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At least 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Self-Assembly of Silica Nanoparticles at Water–Hydrocarbon Interfaces: Insights from In Operando Small-Angle X-ray Scattering Measurements and Molecular Dynamics Simulations

To achieve tunable controls on the interactions of siliceous materials in subsurface environments bearing water and hydrocarbons, it is essential to determine the influence of hydrocarbon–water interfaces on the self-assembly of the silica particles. The hydrophilic silica particle have a tendency to aggregate on the water front of the interface. Self-assembly of silica nanoparticles proceeds via the migration of these nanoparticles to the water–hydrocarbon interface followed by aggregation at the interface. Fractal-like morphologies of assembled silica nanoparticles at water–hydrocarbon interfaces are observed. Rapid assembly of the hydrophilic silica nanoparticles at water–hydrocarbon interfaces corresponds to an overall reduction in the surface tension of water–toluene and water–heptane systems. These studies demonstrate the silica aggregation that is the precursor of silica polymerization, or nucleation and growth is influenced by the presence of hydrocarbons in subsurface geologic environments. These insights were derived from in operando ultrasmall and small-angle X-ray scattering (USAXS/SAXS) measurements, cryo-scanning electron microscopy (Cryo-SEM) imaging, and classical molecular dynamics simulations. As a result, these studies are intended to inform current and future efforts aimed at tuning silica reactivity in subsurface geologic environments to enhance permeability, the design and use of silica-based proppants to enhance fractures, and the development of effective strategies to control silica-based scaling behavior in subsurface reservoirs.

42 ENGINEERING↗
DOE OSTI2022

Impact of Concurrent Solubilization and Fines Migration on Fracture Aperture Growth in Shales during Acidized Brine Injection

Acidic hydraulic fracturing fluid chemically and physically alters shale rock fabric during injection and shut-in, creating a “reaction-altered zone” along the fracture faces. To better characterize the variable thickness and composition of this reaction-altered zone under advective flow, we take a coupled experimental and modeling approach. A fluidic cell, with six fiducial markers, is first fabricated to keep the rock sample in place during the core floods and to allow image alignment of acquired images. Then, we conduct a series of reactive core floods in a clay-rich siliceous Wolfcamp shale sample with 10 wt % carbonate, using a synthetic fracturing fluid under no confining stress and at room temperature. High-resolution computed tomography (CT) scans are periodically conducted to observe the spatial alteration of the fracture network. We then perform scanning electron microscopy (SEM-EDS) on the two orthogonal surfaces (fracture surface and freshly cut profile face) to generate high-resolution elemental maps that show the change in mineralogy, both with distance along a given flow path along a fracture surface and with depth from the fracture surface into the shale matrix. These results are contrasted against a two-dimensional (2D) advection-diffusion reaction model developed previously for batch reactions between shale and synthetic fracturing fluids. The model simulates the geochemical interaction occurring at the fracture/matrix interface and penetrating into the shale matrix during the reactive core flood. Both model and experimental results show that the acidic brine is neutralized during the core flood, corresponding to an increase in fracture aperture as a function of fluid volume injected with the greatest change near the inlet. SEM-EDS scans reveal significant dissolution of carbonates on the fracture surface without pyrite oxidation. The reactive transport model indicates that carbonate depletion into the shale interior should be observable, yet SEM-EDS shows no discernible loss of carbonate in the orthogonal profile face. The combination of these observations suggests an additional fracture evolution mechanism in the reactive system, i.e., fines migration. We show that fines migration enhances the access of fracturing fluid to the matrix resulting in a more pronounced fracture widening. Finally, we conclude that coupled mineral dissolution and fines migration govern fracture aperture growth during acidized brine injection. In this work, we effectively show the underlying risk of relying solely on models that do not include an important (transport) process that can alter the system significantly and propose a combined chemomechanical mechanism for fracture evolution appropriate for this shale mineralogy.

