Geochemical evidence suggests that arsenic is a crucial factor reinforcing gold mineralization in pyrite. The systematic presence of Au-bearing arsenian pyrite in hydrothermal systems suggests a coupled Au-As geochemical behavior in various physico-chemical conditions. However, there is a lack of understanding of elemental interactions at the atomic scale during gold mineralization, leaving unresolved which specific structural configurations host Au-As interactions and whether other trace elements can exert effects similar to arsenic. In this study, we use ab initio simulations to quantitatively evaluate interactions between elemental impurities in pyrite, specifically focusing on the gold-arsenic relation. We consider a wide range of chemical and structural substitutions, for which we monitor the structural distortions they induce and calculate their formation energies. We show that individual incorporations of impurity atoms are prohibitive. But joint substitutions better accommodate the induced stress, decrease energy barriers, and thereby facilitate gold incorporation. Among various trace elements, arsenic substitution for sulfur is the most favorable for gold incorporation, due to the formation of [AuAsnS6-n] coordination octahedra, with Au substituting Fe. The presence of As effectively alleviates the lattice stress associated with Au substitution, and restricts it within the local coordination polyhedron, thereby facilitating the large-scale, long-term preservation of Au in pyrite. Our study provides a novel insight into co-evolutionary processes in ore deposits, with a focus on pyrite-hosted Au deposits.

Metal partitioning into calcite has important geochemical and environmental implications as its reactive surface structure exerts a strong impact on impurity mobility and distribution. Despite numerous observations that strontium (Sr) incorporation into calcite affects reaction rates, the mechanism driving enhancement/inhibition remains debated and is often described only qualitatively. To address this uncertainty, we quantified the chemical composition and lattice strain of the grown Sr-rich calcite using high-resolution chemical imaging techniques and nanoscale strain mapping. These observations were compared to in situ atomic force microscopy (AFM) measurements of the growth rate of single crystal calcite growth as a function of aqueous [Sr]/[Ca](aq) ratio (0-1) with particular focus on hysteresis in growth rates after the solution composition was changed. Our observations suggest that Sr has a complex influence on calcite growth, affecting growth kinetics, surface topography, crystal structure, and lattice strain. At lower Sr concentrations in solution ([Sr]/[Ca](aq) <= 0.29), step velocity can increase or decrease depending on the concentration. Based on our measured solid (Sr/(Sr+Ca) ratios, we assessed the thermodynamics of the CaxSr1-xCO3 solid solution. We find that it can be described using an ideal solid solution model with a solubility for the virtual R3(-)c SrCO3 endmember of logK(sp) = -8.04. Sr incorporation in the calcite structure (Sr/(Sr+Ca)) exhibits a positive, linear correlation with [Sr]/[Ca](aq) matching the ideal solid solution until Sr incorporation plateaus at similar to 15 at%, at which point a miscibility gap is observed. Calcite growth can recover in subsequent Sr-free solutions at these impurity contents. More importantly, within 10 nm of new growth (similar to half hour), growth rate, step density, net growth rate, and hillock morphology are restored. When [Sr]/[Ca](aq) = 0.39, active growth areas on the crystal are significantly decreased, leading to heterogenous growth with smaller hillocks. A growth rate increase at this ratio is confined to the active growth area, whereas other regions exhibit slower growth. The observed inhomogeneity differs from previous literature observations. As [Sr]/[Ca](aq) >= 0.39, strain becomes the dominant factor, leading to a rapid growth inhibition that can be quantitatively explained by an "incorporation" mechanism. Specially, Sr incorporation above a critical lattice strain threshold of similar to 4% correlates with a swift decline in growth rate. Moreover, the recovery of growth rates and growth morphology after Sr removal depends on the extent of prior strain and creates a substantial recovery lag, sometimes requiring the growth of >200 nm of new calcite before pristine growth rates are restored. Our study also shows that Sr incorporation alters calcite's chemo-mechanical properties, increasing its elastic modulus through CaxSr1-xCO3 solid solution strengthening. These results have implications for our understanding for how impurities inhibit the growth of minerals, as well as an application in the controlled immobilization of Sr in the subsurface, such as legacy waste remediation, where precise control of growth rates and mechanical properties is crucial.

