The distribution of pore sizes in a rock evolves continuously throughout its diagenetic history, strongly affecting flow and storage properties. One of the most important drivers of this evolution is mineral cementation, although existing models have not simultaneously represented the distinct effects of both carbonate and quartz cement within a single framework. Here we develop a modified population dynamics model that represents cementation as a combination of deterministic, size-dependent pore closure and stochastic, size-independent porosity reduction. We test the model against pore size distributions derived from digital rock image analysis of three sandstone samples with contrasting cementation histories, and demonstrate that the hybrid approach outperforms both purely deterministic and purely stochastic models. Carbonate cement preferentially occludes large pores, truncating the tail of the pore size distribution and disproportionately reducing permeability relative to the volume of porosity lost. Quartz cement, by contrast, preferentially reduces the abundance of smaller pores through uniform grain-surface overgrowth, leaving the largest flow pathways largely intact. Analysis of the optimized model parameters reveals that the deterministic component is only well-constrained where carbonate cementation is sufficiently abundant to impose a detectable size-selective signature. Our results demonstrate that the framework provides a computationally efficient basis for connecting cementation history to pore structure evolution in porous media.

Coupled dissolution-precipitation reactions are central to many processes in Earth's crust, significantly impacting natural and engineered systems. During these reactions, secondary mineral coatings often form on primary mineral surfaces, creating armoring layers that passivate the surface and dramatically reduce reaction rates. While the structure of secondary coatings is thought to influence their effect on coupled reactions, few studies have focused on characterizing their porous nature. Here, we investigated the reaction between calcite and sulfuric acid, which causes gypsum to precipitate as an armoring layer on the dissolving calcite surface. Our experiments revealed a two-stage growth pattern: initial rapid formation (<0.5 h) of a similar to 20 mu m thick layer of gypsum, followed by constant growth at a rate of approximately 0.71 mu m h(-1) for 768 h, ultimately producing a layer over 500 mu m thick. Consistent with previous studies, the secondary mineral layer causes the reaction rate to be several orders of magnitude lower than expected given the low pH conditions. High-resolution FIB-SEM imaging revealed a complex heterogeneous and anisotropic structure in the secondary mineral coating. The layer closest to the calcite interface exhibited extremely low porosity (1 %) with poorly connected nanoscale pores, while the outer region showed much higher porosity (31 %) with a well-connected pore network. Notably, numerous parallel cracks extended from the calcite interface into the high-porosity zone, potentially serving as the primary conduits for solute transport through the gypsum layer. A novel diffusion-reaction model containing 2 porous layers with contrasting diffusivities successfully reproduced the reaction rates we observed in our experiments. Our results demonstrate that the micrometer and nanometer scale structure of the coatings surrounding reacting minerals is a crucial factor controlling the rates of coupled dissolution-precipitation reactions in geochemical systems.

Carbon mineralization is a promising method for long-term carbon storage. One way to sustainably mineralize carbon is by utilizing Ca and Mg bearing industrial waste products to form carbonate minerals. However, solid oxalates could potentially store twice the amount of carbon as carbonates. Here we provide a proof of concept for the combined utilization of both industrial and agricultural waste products to form carbon-storing oxalate minerals. We tested the reaction between plant-based oxalic acid, extracted from Rumex and rhubarb leaves, with two calcium-bearing industrial waste products-gypsum and coal fly ash. We demonstrate that the method successfully converts 55 % - 79 % of the oxalate in the extracts into stable calcium oxalate phases. The carbon storage potential of this method was calculated to be similar to 9 g of COQ per kg of Rumex leaves and 6 g per kg of rhubarb leaves. While the carbon storage potential is currently low relative to global removal requirements, this method could potentially be upscaled with technological enhancements, including genetic modification and crop manipulation.

