Anorthite Dissolution Research Articles

Kinetics of anorthite dissolution in basaltic melt

We report convection-free anorthite dissolution experiments in a basaltic melt at 1280–1500°C and 0.5GPa on two different crystallographic surfaces, (121¯) and (3¯02) to investigate dissolution kinetics. The anisotropy of the anorthite dissolution rate along these two surfaces is negligible. Time series experiments at ∼1280°C show that anorthite dissolution is mainly controlled by diffusion in the melt within experimental uncertainty. Analytical solutions were used to model the dissolution and diffusion processes, and to obtain the diffusivities and the saturation concentrations of the equilibrium-determining component (Al2O3) for anorthite dissolution into the basaltic melt. For the first time, we are able to show the physical and chemical characteristics of quench growth effect on the near-interface melt using high spatial resolution (0.3μm) EDS analyses. For anorthite (An#⩾90) saturation in a melt with 39–53wt% SiO2 and ⩽0.4wt% H2O, the concentration of Al2O3 in wt% depends on temperature as follows:lnCs,Al2O3=8.032(±0.192)-7882(±287)T,where T is temperature in K, and errors are given at 1σ level. Al2O3 diffusivity in basaltic melt during plagioclase dissolution can be expressed as:lnDAl2O3EBD,plag diss=-13.69(±1.57)-19,313(±2485)T,where D is in m2/s, and the activation energy is 161±21kJ/mol. These results are applied to model the convective dissolution of anorthite in basaltic melts. The model indicates that though anorthite crystals can survive for a longer time compared to olivine crystals of the same size in the same melt, the former would rise for a much smaller distance compared to the olivine sinking distance, due to the smaller density contrast between anorthite and melt.

Uncertainties of geochemical codes and thermodynamic databases for predicting the impact of carbon dioxide on geologic formations

Numerical codes are applied to calculate chemical reactions following geologic carbon sequestration in deep formations and CO2 leakage in shallow formations. However, using different thermodynamic databases generates variations in the simulation results, which are referred to as the model uncertainty. The PHREEQC and The Geochemist's Workbench codes were used to simulate anorthite dissolution for storage, retention, transfer, and near-surface formation waters in the respective geological units. For each of the formation waters, a simple one-dimensional scenario was simulated using eight different thermodynamic databases. Groundwaters in shallow aquifers commonly exhibit low ionic strengths (<0.5 mol/kgw) and low temperatures, whereas storage formation waters are characterized by high ionic strength (>1.0 mol/kgw) and high temperatures. In storage formations, mineral trapping is the most efficient process for long-term CO2 storage. However, with respect to the geological formations and the time needed for anorthite dissolution, the model uncertainties associated with using different combinations of numerical codes and thermodynamic databases were largest (∼90%) for the storage formation waters at 58 °C and I = 6.5 mol/l. Conversely, in near-surface formation waters, the model uncertainty was less than 1%. Due to CO2 dissolution, the calculated pH of the formation waters decreased to a range between pH 4.0 and 5.5. In this pH range, the dissolution mechanism of anorthite switches from the slow neutral mechanism to the faster acid mechanism, causing dissolution time length variations. The calculated pH variation further increased with rising ionic strength. A detailed examination of the reasons revealed the activity coefficient calculation method of the main aquatic species to have the largest impact on the simulated model results. The calculation method of the CO2 activity coefficient had the second largest impact. Via calibration with the experimental data, a specific thermodynamic database can be chosen to represent these experimental results. However, the calibration of thermodynamic databases is not possible for all potential reactions in more complex geological systems at large ranges of temperature, ionic strength and pressure conditions. The uncertainties associated with using thermodynamic databases quantified in this study for CO2 storage systems will therefore persist independently from previously conducted calibrations of thermodynamic databases with experimental or field data. In view of these model uncertainties, the modeller is encouraged to include a routine in the simulations for quantification of the model uncertainty depending on the specific scenario or to assess the simulation results as a range of values that represent a soft outcome.

