially and from 0.04% to 0.07% per day axially. Adding CaCl2 to the MgCl2 brine reduced the average compaction rate from 0.05% per day during MgCl2 injection to 0.02% per day both radially and axially, comparable to reports from outcrop chalk experiments. The brine composition-dependent creep compaction in the core flooding experiments was explained by dissolution of primary calcite, confirmed by Ion Chromatography, and precipitation of secondary Mg-bearing minerals, seen by Scanning Electron Microscopy (SEM) analytics.Generally, the aforementioned results describing geomechanical behaviors of oil-bearing reservoir chalk cores under hydrostatic stress and thermochemical influence in this study are comparable to those from previous studies on outcrop chalk, thus supporting many years of laboratory research as applicable in the reservoir chalk context.

Optimization of microstructure of basalt-based fibers intended for improved thermal and acoustic insulations

Batch compositions of basalt and dolomite were optimized to produce high-quality fibers and wool boards applying Cupola technology. The raw batches, cinders, single fibers and fabricated wool boards have been characterized using XRD, XRF, SEM-EDAX, μ-X-ray CT, Lambda meter, mechanical and acoustic testing procedures according to ASTM and EN standards.The CaO/MgO ratio, acidity, basicity and melting moduli of the basaltic cinders are mainly related to the basalt low content of Mg-bearing minerals (19.15 wt%). The single fibers diameter (8.60 μm), non-fibrous shots (13.00 wt%) and Al2O3 (14.08 wt%) contents together with the quasi-horizontal macrostructure (SL-C = 0.60) improve the wool boards compressive (14.29Kpa) and tensile (3.00Kpa) strength as well as point load (140.00 N) properties.The single fibers microchemistry shows their enrichment with thermal conductivity ceasing oxides (∑ = 65.02 wt%) which together with the average porosity (92.30%) enhance the thermal (32.43 mW/mK and 6.17m2K/W) and acoustic (1.00 αs and 1.05 NRC) wool board insulations.The fabricated wool boards mechanical, thermal, acoustic and fire insulation characteristics are improved 28.00-, 2.35-, 1.40- and 45.45-times greater than the adopted EN standards, respectively.

Effect of waste rock dilution on spodumene flotation

The effect of waste rock minerals introduced by dilution on the flotation of spodumene was investigated. The objectives of this work were to investigate how flotation yield and grades deteriorate mainly by Mg-bearing biotite and amphiboles that are introduced to the system by waste rock dilution and find the possible ways to improve the flotation performance regarding these Mg-bearing minerals.Laboratory flotation tests were conducted with modifications to the standard procedure and new conditions. Analytical methods used in this work include mineralogical tests like electron probe micro-analyzer (EPMA) to find out accurate elemental assays for the minerals in the deposit, especially biotite. X-ray fluorescence (XRF) and atomic absorption spectroscopy (AAS) to analyse flotation products contents. Mineral liberation analysis (MLA) for liberation, associations, Mg-distribution and modal mineralogy. Information about mineral recoveries in different flotation stages was obtained from element to mineral conversion calculations with HSC Chemistry software.The target grade and recovery of Li2O for the Rapasaari ore were established on 4.5% and 84.18% (as a previous standard test). The use of Na2CO3 and starch modifiers gave promising results slightly improving the recoveries to 85.29% and 85.11% respectively while reaching almost 4.5% grade in final concentrate.Mineral liberation analysis (MLA) showed that biotite contains most of the Mg (80–90%), other minerals are amphiboles (mainly edenite and hornblende) and in less extent chlorite and clays. Biotite and amphiboles end up in the final concentrate mostly fully liberated while apatite is mostly coarse mixed grains. Mg-bearing minerals float into the concentrate because of similar surface properties with spodumene. Those similar properties may be due to chemically active Al-O sites that form on the mineral surfaces when the mineral breaks through its cleavage planes during comminution. Al-O sites act as collector attachment spots in fatty acid flotation and compete for collector adsorption.