Magnesium oxide (MgO) is considered as a potential sorbent for direct air capture of carbon dioxide in a looping process. Previous research on mineral looping for carbon capture from flue gas using MgO has shown deactivation of the sorbent with repeated cycles but repeated cycling for carbonation at ambient conditions has not been yet investigated. Here, we tested three cycles of carbonation for MgO nano-scale powders with different starting surface area. We find that carbonation efficiency is higher at higher surface area. No decrease in carbonation efficiency was observed with cycling, instead we find that carbonation efficiency and surface area are stable or even increasing with cycling. Based on our experimental data, we hypothesize that the carbonation in presence of relative humidity leads to first hydration of MgO, leading to formation of brucite (Mg(OH)2). This formation of Mg(OH)2 is a volume-increasing reaction, which leads to fracturing of MgO particles and results in an increase in surface area. We observed formation of amorphous and crystalline hydrated carbonates. The crystalline phase observed was nesquehonite (MgCO3 ⋅ 3H2O). Our results show that MgO does not show any deactivation with repeated cycling for carbon capture at ambient conditions and in the presence of humidity. These findings therefore indicate that MgO is a suitable candidate as sorbent material for direct air capture of carbon dioxide.

Ion sorption extent and mechanism depend in part on the mineral surface termination, which can be highly complex. Variations in surface functional groups, particularly with defect density and surface roughness, influence mineral reactivity towards solutes. In this work, we investigate the adsorption of a rubidium cation (Rb+) at pristine and defect quartz (101) surface sites using well-tempered metadynamics, based on simulations with the quantum chemical density-functional tight-binding (DFTB) method. We compare the relative energetics of Rb+ adsorption across selected sites for each surface, with nanosecond-level sampling, highlighting similarities between vicinal and geminal silanol sites. We find that the positive Rb+ partial atomic charge can increase by as much as 0.5e as it approaches the surface, with implications for modulation of ion adsorption strength and extent at the quartz (101) surface with surface vacancies, silanol coverage, and charge.

Structural defects and lattice strain are intrinsic to many crystalline materials, yet their roles in controlling chemical reaction mechanisms and directing crystallization pathways remain poorly understood. Here, we revealed the three-dimensional evolution of strain and dislocation defects at the nanoscale during the growth of heterogeneously nucleated barite (BaSO4) and calcite (CaCO3) crystals by using coherent X-ray scattering, electron microscopy, and molecular simulations. Unlike barite, which formed with minimal internal strain, calcite developed dislocation defects and exhibited spatially varying strain that increased during growth. During growth in Sr-rich solutions, calcite likely incorporates Sr2+ into the defects, which further modulates the local lattice structure and increases both the compressive and tensile strain. These findings suggest that calcite crystallization was likely dominated by attachment of precursor phases, which gave rise to defect-enriched domain structures not predicted by classical growth models. By linking defect formation to ion incorporation and growth dynamics, this work provides fundamental insight into how lattice-level strain heterogeneity governs the chemical reactivity of ionic crystals.

Recovery of magnesium from brines can potentially be used to source MgO (periclase) as a CO2 sorbent or for Mg-based cements. However, it is not clear how common impurities in brines, such as lithium, affect the resulting MgO reactivity. To test the effect of lithium incorporation on MgO reactivity for hydration and carbonation, we combined computational simulations with experiments. Experimentally altered (Mg,Li)O with a low dopant concentration (0.012 +/- 0.002% w/w Li) was characterized using synchrotron-based X-ray scattering and high-resolution electron microscopy to measure reaction layer formation on (Mg,Li)O. Single-crystal X-ray diffraction analysis of (Mg,Li)O demonstrates that the incorporation of lithium leads to the formation of oxygen vacancies. The presence of vacancies is likely causing faster hydroxylation rates as predicted by ab initio molecular dynamics simulations. However, the faster hydroxylation rates likely lead to faster passivation of the surface because we observe thinner reaction layers on (Mg,Li)O samples both over short time periods (30 days) and over long time periods (28 years). After 28 years, the reaction layer on the (Mg,Li)O sample was less than one-third of the thickness of that of the pure MgO sample. In addition, over 30 days, reaction layers on (Mg,Li)O samples primarily formed at steps rather than on terraces, in contrast to our previous observations on MgO. Based on our results, naturally occurring impurities in MgO modify its reactivity even at very low concentrations and need to be considered for accurate reaction rate prediction for application of MgO as a CO2 sorbent or in cements.

