Kinetics and mechanisms of the leaching of low Fe sphalerite

The surface speciation and leaching kinetics of 38- to 75-μm sphalerite (0.45 wt.% Fe) particles reacted in O₂ purged perchloric acid (at pH 1.0) at 25, 40, 60, and 85 °C over a leach period of 144 h were investigated. In all cases, an initial rapid leach rate is observed followed by a slower leach rate. These two leach regimes can each be adequately modeled using straight-line interpolation, and thus two activation energies (E_a) have been derived. E_a for the fast and slow Zn dissolution rates were 33 ± 4 kJ mol⁻¹ and 34 ± 4 kJ mol⁻¹ respectively, suggesting the same rate-determining step.     

Effect of solution saturation state and temperature on diopside dissolution

Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175°C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175°C. At 175°C, both models over predict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields where the Mg-H exchange coefficient, n = 1.39, the apparent activation energy, E _a = 332 kJ mol^-1, and the apparent rate constant, k = 10^41.2 mol diopside cm^-2 s^-1. Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Rate expressions for each mechanism (i) were fit to where the step edge energy (α) for homogeneously nucleated pits were higher (275 to 65 mJ m^-2) than the pits nucleated at defects (39 to 65 mJ m^-2) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (E_{b-homogeneous} = 2.59 × 10^-16 mJ K^-1) were lower than the pits nucleated at defects (E_{b-defect assisted} = 8.44 × 10^-16 mJ K^-1).     

Does reactive surface area depend on grain size? Results from pH 3, 25 °C far-from-equilibrium flow-through dissolution experiments on anorthite and biotite

Laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems. The principle normalisation terms used in the literature are mass, and geometric- and BET specific surface area (SSA). The purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size. Different size fractions of anorthite and biotite ranging from 180-150 to 20-10 μm were dissolved in pH 3, HCl at 25 °C in flow through reactors under far from equilibrium conditions. Steady state dissolution rates after 5376 h (anorthite) and 4992 h (biotite) were calculated from Si concentrations and were normalised to initial- and final- mass and geometric-, geometric edge- (biotite), and BET SSA. For anorthite, rates normalised to initial- and final-BET SSA ranged from 0.33 to 2.77 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹, rates normalised to initial- and final-geometric SSA ranged from 5.74 to 8.88 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹ and rates normalised to initial- and final-mass ranged from 0.11 to 1.65 mol_feldspar g⁻¹ s⁻¹. For biotite, rates normalised to initial- and final-BET SSA ranged from 1.02 to 2.03 × 10⁻¹² mol_biotite m⁻² s⁻¹, rates normalised to initial- and final-geometric SSA ranged from 3.26 to 16.21 × 10⁻¹² mol_biotite m⁻² s⁻¹, rates normalised to initial- and final-geometric edge SSA ranged from 59.46 to 111.32 × 10⁻¹² mol_biotite m⁻² s⁻¹ and rates normalised to initial- and final-mass ranged from 0.81 to 6.93 × 10⁻¹² mol_biotite g⁻¹ s⁻¹. For all normalising terms rates varied significantly (p ? 0.05) with grain size. The normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial BET SSA and initial- and final-geometric SSA. This is consistent with: (1) dissolution being dominated by the slower dissolving but area dominant non-etched surfaces of the grains and, (2) the walls of etch pits and other dissolution features being relatively unreactive. These steady state normalised dissolution rates are likely to be constant with time. Normalisation to final BET SSA did not give constant ratios across grain size due to a non-uniform distribution of dissolution features. After dissolution coarser grains had a greater density of dissolution features with BET-measurable but unreactive wall surface area than the finer grains. The normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial BET SSA. Initial- and final-geometric edge SSA and final BET SSA gave the next least varied rates. The basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge SSA normalised dissolution rates showing the least variation. Simple modelling indicated that biotite grain edges dissolved 71-132 times faster than basal surfaces. In this experiment, initial BET SSA best integrated the different areas and reactivities of the edge and basal surfaces of biotite. Steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area. Therefore they would be more properly termed pseudo-steady state rates, only appearing constant because the time period over which they were measured (1512 h) was less than the time period over which they would change significantly.     

