The reaction kinetics of reductive pyrite (FeS2) dissolution by H-2 is important to predict the long-term aqueous and gaseous sulfide release in underground hydrogen storage and other engineered subsurface sites. We investigated the rates of pyrite reduction based on sulfide formation as a function of temperature (60-150 degrees C), H-2 partial pressure (0-150 bar), and pH(in situ) (similar to 4-10) in deoxygenated 0.03 M NaCl solutions. The experiments used natural pyrite powder (& oslash; = 50-100 mu m) and were run in hydrothermal batch reactors made of either titanium or Dursan (R)-coated 316L stainless steel. After experimental durations of similar to 700-900 h, dissolved S-(aq)(-II) concentrations measured by methylene blue spectrophotometry ranged from 10(-5) to 10(-3) M. Long-term pyrite dissolution and concomitant elevated S-(aq)(-II) concentrations were controlled by the formation of secondary pyrrhotite (Fe1-xS) and magnetite (Fe3O4), which prevented the ion activity product of FeS2 from achieving rapid saturation. Conversely, in the absence of pyrrhotite and magnetite precipitation, the solutions rapidly equilibrated with respect to pyrite, resulting in low S-(aq)(-II) concentrations. Rates of reductive pyrite dissolution were determined at 12 h from total S-(aq+gas)(-II) concentration vs. time data and were found to increase with temperature and H-2-partial pressure. The rate dependency on pH at 90 degrees C, 7 bar P-H2 was regressed either with an asymmetric 'V'-shaped (two rate-pH domains) or a 'U'-shaped (three rate-pH domains) relation. For this reason, two separate kinetic rate laws were derived, based on the 'U'-regression (E-a = 35.2 kJ mol(-1)): r=10(e)(-5.42)(-35208/RT)(P-H2)(0.37)(1Q/K-eq) or the 'V'-regression (E-a = 29.4 kJ mol(-1)): r=10(-5.13)e(-29370/RT)(a(H+))(0.13)(P-H2)(0.37)(1Q/K-eq) The performance of both rate laws was validated by reproducing the experimental S-(aq)(-II) concentrations in kinetic models. This comparison determined their applicability to be valid from 60 to 120 degrees C, 0-150 bar P-H2, and pH similar to 6.7-8.6 ('U' rate law) or pH similar to 4.1-8.6 ('V' rate law). The rate laws are compatible with geochemical reactive transport codes and will enhance the understanding of geochemical fluid-rock-gas interactions in H-2-bearing subsurface environments.

Approximately 200,000 m3 of legacy radioactive waste from plutonium production stored at U.S. Department of Energy’s Hanford site will be immobilized in glass for disposal. The glass must limit radionuclide release into the environment for thousands of years, which is challenging to assess in laboratory experiments. Long-term alteration signatures on analog glasses can approximate how radioactive waste glass will perform over extended periods. Different glasses buried for tens to thousands of years at sites subject to variable climates and environments were selected for analysis. Surface altered layers that formed during glass corrosion were characterized. The thickness, chemistry, and morphology of surficial layers are discussed in terms of glass chemistry and burial conditions. Glass from arid environments, e.g., Timna (Israel), exhibited thinner surface layers (~2 µm) compared to glasses altered in humid conditions, e.g., Dobkowice (Poland: up to 59 µm), suggesting a role of burial environment and climate in long-term durability.

Multiple-step nucleation pathways have been observed during mineral formation in both inorganic and biomineral systems. These pathways can involve precursor aqueous species, amorphous intermediates, or metastable phases. Despite the widespread occurrence of these processes, elucidating the precise nucleation steps and the transformation mechanisms between each step remains a challenging task. Using a suite of potentiometric, microscopic, and spectroscopic tools, we studied the nucleation pathway of SrSO4 as a function of the physico-chemical solution parameters. Our observations reveal that below a threshold supersaturation, nucleation is driven by bound species, akin to the prenucleation cluster model, which directly leads to the formation of the stable phase celestine, SrSO4. At higher supersaturations, this situation is altered, with nucleation dominated by the consumption of free ions. Importantly, this change in nucleation mechanism is coupled to the formation of a hemihydrate metastable phase, SrSO4 · 1/2H2O, which eventually transforms into celestine, adhering to Ostwald's rule of stages. This transformation is a solution-mediated process, also occurring in the presence of a fluid film and is controlled by the physico-chemical parameters of the surrounding environment. It proceeds through the dissolution of the metastable phase and the de novo crystallization of the final phase. Overall, our results reveal that ion association taking place during the prenucleation stage dictates whether the nucleation pathway goes through an intermediate phase or not. This also underlines that although Ostwald's rule of stages is a common process, it is not a prerequisite for mineral formation-even in systems where it can occur.