Extreme Mg and Zn isotope fractionation recorded in the Himalayan leucogranites

High-silica granites (typically > 70 wt.% SiO2) represent the products of extreme crustal differentiation, but whether their distinctive chemical compositions reflect source lithology or are produced via magmatic differentiation is commonly difficult to discriminate. To provide new insights into this issue, here we present high-precision Mg and Zn isotope data for high-silica, peraluminous leucogranites from the Himalayan orogen. The samples are subdivided into two-mica leucogranite, tourmaline leucogranite and garnet-bearing leucogranite based on their mineral assemblages. The two-mica leucogranites, representing the least evolved Himalayan leucogranites, have similar δ66ZnJMC 3-0749L (mean = 0.31 ± 0.06‰) but heavier δ26MgDSM-3 values (mean = −0.01 ± 0.12‰) relative to more mafic igneous rocks. This indicates that they were formed by anatexis of weathered silicates, consistent with the well-acknowledged metasedimentary source of the Himalayan leucogranites. In contrast, the more evolved tourmaline leucogranites and garnet-bearing leucogranites have lower δ26Mg and higher δ66Zn values compared with the two-mica leucogranites. The extremely low δ26Mg (−1.32‰ to −0.54‰) and high δ66Zn values (0.35–0.69‰) of garnet-bearing leucogranites vary systematically with indices of granitic differentiation (e.g., Zr/Hf, K/Rb, Eu/Eu*, 1/TiO2). Although exsolution of chlorine-rich fluid may result in elevated δ66Zn values, it is unlikely to explain the low δ26Mg signatures of the same samples. Analysis of major Mg-bearing minerals suggests that substantial segregation of tourmaline and/or Fe-Ti oxide could have driven the differentiated leucogranites towards very low δ26Mg values. In this regard, the slightly lower δ26Mg values (mean = −0.17 ± 0.06‰) of tourmaline leucogranites relative to the two-mica leucogranites may also reflect that the former were more differentiated than the latter. Thus, although source heterogeneity may be responsible for the Mg isotopic variations observed in some high-silica granites, our study implies that high-silica granites could be remarkably heterogeneous in terms of Mg isotopes primarily as a result of prolonged fractional crystallization at the late stage of melt evolution. The anomalously light Mg and heavy Zn isotopic signatures of garnet-bearing leucogranites highlight that Mg and Zn isotopes may be treated as important makers of highly fractionated granites in future studies.

Magnesian calcite solid solution thermodynamics inferred from authigenic deep-sea carbonate

Abstract Magnesian calcite is perhaps the most well studied solid solution in the geosciences due to the widespread use of marine carbonates to reconstruct paleoenvironment. Despite decades of research, the low temperature thermodynamic properties of magnesian calcite in seawater are poorly constrained, largely because very slow reaction kinetics prevent the direct measurement of equilibrium distribution coefficients (KdMg) for anhydrous Mg-bearing minerals. In this study, we use the Mg content of authigenic calcite formed in deep-sea marine carbonate sediments to determine the dependence of KdMg on temperature and aqueous Mg/Ca between ∼2 and 25 °C. We find that the solid activity coefficient of magnesite in Mg-calcite is strongly temperature dependent in this range, leading to predicted exsolution of Mg at low temperatures. At the temperatures typical of ocean bottom water, equilibrium Mg distribution coefficients are at least an order of magnitude lower than values inferred from inorganic calcite growth experiments. Moreover, the equilibrium temperature dependence of KdMg agrees well with field-based paleotemperature calibrations determined for low-Mg benthic and planktonic foraminifera at temperatures

Fluid Chemistry of Mid-Ocean Ridge Hydrothermal Vents: A Comparison between Numerical Modeling and Vent Geochemical Data