CO<Subscript>2</Subscript> content of andesitic melts at graphite-saturated upper mantle conditions with implications for redox state of oceanic basalt source regions and remobilization of reduced carbon from subducted eclogite

We have performed experiments to determine the effects of pressure, temperature and oxygen fugacity on the CO₂ contents in nominally anhydrous andesitic melts at graphite saturation. The andesite composition was specifically chosen to match a low-degree partial melt composition that is generated from MORB-like eclogite in the convective, oceanic upper mantle. Experiments were performed at 1–3 GPa, 1375–1550?°C, and fO₂ of FMQ ?3.2 to FMQ ?2.3 and the resulting experimental glasses were analyzed for CO₂ and H₂O contents using FTIR and SIMS. Experimental results were used to develop a thermodynamic model to predict CO₂ content of nominally anhydrous andesitic melts at graphite saturation. Fitting of experimental data returned thermodynamic parameters for dissolution of CO₂ as molecular CO₂: ln(K ⁰) = ?21.79?±?0.04, ΔV ⁰?=?32.91?±?0.65 cm³mol^?1, ΔH ⁰?=?107?±?21 kJ mol^?1, and dissolution of CO₂ as CO₃ ^2?: ln(K ⁰ ) = ?21.38?±?0.08, ΔV ⁰?=?30.66?±?1.33 cm³ mol^?1, ΔH ⁰?=?42?±?37 kJ mol^?1, where K ⁰ is the equilibrium constant at some reference pressure and temperature, ΔV ⁰ is the volume change of reaction, and ΔH ⁰ is the enthalpy change of reaction. The thermodynamic model was used along with trace element partition coefficients to calculate the CO₂ contents and CO₂/Nb ratios resulting from the mixing of a depleted MORB and the partial melt of a graphite-saturated eclogite. Comparison with natural MORB and OIB data suggests that the CO₂ contents and CO₂/Nb ratios of CO₂-enriched oceanic basalts cannot be produced by mixing with partial melts of graphite-saturated eclogite. Instead, they must be produced by melting of a source containing carbonate. This result places a lower bound on the oxygen fugacity for the source region of these CO₂-enriched basalts, and suggests that fO₂ measurements made on cratonic xenoliths may not be applicable to the convecting upper mantle. CO₂-depleted basalts, on the other hand, are consistent with mixing between depleted MORB and partial melts of a graphite-saturated eclogite. Furthermore, calculations suggest that eclogite can remain saturated in graphite in the convecting upper mantle, acting as a reservoir for C.     

An evaluation of rate equations for calcite precipitation kinetics at pCO2 less than 0.01 atm and pH greater than 8

We used a reproducible seeded growth technique with a pH-stat to study the kinetics of calcite precipitation at 25°C. We performed different experiments at initial Ca²⁺ and HCO₃^? concentrations ranging from 0.7–2 and 4–7 mmol L^?1, pH values ranging from 8.25 to 8.70, $\text{pCO}{}_2$ values ranging from 0.0006 to 0.01 atm, and ionic strengths ranging from 0.015 to 0.10 mol L^?1. With this experimental data set, we used initial rate measurements and integral methods to test several precipitation rate equations. Rate equations that possess a disequilibrium functional dependence, such as the BURTON et al. (1951) dislocation model, forms of the Davies and Jones (1955) model, and the model used by Reddy and Nancollas (1973), did not adequately describe the kinetics of calcite precipitation at pH greater than 8 and $\text{pCO}{}_2$ less than 0.01 atm. Rate equations that describe independent dissolution and precipitation mechanisms with elementary reactions, such as the equation presented by Plummeret al. (1978), and nancollas and Reddy (1971) were more successful. However, Plummer's model did not adequately describe the rate of all experiments due to the presence of an OH^? surface term in the precipitation rate equation. The elementary reaction of the Nancollas and Reddy model is written in terms of bulk Ca²⁺ and CO₃^? concentrations, and appears to be the most successful model which describes calcite precipitation at pH > 8 and $\text{pCO}{}_2\text{< 0.01}$ atm. The Nancollas and Reddy model, altered to account for varying ionic strengths, adequately described the rate of all experiments and yielded a precipitation rate constant of $\text{118.2 ± 13.9}$ dm⁶ mol^?1 m^?2 s^?1, with an apparent Arrhenius activation energy of 48.1 kJ mol^?1.     