Sulfur isotopic values (S34S) of pyrite and organic matter (OM) in sediments are widely used for the reconstruction of the sulfur and oxygen cycles as well as pathways of OM preservation. Currently, significant uncertainties persist regarding the mechanism by which sulfur is partitioned between pyrite and OM and its effect on the S34S record of rocks. Here, we present experimental analysis of iron, carbon, sulfur and S34S values in marine and lacustrine rock samples. The experimental data was compared with published data of rock samples from the Mesoproterozoic era (1.6 Ga) to the Paleogene period (23 Ma). We also developed a kinetic model to simulate the evolution of S34S of pyrite and OM under different environmental conditions. Our analysis reveals linear relationships between S34S value of pyrite and its isotopic difference from S34S value of organic sulfur (R2 ranging from 0.43 to 0.96) for large S34S ranges of pyrite (110 %o) and organic sulfur (93 %o). These ranges and linear trends cannot be explained by variations in the S34S value of seawater sulfate or by the isotopic fractionation associated with microbial sulfate reduction. Rather, local conditions that change the ratio between reactive iron and sulfate are shown to control the S34S values of organic sulfur and pyrite and their isotopic gap. This is because reactive iron pyritization rapidly captures isotopically light H2S generated by microbial sulfate reducers in the early stages of diagenesis, leaving behind heavy H2S that reacts with OM. In some environments, the isotopic gap between organic sulfur and pyrite correlates with the OM content in the rock, reflecting the critical role of reactive iron in OM preservation via sulfurization. Hence, the role of iron on the partitioning of sulfur between OM and pyrite in sedimentary environments is essential for reconstructing the sulfur cycle and its interaction with the carbon cycle in geological sequences.

Coalbed methane (CBM) has emerged as a significant unconventional resource, and understanding the interactions between adsorbed water and methane in coal is crucial for assessing coalbed methane reserves and productivity. To elucidate how pre-adsorbed water influences methane adsorption, we conducted high-pressure methane adsorption experiments on both dry and wet coals with various maturities. By employing a combination of theoretical modeling and experimental analyses, we have identified that adsorbed water could markedly decrease the amount of methane sorption, and changes in water coverage are responsible for the observed negative correlation between water coverage and methane pressure for these samples. Further investigations involving wettability measurements and low-pressure N2 adsorption demonstrate that adsorbed water exhibits weaker interaction with highly mature coal compared to the low-maturity coal. Our analyses suggest that adsorbed water can decrease methane adsorption quantity in coal by up to 60%. Moreover, the adsorption and mass transfer rates of methane can be reduced by up to 90% and 80%, respectively, due to the presence of adsorbed water. Additionally, our results indicate that methane can displace adsorbed water in highly mature coal significantly, decreasing water coverage from 63% to 22% with increasing methane pressure.

Fluid transport in cementitious matrices and materials is fundamental for many engineering and environmental applications. However, observing and measuring transport processes inside opaque porous solids, such as cement, is technically challenging. We tested the feasibility of using Positron Emission Tomography (PET) for insitu tracking of fluid being spontaneously imbibed into cementitious matrices. We found that the I-124 radiotracer enables tracking of the 3D spatiotemporal distribution of the fluids moving through the solid phase. Our measurements show a similar transport rate for Ordinary Portland (CEM & Iukcy;) and slag (CEM & Iukcy;& Iukcy;& Iukcy;/B) cement pastes. For both cement types, we found that the fluid penetration depth is proportional to t(0.25). Such a relationship indicates anomalous transport and agrees with standard imbibition/sorptivity experiments. This result confirms that the method offers a reliable technique to explore transport phenomena in cementitious materials.

The interaction between shale and water in the environment can lead to the mobilization of micrometer-sized iron sulfide (pyrite) grains. Because these grains often contain high concentrations of toxic elements, including arsenic, this process can pose a significant hazard. While previous studies have suggested that calcite dissolution plays a major role in iron sulfide mobilization, the relationship between rock composition and the release of particulate matter is unclear. Here, we performed laboratory experiments that simulated water–rock interaction in the subsurface on 5 different shale formations: Eagle Ford, Marcellus, Mancos, and Barnett from the USA, and Ein Zeitim from Israel. We used high resolution imaging to evaluate the impact of a reactive fluid on the shale surface, and used image analysis software to determine the rate of iron sulfide grain mobilization. Comparison of the shale surfaces before and after the experiments showed that the dissolution of calcite cement had a major impact on sulfide mobilization: grains that were primarily surrounded by calcite cement were up to 85 times more likely to be mobilized than grains embedded in the shale matrix, which compromises a complex mixture of submicrometer-scale phases, including phyllosilicates, organics, and carbonates. By contrast, iron sulfide embedded in organic matter typically remained cemented in place. This suggests that while calcite dissolution is a crucial phase in facilitating iron sulfide detachment in shales, organic matter might act as an adhesive that suppresses particulate mobilization. However, the adhesive effect of organic matter is only likely to be of secondary importance. Overall, our results indicate that during hydraulic fracturing operations, which involve the injection of fluid into shales in the subsurface, formations with high levels of carbonate cement are more likely to release particulate iron sulfide, thereby reducing the quality of flowback water and potentially increasing treatment costs.

