Recent studies of carbonate surface reactivity have underscored the fundamental variability of dissolution rates and the heterogeneous distribution of the reaction over the mineral surface due to the inhomogeneous distribution of surface energy. Dolomite dissolution rates relative to different cleavage planes (r-planes) and surfaces cut approximately perpendicular to the c-axis (c-planes) were studied at 50 °C as a function of pH (3.4 ≤ pH ≤ 9.0) and solution composition by vertical scanning interferometry (VSI) and atomic force microscopy (AFM), with the aim of providing an estimate of the intrinsic rate variation of dolomite single crystals and describing the surface reaction distribution and the rate controlling mechanisms. Surface normal retreat rates measured under acidic conditions increased linearly with time and were not visibly affected by the parallel increase of surface roughness. Mean total dissolution rates of r-planes decreased by over 200 times from pH 3.4 to pH 9.0 and CO32–-rich solutions, whereas corresponding rate variations spanned over 3 orders of magnitude when also c-plane rate distributions were included in the analysis. At acid to near neutral pH, c-planes dissolved ∼ three times faster than the adjoining r-planes but slower at basic pH and high total carbon concentration, displaying a distinctive morphologic evolution in these two regimes. The comparison of polished and unpolished crystals showed that polished cleavage planes dissolved about three times faster than the unpolished counterpart at near neutral to basic conditions, whereas no significant difference in reactivity was observed at pH < 5. Although experimental data and observations indicate a tendency of dolomite faces to reach a low-energy topography over the course of the reaction, the evolution of the entire crystal morphology depends also on the reactivity of edge and corner regions, whose contribution to measured rates is not generally taken into account by laboratory experiments. The study of time-dependent mineral morphology and reactivity requires an integrated approach of kinetic modeling and experimentation, where measured rate variance and observed reaction mechanisms represent fundamental parameters for the improvement of geochemical models in predicting long-term reaction rates in a wide range of environmental conditions.

We have investigated the hydrothermal alteration of polished wafers of tuff reacted with dilute groundwater at 90 °C, 150 °C and 250 °C for time periods ranging from 2 months to nearly 1 year. The polished rock wafer provided a convenient surface upon which to grow secondary minerals. Reaction product minerals were identified and analyzed at the end of each experiment and, along with the evolving fluid chemistry, were compared to computational results from corresponding reaction progress models. At 250 °C after a few months the run products in the experiment were dominantly the mordenite group zeolite minerals: dachiardite (a Ca-rich variety) and mordenite, itself. At 150 °C after a few months of reaction only minor amounts of clay were produced, but after 1 year of reaction at this temperature both mordenite group zeolites were again present. At this lower temperature the total amount of run products was much smaller. At 90 °C no run products could be seen at all, even after 1 year of reaction. The reaction progress modeling results for reaction products were in good relative agreement with the experimental results. The higher the temperature, and the greater the extent of reaction, the better the fluid phase modeling results agreed with the actual experimental results. At 250 °C the agreement was good for nearly all elements. At 150 °C agreement for pH, SiO2, Na and K were good, but less good for Al, Mg and Ca, especially after short reaction times. At 90 °C agreement for pH, SiO2 and Na was reasonable, but not as good for the other elements, and all modeling results for short reaction times did not match experimental results as well as the longer time results. This study demonstrates that reaction progress modeling provides a powerful tool for predicting hydrothermal rock-water interactions, with results expected to improve, as more and better quality thermodynamic and kinetic data become available and as process-oriented simulators incorporate better and more comprehensive sub-models for mineral dissolution and growth.