Thermodynamic properties of copper carbonates — malachite Cu2(OH)2CO3 and azurite Cu3(OH)2(CO3)2

The thermodynamic properties of the copper carbonates malachite and azurite have been studied by adiabatic calorimetry, by heat-flux Calvet Calorimetry, by differential thermal analysis (DTA) and by thermogravimetrie (TGA) analysis. The heat capacities, C _p ⁰ of natural malachite and azurite have been measured between 3.8 and 300 K by low-temperature adiabatic calorimetry. The heat capacity of azurite exhibits anomalous behavior at low temperatures. At 298.15 K the molar heat capacities C _p ⁰ and the third law entropies S _298.15 ⁰ are 228.5±1.4 and 254.4±3.8 J mol^?1 K^?1 for azurite and 154.3±0.93 and 166.3±2.5 J mol^?1 K^?1 for malachite. Enthalpies of solution at 973 K in lead borate 2PbO·B₂O₃ have been measured for heat treated malachite and azurite. The enthalpies of decomposition are 105.1±5.8 for azurite and 66.1±5.0 kJ mol^? for malachite. The enthalpies of formation from oxides of azurite and malachite determined by oxide melt solution calorimetry, are ?84.7±7.4 and ?52.5±5.9 kJ mol^?1, respectively. On the basis of the thermodynamic data obtained, phase relations of azurite and malachite in the system Cu²⁺-H₂O-CO₂ at 25 and 75 °C have been studied.     

Metal release from dolomites at high partial-pressures of CO2

The potential for metal release associated with CO₂ leakage from underground storage formations into shallow aquifers is an important consideration in assessment of risk associated with CO₂ sequestration. Metal release can be driven by acidification of groundwaters caused by dissolution of CO₂ and subsequent dissociation of carbonic acid. Thus, acidity is considered one of the main drivers for water quality degradation when evaluating potential impacts of CO₂ leakage. Dissolution of carbonate minerals buffers the increased acidity. Thus, it is generally thought that carbonate aquifers will be less impacted by CO₂ leakage than non-carbonate aquifers due to their high buffering potential. However, dissolution of carbonate minerals can also release trace metals, often present as impurities in the carbonate crystal structure, into solution. The impact of the release of trace metals through this mechanism on water quality remains relatively unknown. In a previous study we demonstrated that calcite dissolution contributed more metal release into solution than sulfide dissolution or desorption when limestone samples were dissolved in elevated CO₂ conditions. The study presented in this paper expanded our work to dolomite formations and details a thorough investigation on the role of mineral composition and mechanisms on trace element release in the presence of CO₂. Detailed characterization of samples from dolomite formations demonstrated stronger associations of metal releases with dissolution of carbonate mineral phases relative to sulfide minerals or surface sorption sites. Aqueous concentrations of Sr²⁺, CO²⁺, Mn²⁺, Ni²⁺, Tl⁺, and Zn²⁺ increased when these dolomite rocks were exposed to elevated concentrations of CO₂. The aqueous concentrations of these metals correlate to aqueous concentrations of Ca²⁺ throughout the experiments. All of the experimental evidence points to carbonate minerals as the dominant source of metals from these dolomite rocks to solution under experimental CO₂ leakage conditions. Aqueous concentrations of Ca²⁺ and Mg²⁺ predicted from numerical simulation of kinetic dolomite dissolution match those observed in the experiments when the surface area is three to five orders of magnitude lower than the surface area of the samples measured by gas adsorption.     

Heat of formation of petalite, LiAlSi4O10

The enthalpy of formation of petalite, LiAlSi₄O₁₀, has been measured using high-temperature solution calorimetry. The measurements were carried out in a Calvet-type twin micro calorimeter at 728?°C. A 2PbO?·?B₂O₃ melt was used as a solvent. Tabulated heats of formation of the components and tabulated heat capacities of the reactants and the product (Robie and Hemingway 1995) were used to calculate the standard heat of formation of petalite from the measured heats of solution. The calculations yielded a mean value of Δ _f H ^pet _298.15=?4872±5.4 kJ mol^?1. This value may be compared to the heat of formation of Δ _f H ^pet _298.15= ?4886.5±6.3 kJ mol^?1 determined by the HF solution calorimetry by Bennington et?al. (1980). Faßhauer et?al. (1998) combined thermodynamic data with phase-equilibrium results to obtain best-fit thermodynamic results using the Bayes method, in order to derive an internally consistent dataset for phases in the NaAlSiO₄– LiAlSiO₄–Al₂O₃–SiO₂–H₂O system. They determined ?4865.6?±?0.8?kJ?mol^?1 as the enthalpy of formation of petalite, a value that is appreciably closer to the enthalpy found in this work.     