The most active icy worlds such as Europa or Enceladus are predicted to host extensive aqueous alteration driven by water–rock interactions at elevated temperatures1–3. On the other hand, it is assumed that such alteration is kinetically inhibited at the subzero temperatures of other icy worlds, such as the mid-sized moons of Saturn and Uranus or trans-Neptunian objects1,4. Here we perform aqueous alteration experiments on a chondrite-analogue material (olivine) and find that chemical alteration processes are still efficient at temperatures as low as −20 °C, as the presence of an unfrozen water film still allows olivine to dissolve in partially frozen alkaline solutions. We infer that aqueous alteration may be enhanced by salts and ammonia present in icy worlds, and therefore remains a geologically rapid process even at subzero temperatures. Our results imply that the primary chondritic minerals in most icy bodies exceeding 400–500 km in diameter will be completely altered to hydrous secondary minerals early in their evolutionary histories. Alkaline conditions in icy worlds favour aqueous alteration processes even at subzero temperatures. As a consequence of geologically rapid alterations, the rocky cores of all icy bodies larger than 500 km in diameter should be largely composed by altered hydrated minerals formed within a few hundred million years after formation.

The application of rock powder on agricultural land to ameliorate soils and remove carbon dioxide (CO2) from the air by chemical weathering is still subject to many uncertainties. To elucidate the effects of grain size distribution and soil partial pressure of carbon dioxide (pCO2) levels on CO2 uptake rates, two simple column experiments were designed and filled nearly daily with an amount of water that simulates humid tropical conditions, which prevail in areas known for being hotspots of weathering. Multiple materials (dunite, basanite, agricultural oxisol, a combination of the latter two, and loess) were compared under ambient and 100% CO2 atmosphere. In a second series, single material columns (dunite) were filled with three different grain size distributions. Total alkalinity, pH, major ions, and dissolved silica were determined in the outflow water of the columns for about 300 days. Under ambient atmospheric conditions, the CO2 consumption was the lowest in the oxisol column, with 100 t CO2 km−2 year−1, while dunite and basanite showed similar consumption rates (around 220 t CO2 km−2 year−1). The values are comparable to high literature values for ultramafic lithologies. Interestingly, the mixture of basanite and oxisol has a much higher consumption rate (around 430 t CO2 km−2 year−1) than the basanite alone. The weathering fluxes under saturated CO2 conditions are about four times higher in all columns, except the dunite column, where fluxes are increased by a factor of more than eleven. Grain size distribution differences also play a role, with the highest grain surface area normalized weathering rates observed in the columns with coarser grains, which at first seems counterintuitive. Our findings point to some important issues to be considered in future experiments and a potential rollout of EW as a carbon dioxide removal method. Only in theory do small grain sizes of the spread-material yield higher CO2 drawdown potentials than coarser material. The hydrologic conditions, which determine the residence times in the pore space, i.e., the time available for weathering reactions, can be more important than small grain size. Saturated-CO2 column results provide an upper limit for weathering rates under elevated CO2.

Chemical alteration of basalt glasses in seawater ranks as one of the most important global Earth processes, playing a significant role in the terrestrial carbon cycle, the chemical composition of the oceans, and the cycling of major, minor, and trace elements between the crust and the mantle. This chapter examines abiotic and biotic processes that drive seafloor alteration of basaltic glasses. Biotic alteration depends on specific metabolic processes that supply energy to microorganisms, and at the same time, serve to breakdown the glass structure. Representative studies illustrate current models of abiotic and biotic basalt glass dissolution. In most cases, the predominant alteration product is palagonite, a secondary surficial phase that grows at the expense of the glass. Its physical and chemical properties indicate how it forms, which, in turn, is linked to the glass corrosion mechanism. Each study is discussed in light of the two currently accepted corrosion mechanisms: the cation-depleted leached layer model and the coupled interfacial dissolution–reprecipitation (CIDR) model.