58 GEOSCIENCES↗
DOE OSTI2026

Impact of Rock Wettability and Mineralization on CO 2 Storage Efficiency in Basalt Reservoirs

Basalt formations have emerged as highly promising targets for geological CO 2 storage due to their abundance of reactive silicate minerals that can rapidly convert dissolved CO 2 into stable carbonate minerals. The efficiency of CO 2 trapping in basalts, however, depends not only on their geochemical reactivity but also on reservoir wettability, which governs how CO 2 partitions among structural, residual, solubility, and mineral trapping mechanisms. However, wettability and mineral kinetics have largely been examined separately, wettability for plume migration and kinetics in terms of mineralization, leaving their coupled impact unquantified. Here, we systematically integrate th ese processes in a reservoir-scale reactive transport model to quantify the impact of three distinct wettability conditions (water-wet, mixed-wet, and CO 2 -wet) on plume migration, mineral precipitation, pore-scale blockage, and long-term immobilization. Results show that under ideal conditions (no mineralization), water-wet systems exhibited plume aspect ratios up to 28% higher than CO 2 -wet systems and promoted at least 60% greater residual trapping. When mineralization was incorporated, mineral trapping accounted for 57%, 53%, and 46% of the total immobilized CO 2 in water-wet, mixed-wet, and CO 2 -wet systems, respectively. Dissolved CO 2 contributed 43-46%, while residual trapping remained below 5%, and mobile CO 2 was substantially reduced due to mineralization, demonstrating that as the system becomes more water-wet, CO 2 is more effectively converted into stable carbonate minerals, although extensive mineral precipitation may lead to pore clogging. A global Sobol-Morris sensitivity analysis further reveals that the kinetic parameters of reactive minerals are the most influential factor, followed by wettability, while salinity has only a minor influence. Furthermore, these findings highlight that wettability is not merely a flow property but a fundamental control on CO 2 mineralization in basalts, with direct implications for site selection for storage projects.

Basalts↗
DOE OSTI2023

Resolving Nanoscale Processes during Carbon Mineralization Using Identical Location Transmission Electron Microscopy

Basalt reservoirs offer the potential for carbon mineralization, aiding in achieving net-zero emissions. However, debates persist about microscopic crystallization mechanisms due to limited characterization techniques under high-temperature and pressure conditions. By using Identical Location Transmission Electron Microscopy (IL-TEM) and cryo-TEM, this study reveals nanoscale interfacial carbonation processes of forsterite and diopside nanoparticles in water-saturated supercritical carbon dioxide under realistic reservoir conditions. Further, both minerals undergo preferential metal cation dissolution into a thin water film, forming porous Si-rich amorphous layers, supporting the leached layer mechanism as the dominant mineral reactivity process. Diopside’s amorphous layer has lower porosity and growth rate relative to forsterite, likely related to the connectivity of silicate tetrahedra. Kinetically favorable nesquehonite and aragonite nanocrystals form on the amorphous layers. These findings support the development of accurate reservoir simulations and help enable commercial-scale carbon storage deployment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Hierarchical Reinforcement Learning of a Short-Range Bond-Order Potential for Silica: Analytic Embedding of Coordination with Classical Efficiency

Reinforcement learning (RL) has recently emerged as a data-efficient strategy to parametrize short-range interatomic potentials. Building on our past RL optimization of pairwise silica models, we extend the framework to a bond-order (Tersoff-type) potential that provides an analytic embedding of local coordination through a three-body term. A hierarchical RL workflow combining continuous-action Monte Carlo Tree Search and property-based rewards efficiently explores the 26-dimensional parameter space, sequentially optimizing lattice parameters, densities, angles, and cohesive energies of 21 silica polymorphs. The resulting models, Q-Tersoff and ML-Tersoff, reproduce the energetic ordering of low-energy phases and capture the angular correlations and amorphous structure factors of silica with improved fidelity over pairwise force fields, while remaining orders of magnitude faster than high-dimensional machine-learned potentials. Both models underperform for elastic constants and high-energy frameworks, delineating the limits of the current analytic form. The approach establishes a general and interpretable route to angle-aware, short-range potentials that bridge physics-based and machine-learned descriptions of silicate materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Machine Learning Full NMR Chemical Shift Tensors of Silicon Oxides with Equivariant Graph Neural Networks

