Jacques Schott

The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions

Abstract Analysis of aluminosilicate steady-state dissolution/precipitation rates indicate that in contrast to what is commonly assumed, the constant pH rates are not independent of chemical affinity at far from equilibrium conditions. Rather, the logarithm of these rates for albite and kaolinite are linear functions of the logarithm of aqueous Al concentration over wide ranges of saturation states. Consideration of both the steady-state rates and the surface chemistry of these minerals following dissolution indicates that these rates are consistent with their control by the decomposition of an Al-deficient, silica-rich surface precursor complex. Taking account of reactions written to form this complex leads to a rate equation for the dissolution/precipitation of these minerals that accurately describes their variation on pH, aqueous Al concentration, and chemical affinity. By analogy, it appears likely that the rates of numerous other aluminosilicate dissolution/crystallization reactions are also consistent with their control by the decomposition of similar precursor complexes. It follows from these observations that 1. (1) the generation of steady state dissolution rate constants from experiments performed in batch type reactors, 2. (2) the interpretation of the pH dependence of aluminosilicate dissolution reactions requires explicit account of the effects of aqueous Al concentration on these rates.

Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and the 87Sr/86Sr ratio of seawater

Abstract The impact of the Deccan Traps on chemical weathering and atmospheric CO2 consumption on Earth is evaluated based on the study of major elements, strontium and 87Sr/86Sr isotopic ratios of the main rivers flowing through the traps, using a numerical model which describes the coupled evolution of the chemical cycles of carbon, alkalinity and strontium and allows one to compute the variations in atmospheric pCO2, mean global temperature and the 87Sr/86Sr isotopic ratio of seawater, in response to Deccan trap emplacement. The results suggest that the rate of chemical weathering of Deccan Traps (21–63 t/km2/yr) and associated atmospheric CO2 consumption (0.58–2.54×106 mol C/km2/yr) are relatively high compared to those linked to other basaltic regions. Our results on the Deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of CO2 consumption during weathering of basalts, according to the relationship: f=R f ×C 0 exp −Ea R 1 T − 1 298 where f is the specific CO2 consumption rate (mol/km2/yr), Rf is runoff (mm/yr), C0 is a constant (=1764 μmol/l), Ea represents an apparent activation energy for basalt weathering (with a value of 42.3 kJ/mol determined in the present study), R is the gas constant and T is the absolute temperature (°K). Modelling results show that emplacement and weathering of Deccan Traps basalts played an important role in the geochemical cycles of carbon and strontium. In particular, the traps led to a change in weathering rate of both carbonates and silicates, in carbonate deposition on seafloor, in Sr isotopic composition of the riverine flux and hence a change in marine Sr isotopic composition. As a result, Deccan Traps emplacement was responsible for a strong increase of atmospheric pCO2 by 1050 ppmv followed by a new steady-state pCO2 lower than that in pre-Deccan times by 57 ppmv, implying that pre-industrial atmospheric pCO2 would have been 20% higher in the absence of Deccan basalts. pCO2 evolution was accompanied by a rapid warming of 4°C, followed after 1 Myr by a global cooling of 0.55°C. During the warming phase, continental silicate weathering is increased globally. Since weathering of continental silicate rocks provides radiogenic Sr to the ocean, the model predicts a peak in the 87Sr/86Sr ratio of seawater following the Deccan Traps emplacement. The amplitude and duration of this spike in the Sr isotopic signal are comparable to those observed at the Cretaceous–Tertiary boundary. The results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on Earth.

Mechanism of pyroxene and amphibole weathering—I. Experimental studies of iron-free minerals