Seawater-basalt interaction taking place at mid-ocean ridges was studied using numerical modeling to determine the compositional evolution of hydrothermal fluids and associated alteration mineralogy forming within newly emplaced crustal material. Geochemical modeling was carried out in a closed seawater-basalt system at discrete temperature intervals between 2 and 400°C at 500 bars, varying fluid/rock ratios, and secondary mineral assemblages representative of basalt alteration in natural systems. In addition to temperature, the fluid/rock ratio has a fundamental control on the resulting system chemistry. At rock-buffered conditions (low fluid/rock ratios), the mineral-solution equilibrium was characterized by high cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and very low dissolved Mg concentrations due to the precipitation of smectites and chlorite. A complex secondary mineral alteration assemblage dominated by Ca- and Na-bearing minerals including zeolites, calcite, epidote, prehnite, clinozoisite, and albite was predicted to form. The resulting fluid composition was alkaline and reduced relative to ambient seawater, with Eh values ranging between −0.2 and −0.6 V. In contrast, seawater-buffered conditions (high fluid/rock ratios) resulted in lower cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and higher dissolved Mg concentrations comparable to the value of this element in ambient seawater. A more simple mineral assemblage was predicted to form at these conditions with the predominance of Al-Si- and Mg-bearing minerals including kaolinite, quartz, and talc in addition to large amounts of anhydrite. The resulting fluid composition was mildly acidic and oxidized relative to seawater with Eh values ranging between −0.2 and 0 V. These modeling results were compared to a compilation of submarine hydrothermal vent fluid compositions from mid-ocean ridge settings and analogous basalt-dominated environments. The agreement obtained between the simulations and the compiled fluid data indicates that mid-ocean ridge hydrothermal processes can be closely reproduced by mineral-solution equilibria for a broad range of temperatures and fluid/rock ratios.

Magnesium isotope fractionation during granite weathering

The Mg isotope compositions of weathering profiles developed on granite and granodiorite bedrock have been analyzed to investigate Mg isotope behavior during the weathering of felsic rocks in a continental environment. δ26Mg generally exhibits a small increase from the bedrock values, which range from −0.17 ± 0.06‰ to +0.07 ± 0.06‰, until the upper two meters in which a decrease is observed, producing near surface values between −0.14 ± 0.06‰ and +0.09 ± 0.06‰. The small observed increase in isotopic composition below 2 m is likely due to Mg loss to the hydrosphere during biotite weathering and in situ illite production, while the decrease at depths <2 m is likely due to illite weathering near the surface, with a possible minor contribution from downslope deposition of less weathered material. One hydrothermally altered profile shows more extreme fractionation, reaching δ26Mg values as heavy as +0.55 ± 0.06‰ in the near surface component of the section. The fractionation associated with Mg loss during weathering can be modeled by Rayleigh distillation with fractionation factors between the residual rock and fluid ranging from α = 1.00001 to 1.00040. The Mg isotope composition is primarily controlled by the behavior of illite in the weathering profile, as Mg isotope composition is positively correlated with the illite fraction of major Mg-bearing minerals. A thorough understanding of Mg isotope systematics in these complex clay minerals may permit the use of Mg isotopes in paleosols as paleoclimate indicators.