The nuclear magnetic resonance (NMR) chemical shift tensor is a highly sensitive probe of the electronic structure of an atom and furthermore its local structure. Recently, machine learning has been applied to NMR in the prediction of isotropic chemical shifts from a structure. Current machine learning models, however, often ignore the full chemical shift tensor for the easier-to-predict isotropic chemical shift, effectively ignoring a multitude of structural information available in the NMR chemical shift tensor. Here we use an equivariant graph neural network (GNN) to predict full 29 Si chemical shift tensors in silicate materials. The equivariant GNN model predicts full tensors to a mean absolute error of 1.05 ppm and is able to accurately determine the magnitude, anisotropy, and tensor orientation in a diverse set of silicon oxide local structures. When compared with other models, the equivariant GNN model outperforms the state-of-the-art machine learning models by 53%. The equivariant GNN model also outperforms historic analytical models by 57% for isotropic chemical shift and 91% for anisotropy. The software is available as a simple-to-use open-source repository, allowing similar models to be created and trained with ease.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Insights into Sorption and Molecular Transport of Aqueous Glucose into Zeolite Nanopores

Liquid-phase heterogeneous catalysis using zeolites is important for biomass conversion to fuels and chemicals. There is a substantial body of work on gas-phase sorption in zeolites with different topologies; however, studies investigating the diffusion of complex molecules in liquid medium into zeolitic nanopores are scarce. Here, we present a molecular dynamics study to understand the sorption and diffusion of aqueous β-d-glucose into β-zeolite silicate at T = 395 K and P = 1 bar. Through 2-μs-long molecular dynamics trajectories, we reveal the role of the solvent, the kinetics of the pore filling, and the effect of the water model on these properties. We find that the glucose and water loading is a function of the initial glucose concentration. Although the glucose concentration increases monotonically with the initial glucose concentration, the water loading exhibits a nonmonotonic behavior. At the highest initial concentration (~20 wt %), we find that the equilibrium loading of glucose is approximately five molecules per unit cell and displays a weak dependence on the water model. Glucose molecules follow a single-file diffusion in the nanopores due to confinement. Here, the dynamics of glucose and water molecules slows significantly at the interface. The average residence time for glucose molecules is an order of magnitude larger than that in the bulk solution, while it is about twice as large for the water molecules. Our simulations reveal critical molecular details of the glucose molecule’s local environment inside the zeolite pore relevant to catalytic conversion of biomass to valuable chemicals.

09 BIOMASS FUELS↗
DOE OSTI2020

An Unexpected Role of H During SiC Corrosion in Water

During aqueous corrosion, atoms in the solid react chemically with oxygen, leading either to the formation of an oxide film or to the dissolution of the host material. Commonly, the first step in corrosion involves an oxygen atom from the dissociated water that reacts with the surface atoms and breaks near-surface bonds. In contrast, hydrogen on the surface often functions as a passivating species. Here, we discovered that the roles of O and H are reversed in the early corrosion stages on a Si-terminated SiC surface. O forms stable species on the surface, and chemical attack occurs by H that breaks the Si–C bonds. Here, this so-called hydrogen scission reaction is enabled by a newly discovered metastable bridging hydroxyl group that can form during water dissociation. The Si atom that is displaced from the surface during water attack subsequently forms H 2 SiO 3 , which is a known precursor to the formation of silica and silicic acid. This study suggests that the roles of H and O in oxidation need to be reconsidered.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Multiscale Investigation of the Mechanisms Controlling the Corrosion of Borosilicate Glasses in Hyper-Alkaline Media