Coalbed methane has emerged as a significant unconventional resource, and understanding the interactions between adsorbed water and methane in coal is crucial for assessing coalbed methane reserves and productivity. To elucidate how pre-adsorbed water influences methane adsorption, we conducted high-pressure methane adsorption experiments on both dry and wet coals with various maturities. By employing a combination of theoretical modeling and experimental analyses, we have identified that adsorbed water could markedly decrease the amount of methane sorption, and changes in water coverage are responsible for the observed negative correlation between water coverage and methane pressure for these samples. Further investigations involving wettability measurements and low-pressure N2 adsorption demonstrate that adsorbed water exhibits weaker interaction with highly mature coal compared to the low maturity coal. Our analyses suggest that adsorbed water can decrease methane adsorption quantity in coal by up to 60%. Moreover, the adsorption and mass transfer rates of methane can be reduced by up to 90% and 80%, respectively, due to the presence of adsorbed water. Additionally, our results indicate that methane can displace adsorbed water in highly mature coal significantly, decreasing water coverage from 63% to 22% with increasing methane pressure.

Carbon capture and storage are likely to be critical components in lowering atmospheric CO2 levels. Mineralization is often proposed as a method to store carbon and typically involves reacting CO2 directly with silicate minerals, such as forsterite, to form carbonate minerals. However, this reaction is slow under standard conditions, so that sequestering significant amounts of carbon can take years or decades. Here, we demonstrate the feasibility of using a reaction between oxalic acid and forsterite to create stable carbon-bearing oxalate minerals. We performed a series of batch experiments at room temperature and pressure to quantify the forsterite dissolution rate and the efficiency of Mg utilization. Our results show that conversion of forsterite to Mg and Fe oxalate is achieved rapidly: after 30 days, 52% of Mg was converted to Mg oxalate so that 1 t of forsterite can be used to store 177 kg of carbon. Our calculations show that reacting ultramafic mine tailings with oxalic acid has the potential to make a significant contribution toward the global target for CO2 removal by carbon capture and storage.

Coal combustion is one of the leading sources of CO2 emissions, and it is predicted to remain so for the foreseeable future. The environmental effects of coal burning can be partially offset by utilizing fly ash, which is a combustion byproduct, to mineralize and store carbon. Our study tests a novel method for carbon storage through the reaction of fly ash with oxalic acid (H2C2O4), creating durable solid oxalate phases. Our results show that whewellite (CaC2O4 center dot H2O) and weddellite (CaC2O4 center dot(2 + x)H2O, x <= 0.5) are formed when fly ash reacts with H2C2O4 at ambient temperature and pressure. We examined 2 types of ash and found that the reaction occurs relatively rapidly, reaching completion within 4 days. Moreover, the reacted material comprised similar to 18% Ca oxalate. During the reaction, portlandite, the primary calcium-bearing mineral in the ash, was dissolved entirely, although mass balance calculations indicate that amorphous phases also serve as an important source of Ca for the oxalate minerals. Reaction modeling suggests that Ca is released by two phases that dissolve at different rates, with the rapidly dissolving phase releasing Ca at a rate 40 times faster than that of the slow phase. Based on our calculations, 1 tonne of reacted coal fly ash could store over 34 kg of carbon, and the method has the potential to store more than 35 Mt of carbon per year on a global scale. Thus, our findings indicate that reacting fly ash with oxalic acid could reduce the environmental impact of coal burning, and adapting the technique for use with other alkaline solid wastes may represent a critical green technology.