Estimation of dolomite formation: Dolomite precipitation and dolomitization

Reactive-transport models are developed here that produce dolomite via two scenarios: primary dolomite (no CaCO₃ dissolution involved) versus secondary dolomite (dolomitization, involving CaCO₃ dissolution). Using the available dolomite precipitation rate kinetics, calculations suggest that tens of meters of thick dolomite deposits cannot form at near room temperature (25-35°C) by inorganic precipitation mechanism, though this mechanism will provide dolomite aggregates that can act as the nuclei for dolomite crystallization during later dolomitization stage. Increase in supersaturation, Mg⁺²/Ca⁺² ratio and CO₃^-2 on the formation of dolomite at near room temperature are subtle except for temperature.This study suggests that microbial mediation is needed for appreciable amount of primary dolomite formation. On the other hand, reactive-transport models depicting dolomitization (temperature range of 40 to 200°C) predicts the formation of two adjacent moving coupled reaction zones (calcite dissolution and dolomite precipitation) with sharp dolomitization front, and generation of >20% of secondary porosity. Due to elevated temperature of formation, dolomitization mechanism is efficient in converting existing calcite into dolomite at a much faster rate compared to primary dolomite formation.     

Comparative study of dissolution rate-determining mechanisms of limestone and dolomite

The dissolution rate-determining processes of carbonate rocks include: (1) heterogeneous reactions on rock surfaces; (2) mass transport of ions into solution from rock surfaces via diffusion; and (3) the conversion reaction of CO₂ into H⁺ and HCO ₃ ⁻ . Generally, it is the slowest of these three processes that limits the dissolution rate of carbonate rock. However, from experiment and theoretical analysis under similar conditions not only were the initial dissolution rates of dolomite lower by a factor of 3–60 than those of limestone, but also there are different dissolution rate-determining mechanisms between limestone and dolomite. For example, for limestone under the condition of CO₂ partial pressures dissolution rates increased significantly by a factor of about ten after addition of carbonic anhydrase (CA) into solution, which catalysed the conversation reaction of CO₂, whereas CA had little influence on dolomite dissolution. For dolomite, the increase of dissolution rate after addition of CA into solution appeared at Moreover, the enhancement factor of CA on dolomite dissolution rate was much lower (by a factor of about 3). In addition, when dissolution of both limestone and dolomite was determined by hydrodynamics (rotation speed or flow speed), especially under the dissolution of limestone was more sensitive to hydrodynamic change than that of dolomite. These findings are of significance in understanding the differences in karstification and relevant problems of resource and environment in dolomite and limestone areas.     

Interferometric study of the dolomite dissolution: a new conceptual model for mineral dissolution

The dissolution rate and mechanism of three different cleavage faces of a dolomite crystal from Navarra (near Pamplona), Spain, were studied in detail by vertical scanning interferometry techniques. A total of 37 different regions (each about 124 × 156 μm in size) on the three sample surfaces were monitored as a function of time during dissolution at 25°C and pH 3. Dissolution produced shallow etch pits with widths reaching 20 μm during 8 h of dissolution. Depth development as a function of time was remarkably similar for all etch pits on a given dolomite surface.On the basis of etch pit distribution and volume as a function of time, the calculated dissolution rate increases from near zero to 4 × 10⁻¹¹ mol cm⁻² s⁻¹ over 5 h. The time variation is different for each of the three cleavage surfaces studied. In addition, the absolute dissolution rates of different parts of the dolomite crystal surface can be computed by using a reference surface. The different surfaces yield an “average” rate of 1.08 × 10⁻¹¹ mol cm⁻² s⁻¹ with a standard deviation of 0.3 × 10⁻¹¹ mol cm⁻² s⁻¹ based on about 60 analyses. The mean absolute rate of the dolomite surface is about 10 times slower than the rate calculated from etch pit dissolution alone. On the other hand, earlier batch rate data that used BET surface areas yield rates that are at least 30 to 60 times faster than our directly measured mean dissolution rate for the same pH and temperature.A conceptual model for mineral dissolution has been inferred from the surface topography obtained by the interferometry investigations. In this model, mineral dissolution is not dominated by etch pit formation itself but rather by extensive dissolution stepwaves that originate at the outskirts of the etch pits. These stepwaves control the overall dissolution as well as the dependence on temperature and saturation state.     