The overarching goal of the present multiscale investigation is to unearth the kinetics and mechanisms of corrosion of borosilicate glasses in hyper-alkaline (pH = 13) environments as a function of their chemical composition. Accordingly, a series of 3- to 6-component borosilicate glasses have been designed starting from Na 2 O–B 2 O 3 –SiO 2 ternary, wherein the compositional complexity has been added in a systematic tiered approach, finally resulting in the composition of the well-known international simple glass (ISG). Tetramethylammonium hydroxide (TMAH), one of the most widely used alkaline etchant in the glass and electronics industry, has been used as the corrosion media. A series of state-of-the-art characterization techniques including magic angle spinning nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectroscopy, elastic recoil detection analysis, and high-resolution transmission electron microscopy have been employed to unearth the compositional dependence of glass corrosion in hyper-alkaline environments. The glass compositions underwent congruent corrosion in the forward rate regime, whereas the controlling mechanism of corrosion in the residual rate regime depends on the presence/absence of Ca in the surrounding environment and can be explained on the basis of the dissolution–reprecipitation model. The dependence of corrosion kinetics and the chemistry of alteration products (in the residual rate regime) on the glass composition have been discussed. Here, the results presented in this contribution will ultimately supplement the scientific literature attempting to understand the fundamental science governing the aqueous corrosion of silicate-based glass chemistries and add to the growing database required to develop non-empirical predictive models for designing glasses with controlled dissolution rates.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2024

Variation in Cation Adsorption Mechanism Controlled by Chemical and Structural Heterogeneities at the Quartz (101)–Water Interface

Mineral–water interfacial reactions are central to chemical processes that control the fate of nutrients and contaminants in natural environments. Mineral surfaces commonly have complex structures and compositions whose impact on interfacial reactivity is poorly understood. Here, in this work, we investigated the effects of surface heterogeneities on Rb + sorption at the quartz (101)–10 mM RbCl solution interface at pH 9.8 using in situ high-resolution X-ray reflectivity. Two surface locales (i.e., Spots A and B) having distinct interfacial structures were chosen: Spot A was characterized by its low defect density (≤20% topmost Si vacancies) and Rb + adsorption occurred predominantly as an inner-sphere complex. In comparison, Spot B had a higher defect density (~50% vacancies) and was covered with poorly crystalline SiO 2 . A substantially larger Rb + uptake (i.e., 7-times higher coverage) was observed on this defective surface where Rb + incorporated in the vacancy sites (confirmed by density functional tight binding-based molecular dynamics simulations) or adsorbed directly on the disordered film. These results provide a direct quantification of how surface heterogeneity influences the geochemical behavior of mineral–water interfaces, in particular highlighting the important role of chemical and structural defects on the sorbate speciation and coverage at silicate mineral surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Carbonation and Sulfidation of Mg- and Ni-Containing Solutions: Implications for Carbon Mineralization and Critical Element Recovery

Carbonation of alkaline earth metals ( e.g. , magnesium (Mg)) and sulfidation of nickel (Ni) are promising methods to achieve concurrent carbon mineralization and selective Ni recovery. However, the coexistence of alkaline earth metals and Ni from silicate ores or mining wastewater complicates the carbonation and sulfidation owing to cation coprecipitation. To better understand simultaneous metal carbonation and Ni-sulfide formation, we used Mg- and Ni-containing solutions and systematically investigated the Mg and Ni coprecipitates’ phase transformation during sequential/concurrent carbonation and sulfidation. During a single carbonation process, hydromagnesite dehydrated and formed magnesite over time. Nickel bicarbonate formed and became a Mg–Ni carbonate solid solution because of their similar ionic radii. During a single sulfidation process, the pH did not affect Ni-sulfide formation, but it controlled Mg behavior. Specifically, at pH 9.6, brucite formed, while at pH 7.8, Mg 2+ remained in the solution. For the sequential carbonation–sulfidation process, Ni-carbonate formed during carbonation converted to Ni-sulfide because of the low Ni-sulfide K sp . For the sulfidation–carbonation process, Ni-sulfide remained the same even after carbonation and Mg-carbonate precipitates. For the concurrent carbonation and sulfidation process, Mg-carbonate and Ni-sulfide formed simultaneously. Finally, this study develops a scientific foundation of carbonation and sulfidation processes, benefiting coupled CO 2 storage and sulfide-enabled resource recovery.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2026