We demonstrate how the spatial distribution of water on the MgO single crystal surfaces controls the texture of a new phase, Mg(OH)2, formed during a dissolution-precipitation reaction. Modeled liquid-vapor phase separation near the solid-liquid interface demonstrates the formation of a homogeneous film as opposed to isolated droplets depending on the surface wettability properties. Modeling results confirm that our experimental observation of droplet-like features during formation of Mg(OH)2 on the MgO surface is driven by the temperature-dependent interfacial properties.

Brucite [Mg(OH)2] is a promising sorbent for carbon dioxide removal (CDR) due to its availability and low calcination temperatures. However, natural and synthetic brucites tend to contain metal impurities, such as iron or manganese, and how these impurities affect the interfacial chemical reactivity is uncertain. Here, the impact of low concentrations of iron and manganese impurities on the carbonation efficiency of Mg(OH)2 was examined. Mg(OH)2 with small amounts (1-5 mol %) of Fe and Mn was synthesized. The increasing substitution of Fe into Mg(OH)2 was accompanied by the oxidation of Fe. The phase transformation sequence during the carbonation was found to be brucite [Mg(OH)2] -> amorphous magnesium carbonate (MgCO3nH2O) -> nesquehonite (MgCO33H2O), regardless of impurity concentration. Both the Fe- and Mn-doped Mg(OH)2 samples were more reactive than endmember Mg(OH)2, possibly due to their higher surface areas and lower stabilities. During carbonation, 3 mol % Fe- and Mn-doped Mg(OH)2 showed the highest reactivity. The variance in reactivity for Mn-doped Mg(OH)2 was less than that of Fe-doped Mg(OH)2. These results suggest that natural or industrial waste Mg(OH)2 with less than 5 mol % Fe and Mn impurities may be targeted as more effective CDR sorbents than endmember Mg(OH)2.

Experimental measurements of the heterogeneous nucleation rate rely on counting the number of nuclei with time. However, the size of a thermodynamically stable nucleus is often a few nanometers in diameter and is below the resolution of most (in situ) measurement techniques that provide a statistically valid sample. Due to the finite resolution of the instruments and analysis methods, it is challenging to capture the incipient nuclei and the subsequent evolution of nuclei density over time. In this work, we demonstrate the impact of instrument resolution on observed nuclei densities by comparing numerical modeling with experimental results. To achieve this, we implemented heterogeneous nucleation within the pore-scale reactive transport modeling framework using classical nucleation theory (CNT). We compared the modeling results with nucleation rates measured using X-ray nanotomography (XnT) and evaluated how these impact the apparent values of the prefactor and interfacial energy based on CNT and the crystal growth rate. Specifically, we applied a resolution threshold (artificial resolution limit) in the model during nuclei counting to resemble an experimental resolution, ranging from 15 to 500 nm. The findings reveal that the instrument resolution significantly impacts the apparent prefactor and interfacial energy. Both apparent prefactor and interfacial energy decrease with a decrease in the instrument resolution. While deviation in the prefactor due to resolution is anticipated, those in the interfacial energy are unexpected. The approach described here allows one to correct apparent nucleation rates that depend on the instrument's resolution to derive "intrinsic" CNT parameters for the prefactor and interfacial energy.

The hydroxylation of periclase (MgO) to brucite (Mg(OH)2) is thought to be an important intermediate step when using MgO to capture CO2 from the atmosphere. However, the mechanism of hydroxylation of MgO to form Mg(OH)2 is poorly understood. In this work, we used atomic-scale density functional tight binding simulations coupled with the metadynamics rare event method to analyze the surface chemistry of MgO and the acid dissociation equilibrium constants (pKa) of its surface sites. The method and parameters were validated by calculating the pKa for hydroxylation of the first shell water bound to aqueous Mg2+ ion. The pKa value derived using a probabilistic method was 12.3, which is in fair agreement with the accepted value of 11.4, with the difference between them equal to a similar to 5 kJ mol-1 error in the calculations. We then extended these pKa calculations to probe the hydroxylation reactions of the surface sites of the MgO(100)-water interface, arriving at pKas of 5.4 to deprotonate terminal water molecules bound to the surface magnesium sites (eta-OH2 or > MgOH2), and 13.9 to deprotonate hydroxylated bridging oxygen sites (mu 5-oxo or > O). Hydroxide (OH-) adsorption on the surface was also probed and found to be less thermodynamically favorable than deprotonation of the terminal water molecule. The plausibility of the computed pKas was verified using an activity-based speciation model and compared to pH measurements of water equilibrated with MgO nanoparticles and single crystals. The model predicted a solution pH of 7.1 when surface sites buffered and the pH of 12.0 when MgO dissolution dominated. These are close to the experimental initial solution pHs of 7-7.5 and the long term pHs of similar to 10.5. The similarity suggests that the calculated pKa values from the DFTB+/metadynamics simulations are plausible and that these methods can be a useful tool to probe reaction mechanisms involving covalent bonds.