Potassium feldspars (KAlSi3O8) are ubiquitous minerals in the Earth's upper crust. This family of minerals has been the subject of numerous experimental and theoretical investigations concerning their dissolution kinetics and the mechanisms controlling chemical alteration at acid and neutral pH, and at temperatures ranging from ambient to hydrothermal conditions. On the other hand, considerably less research on the dissolution behavior of K-feldspars has been carried out at alkaline conditions, in particular at pH > 9 and elevated temperatures. Filling in this gap in knowledge is the major motivation for this study. More specifically, we wanted to document and understand how the K-feldspar interface structurally and chemically evolves during alteration in order to determine the mechanism of dissolution. In this study we examined interfaces of orthoclase samples that were altered in separate experiments in a Ca(OH)(2)-H2O solution (pH(25 degrees C) 12.4) at 190 degrees C for 24 h. We used a combination of focused ion beam (FIB) milling and advanced analytical transmission electron microscopy (TEM) techniques to investigate the structure and chemistry of the near surface region of post-reaction grains, with particular attention being given to the fluid-solid interface. Even though each grain diminishes in volume due to dissolution, high-resolution TEM imaging indicates that the feldspar structure itself remains completely intact and crystalline, as evidenced by lattice fringes that abruptly terminate at the grain edge. Nanometer-scale chemical composition measurements and mapping by TEM-EDXS (energy dispersive X-ray spectroscopy) and EFTEM (energy filtered TEM) show that the chemistry of the parent feldspar also remains unchanged at the interface. In particular, there is no evidence for the incursion of Ca from the fluid solvent into the structure, either by interdiffusion or by a replacement process. Taken together, the TEM observations point to a sharp chemical reaction front characterized by the congruent (i.e. stoichiometric) release of all elements from the feldspar structure. Nanometer-scale measurements by high resolution analytical TEM also reveal that a surface alteration layer (SAL) of amorphous material forms in situ at the expense of the feldspar structure. The interface demarcates a spatially coincident and nm-sharp chemical and structural discontinuity between the parent feldspar and the amorphous phase. The amorphous SAL has a variable thickness, from under 10 nm up to similar to 200 nm. This is likely one of the first observed occurrences of a significant surface amorphous layer on feldspar due to alteration in an alkaline solvent. The lack of a gap between the two phases points to an interfacial dissolution-reprecipitation process that continuously operates during hydrothermal alteration, and mostly likely right from the onset of contact with the fluid. After the initial formation of the amorphous layer, a 1-2 mu m-thick porous amalgam of secondary crystalline phases comprised of calcite, tobermorite, and hydrogrossular, as well as other minor phases, precipitated over the SAL. These authigenic crystalline minerals formed during the experiment (hydrothermal alteration, followed by fluid loss due to evaporation) by a classical thermodynamically-controlled precipitation process as the reactor bulk fluid became increasingly concentrated. We propose that a coupled interfacial dissolution-reprecipitation (CIDR) mechanism best explains the chemical and structural properties of the interface and the formation of an amorphous surface layer. In fact, many recent studies postulate that a CIDR process controls feldspar dissolution and the formation of SALs at acid and circumneutral pH over a wide range of temperatures. Combining these previous results with our new observations supports the idea that a unique and unifying mechanism likely controls chemical alteration of feldspars in all aqueous fluids.

Global food security concerns have spurred increasing demand for locally sourced and produced K-fertilizers. Various processes have been explored for more than a century; one promising solution is based on the alkaline aqueous alteration of feldspar-rich rocks at elevated temperatures. However, knowledge of the overall physicochemical reactions comprising dissolution of feldspar and precipitation of secondary phases is still rudimentary, in particular how the feldspar structure evolves at the nm-scale during hydrolysis at alkaline conditions. Here we report on the results of a study aimed at converting potassium feldspars to K-rich fertilizer based on the alteration of sanidine and microcline samples at 190 degrees C in pH 12 Ca(OH)(2) solutions for 24 h. Based on X-ray diffraction and Rietveld refinement, the secondary authigenic minerals that precipitated are primarily composed of Cacarbonate (calcite, vaterite), and Ca-(Al)-silicates, such as tobermorite and hydrogrossular. Short-term bench top leaching experiments in water prove that the hydrothermal product releases up to two orders of magnitude more K than the unaltered K-feldspar starting material, pointing to its application as a ready-to-use fertilizer for K-deficient soils. Detailed chemical mapping and energy dispersive X-ray spectroscopy (FESEM- and TEM-EDXS) analyses of the precipitates at the um to nm-scale show that the distribution of K associated with the secondary phases is very heterogeneous, both spatially and in terms of concentrations. Using various analytical transmission electron microscopy (TEM) techniques, e.g., HRTEM, TEM-EDXS, EFTEM, to investigate the structure and chemistry of the feldspar interface, we find no evidence for a change in chemistry or structure at the nm-scale, even though dissolution continuously decreases the volume of each grain. Our observations also show the existence of an amorphous surface altered layer (SAL) of variable thickness (10-similar to 100 nm) forming at the feldspar interface. Nanometer-scale chemical measurements show that this amorphous SAL is rich in K, and therefore may also be an important reservoir of easily leachable K. We hypothesize that it forms continuously and in situ at the expense of the feldspar by a coupled interfacial dissolution-reprecipitation process (CIDR).