Microstructure, Transport, and Mechanics of Compacted Clay Simulated at the 0.1 μm Scale (1400 Smectite Clay Particles) Using a Coarse-Grained Model with Explicit Counterions

Clay-rich geomaterials play a critical role in many subsurface systems. The macroscale properties of these materials (low permeability, high ionic conductivity, high swelling pressure, etc.) are sensitive to molecular-level adsorption and hydration interactions at clay−water interfaces. Efforts to develop multiscale simulation approaches to predict these properties reveal a scale gap between atomistic simulations (typically limited to systems smaller than 10 nm) and continuum-scale models (which use computational grid elements with dimensions ≳ 10 μm). In this study, we present a coarse-grained (CG) framework that partly bridges this gap by simulating compacted smectite clay assemblages with dimensions of 0.1 μm containing 1,400 clay particles across a range of dry densities (1,050 to 1,850 kg·m −3 ) and Na/Ca counterion compositions (Na fraction ranging from 0.2 to 1). The simulated systems, along with their reconstructed binary three-dimensional pore networks, are used to evaluate the microstructure, pore size distribution, tortuosity, ion diffusivity, and swelling pressure of compacted smectite clay. Results show that our approach captures important features of the mesoscale heterogeneity of compacted clays, including tactoid formation, hierarchical porosity, and anisotropic pore networks. Results also reveal how compaction and counterion composition govern emergent behaviors, including dominant pore sizes, directional transport, and electrochemical response. This work highlights the potential of CG simulations to bridge molecular and continuum scales and to advance geotechnical and environmental applications involving clay-rich materials as well as related nanoporous media such as geopolymers and calcium-silicate-hydrate. However, the results also suggest that accurate prediction of certain microstructural and mechanical properties (e.g., swelling pressure) may require even larger-scale systems on the order of 1 μm.

02 PETROLEUM↗
DOE OSTI2025

Linking Manganese Fractions and Organic Carbon in Soils of Contrasting Land Use Systems

Manganese (Mn) is an essential micronutrient that influences carbon (C) cycling by binding or oxidizing soil organic matter. Mn fractions in soil and their plant availability depend largely on soil pH, which is commonly altered by agricultural practices. Fractions of Mn in soils range from readily available (e.g., bioavailable Mn, exchangeable Mn) to stabilized (e.g., Mn-oxide minerals, Mn contained in silicates). Furthermore, the distribution of soil Mn with depth was assessed in contrasting land use systems (organic agriculture, conventional agriculture, and unmanaged forest) using a sequential extraction method that targets Mn fractions ranging in bioavailability. Both agricultural sites had similar amounts of total Mn but had available Mn (1–7% of total Mn) lower than that of the unmanaged forested site (15% of total Mn). Manganese and organic C were generally positively correlated, but this relationship depended on soil depth, Mn fractions, and concentrations, while land management change had little influence.