The weathering of carbonate rocks plays a significant role in the evolution of Earth’s surface. Such weathering is often accelerated by the presence of stylolites, which are rough, serrated surfaces that form by dissolution under burial or tectonic stresses. Stylolites are thought to represent zones of mechanical weakness in rocks, as well as regions in which chemical weathering is enhanced. However, a quantitative framework capable of predicting how stylolites accelerate weathering in carbonates has yet to be achieved. In this study, we first used scanning electron microscopy and wavelength dispersive spectroscopy to characterize the way in which the two sides of individual stylolites connect at the micrometer scale. In the samples we examined, we found that tiny calcite bridges span the opposing sides of the stylolites, effectively cementing the rock together. This cement filled 1–30% of the stylolite volume. We then used a numerical cellular automaton model to simulate the effect that different degrees of carbonate cementation have on stylolitic carbonate rock weathering. Our results show that weathering rates decrease non-linearly as the degree of stylolite cementation increases. The effect on overall rock weathering rates is significant: stylolite-bearing rocks with 1% cementation weathered as much as 37 times faster than limestone without stylolites, primarily because of accelerated mechanical erosion. Our results indicate that stylolites could be as important as joints and fractures in accelerating carbonate rock weathering and in the development of karst landscapes, potentially making a major contribution to global carbonate weathering.

Accuratecharacterization of the pore structure of coal is criticalfor predicting its behavior in applications, such as CO2 storage and coal bed methane production. Focused ion beam scanningelectron microscopy (FIB-SEM) is an ultra-high resolution imagingtechnique that is widely used to visualize the internal 3D structureof coal samples at the microscale and nanoscale. To reconstruct thepore structure, image segmentation algorithms are used to identifypores in the FIB-SEM images. However, traditional grayscalethreshold segmentation can lead to significant errors due to imageartifacts, including the pore-back effect, curtaining, and charging.In this study, we present a novel workflow for improving the accuracyof pore segmentation in coal that applies interactive machine learningsoftware to FIB-SEM images. We analyzed a FIB-SEM dataset for an anthracite coal sample with a vitrinite reflectance of2.7%. We trained machine learning software (Ilastik) by manually labelingthe pores in 1.25% of the 1200 images. We selected three features(intensity, edge, and texture) on different scales to train the classifier.Once trained, we then applied the classifier to the remaining images.Our results showed that the machine learning method segments the imagesfar more accurately than grayscale segmentation: the comprehensivesegmentation quality (F (1) score) of themachine learning approach (0.89-0.94) is approximately twicethat of the grayscale threshold method (0.32-0.55). Crucially,relative to the machine learning method, grayscale threshold segmentationleads to estimates for porosity that are lower by up to 73%. Our resultshighlight the potential of using machine learning techniques to morereliably characterize the pore structure of coal and other geologicalmaterials.

One of the mechanisms influencing weathering rates is mineral armoring, which is the build-up of insoluble coatings on primary mineral phases. Such armoring is often the result of coupled dissolution-precipitation reactions that occur during weathering. While the role of armoring in reducing dissolution rates is widely recognized, a quantitative mechanistic description of the way it impacts mineral weathering rates has not yet been developed. This paper demonstrates how a diffusion boundary layer model can simulate the effect of a porous secondary mineral layer on the rate of primary mineral dissolution under acidic conditions. In the model, the rates are affected by 3 parameters: layer thickness, the ratio of porosity to tortuosity, and the Biot number, which defines the ratio of dissolution to diffusion. Numerical solutions to the equations show that when the porosity/tortuosity ratio is low (~0.01), dissolution rates can be reduced by several orders of magnitude relative to armor-free minerals. Biot numbers greater than 1 were also found to lead to a significant reduction in the dissolution rate. Moreover, comparison with reported experimental data shows that the model can accurately simulate the change in thickness of the secondary mineral coating with time. The dissolution rates of pristine coating-free minerals in laboratory experiments can be orders of magnitude higher than those observed under field conditions, and the results suggest that mineral armoring can account for much of this discrepancy.

Subsurface barriers are often used to mitigate the spread of contaminants in groundwater. Current methods typically involve the injection of slurries into aquifers, a process that clogs porosity and reduces hydraulic conductivity. An alternative way to achieve this effect in calcareous aquifers could be to inject a fluid that dissolves the existing carbonate minerals and induces the formation of phases that have higher molar volumes. While such mineral replacement reactions are known to affect hydraulic conductivity, their ability to impede contaminant transport has not been tested. In this study, we injected oxalic acid into a pseudo-2D flow through cell containing calcitic sand, which simulated a calcareous aquifer. The oxalic acid causes the dissolution of calcite, as well as the precipitation of calcium oxalate minerals, in a reaction zone that spreads out from the point of injection. We found that this process reduces the water discharge in the cell by up to 80%, and caused the flow velocity around the reaction zone to drop from 20 cm h-1 to <0.1 cm h-1. Mineralogical analyses and electron microscopy indicate that permeability is reduced primarily by partitioning of the intergranular voids by micrometer sized calcium oxalate crystals. Our results suggest that coupled dissolution-precipitation reactions could be a feasible method for deploying subsurface barriers in aquifers.