Direct air capture (DAC) may be feasible to remove carbon dioxide (CO2) from the atmosphere at the gigaton scale, holding promise to become a major contributor to climate change mitigation. Mineral looping using magnesium oxide (MgO) is potentially an economical, efficient, and sustainable pathway to gigaton-scale DAC. The hydroxylation and carbonation of MgO determine the efficiency of the looping process, but their rates and mechanisms remain uncertain. In this work, MgO single crystals were reacted in air or CO2 at varying humidities and characterized by X-ray scattering, microscopy, and vibrational spectroscopy. Results show that the hydroxylation formed a brucite (Mg(OH)2)-like layer immediately after crystal cleaving. Concurrently, the carbonation formed hydrated magnesium carbonate phases, including barringtonite (MgCO3·2H2O) and nesquehonite (MgCO3·2H2O), in the layer. Rapid initial growth of the layer is also manifested in short-range bending/warping of nanocrystallites, resulting in multiple orientations of the same phases on the surface. The layer growth slowed down over time, indicating surface passivation. The formation of barringtonite and nesquehonite with 1:1 CO3/Mg ratio indicates an efficient carbonation when compared to other magnesium carbonate phases of lower ratio. Our results are essential for understanding surface passivation mechanisms and tackling the passivation issue of mineral looping DAC technology.

Thin fluid layers confined between nanoparticles play an important role in several natural and industrial systems, including radioactive wastes stored in tanks at the U.S. Department of Energy's Hanford site. Multimodal neutron and computational approaches have been integrated to examine the properties of one, two, and four layers of water (H2O or D2O) on nanoparticulate, hydrous, or deuterated boehmite (γ-AlOOH or γ-AlOOD). Exposure of deuterated boehmite to H2O at 90 °C yielded rapid H/D isotopic exchange likely driven by a Grotthus-like proton-hopping mechanism. The in-plane and out-of-plane vibrations of the structural hydroxyls involved in this exchange were observed for both the hydrated and deuterated boehmite. These observations were confirmed by molecular dynamics simulations that also showed that single water molecules on the (010) surface bond to the structure via four bonds: two from hydrogens (deuteriums) on the water to surrounding oxygens and two from the water's oxygen to surrounding OD/OH. Bulk water-like properties began to appear once four monolayers of water had been added, but steric crowding limited water diffusion rates once two monolayers had been added. The super-Arrhenius temperature dependence observed at four monolayers indicated glass-like behavior in a well-formed hydrogen-bonding network. Such a network is not sufficiently developed, however, when the surface water coverage is less than four layers. The unique nature of these layers can provide critical information for understanding forces between particles in proximity, and resultant effects on suspension rheology.

Catalyst size, morphology, and crystal structure play crucial roles in determining the activity and selectivity of electrochemical CO2 reduction reactions, which are known to change during the reaction process. A comprehensive understanding of how, when, and why these parameters evolve under operational conditions is essential for developing stable, efficient, and selective catalysts. In this study, we reveal that formate, one of the reaction products, contributes to the degradation of copper catalysts through a ligand-assisted dissolution mechanism. Utilizing in situ electrochemical atomic force microscopy and ex-situ scanning and transmission electron microscopies, we observed a significant reduction in the size of copper nanoparticles, which decreased from over 30 nm to less than 10 nm in diameter within 60 min of CO2RR. The temporal production of formate correlated with the particle size changes. Furthermore, analysis of the electrolyte using inductively coupled plasma optical emission spectroscopy confirmed the dissolution of copper nanoparticles. Control experiments involving various reaction products (H2, CO, and HCOO-) demonstrated that formate significantly promotes copper dissolution, thereby highlighting its role in the ligand-assisted dissolution mechanism of copper electrocatalysts. Our findings provide critical insights into copper catalyst behavior during electrochemical CO2 reduction, facilitating the design of more resilient and effective electrocatalysts.