Relevant modeling of mass and energy fluxes involved in pedogenesis, sequestration of atmospheric CO2 or geochemical cycling of elements partly relies on kinetic rate laws of mineral dissolution obtained in the laboratory. Deriving an accurate and unified description of mineral dissolution has therefore become a prerequisite of crucial importance. However, the impact of amorphous silica-rich surface layers on the dissolution kinetics of silicate minerals remains poorly understood, and ignored in most reactive transport codes. In the present study, the dissolution of oriented cleavage surfaces and powders of labradorite feldspar was investigated as a function of pH and time at 80 °C in batch reactors. Electron microscopy observations confirmed the formation of silica-rich surface layers on all samples. At pH = 1.5, the dissolution rate of labradorite remained constant over time. In contrast, at pH = 3, both the dissolution rates at the external layer/solution interface and the internal layer/mineral interface dramatically decreased over time. The dissolution rate at the external interface was hardly measurable after 4 weeks of reaction, and decreased by an order of magnitude at the internal interface. In another set of experiments conducted in aqueous silica-rich solutions, the stabilization of silica-rich surface layers controlled the dissolution rate of labradorite at pH = 3. The reduction of labradorite dissolution rate may result from a gradual modification of the textural properties of the amorphous surface layer at the fluid/mineral interface. The passivation of the main cleavage of labradorite feldspar was consistent with that observed on powders. Overall, our results demonstrate that the nature of the fluid/mineral interface to be considered in the rate limiting step of the process, as well as the properties of the interfacial layer (i.e. its chemical composition, structure and texture) to be taken into account for an accurate determination of the dissolution kinetics may depend on several parameters, such as pH or time. The dramatic impact of the stabilization of surface layers with increasing pH implies that the formation and the role of surface layers on dissolving feldspar minerals should be accounted for in the future.

The (001) orthoclase surface was dissolved at 180 °C and at far from equilibrium conditions with an alkaline solution (pH180 °C = 9) in a titanium open flow reactor. Vertical scanning interferometer (VSI) and atomic force microscope (AFM) surface monitoring were periodically used during the reaction process in order to quantify the surface topography evolution. The dissolution of the (001) orthoclase face occurs with the formation of diamond shape etch pits. Diamond pit diagonals are parallel to the [100] and [010] axes, and the pit walls are parallel to (6 5 6), 65¯6, (6¯511) and 6¯5¯11 planes. The etch pit size and global surface retreat of the (001) surface were found to increase linearly with time. Based on statistical treatments of etch pit development monitoring by AFM, we designed a numerical model aimed at reproducing and quantifying the total surface evolution. Numerical results show that the stabilization of etch pits doubles the calculated dissolution rate, partly due to the intrinsically higher reactivity of pit walls, consistent with a dissolution process in line with the periodic bond chain (PBC) theory. In addition, normalizing the dissolution rate by the initial surface area of the (001) orthoclase surface induces a 20% overestimation of the calculated dissolution rate, while the total surface area of the dissolving face reaches a steady state after a few days of reaction. Additional simulations conducted to assess the impact of defect parameters revealed a weak dependence of the dissolution rate on dislocation density, consistent with previous experimental observations. Overall, the combined effect of the various defect parameters does not affect the dissolution rate by more than an order of magnitude, and probably contributes to a moderate extent to the dispersion of mineral dissolution rate data reported in the literature.

At equilibrium, the D/H isotope fractionation factor between H2 and H2O (αH2O-H2(eq)) is a sensitive indicator of temperature, and has been used as a geothermometer for natural springs and gas discharges. However, δDH2 measured in spring waters may underestimate subsurface temperatures of origin due to partial isotopic re-equilibration during ascent and cooling. We present new experimental data on the kinetics of D–H exchange for H2 dissolved in liquid water at temperatures below 100 °C. Comparing these results with published exchange rates obtained from gas phase experiments (100–400 °C), we derive a consistent activation energy of 52 kJ/mol, and the following rate expressions; lnk=9.186-6298/Tandk1=9764.61[H2O]e-6298/T where T is absolute temperature (K), k is the universal rate constant ([L/mol]/hr), and k1 is a pseudo-first-order constant (hr−1) applicable to water-dominated terrestrial systems by constraining [H2O] as the density of H2O (in mol/L) at the P-T of interest. The density-dependent rate constant accounts for the kinetic disparity of D–H exchange with H2 when dissolved in liquid H2O relative to a gas/steam phase, exemplifed by 1/k1 at 100 °C of ∼2 days in liquid, versus ∼7 yrs in saturated steam. This difference may explain the high variability of δDH2 observed in fumarolic gases. Fluids convecting in the crust frequently reach T > 225 °C, where isotopic equilibrium is rapidly attained (<1 hr). We compare fractionation factors measured in natural fluids (αOBS) with values expected for equilibrium at the T of acquisition. Where these values differ, we use kinetic models to estimate cooling rates during upward advection that account for the observed disequilibrium. Models fit to fluids from Yellowstone Park and the Lost City (deep-sea) vent field, both recovered at ∼90 °C, require respective transit times of ∼7 hrs and ∼11 days between higher temperature reaction zones and the surface. Using estimates of subsurface depths of origin, however, suggests similar mean fluid flow rates (10 s of meters/hr). Additional complications must be considered when interpreting the δDH2 of lower-temperature effluent. When applied to data from deep-sea hydrothermal systems, our kinetic models indicate microbial catalysis accelerates D–H exchange once fluids cool below ∼60 °C. The H2 measured in both continental alkaline springs and fracture fluids from Precambrian shield rock is likely produced at T < 100 °C, through processes such as serpentinization. In these settings, δDH2 values appear closer to equilibrium with H2O than those from geothermal systems. Considering kinetic isotope effects may yield H2 that is out of equilibrium when generated at lower temperatures, we calculate maximum (isothermal) times to apparent isotopic equilibrium of 1.3 yrs at 50 °C, 9 yrs at 25 °C, and 35 yrs at 10 °C. A similar calculation applied to Antarctic brines (−13 °C), where measured δDH2 is far from equilibrium, yields ∼350 yrs. This time is shorter than the fluids have been isolated (2.8 ka), suggesting kinetic isotope effects associated with H2 destruction or loss via diffusion may also be possible.