Synthesis of zeolitic material from green tuff stone cake and its adsorption properties of silver (I) from aqueous solution

A zeolitic adsorbent was synthesized from waste green tuff stone cake by hydrothermal treatment, and its adsorption of silver ions from aqueous solution was examined for application in silver ion removal from polluted industrial waters and as a bactericide. The waste cake was composed mainly of SiO2 and Al2O3 in the form of silicate and aluminosilicate minerals, such as quartz, anorthite, muscovite and clinochlore. The cake was treated with 0–10 M NaOH solution at 80, 120 and 160 °C for 24 h, and analcime, hydroxysodalite and hydroxycancrinite were synthesized. A product with high silver adsorption ability, including analcime, was obtained in 4 M NaOH solution at 160 °C. The concentrations of Si and Al in the solution during the reaction in 4 M NaOH solution at 160 °C explain the synthesis of analcime zeolite phase due to the dissolution of quartz and anorthite in the cake, and Ag adsorption of the product depends on analcime zeolite crystals in the product. The optimal pH for silver adsorption for the product was found to be around 5. The equilibrium silver adsorption capacity was measured and extrapolated using the Langmuir and Freundlich isotherm models, and experimental data were found to fit the Langmuir model. The calculated maximum adsorption capacity of 1.10 mmol/g shows that this product is suitable for silver uptake from aqueous media. Adsorption kinetics was tested for pseudo-first- and -second-order reactions, and the rate constants of adsorption for these kinetic models were calculated. Adsorption experiments demonstrated that the adsorption process corresponds to a pseudo-second- rather than -first-order kinetics model. With decreasing aqueous solution temperature, the adsorption kinetics become slower and the amount of silver ion adsorbed increases. The thermodynamic values, ΔG°, ΔH° and ΔS°, indicated that adsorption was an exothermic and spontaneous process.

Mineral dissolution and precipitation during CO2 injection at the Frio-I Brine Pilot: Geochemical modeling and uncertainty analysis

During the Frio-I Brine Pilot CO2 injection experiment in 2004, distinct geochemical changes in response to the injection of 1600tons of CO2 were recorded in brine samples collected from the monitoring well. Previous geochemical modeling studies have considered dissolution of calcite and iron oxyhydroxides, or release of adsorbed iron, as the most likely sources of the increased ion concentrations. In this modeling study we explore possible alternative sources of the increasing calcium and iron, based on the data from the detailed petrographic characterization of the Upper Frio Formation “C”. Particularly, we evaluate whether dissolution of pyrite and oligoclase (anorthite component) can account for the observed geochemical changes. Due to kinetic limitations, dissolution of pyrite and anorthite cannot account for the increased iron and calcium concentrations on the time scale of the field test (10 days). However, dissolution of these minerals is contributing to carbonate and clay mineral precipitation on the longer time scales (1000 years). We estimated that during the field test dissolution of calcite and iron oxide resulted in ∼0.02wt.% loss of the reservoir rock mass. The reactive transport models were constructed for 25 and 59°C temperature and using Pitzer and B-dot activity correction methods. These models predict carbonate minerals, dolomite and ankerite, as well as clay minerals kaolinite, nontronite and montmorillonite, will precipitate in the Frio Formation “C” sandstone as the system progresses toward chemical equilibrium during a 1000-year period. Cumulative uncertainties associated with using different thermodynamic databases, activity correction models (Pitzer vs. B-dot), and extrapolating to reservoir temperature, are manifested in the difference in the predicted mineral phases. However, these models are consistent with regards to the total volume of mineral precipitation and porosity values which are predicted to within 0.002%.

The origin of groundwater composition in the Pampeano Aquifer underlying the Del Azul Creek basin, Argentina