Magnesium isotopic composition of altered oceanic crust and the global Mg cycle

To investigate the behavior of Mg isotopes during low-temperature alteration of oceanic crust and to further understand its role in the global Mg cycle, we measured the Mg isotopic compositions (25Mg/24Mg and 26Mg/24Mg) of a set of samples of altered oceanic crust (AOC) recovered from the Ocean Drilling Program Hole 801C, the reference site for old crust (∼170 Ma) subducting in the Pacific. The measured δ26Mg values range from −1.70‰ to 0.21‰, deviating from that of pristine oceanic basalts (−0.25 ± 0.07‰). Composite samples of volcanoclastic breccia that have experienced relatively intense alteration have larger variation in δ26Mg values (−1.01‰ to 0.15‰) than composite samples of massive basaltic flows (−0.53‰ to −0.04‰), indicating significant Mg isotope fractionation during low-temperature alteration of the oceanic crust. Moreover, the upper off-axis basement has on average lower δ26Mg values (−1.70‰ to −0.04‰) than the lower on-axis basement (−0.16‰ to 0.21‰). These findings, combined with the co-variations between MgO content and FeO∗/CaO ratio and between δ26Mg and FeO∗/CaO ratio, suggest that formation of Mg-bearing minerals (i.e., saponite and calcite) during low-temperature alteration of the oceanic crust accounts for the highly variable δ26Mg of AOC. Early formation of saponite under anoxic condition preferentially takes up heavy Mg isotopes and accounts for Mg enrichment and relatively high δ26Mg in the on-axis basement. Subsequent precipitation of carbonates results in the dilution of Mg and relatively low δ26Mg in the off-axis basement. In addition, accumulation of carbonate-rich interflow sediments in the upper basement may contribute further to the low δ26Mg. A weighted average δ26Mg value of 0.00 ± 0.09‰ is estimated for the AOC at Site 801, implying that low-temperature alteration of oceanic crust drives the ocean to a lighter Mg isotopic composition, and thus requires additional carbonate precipitation to maintain a steady-state Mg isotopic composition of seawater. A mass balance calculation suggests that the Mg output flux due to low-temperature alteration of the oceanic crust equals ∼12% of the annual Mg riverine input, indicating that AOC is a significant sink of Mg in seawater. Our study further highlights that recycling of AOC with highly variable δ26Mg along with overlying marine sediments into the mantle through subduction may generate Mg isotopic heterogeneity in the mantle at small scales.

Temperature effects on rock engineering properties and rock-fluid chemistry in opal-CT-bearing chalk

In this study, eight tri-axial tests on Cretaceous age outcrop chalk from Aalborg have been performed systematically by injecting MgCl2 for the first time at different temperatures (25, 60, 92, 110 and 130 °C) and for comparison, NaCl at 130 °C. Whole-rock geochemistry, stable isotope measurements, pycnometry, Field Emission Gun Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy, X-Ray Diffraction (XRD) and measurements of Specific Surface Area (Brunauer-Emmett-Teller theory (N2)) were applied to analyse unflooded and flooded cores. Based on analyses of changes in brine composition, mineralogy, specific surface area, solid density, porosity and permeability some conclusions can be drawn on temperature effects on rock engineering properties and rock-fluid chemistry.The MgCl2 flooded cores show systematically higher creep rates at higher temperature and the cores tested at 25 and 60 °C show similar creep rates as the two NaCl flooded cores at 130 °C. All fluid-rock interactions were more pronounced at higher temperature. After flooding with MgCl2 at 110 and 130 °C newly formed magnesite is observed. In the cores tested at 25, 60 and 92 °C magnesite crystals have not been positively identified, but minute increases in MgO in whole-rock geochemistry analyses are seen. Si4+ originating from the dissolution of silica bearing phases (mainly diagenetic opal-CT), has taken part in the re-precipitation of Si-Mg-bearing minerals during MgCl2 injection from 25 to 130 °C, leading to an increase of the specific surface area. This is partly balanced by opal-CT dissolution, which should lower the specific surface area. The flaky Si-Mg-bearing minerals, covering significant portions of the pore surfaces and throats are the main drivers to reduce permeability in cores flooded at high temperatures. Additionally, in NaCl flooded cores where mineralogical changes are minute, the dissolution of parts of the existing opal-CT has lowered the SSA. At high temperatures, the following chemical changes must be carefully acknowledged when porosity reduction is calculated: calcite and opal-CT dissolution and precipitation of new minerals, particularly Mg-bearing minerals. The presence of opal-CT strongly influences the mineralogical alterations in flooded cores, hence the geo-mechanical evolution.