The hydrogen isotopic composition of methane (CH4) is used as a fingerprint of gas origins. Exchange of hydrogen isotopes between CH4 and liquid water has been proposed to occur in both low- and high-temperature settings. However, despite environmental evidence for hydrogen isotope exchange between CH4 and liquid water, there are few experimental constraints on the kinetics of this process. We present results from hydrothermal experiments conducted to constrain the kinetics of hydrogen isotope exchange between CH4 and supercritical water. Seven isothermal experiments were performed over a temperature range of 376–420 °C in which deuterium-enriched water and CH4 were reacted in flexible gold reaction cell systems. Rates of exchange were determined by measuring the change in the δD of CH4 over the time course of an experiment. Regression of derived second order rate constants (kr) vs. 1000/T (i.e., an Arrhenius plot) yields the following equation: ln(kr) = −17.32 (±4.08, 1 s.e.) × 1000/T + 3.19 (±6.01, 1 s.e.) (units of kr of sec−1 [mol/L]−1), equivalent to an activation energy of 144.0 ± 33.9 kJ/mol (1 s.e.). These results indicate that without catalysts, CH4 will not exchange hydrogen isotopes with liquid water on a timescale shorter than the age of the Earth (i.e., billions of years) at temperatures below 100–125 °C. Exchange at or below these temperatures is thought to occur due to the activity of life, and thus hydrogen isotopic equilibrium between methane and water may be a biosignature at low temperatures on Earth (in the present or the past) and on other planetary bodies. At temperatures ranging from 125 to 200 °C, hydrogen isotope exchange between CH4 and liquid water can occur on timescales of millions to hundreds of thousands of years, indicating that in thermogenic natural gas systems CH4 may isotopically equilibrate with water and achieve equilibrium isotopic compositions. Finally, the kinetics indicate that in deep-sea hydrothermal systems, the hydrogen (and thus clumped) isotopic composition of CH4 is likely set by formation and/or storage conditions isolated from the active flow regime. The determined kinetics indicate that once methane is entrained in circulating fluids, the expected time-temperature pathways are insufficient for measurable hydrogen isotope exchange between CH4 and water to occur.

Corrosion is a major obstacle to a safe implementation of geotechnical applications. Using a novel approach that includes vertical scanning interferometry (VSI) and electrochemical impedance spectroscopy (EIS) we discuss time-dependent carbon steel corrosion and film formation at geothermally relevant temperatures (80-160 °C) in CO₂-saturated mildly acidic NaCl brine. Iron dissolution kinetics follows a logarithmic rate at 80 and 160 °C and a linear rate at 120 °C. At 80 °C, high initial corrosion rates (first 24 h) generate H₂ at a minimum rate of 12 μmol h^-1 cm^-2 and lead to the formation of a continuous ~100 μm thick porous corrosion film. It exhibits a duplex structure with a crystalline outer FeCO₃ layer and an inner layer composed of a skeletal network of Fe₃C impregnated with FeCO₃. Being an electrical conductor we hypothesize the Fe₃C to strongly enhance corrosion rates by providing additional cathodic sites. Pseudo-passivity due to an anodic film-forming reaction (presumably Fe-oxide) was observed at 120 and 160 °C, soon followed by the initiation of pitting at 120 °C. Steady-state corrosion rates at 160 °C are at least one order of magnitude lower than for 120 °C. Our experimental approach demonstrated potential for general applicability in studying corrosion-related phenomena.