Crops↗
DOE OSTI2025

Understanding the Dissolution and Passivation of an Aluminum Electrode during Electrocoagulation of Groundwater Using Neutron and X-ray Reflectometry

An aluminum (Al)-based electrocoagulation (EC) system can effectively remove dissolved silica and hardness in groundwater. The effectiveness of Al-EC in terms of pollutant removal, Faradaic efficiency, and energy consumption depends on the interfacial electrolysis or passivation of the electrode in water. Thus, understanding the electrolysis reaction at the liquid/electrode interface during operation is important for sustainable EC deployment. Here, a continuous flow-through Al-EC system was tested with various groundwater simulants, i.e., chloride (Cl – )-based, sulfate (SO 4 2– )-based, and mixed solutions. High pollutant removal with low energy consumption was observed in Cl – -based groundwater treatment, while low pollutant removal with high energy consumption was observed in SO 4 2– -based groundwater. For example, the required energy per unit mass of Al dosing in SO 4 2– -based groundwater is three times higher than that in Cl – -based groundwater at 10 mA/cm 2 . However, increasing the Cl – concentration significantly reduces this energy demand. In SO 4 2– -based groundwater, the silicate removal efficiency drops from 85.1% to 24.0% compared to that for Cl – -based groundwater, while Mg 2+ and Ca 2+ removal efficiencies decrease to 0.6% from 15.8% and 5.7% from 44.8%, respectively. To better understand this EC performance, we used in situ neutron reflectometry (NR) to examine the interfacial dynamics of Al dissolution and passivation at a 100 nm scale occurring on the surface of the sacrificial Al electrodes during EC. Ex situ X-ray reflectometry (XRR) was also used to support the in situ NR results. Both NR and XRR results revealed that Al dissolution is influenced by the presence of Cl – in the simulants, while a passivating layer forms on the electrode in a SO 4 2– -based solution. In the Cl – -based solution, anodic Al dissolution occurred locally and inhomogeneously across the surface of the Al anode film, resulting in a localized thickness reduction over time. In the SO 4 2– -based solution, no apparent dissolution of the Al anode was identified. Instead, Al underwent oxidation, forming an amorphous Al 2 O 3 surface layer within the Al electrode film that increased in thickness over time. In the mixed solution, both anodic Al dissolution and surface Al 2 O 3 layer formation occurred, indicating that Al dissolution and surface Al 2 O 3 layer formation are attributable to the Cl – and SO 4 2– ions, respectively.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Active Sites in the Dealuminated Beta Zeolite-Supported Cobalt Catalyst for Non-Oxidative Ethane Dehydrogenation

Dispersed metal species in siliceous zeolites have been actively studied for non-oxidative dehydrogenation of ethane (NDE). Fundamental insights into the dynamics of metal species in zeolites under reaction conditions have rarely been explored. Herein, we report an atomic level understanding of the dynamics and activity of cobalt (Co) sites in dealuminated Beta zeolite (DeAl-BEA) for NDE during induction and reaction conditions with extensive characterization techniques such as diffuse reflectance UV–vis, solid state nuclear magnetic resonance and X-ray photoelectron, X-ray diffraction along with in situ Fourier transform infrared and X-ray absorption spectroscopy. For a catalyst with 0.5 mass % Co loading, tetrahedral Co 2+ mononuclear sites, di-coordinated to the zeolite framework and with two silanol groups in vicinity (i.e., (≡SiO) 2 Co(HO–Si≡) 2 ), form upon exposure to hydrogen during induction and persist through the NDE reaction. Increasing the Co loading to 3.0 mass % yielded Co sites with similar electronic and coordination structures but slightly elongated Co–O bonds. Upon cooling to room temperature, the Co sites persisted in the same coordination environment, though the disappearance of a feature in the Co K-edge near-edge region revealed changes in the active site’s electronic structure coinciding with modest shifts in bond lengths. The electronic structure and activity of (≡SiO) 2 Co(HO–Si≡) 2 sites were studied comparatively to a few other hypothetical Co 2+ coordination structures, using electronic structure calculations and microkinetic simulations. The simulations showed that NDE is controlled by β-hydride elimination following C–H bond activation and that Co-sites possessing flexibility because of neighboring silanol defects are more active. Interestingly, dinuclear Co–O–Co sites (i.e., (≡SiO)Co(HO–Si≡) 2 –O–(HO–Si≡) 2 Co(≡SiO)) were more active than the mononuclear (≡SiO) 2 Co(HO–Si≡) 2 sites because of favorable hydrogen bonding with the vicinal silanol groups. In conclusion, the present study bridges the gap between the knowledge acquired by ex-situ characterizations and the active sites under the reaction conditions in alkane dehydrogenation chemistry.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Pore-Scale Microenvironments Control Anthropogenic Carbon Mineralization Outcomes in Basalt