Rock texture has a critical influence on the way rocks weather. The most important textural factors affecting weathering are grain size and the presence of cracks and stylolites. These discontinuities operate as planes of mechanical weakness at which chemical weathering is enhanced. However, it is unclear how different rock textures impact weathering rates and the size of weathered grains. Here, we use a numerical model to simulate weathering of rocks possessing grain boundaries, cracks, and stylolites. We ran simulations with either synthetic or natural patterns of discontinuities. We found that for all patterns, weathering rates increase with discontinuity density. When the density was <~25%, the weathering rate of synthetic patterns followed the order: grid >honeycomb >Voronoi >brick-wall. For higher values, all weathering rates were similar. We also found that weathering rates decreased as the tortuosity of the pattern increased. Moreover, we show that textural patterns strongly impact the size distributions of detached grains. Rocks with an initial monomodal grain size distribution produce weathered fragments that are normally distributed. In contrast, rocks with an initial log-normal size distribution produce weathered grains that are log-normally distributed. For the natural patterns, weathering produced lower modality distributions.

Gypsum (CaSO4·2H2O) is the most common sulfate mineral on Earth and is also found on Mars. It is an evaporitic mineral that predominantly precipitates from brines. In addition to its precipitation in natural environments, gypsum also forms an undesired scale in many industrial processes that utilize or produce brines. Thus, better insights into gypsum formation can contribute to the understanding of natural processes, as well as improving industrial practices. Subsequently, the thermodynamics, nucleation and crystal growth mechanisms and kinetics, and how these factors shape the morphology of gypsum have been widely studied. Over the last decade, the precipitation of gypsum under saline and hypersaline conditions has been the focus of several studies. However, to date, most of the thermodynamic data are derived from experiments with artificial solutions that have limited background electrolytes and have Ca2+/SO42− ratios that are similar to the 1:1 ratio in the mineral. Moreover, direct observations of the nucleation and growth processes of gypsum are still derived from experimental settings that can be described as having low ionic strength. Thus, the mechanisms of gypsum precipitation under conditions from which the mineral precipitates in many natural environments and industrial processes are still less well known. The present review focuses on the precipitation of gypsum from a range of aspects. Special attention is given to brines. The effects of ionic strength, brine composition, and temperature on the thermodynamic settings are broadly discussed. The mechanisms and rates of gypsum nucleation and growth, and the effect the thermodynamic properties of the brine have on these processes is demonstrated by recent microscopic and macroscopic observations. The morphology and size distribution of gypsum crystals precipitation is examined in the light of the precipitation processes that shape these properties. Finally, the present review highlights discrepancies between microscopic and macroscopic observations, and studies carried out under low and high ionic strengths. The special challenges posed by experiments with brines are also discussed. Thus, while this review covers contemporary literature, it also outlines further research that is required in order to improve our understanding of gypsum precipitation in natural environments and industrial settings.

Textural alignment is the primary cause of anisotropy associated with the permeability and mechanical and acoustic properties of rocks. Although particle alignment in shales is known to be influenced by burial depth, mineralogical composition, and diagenetic history, understanding how these influence image-based measures of compaction at the micrometer to nanometer scale is not well understood. Here, we use a novel method, which combines scanning electron microscopy and image analysis algorithms, to quantify clay alignment in sedimentary rocks. For a given field of view, the alignment intensity in shales from the Podhale Basin in Poland is positively correlated with clay content based on image analysis. However, our analysis also shows that the intensity is strongly dependent on the spatial scale of imaging: as magnification increases and the field of view in the SEM images decreases, the alignment intensity drops. Therefore, in images with small fields of view, clay alignment is determined by the presence of localized equant silicate mineral grains, which effectively hinder the alignment process at the micrometer scale. By contrast, in images with larger fields of view, alignment intensity is determined by the overall compaction state. We also show that when the field of view is constant, alignment intensity decreases with pixel density, and we suggest a range of operating parameters required for obtaining optimum values for alignment intensity from SEM images of shales.