The Pampean plain is the most productive region in Argentina. The Pampeano Aquifer beneath the Pampean plain is used mostly for drinking water. The study area is the sector of the Pampeano Aquifer underlying the Del Azul Creek basin, in Buenos Aires province. The main objective is to characterize the chemical and isotopic compositions of groundwater and their origin on a regional scale. The methodology used involved the identification and characterization of potential sources of solutes, the study of rain water and groundwater chemical and isotopic characteristics to deduce processes, the development of a hydrogeochemical conceptual model, and its validation by hydrogeochemical modelling with PHREEQC. Groundwater samples come mostly from a two-depth monitoring network of the “Dr. Eduardo J. Usunoff” Large Plains Hydrology Institute (IHLLA). Groundwater salinity increases from SW to NE, where groundwater is saline. In the upper basin groundwater is of the HCO3-Ca type, in the middle basin it is HCO3-Na, and in the lower basin it is ClSO4–NaCa and Cl–Na. The main processes incorporating solutes to groundwater during recharge in the upper basin are rain water evaporation, dissolution of CO2, calcite, dolomite, silica, and anorthite; cationic exchange with Na release and Ca and Mg uptake, and clay precipitation. The main processes modifying groundwater chemistry along horizontal flow at 30m depth from the upper to the lower basin are cationic exchange, dissolution of silica and anorthite, and clay precipitation. The origin of salinity in the middle and lower basin is secular evaporation in a naturally endorheic area. In the upper and middle basins there is agricultural pollution. In the lower basin the main pollution source is human liquid and solid wastes. Vertical infiltration through the boreholes annular space during the yearly flooding stages is probably the pollution mechanism of the samples at 30m depth.

Pore-Scale Process Coupling and Effective Surface Reaction Rates in Heterogeneous Subsurface Materials

Heterogeneity in pore structure and reaction properties including grain size and mineralogy, pore size and connectivity, and sediment surface area and reactivity is a common phenomenon in subsurface materials. Heterogeneity affects transport, mixing, and the interactions of reactants that affect local and overall geochemical and biogeochemical reactions. Effective reaction rates can be orders of magnitude lower in heterogeneous porous media than those observed in well-mixed, homogeneous systems as a result of the pore-scale variability in physical, chemical, and biological properties, and the coupling of pore-scale surface reactions with mass-transport processes in heterogeneous materials. Extensive research has been performed on surface reactions at the pore-scale to provide physicochemical insights on factors that control macroscopic reaction kinetics in porous media. Mineral dissolution and precipitation reactions have been frequently investigated to evaluate how intrinsic reaction rates and mass transfer control macroscopic reaction rates. Examples include the dissolution and/or precipitation of calcite (Bernard 2005; Li et al. 2008; Tartakovsky et al. 2008a; Flukiger and Bernard 2009; Luquot and Gouze 2009; Kang et al. 2010; Zhang et al. 2010a; Molins et al. 2012, 2014; Yoon et al. 2012; Steefel et al. 2013; Luquot et al. 2014), anorthite and kaolinite (Li et al. 2006, 2007), iron oxides (Pallud et al. 2010a,b; Raoof et al. 2013; Zhang et al. 2013a), and uranyl silicate and uraninite (Liu et al. 2006; Pearce et al. 2012). Adsorption and desorption at the pore-scale have been investigated to understand the effect of pore structure heterogeneity on reaction rates and rate scaling from the pore to macroscopic scales (Acharya et al. 2005; Zhang et al. 2008, 2010c, 2013b; Zhang and Lv 2009; Liu et al. 2013a). Microbially mediated reactions have also …

Experimental determination of plagioclase dissolution rates as a function of its composition and pH at 22 °C

The steady-state, far-from-equilibrium dissolution rates of nine distinct plagioclases ranging in composition from An2 to An89 were measured in mixed flow reactors at 22±2°C and pH from 2 to 11. The dissolution rates of all plagioclases based on silica release show a common U-shaped behaviour as a function of pH, where rates decrease with increasing pH at acid condition but rise with increasing pH at alkaline conditions. Consistent with literature findings, constant pH plagioclase dissolution rates increase with increasing anorthite content at acidic conditions; measured anorthite dissolution rates are ∼2.5 orders of magnitude faster than those of albite at pH ∼2. Perhaps more significantly, rates are independent of plagioclase composition at alkaline conditions. Interpretation and data fitting suggests that plagioclase dissolution rates are consistent with their control by the detachment of Si-rich activated complexes formed by the removal of Al from the mineral framework. Taking account of this mechanism and transition state theory yields equations describing plagioclase dissolution rates (r+) as a function of both the mineral and aqueous fluid compositions found in natural Earth surface systems. For pH⩾6 rates are consistent with Log(r+/(mol/cm2/s))=0.35Log(aH+3/aAl3+)-11.53 and for pH<6 rates are consistent with Log(r+/(mol/cm2/s))=nacidLog(aH+3/aAl3+)+0.033An%-14.77 where An% represents the percent anorthite in the plagioclase solid solution, ai corresponds to the activity of the ith aqueous species, and nacid is given by nacid=0.004An%+0.05.