Abstract The short term (2–40 days) dissolution of enstatite, diopside, and tremolite in aqueous solution at low temperatures (20–60°C) and pH 1–6 has been studied in the laboratory by means of chemical analyses of reacting solutions for Ca 2+ , Mg 2+ , and Si(OH) 4 and by the use of X-ray photoelectron spectroscopy (XPS) for detecting changes in surface chemistry of the minerals. All three minerals were found to release silica at a constant rate (linear kinetics) providing that ultrafine particles, produced by grinding, were removed initially by HF treatment. All three also underwent incongruent dissolution with preferential release of Ca and/or Mg relative to Si from their outermost surfaces. The preferential release of Ca, but not Mg for diopside at pH 6 was found by both XPS and solution chemistry verifying the theoretical prediction of greater mobility of cations located in M 2 structural sites. Loss mainly from M 2 sites also explains the degree of preferential loss of Mg from enstatite at pH 6; similar structural arguments apply to the loss of Ca and Mg from the surface of tremolite. In the case of diopside and tremolite initial incongruency was followed by essentially congruent cation-plus-silica dissolution indicating rapid formation of a constant-thickness, cation-depleted surface layer. Cation depletion at elevated temperature and low pH (~ 1) for enstatite and diopside was much greater than at low temperature and neutral pH, and continued reaction resulted in the formation of a surface precipitate of pure silica as indicated by solubility calculations, XPS analyses, and scanning electron microscopy. From XPS results at pH 6, model calculations indicate a cation-depleted altered surface layer of only a few atoms thickness in all three minerals. Also, lack of shifts in XPS peak energies for Si, Ca, and Mg, along with undersaturation of solutions with respect to all known Mg and Ca silicate minerals, suggest that cation depletion results from the substitution of hydrogen ion for Ca 2+ and/or Mg 2+ in a modified silicate structure and not from the precipitation of a new, radically different surface phase. These results, combined with findings of high activation energies for dissolution, a non-linear dependence on a H + for silica release from enstatite and diopside, and the occurrence of etch pitting, all point to surface chemical reaction and not bulk diffusion (either in solution or through altered surface layers) as the rate controlling mechanism of iron-free pyroxene and amphibole dissolution at earth surface temperatures.

Kinetics and mechanism of forsterite dissolution at 25°C and pH from 1 to 12

The forward dissolution rate of San Carlos forsterite Fo91 was measured at 25°C in a mixed-flow reactor as a function of pH (1 to 12), ionic strength (0.001 to 0.1 M), ΣCO2 (0 to 0.05 M), aqueous magnesium (10−6 to 0.05 M) and silica (10−6 to 0.001 M) concentrations. In CO2-free solutions, the rates decrease with increasing pH at 1 ≤ pH ≤ 8 with a slope close to 0.5. At 9 ≤ pH ≤ 12, the rates continue to decrease but with a smaller slope of ∼0.1. Addition of silica to solution at pH above 8.8 leads to reduction of up to 5 times in the dissolution rate. Magnesium ions have no effect on forsterite dissolution rate at pH from 3 to 6 and 10−5 10−4 M. In acidic and slightly alkaline solutions, forsterite dissolution is controlled by the decomposition of a silica-rich/magnesium-deficient protonated precursor complex. This complex is formed by exchange of two hydrogen ions for a Mg atom on the forsterite surface followed by polymerization of partially protonated SiO4 tetrahedra and rate-controlling H+ penetration into the leached layer and its adsorption on silica dimers. This accounts for the observed 0.5 order dependence of dissolution rate on H+ activity. In alkaline solutions, dissolution is controlled by the decomposition of Mg hydrated sites in a Mg-rich layer formed by silica preferential release. Within this conceptual model, forsterite forward rate of dissolution can be accurately described for a wide variety of solution compositions assuming two parallel reactions occurring at silica-rich and hydrated Mg surface sites: R+ (mol/cm2/s)=2.38×10−11 {>Si2O-H+}+1.62×10−10 {>MgOH2+} where {>i} stands for surface species concentration (mol/m2). This equation describes the weak dependence of dissolution rates on pH in alkaline solutions and the inhibiting effect of carbonate ions and dissolved silica when the hydration of surface Mg atoms with formation of >MgOH2+ is the rate-controlling step for dissolution. It follows that the decrease of forsterite dissolution rate with increasing carbonate concentration at pH ≥ 9 in natural aquatic systems results in a decrease of atmospheric CO2 consumption, i.e., unlike for feldspars, there is a negative feedback between pCO2 and forsterite weathering rate. This should be taken into account when modeling the effect of mafic mineral weathering on CO2 global balance.

Iron colloids/organic matter associated transport of major and trace elements in small boreal rivers and their estuaries (NW Russia)

Abstract The chemical status of major and trace elements (TE) in various boreal small rivers and watershed has been investigated along a 1500-km transect of NW Russia. Samples were filtered in the field through a progressively decreasing pore size (5, 0.8 and 0.22 μm; 100, 10, and 1 kD) using a frontal filtration technique. All major and trace elements and organic carbon (OC) were measured in filtrates and ultrafiltrates. Most rivers exhibit high concentration of dissolved iron (0.2–4 mg/l), OC (10–30 mg/l) and significant amounts of trace elements usually considered as immobile in weathering processes (Ti, Zr, Th, Al, Ga, Y, REE, V, Pb). In (ultra)filtrates, Fe and OC are poorly correlated: iron concentration gradually decreases upon filtration from 5 μm to 1 kD whereas the major part of OC is concentrated in the

Experimental study of anorthite dissolution and the relative mechanism of feldspar hydrolysis