Despite extensive research on MgO's reactivity in the presence of CO2 under various conditions, little is known about whether impurities incorporated into the solid, such as iron, enhance or impede hydroxylation and carbonation reactions. The purity of the MgO required for the successful implementation of MgO looping as a direct air capture technology affects the deployment costs. With this motivation, we tested how incorporated iron impacts MgO (100) reactivity and passivation layer formation under ambient conditions by using atomic force microscopy, electron microscopy, and synchrotron-based X-ray scattering. Based on electron microprobe analysis, our MgO samples were 0.5 wt % iron, and Mössbauer spectroscopy results indicated that 70% of the iron is present as Fe(II). We find that even these low levels of iron dopants impeded both the hydroxylation at various relative humidities (10%, 33%, 75%, and >95%) and carbonation in CO2 (33%, 75%, and >95%) on the (100) surface. Crystalline reaction products were formed. Reaction layers on the sample were easily removed by exposing the sample to deionized water for 2 min. Overall, our findings demonstrate that the presence of iron dopants slows the reaction rate of MgO, indicating that MgO without incorporated iron is preferable for mineral looping applications.

Volume-increasing replacement reactions can lead either to fracturing of the parent phase or to the formation of cohesive layers that passivate further reaction, but the factors that drive one outcome or the other are not understood. In this experimental study, we investigated the volume-increasing replacement of carbonate rocks (consisting of CaCO3 and CaMg(CO3)2) of different porosity by witherite (BaCO3). Samples were characterized using scanning electron microscopy, Raman spectroscopy, small-angle neutron/X-ray scattering, X-ray tomography, and scanning transmission electron microscopy. We observed the formation of a witherite reaction layer and witherite formation within pores and along grain boundaries. Despite this being a volume-increasing replacement reaction, newly formed witherite was porous, potentially allowing further replacement. Filled fractures were observed in the low-porosity carbonates, whereas witherite formed within pores in high-porosity carbonates. We conclude that fracturing of the parent phase versus passivation is contingent on the initial microstructure of the parent with an optimal degree of porosity required for fracturing.

The connection between solution structure, particle forces, and emergent phenomena at solid-liquid interfaces remains ambiguous. In this case study on boehmite aggregation, we established a connection between interfacial solution structure, emerging hydration forces between two approaching particles, and the resulting structure and kinetics of particle aggregation. In contrast to expectations from continuum-based theories, we observed a nonmonotonic dependence of the aggregation rate on the concentration of sodium chloride, nitrate, or nitrite, decreasing by 15-fold in 4 molal compared to 1 molal solutions. These results are accompanied by an increase in repulsive hydration forces and interfacial oscillatory features from 0.27-0.31 nm in 0.01 molal to 0.38-0.52 nm in 2 molal. Moreover, molecular dynamics (MD) simulations indicated that these changes correspond to enhanced ion correlations near the interface and produced loosely bound aggregates that retain electrolyte between the particles. We anticipate that these results will enable the prediction of particle aggregation, attachment, and assembly, with broad relevance to interfacial phenomena.

MgO (periclase) is a promising material for direct air capture of CO2 using a mineral looping process, but it is unknown how impurities in the environment will affect the CO2 uptake and hence process economics. Here, we investigated the effects of dissolved iron on the extents of MgO hydroxylation and subsequent carbonation reactions to determine if this has a beneficial or detrimental effect. On single-crystal MgO, dissolved iron prevented hydration of MgO to Mg(OH)2 (brucite) and instead formed a shell of lepidocrocite (gamma-FeOOH). This did not passivate the MgO as dissolution below the shell was observed. During hydroxylation of MgO powders in the presence of dissolved iron, formation of brucite containing Fe(II) was observed. In addition, formation of nanoscale iron oxides containing Fe(III) was observed using magnetometry and Mossbauer spectroscopy. Subsequent carbonation experiments showed increased carbonation of MgO hydroxylated in the presence of iron. Our results indicate that the presence of dissolved solute impurities during hydroxylation may be beneficial for carbonation of hydroxylated MgO.

Using neutron scattering dynamics measurements to validate molecular dynamic simulations, it is found that the rates of solvent exchange are not likely to limit the rates of calcite crystal growth and dissolution.