Geochemical reactions can induce significant changes of rock reservoir porosity and permeability via mineral dissolution and precipitation processes, affecting the long-term fluid behaviour within various geological systems. The understanding and quantification of these reactions rely on field and experimental studies and on the predictions of reactive transport models. The present study was aimed at assessing the extent to which current geochemical models integrating available mineral dissolution/precipitation rate equations can reproduce the experimental data obtained from 4 to 17-day long hydrothermal alteration experiments of a muscovite-biotite granite and, thus, help provide an accurate description of the evolution of geothermal systems within granitic reservoirs. The experiments were conducted at a constant temperature of 180 °C and over an aqueous fluid pH range of 2 to 8.5, using both mixed-flow and static batch reactors. Modelled major element (K, Al, Si, Ca, and Mg) concentrations were generally in satisfactory agreement with the corresponding measured elemental fluxes – the differences between modelled and experimental values were generally within a factor of 5 – and the predicted identity and mass of formed secondary phases were consistent with the microscopic observations of the reacted solids. However, larger differences between measured and modelled element concentrations were observed when significant amounts of secondary phases formed, notably at pH 2 to 3, and for longer-term batch experiments. Much of this concentration difference stems from the underestimation of the amounts of Al-phases formed at acid to near-neutral pH. Although an idealized rock composition was considered, the observed mismatch between model calculations and experimental data can be attributed to inadequate mineral precipitation reaction rates and a poor description of reactive surface areas in existing geochemical modelling codes. More accurate quantification of precipitation kinetics, including nucleation and growth, and improved descriptions of the temporal change of mineral surface area would enhance the predictive capabilities of reactive transport models and benefit, particularly, the efforts aimed at increasing the sustainability of EGS reservoirs.

Recent studies investigating carbonation of iron-bearing silicates have shown that the rates of these reactions, although formally not depending on oxygen fugacity, are strongly different at different redox states of the system (Saldi et al. 2013; Sissmann et al. 2013). Here we provide a micro- and nanostructural characterization of the olivine/water interface during the carbonation of forsteritic olivine at 150 °C and pCO2 = 100 bar. When the reaction starts under oxic conditions, the observed temporal sequence of interfacial layers consists of: a hematite/SiO2(am) assemblage, Fe-rich phyllosilicates with mixed Fe valence and a non-passivating Fe-free amorphous SiO2 layer, which allows the formation of ferroan magnesite. In contrast, starting at micro-oxic conditions, carbonation rates are much faster, with no real evidence of interfacial layers. Separate deposits of goethite/lepidocrocite in the early stages of the reaction and then formation of magnetite are observed at these conditions, while precipitation of siderite/magnesite proceeds unhindered. The evolution of the redox conditions during the reaction progress controls the sequence of the observed reaction products and the passivating properties of Fe-Si-rich interfacial layers. These findings have important implications for modeling the carbonation of ultramafic rocks under different oxygen fugacity conditions as well as for understanding the technological implications of adding accessory gases to CO2 in carbon capture and storage mineralization processes involving ultrabasic rocks.

Which conceptual framework should be preferred to develop mineral dissolution rate laws, and how the aqueous mineral reactivity should be measured? For over 30 years, the classical strategy to model solid dissolution over large space and time scales has relied on so-called kinetic rate laws derived from powder dissolution experiments. In the present study, we provide detailed investigations of the dissolution kinetics of K-feldspar as a function of surface orientation and chemical affinity which question the commonplace belief that elementary mechanisms and resulting rate laws can be retrieved from conventional powder dissolution experiments. Nanometer-scale surface measurements evidenced that K-feldspar dissolution is an anisotropic process, where the face-specific dissolution rate satisfactorily agrees with the periodic bond chain (PBC) theory. The chemical affinity of the reaction was shown to impact differently the various faces of a single crystal, controlling the spontaneous nucleation of etch pits which, in turn, drive the dissolution process. These results were used to develop a simple numerical model which revealed that single crystal dissolution rates vary with reaction progress. Overall, these results cast doubt on the conventional protocol which is used to measure mineral dissolution rates and develop kinetic rate laws, because mineral reactivity is intimately related to the morphology of dissolving crystals, which remains totally uncontrolled in powder dissolution experiments. Beyond offering an interpretive framework to understand the large discrepancies consistently reported between sources and across space scales, the recognition of the anisotropy of crystal reactivity challenges the classical approach for modeling dissolution and weathering, and may be drawn upon to develop alternative treatments of aqueous mineral reactivity.