Introduction:The identification of a liquid ocean beneath the ice crust on Saturn's small moon Enceladus has kindled interest in low-temperature water-rock interactions as a driver in developing habitable worlds [1], [2].The preservation of NH 3 and salts that depress the freezing point of water in small, cold satellites suggest that the heat generated from exothermic mineral dissolution at below-0 °C could even initiate the ice melting processes that create liquid oceans [3].However, numerical models of mineral dissolution in these environments often rely on thermodynamic and kinetic data extrapolated from 25 °C or above.Methods: Batch experiments combined San Carlos olivine (Mg 1.83 Fe 0.17 SiO 2 ) with NH 3 -H 2 O solutions, reacted at -20, 4, and 22 °C for up to 442 days.Solution chemistry changes were monitored at intervals to evaluate mineral alteration over time.The ice-fluid-mineral interface was characterized using Raman spectroscopy and geochemical modelling.In addition, olivine surfaces were analyzed at the nanoscale with TEM microscopy.Results: Changes to fluid chemistry over time show that olivine dissolution occurs even in partially frozen solutions.Initial evolution of concentrations of Si and Mg show a surprisingly weak dependence on temperature, pH, and NH 3 , while long-term rates show an apparent weak inverse relationship with temperature.Our findings imply that olivine dissolution is not significantly retarded at -20 °C compared to 22 °C.In addition, high-resolution TEM analysis of the olivine surface after 442 days of reaction shows a thin (< 1 nm) altered layer at the olivine surface, unambiguously demonstrating that secondary reaction products can form in partially frozen solutions even at experimental timescales.These findings are applicable to evaluating mineral weathering processes in frozen or partially-frozen icy worlds such as Enceladus, (1) Ceres, or Uranus' moon Ariel.

Experimental studies over the past decades have highlighted the importance of complex, particularly multiple-step, nucleation pathways during mineral formation in both inorganic and biomineralization environments 1 . Many of these pathways can be catalogued as a distinct two-step process where one phase forms and then transforms into a second phase, something that has been proposed as a means for the concentration, transportation, and/or temporary storage of ions in biomineralization 2,3 but is also used to describe the behavior of (in)organic solutions with high supersaturation 4 . Using strontium sulfate (celestine) and its metastable hydrated precourser phase, we demonstrate how a suite of microscopic and spectroscopic tools can be successfully deployed to study the phases that form during multi-step nucleation pathways and the physico-chemical dependance of the reaction kinetics. In situ experiments track the concentrations of ions (Raman) and solid phases (XRD) during the transformation reaction. SEM, TEM and optical microscopy have shown the macrostructural differences between the phases with optical microscopy allowing for in situ tracking of the phases with spatial and temporal resolution.

The discovery of a liquid ocean on Saturn's small moon Enceladus and evidence of modern hydrothermal activity provide an unexpected new environment in which to expand the search for life. However, as with the age of the moons themselves, the age of the liquid ocean and any hydrothermal activity therein remains an area of debate. Based on physical and chemical observations from the Cassini mission we can apply known mineral dissolution rates, estimated water-rock ratios from Enceladus' density, and variable water flow rates within the rocky core to constrain durations of active serpentinization. On this basis we developed a 1-D reactive transport model to compare the effect of initial olivine percentage, grain size, temperature, and flow rate on timespans of primary olivine alteration in a rocky core the size and density of Enceladus'. In most cases, olivine alteration and precipitation of hydrous secondary minerals results in a water-limited alteration regime. An alteration front that propagates in the direction of water flow then controls the overall rate of olivine alteration. Of the parameters explored, high initial olivine percentages and slow fluid flow rates were the strongest predictors of long serpentinization times, while temperature and grain size had a smaller effect. The annual global H-2 production rate in all model cases (> 1 x 10(12) mol yr(-1)) is several orders of magnitude greater than the minimum H-2 release rate calculated from the observed H-2 in Enceladus' plume (1 x 10(9) mol yr(-1)), suggesting that any ongoing active serpentinization processes in the core are likely nearing completion. The longest timescales indicate the potential for olivine alteration and H-2 production for up to similar to 75 Myr, consistent with weathering rates of terrestrial peridotite massifs. If the H-2 produced from Enceladus is sourced from primary mineral alteration, these results suggest that hydrothermal activity in the core of Enceladus may have developed only very recently - even as recent as within the past 100 Myr.