Groundwater Evolution and Mineral Alteration Reactions in the Basaltic Rock Sequence of Mt. Wasserkuppe, Germany: A Case Study

Groundwater sampling was accomplished in the basaltic sequence of the Rhon mountain range, Germany, in order to investigate hydrochemical groundwater evolution and to delineate mineral alteration reactions involved in natural weathering. The hydrochemical compositions of near-surface groundwaters indicate a Ca/Mg–HCO3 type with near-neutral pH and evolve to a Na–HCO3 type with high pH at greater depth. Column experiments were performed with basaltic and phonolitic rock samples to determine individual mineral alteration reactions. The basic reactions could be related to the alteration of olivine, Ca-pyroxene, plagioclase, pyrrhotite, and feldspathoids under formation of secondary clay minerals (smectites, illite) and goethite. The mineral alteration reactions deduced from the leaching experiments by inverse modelling were found to be consistent with the mineral reactions associated with the natural groundwaters. The reactions calculated for groundwater evolution involve the alteration of primary and secondary minerals to produce low-T mineral phase. The conversion of secondary Na-beidellite to illite occurs at a later stage of groundwater evolution, reducing the concentrations of K+ and Mg2+. Near-surface groundwaters do not indicate significant cation exchange. Initial cation exchange requires elevated pH values, with Mg2+ removed from solution preferred to Ca2+. Na-alkalisation of the groundwaters at greater depth suggests the exchange of Na+ for Mg2+ and Ca2+ on Na-beidellite, supported by cation exchange on coatings of iron hydroxides as alteration products. Among the mature high-pH groundwater at greater depth, the dissolution of anorthite and albite has significant effect on groundwater composition.

Material conversion from paper sludge ash in NaOH solution to synthesize adsorbent for removal of Pb 2+, NH 4+ and PO 43− from aqueous solution

Abstract Material conversion from paper sludge ash (PSA) in NaOH solution was attempted to synthesize the adsorbent for removal of inorganic pollutants, such as Pb 2+, NH 4 + and PO 4 3- from aqueous solution. PSA of 0.5 g was added into 10 mL of 3 mol/L NaOH solution, and then heated at 80, 120, and 160°C for 6-48 hr to obtain the product. PSA mainly composed of two crystalline phases, gehlenite (Ca 2Al 2SiO 7) and anorthite (CaAl 2Si 2O 8), and amorphous phase. Hydroxysodalite (Na 6Al 6Si 6O 24·8H 2O) was formed at 80°C, and anorthite dissolved, whereas gehlenite remained unaffected. Katoite (Ca 3Al 2SiO 4(OH) 8) was formed over 120°C, and hydroxycancrinite (Na 8(OH) 2Al 6Si 6O 24·2H 2O) was formed at 160°C, due to the dissolution of both gehlenite and anorthite. Specific surface areas of the products were almost same and were higher than that of raw ash. Cation exchange capacities (CECs) of the products were also higher than that of raw ash, and CEC obtained at lower temperature was higher. Removal abilities of products for Pb 2+, NH 4 +, and PO 4 3- were higher than that of raw ash. With increasing reaction temperature, the removal efficiencies of Pb 2+ and NH 4 + decreased due to the decrease of CEC of the product, while removal efficiency for PO 4 3- was almost same. The concentrations of Si and Al in the solution and the crystalline phases in the solid during the reaction explain the formation of the product phases at each temperature.

The effects of initial acetate concentration on CO2–brine-anorthite interactions under geologic CO2 sequestration conditions