Thin sections and hand samples from 50 sidewall cores from the Wallula Basalt Pilot Demonstration, a basaltic carbon sequestration demonstration, provided the opportunity for the in-depth analysis of carbon mineralization induced by the injection of supercritical CO 2 . In this study, we used optical petrography and scanning electron microscopy to characterize the physical and chemical characteristics of the basalt components influenced by carbon mineralization reactions from all available hand samples and thin sections within the three CO 2 injection zones and caprock flow interiors. We found extensive carbonate mineralization, mostly in the form of nodules that were shown to be chemically zoned: Ca-dominant in the core regions and Ca-bearing Fe-dominant in the outer regions. Carbonate mineralization also took the form of fracture-filling carbonate cement, and acicular aragonite was also observed. Overall, we clarified the structural and paragenetic relationships between newly formed minerals, identifying a new fibro-palagonite-like, poorly crystalline silicate phase that grew on the carbonate nodules and pore-lining zeolites. Here we observed Fe-dominant carbonate precipitates surrounding acicular aragonite and rhombohedral Ca-carbonate cores, whereas previous studies of these zoned nodules did not observe these structures. A comprehensive accounting of the carbon mineralization products is vital to understand and predict the behavior of supercritical CO 2 in the subsurface given both the diversity of the host rock between and within injection zones, especially considering that the morphology and chemistry of the diverse precipitates are influenced by the pore-scale microenvironments of the basalt.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Carbonate-Metal Reactions in the Lower Mantle

Carbonates are important carbon-bearing phases in the mantle. While their role in upper mantle petrologic processes has been well studied, their effect on phase relations, melting, and transport properties in the lower mantle is less understood. The stability of carbonates in the mantle depends on a host of factors, including pressure, temperature, oxygen fugacity, and reactions with surrounding mantle phases. To understand the stability of carbonates in the presence of metal in the lower mantle, carbonate-metal reaction experiments on the Fe–Si–Ca–Mg–C–O system were conducted up to 124 GPa and 3200 K. We find that carbonates react with iron alloys to form silicates, iron carbides, and oxides. However, the temperature at which these reactions occur increases with pressure, indicating that along a geotherm in the lowermost mantle carbonates are the stable carbon-bearing phase. Carbon is found to be less siderophilic at high-pressure compared to silicon.

58 GEOSCIENCES↗
DOE OSTI2025

Kinetics of Pyrolysis and Thermal Evolution of Negev Desert Lithologies

The Negev desert in Israel is home to large quantities of organic-rich, shallow marine sedimentary lithologies that could potentially accommodate the disposal of spent nuclear fuel. Previous thermal analyses of Negev carbonates have focused on industrially relevant considerations such as natural gas and oil extraction or pyrolysis for recovering hydrocarbon fuels. Here, this study addresses thermal evolution of the Negev organic-rich carbonate, siliceous, and phosphorite rocks and associated chemical, mineralogical, and microstructural changes that may occur under prolonged thermal loading in the vicinity of spent nuclear fuel disposal systems. Our employed methods include high-temperature X-ray diffraction, high-temperature infrared spectroscopy, and thermal analysis integrating thermogravimetry, differential scanning calorimetry, and mass spectrometry. Further, we apply iterative iso-conversional model-free methods to derive kinetic parameters for thermal decomposition of the Negev organic-rich carbonate rocks from 200 to 550 °C. Our results have provided mechanistic insights into the thermal evolution encompassing water desorption, decomposition of organic matter, and decarbonation of carbonate phases.

58 GEOSCIENCES↗