Energetics of multicomponent diffusion in molten CaO-Al2O3-SiO2

The energetics of multicomponent diffusion in molten CaO-Al₂O₃-SiO₂ (CAS) were examined experimentally at 1440 to 1650°C and 0.5 to 2 GPa. Two melt compositions were investigated: a haplodacitic melt (25 wt.% CaO, 15% Al₂O₃, and 60% SiO₂) and a haplobasaltic melt (35% CaO, 20% Al₂O₃, and 45% SiO₂). Diffusion matrices were measured in a mass-fixed frame of reference with simple oxides as end-member components and Al₂O₃ as a dependent variable. Chemical diffusion in molten CAS shows clear evidence of diffusive coupling among the components. The diffusive flux of SiO₂ is significantly enhanced whenever there is a large CaO gradient that is oriented in a direction opposite to the SiO₂ gradient. This coupling effect is more pronounced in the haplodacitic melt and is likely to be significant in natural magmas of rhyolitic to andesitic compositions. The relative magnitude of coupled chemical diffusion is not very sensitive to changes in temperature and pressure.To a good approximation, the measured diffusion matrices follow well-defined Arrhenius relationships with pressure and reciprocal temperature. Typically, a change in temperature of 100°C results in a relative change in the elements of diffusion matrix of 50 to 100%, whereas a change in pressure of 1 GPa introduces a relative change in elements of diffusion matrix of 4 to 6% for the haplobasalt, and less than 5% for the haplodacite. At a pressure of 1 GPa, the ratios between the major and minor eigenvalues of the diffusion matrix λ₁/λ₂ are not very sensitive to temperature variations, with an average of 5.5 ± 0.2 for the haplobasalt and 3.7 ± 0.6 for the haplodacite. The activation energies for the major and minor eigenvalues of the diffusion matrix are 215 ± 12 and 240 ± 21 kJ mol⁻¹, respectively, for the haplodacite and 192 ± 8 and 217 ± 14 kJ mol⁻¹ for the haplobasalt. These values are comparable to the activation energies for self-diffusion of calcium and silicon at the same melt compositions and pressure. At a fixed temperature of 1500°C, the ratios λ₁/λ₂ increase with the increase of pressure, with λ₁/λ₂ varying from 2.5 to 4.1 (0.5 to 1.3 GPa) for the haplodacite and 4 to 6.5 (0.5 to 2.0 GPa) for the haplobasalt. The activation volumes for the major and minor eigenvalues of the diffusion matrix are 0.31 ± 0.44 and 2.3 ± 0.8 cm³ mol⁻¹, respectively, for the haplodacite and −1.48 ± 0.18 and −0.42 ± 0.24 cm³ mol⁻¹ for the haplobasalt. These values are quite different from the activation volumes for self-diffusion of calcium and silicon at the same melt compositions and temperature. These differences in activation volumes between the two melts likely result from a difference in the structure and thermodynamic properties of the melt between the two compositions (e.g., partial molar volume).Applications of the measured diffusion matrices to quartz crystal dissolution in molten CAS reveal that the activation energy and activation volume for quartz dissolution are almost identical to the activation energy and activation volume for diffusion of the minor or slower eigencomponent of the diffusion matrix. This suggests that the diffusion rate of slow eigencomponent is the rate-limiting factor in isothermal crystal dissolution, a conclusion that is likely to be valid for crystal growth and dissolution in natural magmas when diffusion in liquid is the rate-limiting factor.     

The dissolution kinetics of shallow marine carbonates in seawater: A laboratory study

The dissolution kinetics of shallow water marine carbonates (low-Mg calcite, aragonite and Mg-calcites) were investigated in seawater (S = 35) at 25°C and a P_CO2 of 10^?2.5 atm. using the pH-stat method. Carbonate dissoluton rates (μmoles g^?1 hr^?1) fit the empirical kinetic expression, R = k(1 - Ω)ⁿ, where R = dissolution rate, k = rate constant, Ω = saturation state, and n = order of reaction. Reaction orders were near 2.9 for low-Mg calcites, 2.5 for aragonites and 3.4 for Mg-calcites.The rate constant, k, expressed as μmoles g^?1 hr^?1, varied by nearly a factor of ten for the different samples, reflecting differences in amount of reactive surface area. Reactive surface area of the biogenic phases ranged from 0.3% to 66% of the total surface area determined by the BET gas adsorption method. The discrepancy between reactive and total surface area was greatest for samples with high BET surface areas (> 1 m² g^?1) and delicate microstructures.Relative dissolution rates of the various biogenic carbonates as a function of seawater calcium carbonate ion molal product (IMP) were related to both mineral stability and grain microstructure. In seawater undersaturated with respect to aragonite, finely crystalline aragonites dissolved more rapidly than thermodynamically less stable high Mg-calcites (15–18 mole% MgCO₃) with lower reactive surface areas. Therefore, under certain conditions, differences in grain microstructural complexity can override thermodynamic constraints and lead to selective dissolution of a thermodynamically more stable mineral phase.     