Acetate is one of the most abundant organic compounds in many formation waters and is likely to be present in deep saline aquifers suitable for geologic CO2 sequestration (GCS). This work studied the effect of initially present acetate on the dissolution of anorthite (CaAl2Si2O8) and on subsequent secondary mineral precipitation under GCS conditions (35 °C and 74.8 atm). Anorthite was chosen as a model mineral because of the abundance of feldspar in clayey sandstones and the possibility of metal carbonation. In this study, acetate was found to decrease the cumulative aqueous concentrations of Al, Si, and Ca upon CO2 injection by inhibiting anorthite dissolution and increasing the amount of secondary mineral precipitates. The extent of the effect of acetate on metal concentration changes was element-specific (Al > Si > Ca), and the effect was found to be more significant in systems with lower salinity and lower pH. For anorthite dissolution, acetic acid inhibited the proton-mediated decomposition of the Al/Si-containing feldspar framework, while acetate anions may have facilitated the ion-exchange between interstitial Ca and aqueous cations. For secondary mineral precipitation, stoichiometry analysis of aqueous metal concentrations suggested the formation of Al-containing mineral(s). The presence of kaolinite as a secondary mineral was confirmed using high resolution transmission electron microscopy's electron diffraction data. An increase in the relative amount of precipitation due to the initial presence of acetate was suggested by mass balancing and verified on the cleaved anorthite surfaces by atomic force microscopy analysis. These results provide new insights for understanding and predicting GCS system evolution upon scCO2 injection in the initial presence of acetate.

Modeling diffusive dissolution in silicate melts

Here empirical models for calculating self-diffusion coefficients and diffusion matrices are combined with MELTS, a thermodynamic model for silicate minerals and melts, to estimate diffusive dissolution rates, interface melt compositions and melt diffusivities. Simulations of olivine dissolution experiments in basalt show that the overall model is capable of accurately reproducing diffusive dissolution rates, and the resulting diffusion profiles, over a range of pressures and temperatures. However, the overall model is less successful at reproducing olivine dissolution in andesite, diopside dissolution in either basalt or andesite, or anorthite dissolution in picrite. Yet, even for these systems the predicted dissolution rates are generally within about a factor of two of the measured ones. Comparisons between simulations and experiments suggest that errors in the self-diffusion and thermodynamic models are responsible for the differences, and show that dissolution experiments could be a powerful way of testing and calibrating these and similar models. The overall model will also be a useful tool for designing future experiments, and for identifying the parameters that control diffusive dissolution (and crystallization) in silicate melts under a wide range of conditions.

Anorthite dissolution promoted by bacterial adhesion: Direct evidence from dialytic experiment

Bacterial adhesion is crucial to the dissolution of minerals and rocks. By employing dialysis method, we designed comparative experiments to investigate the dissolution behavior of anorthite with the mediation of both adhered and non-adhered Paenibacillus polymyxa. The results show that during 10 experimental days, the dissolution of anorthite was promoted considerably by P. polymyxa and metabolites. Nanoscale precipitates were formed in contact experiment. Anorthite has a higher release of Ca and lower releases of Al and Si in dialysis experiment than in contact experiment. The difference implies that the release of Ca is controlled by mechanism of proton-exchange, whereas those of Al and Si are controlled by ligand-complexation in which Al-O-Si bonds are destroyed first. Kinetically the release of Ca is controlled mainly by surface reaction and leached-layer diffusion. Both of them are restrained by adhered bacteria. The releases of Al and Si are controlled mainly by surface reaction, which is accelerated by adhered bacteria.

Groundwater flow modeling and particle tracking for chemical transport in the southern Voltaian aquifers

Groundwater is the major source of water supply for most uses in the rural settlements in Ghana. A groundwater flow model was calibrated for some aquifers of the southern Voltaian sedimentary system under steady-state conditions. The objective was to determine estimates of the hydraulic conductivities of the different hydrostratigraphic units of the southern Voltaian, and the distribution of recharge from precipitation. Data on the stable isotopes of oxygen and hydrogen from the study area suggest that groundwater recharge in the area is from rainfall. The calibrated steady-state model suggests that aquifer hydraulic conductivities in the study area range from 1.19 to 6.3 m/day. Hydrostratigraphic unit specific hydraulic conductivities are discussed. The calibrated recharge ranges from 3.81e−05 m/day to 6.0e−04 m/day, which represents 0.9–6% of the precipitation in the form of rainfall. Six distinct flowpaths have been defined using particle tracking. The particle tracking simulation suggests travel times in the range of 380 to 5,199 years from recharge areas to discharge areas along the flowpaths identified. A contaminant dropped at the recharge areas in the central sections of the model area would travel at these rates along the flowpaths, assuming that advection is the dominant transport process. Inverse geochemical modeling indicates the dissolution of albite, K-feldspars and anorthite, respectively, along flowpaths I and IV. The inverse modeling along flowpaths I and IV suggest the dissolution of albite, K-feldspar and anorthite, respectively, at 1.085e−06, 3.16e−08 and 3.168e−07 mmol/year.