Chemo-morphological coupling during serpentine heat treatment for carbon mineralization

One of the safest and permanent routes for potentially capturing and storing CO2 is via carbon mineralization which involves converting CO2 into environmentally benign and thermodynamically stable calcium and magnesium carbonates. One of the most abundant Mg-bearing minerals in the world suitable for carbon mineralization is serpentine (Mg3Si2O5(OH)4). To enhance the reactivity of serpentine with CO2, dehydroxylation by heating to temperatures in the range of 600 °C–700 °C was proposed. To establish a fundamental structural and morphological basis for the enhanced reactivity of serpentine (e.g. lizardite) on heating to temperatures in this range, in-operando synchrotron multi-scale X-ray scattering measurements were performed, with complementary analyses of the changes in the porosity, particle size, and surface morphology. The detailed transformation of lamellar serpentine to a pseudo-amorphous state on heating to temperatures in the range of 600 °C–700 °C, and the subsequent conversion to denser crystalline phases such as Al2.35Si0.64O4.82 (mullite), SiO2 (cristobalite), Mg2SiO4 (forsterite), and Fe0.3Mg0.7(SiO3) (enstatite) was determined from the wide and small angle X-ray scattering measurements. Increasing roughness of the pore-solid interface on heating to 700 °C followed by enhanced smoothness due to the formation of crystalline phases at higher temperatures was established from the combined ultra small and small angle X-ray scattering measurements. The transformation of lamellar lizardite to its pseudo-amorphous state corresponded to an increase in the porosity and surface area, while the formation of crystalline phases reduced the porosity and surface area while increasing the particle size.

Comparative Study of Five Outcrop Chalks Flooded at Reservoir Conditions: Chemo-mechanical Behaviour and Profiles of Compositional Alteration

This study presents experimental results from a flooding test series performed at reservoir conditions for five high-porosity Cretaceous onshore chalks from Denmark, Belgium and the USA, analogous to North Sea reservoir chalk. The chalks are studied in regard to their chemo-mechanical behaviour when performing tri-axial compaction tests while injecting brines (0.219 mol/L $$\hbox {MgCl}_{2}$$ or 0.657 mol/L NaCl) at reservoir conditions for 2–3 months (T = 130 $$^\circ \hbox {C}$$ ; 1 PV/d). Each chalk type was examined in terms of its mineralogical and chemical composition before and after the mechanical flooding tests, using an extensive set of analysis methods, to evaluate the chalk- and brine-dependent chemical alterations. All $$\hbox {MgCl}_{2}$$ -flooded cores showed precipitation of Mg-bearing minerals (mainly magnesite). The distribution of newly formed Mg-bearing minerals appears to be chalk-dependent with varying peaks of enrichment. The chalk samples from Aalborg originally contained abundant opal-CT, which was dissolved with both NaCl and $$\hbox {MgCl}_{2}$$ and partly re-precipitated as Si-Mg-bearing minerals. The Aalborg core injected with $$\hbox {MgCl}_{2}$$ indicated strongly increased specific surface area (from 4.9 $$\hbox {m}^{2}\hbox {/g}$$ to within 7–9 $$\hbox {m}^{2}\hbox {/g}$$ ). Mineral precipitation effects were negligible in chalk samples flooded with NaCl compared to $$\hbox {MgCl}_{2}$$ . Silicates were the main mineralogical impurity in the studied chalk samples (0.3–6 wt%). The cores with higher $$\hbox {SiO}_{2}$$ content showed less deformation when injecting NaCl brine, but more compaction when injecting $$\hbox {MgCl}_{2}$$ -brine. The observations were successfully interpreted by mathematical geochemical modelling which suggests that the re-precipitation of Si-bearing minerals leads to enhanced calcite dissolution and mass loss (as seen experimentally) explaining the high compaction seen in $$\hbox {MgCl}_{2}$$ -flooded Aalborg chalk. Our work demonstrates that the original mineralogy, together with the newly formed minerals, can control the chemo-mechanical interactions during flooding and should be taken into account when predicting reservoir behaviour from laboratory studies. This study improves the understanding of complex flow reaction mechanisms also relevant for field-scale dynamics seen during brine injection.