Solubility and near-equilibrium dissolution rates of quartz in dilute NaCl solutions at 398-473 K under alkaline conditions

The dissolution-precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH)₃ solutions with pH 8.2-9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol⁻¹ of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol⁻¹ are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at Q¹ or Q² silicon sites, respectively. The dissolution mechanism varies with ΔG such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at Q¹ sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by Bickmore et al. (2008). However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated Q¹ reaction rate on pH, and a poorly-constrained pH dependence of the postulated Q² rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at Q³ sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.     

Diagenetic crystallization and oxidation of siderite in red bed (Buntsandstein) sediments from the Central Iberian Chain,Spain

Compactional deformation facilitated replacement of dolomite and calcite by siderite and its subsequent oxidation in carbonate cemented red beds of the Triassic Buntsandstein in the Iberian Chain. Locally, the sedimentary clasts were cemented by carbonate that was derived from dissolution of locally exposed dolomite in the basement. Microstructures indicate that during sedimentation of the rocks, oxidizing conditions prevailed in the sediments and the basement was reddened by impregnation of hematite. Reducing conditions prevailed during deformation of the sediments. Ferric iron was reduced to Fe²⁺, that reacted with deformed dolomite and calcite cement to produce fine grained siderite. At a later stage, siderite crystallites were (partly) oxidized to form a secondary phase of brown ferric oxide (goethite). Locally, goethite transformed to fine grained hematite that caused secondary reddening of the sediments. The reactions are associated with a combined volume loss of the solid phases of c. 50% per reaction mol; this was accommodated by the formation of pores. Oxidation of siderite was associated with release of CO₂; localized dissolution took place of feldspar and concurrently growth of kaolinite occurred by acidifying condition during release of CO₂. The relation of redox reactions and deformation is comparable to those in red bed conglomerates in the region. Reductive dissolution occurred at sites of stress concentration, particularly at contact points of pebbles. Late stage precipitation of ferric oxides and pyrolusite took place at oxidizing conditions in association with uplift.     

Accumulation of acid sites on natural clinoptilolite under recurring dehydration

A new ¹H NMR approach was applied to study the influence of recurring dehydration on the acidity of a natural Ca-rich zeolite clinoptilolite. It has been found that thermal cycling progressively increases the rate of the proton exchange between water molecules in the rehydrated state. The observed effect is interpreted as a result of the irreversible accumulation of specific structural defects represented by Brønsted acid sites: each dehydration at 720 K adds to the acid sites in an amount of the order of 10^?3 per unit cell. The number of these defects, detected by NMR in hydrated mineral, is in reasonable agreement with their amount estimated for the dehydrated state with an IR-spectroscopy CO-probe method. A comparison of the results obtained for two distinct zeolite samples shows that the Ca²⁺ ions are of first importance in the dehydration-induced formation of the active acid sites. The barrier for the proton-transfer reaction between the acid sites and H₂O molecules in hydrated clinoptilolite is found to be 46 kJ mol^?1, which is not too different from the value of 54 kJ mol^?1 reported recently for natural chabazite.     