Dissolution kinetics of anorthite in a supercritical CO2-water system

The Gibbs free energy change, Δ G r , dependence of the anorthite dissolution rate in a supercritical CO2–water system was measured as part of a geochemical assessment of CO2 geological sequestration. Being bounded at Δ G r crit, the critical Δ G r required for an opening of the etch pit at a screw dislocation, it has been accepted that mineral dissolution follows etch pit formation assisted by dislocations below Δ G r crit, whereas the horizontal step retreats without the etch pit above Δ G r crit. The experiment described herein, however, revealed that another mode of dissolution occurs more distant from equilibrium by spontaneous formation of the etch pit over the entire surface, as observed on calcite. The dissolution rate is higher by more than one order of magnitude than that in the dislocation-assisted mode. Therefore, including the rate gap at Δ G r crit, a nonlinear curve with three steps instead of a sigmoidal curve is proposed for the Δ G r dependence of the anorthite dissolution rate. Extremely slow rates were observed depending on observed points for the same Δ G r condition. Although the reason for such a rate difference remains unknown, it is likely related to the defect density on the crystal surface. It is possible that initial spreading of the dissolved surface attributable to the etch pit formation assisted by defects provides some trigger for subsequent explosive etch pit formation. These findings suggest that the initial transient process can strongly influence the kinetics of geochemical reactions that occur during CO2 geological sequestration.

Saturation states dependence of feldspar dissolution rate: Sensitivity to the timescale of geochemical processes on CO2 geological sequestration

Abstract For the long-term assessment of CO 2 geological sequestration, the dissolution rate of feldspar was measured under various degrees of saturation based on observations of the time-course change of dissolved crystal surfaces on a nanoscale. We identified that the dependence of solution saturation state on anorthite dissolution rate follows to a L-shaped function, rather than to a linear function. Compared with a linear assumption, such a nonlinear form provides a much lower estimate of dissolution rate. This implies that the difference of rate formula can cause meaningful errors in the timescale evaluation of various geochemical processes. In this study, therefore, the effect of the function form of dissolution rate was also checked using the geochemical simulation code to analyze CO 2 geological sequestration.

CO2 地中貯留における灰長石溶解プロセスの速度論的考察

The Gibbs free energy change, ΔGr, dependence of the anorthite dissolution rate under a supercritical CO2-water system was measured as part of a long-term assessment of CO2 geological sequestration. The surface observation on a nanoscale described herein showed that the ΔGr dependency follows to the rate function with three steps (i.e., the horizontal step retreats without the etch pit, the etch pit formation assisted by dislocations, and the spontaneous formation of the etch pit) instead of transient state theory and a traditional sigmoidal curve, where the rate increases abruptly more distant from equilibrium by spontaneous formation of the etch pit over the entire surface. Such a variation of the dissolution rate can affect largely not only the change in porosity or permeability of reservoir rocks and the resultant flow property of formation water, but also the precipitation rate of secondary minerals. Our preliminary numerical simulation revealed that the difference of the rate function form according to ΔGr dependence produces the time gap more than two orders on both the dissolution of anorthite and the precipitation of secondary minerals. On the other hand, extremely slow rates were observed depending on surface conditions for the same ΔGr condition. Therefore, understanding of transient processes before a steady state is also necessary for evaluation of the mineral dissolution rate under natural conditions including CO2 geological sequestration.

Modeling of water–rock interactions in an aquitard of sandy clay in the coastal area near Beihai, China

Unconsolidated sand, gravel and clay deposits near Beihai and in the Leizhou Peninsula in southern China form an unconfined aquifer, aquitard and a confined aquifer. Water and soil samples were collected from the two aquifers in the coastal Beihai area for the determination of chemical compositions, minerals and soluble ions. Hydrogeochemical modeling of three flow paths through the aquitard are carried out using PHREEQC to determine water–rock interactions along the flow paths. The results indicate that the dissolution of anorthite, fluorite, halite, rhodochrosite and CO2, and precipitation of potash feldspar and kaolinite may be occurring when groundwater leaks through the aquitard from the unconfined aquifer to the confined aquifer. Cation exchanges between Na and Ca can also happen along the flow paths.