Thermodynamics and crystal chemistry of the hematite–corundum solid solution and the FeAlO3 phase

High-temperature oxide-melt calorimetry and Rietveld refinement of powder X-ray diffraction patterns were used to investigate the energetics and structure of the hematite–corundum solid solution and ternary phase FeAlO₃ (with FeGaO₃ structure). The mixing enthalpies in the solid solution can be described by a polynomial ΔH_mix=WX _hem(1?X _hem) with W=116 ± 10 kJ mol^?1. The excess mixing enthalpies are too positive to reproduce the experimental phase diagram, and excess entropies in the solid solution should be considered. The hematite–corundum solvus can be approximately reproduced by a symmetric, regular-like solution model with ΔG _excess=(W _H ?TW _S )X _hem X _cor, where W _H= 116 ± 10 kJ mol^?1 and W _S =32 ± 4 J mol^?1 K^?1. In this model, short-range order (SRO) of Fe/Al is neglected because SRO probably becomes important only at intermediate compositions close to Fe:Al=1:1 but these compositions cannot be synthesized. The volume of mixing is positive for Al-hematite but almost ideal for Fe-corundum. Moreover, the degree of deviation from Vegard's law for Al-hematite depends on the history of the samples. Introduction of Al into the hematite structure causes varying distortion of the hexagonal network of oxygen ions while the position of the metal ions remains intact. Distortion of the hexagonal network of oxygen ions attains a minimum at the composition (Fe_0.95Al_0.05)₂O₃. The enthalpy of formation of FeAlO₃ from oxides at 298 K is 27.9 ± 1.8 kJ mol^?1. Its estimated standard entropy (including configurational entropy due to disorder of Fe/Al) is 98.9 J mol^?1 K^?1, giving the standard free energy of formation at 298 K from oxides and elements as +19.1 ± 1.8 and ?1144.2 ± 2.0 kJ mol^?1, respectively. The heat capacity of FeAlO₃ is approximated as C _p (T in K)= 175.8 ? 0.002472T ? (1.958 × 10⁶)/T ²? 917.3/T ^0.5+(7.546 × 10^?6) T ² between 298 and 1550 K, based on differential scanning calorimetric measurements. No ferrous iron was detected in FeAlO₃ by Mössbauer spectroscopy. The ternary phase is entropy stabilized and is predicted to be stable above about 1730 ± 70 K, in good agreement with the experiment. Static lattice calculations show that the LiNbO₃-, FeGaO₃-, FeTiO₃-, and disordered corundum-like FeAlO₃ structures are less stable (in the order in which they are listed) than a mechanical mixture of corundum and hematite. At high temperatures, the FeGaO₃-like structure is favored by its entropy, and its stability field appears on the phase diagram.     

某地区白云岩室内溶蚀试验及微观溶蚀机理研究

揭示白云岩溶蚀砂化形成机理。开展室内不同溶液条件、不同结构白云岩溶蚀试验,对比分析溶蚀速率控制因素,采用铸体薄片、扫描电镜分析不同溶蚀阶段白云岩微观结构变化特征。结果表明:岩石微观结构对溶蚀速率起控制性作用,不同结构白云岩溶蚀速率关系为:中-细晶白云岩＞细-粉晶白云岩＞粉-泥晶白云岩; 比表面积与总溶蚀量有关,与单位面积溶蚀速率无关; 溶蚀起始于粒间、晶间孔隙、构造微裂隙与解理,并逐步扩展使孔隙、裂隙相互连通; 溶蚀使白云岩微观结构逐步劣化,晶体联结力减弱,伴随晶体崩解脱落,最终分解成细小颗粒而形成白云岩溶蚀砂化。     

A simulation experimental study on oxidative kinetics of sphalerite under hypergene condition

The sphalerite oxidative kinetics under hypergene condition was simulated and studied by means of a mixed flow reactor over a pH range of 1.0 7.8,and at dissolution temperatures from 20 to 55℃,ferric ion concentrations from 1.0×10-5 to 1.0×10-2 mol/L,O 2 flux of 0.5 L/min,and oxidants of ferric ion and O 2.It is indicated that with ferric ion as oxidant,the oxidation rate of sphalerite increases with increasing ferric ion concentration,temperature and decreasing pH value,and under the studied conditions,the dissolution rates of Zn and Cd are approximately identical,with the values of activation energy being 41.75 and 42.51 kJ·mol-1,respectively,suggesting that the oxidation rate of sphalerite is controlled by chemical reactions on mineral surface.However,with O 2 as oxidant,the oxidation mechanism of sphalerite varies with pH value.Oxidation rate decreases with increasing pH value when pH is lower than 5.95,whereas the increase of pH value results in an increase in oxidation rate when pH value is higher than 7.The oxidation rate of sphalerite can be expressed as:R Zn =10 1.1663 [Fe3+] 0 0.154 ·[H+] 0.2659 ·e-41.75/RT or R Cd =10 1.7292 [Fe3+] 0 0.170 ·[H+] 0.2637 ·e-42.51/RT