Applicability of averaged concentrations in determining geochemical reaction rates in heterogeneous porous media

This work examines the applicability of averaged concentrations, a mathematical analog of field-measured solute concentrations averaged over a large number of pores, in determining mineral reaction rates in heterogeneous porous media. Pore-scale network models were used to represent sandstones with anorthite and kaolinite as reactive minerals that are heterogeneously distributed in space. Reaction rates calculated from averaged concentrations were compared to true reaction rates that take into account variabilities in individual pore properties. Simulations were run under the highly acidic conditions relevant to geological CO2 sequestration in deep brine formations under various mineralogical and flow conditions. Results show that, under conditions where incomplete mixing arises, the averaged concentrations and analogously the field-measured concentrations, do not accurately reflect reaction progress. Over the length scale of several millimeters, the anorthite dissolution rates can be overestimated by a factor of three. For kaolinite, due to its highly nonlinear reaction rate law, even the reaction direction may be incorrectly determined, with precipitation predicted as dissolution. The extent of errors introduced depends on the extent of incomplete mixing. Conditions that homogenize the concentration fields, such as small reactive mineral clusters, abundant reactive minerals, and very fast or very slow flow rates, minimize errors introduced from averaging. These results indicate that the averaging scheme may partly contribute to the often-cited laboratory-field rate discrepancy and have important implications for the interpretation of concentration data obtained from field investigation.

Morphological analyses of minute crystals by using stereo‐photogrammetric scanning electron microscopy and electron back‐scattered diffraction

We present a new method for the morphological analyses of minute faceted crystals by combining stereo-photogrammetric analysis of scanning electron microscope images and electron back-scattered diffraction. Two scanning electron microscope images of the same crystal, recorded at different tilt angles of the specimen stage, are used to determine the orientations of crystal edges in a specimen-fixed coordinate system. The edge orientations are converted to the indices [uvw] in the crystal system using the crystal orientation determined by electron back-scattered diffraction analysis. The Miller indices of crystal facets are derived from the indices of the edges surrounding the facets. The method is applicable to very small crystal facets. The angular error, as derived from tests using a calcite crystal of known morphology, is a few degrees. To demonstrate the applicability of the method, the morphology of boehmite (gamma-AlOOH) precipitated from solution during the dissolution of anorthite was analyzed. The micrometre-sized boehmite crystals are surrounded by two {010} basal facets and eight equivalent side facets that can be indexed equally well as {323}, {434} or {545}. We suggest that these side facets are in fact {111}, the morphology having been modified slightly (by a few degrees) by a small extension associated with opening along (010) microcleavage planes. Tiny {140} facets are also commonly observed.

Feldspar dissolution rates measured using phase-shift interferometry: Implications to CO2 underground sequestration

To assess CO 2 underground sequestration from a geochemical viewpoint, the anorthite dissolution rate, which is an important parameter of risk analysis, was measured in a CO 2 –water system. The authors sought to obtain precise dissolution rate data in a short time observing a crystal surface on a nanoscale. For this purpose, phase-shift interferometry was applied. Using this method, uncertainty of the reactive surface area that is imparted on calculation of the dissolution rate constant can also be avoided. The time-course profile of vertical retreat of the surface revealed that the anorthite dissolution process changes from the initial transient state to a later steady state, which is consistent with results of numerous precedent studies. The transient dissolution rate depends strongly on local features (e.g., density of defects, variation of chemical compositions) of the crystal surface, rather than on temperature. Therefore, it is very important to determine the original properties of the anorthite surface for the examination of subsequent dissolution process. Contrary to general expectations, the anorthite dissolution can alter the physical properties of reservoir rock immediately after CO 2 injection. The simple estimation using the anorthite dissolution rate obtained in this study, which was done as a test case for the CO 2 underground sequestration project conducted by RITE, revealed that porosity of reservoir rock increased about 2% (23–23.4%) of initial values during 60 a. That change in physical property in such a short time might enhance the diffusion of injected CO 2 and formation water, and therefore accelerate further geochemical reactions. Results of this study demonstrate that the geochemical water–rock interaction, which is generally regarded as a longer-term phenomenon than various physical processes, can also affect the reservoir system from the initial stage.