Steady-state dissolution rates of anorthite (An96) were measured as a function of aqueous Si, Al, and Ca concentration at temperatures from 45 to 95°C and over the pH range 2.4 to 3.2 using a Ti mixed-flow reactor. All dissolution experiments exhibited stoichiometric dissolution. The concentration of aqueous Si, Al, and Ca ranged from ∼7 X× 10−5 to ∼1 × 10−3 molal, ∼6 × 10−5 to ∼3.4 × 10−3 molal, and ∼5 × 10−5 to ∼0.1 molal, respectively, corresponding to calculated anorthite chemical affinities ranging from ∼ 115 to ∼65 kJ/mol. Measured anorthite dissolution rates at constant temperature are proportional to αH+1.5, where αH+ designates the activity of the hydrogen ion, and consistent with an apparent activation energy of 18.4 kJ/mol. Anorthite dissolution rates are independent of aqueous Al concentration, which is in contrast with the alkali feldspars, whose constant pH, far from equilibrium rates are proportional to αAl+3−0.33 (Oelkers et al., 1994; Gautier et al., 1994; E. H. Oelkers and J. Schott, unpubl. data). This difference suggests a distinctly different dissolution mechanism. For the case of both types of feldspars it appears that Al is more readily removed than Si from the aluminosilicate framework. Because it has a SiAl ratio of 3, the removal of Al from the alkali feldspar framework leaves partially linked Si tetrehedra. Removal of Si still requires the breaking of SiO bonds, and thus the overall alkali feldspar dissolution rate is controlled by the decomposition of a silica-rich surface precursor. The variation of alkali feldspar dissolution rates with aqueous Al activity stems from the fact that the formation of this precursor requires the removal of Al. In contrast, because it has a SiAl ratio of 1, the removal of Al from the anorthite framework leaves completely detached Si tetrehedra. As a result, the removal of Si does not require the breaking of SiO bonds, the rate controlling precursor complex is not formed by the removal of Al, and the overall dissolution rate is independent of aqueous Al concentration at far from equilibrium conditions. It can be inferred from these results that the variation of far from equilibrium aluminosilicate dissolution rates on aqueous Al depends on the number and relative strength of different bond types that need to be broken for mineral hydrolysis.

Are quartz dissolution rates proportional to B.E.T. surface areas

Abstract A single quartz powder was dissolved at 200°C and 250°C under far from equilibrium conditions in atmosphere-equilibrated deionized water during a sequential series of experiments performed over one year in a titanium open system mixed flow reactor. Scanning electron microscope (SEM) photomicrographs show that morphological changes of this powder were dominated by grain edge rounding and etch pit formation. Etch pit walls rapidly evolve into unreactive negative crystal faceted forms; etch pit density and diameter are essentially constant during the experiments, indicating that dissolution predominantly deepened rather than widened etch pits. Measured B.E.T. surface areas increase linearly with mass of quartz dissolved to a value 5.6 times greater than that of the initial powder during the course of the experiments. Nevertheless, measured 200°C far from equilibrium mass normalized dissolution rates remained constant within experimental uncertainty. It is concluded that the bulk of the observed increase in B.E.T. surface areas during dissolution consisted of essentially unreactive etch pit walls which contribute negligibly to mineral dissolution. As similar etch pit development is common in natural systems, geometric rather than B.E.T. surface areas may provide a more accurate parameter for estimating dissolution rates.

Experimental study of K-feldspar dissolution rates as a function of chemical affinity at 150°C and pH 9

Steady state dissolution rates of a K-rich feldspar (K0.81Na0.15Ba0.03Al1.05Si2.96O8) were measured as a function of chemical affinity and aqueous Si and Al concentration in solutions containing 5 × 10−3 m total K using a titanium mixed flow reactor at a temperature of 150°C and pH of 9.0. All dissolution experiments exhibited stoichiometric dissolution with respect to Al and Si. The concentration of aqueous silica and Al ranged from 1 × 10−6to 5 × 10−4mol/kg and 4 × 10−7to 5 × 10−4mol/kg, respectively, corresponding to K-feldspar chemical affinities ranging from ~90 to ~5 kJ/mol. Logarithms of measured dissolution rates are an inverse linear function of aqueous aluminum concentration, but independent of aqueous silica concentration at all chemical affinities greater than ~20 kJ/mol. These rates become increasingly controlled by chemical affinity as equilibrium is approached. This variation of steady state dissolution rates is consistent with their control by the decomposition of silica rich/aluminum deficient surface precursor complex. Taking account of transition state theory and the identity of reactions to form this precursor complex, an equation was derived to describe the steady state dissolution rates over the full range of chemical affinity. A simplified but less general version of this equation, which can be used to describe the steady state rates (r) obtained in the present study can be expressed as r=k+1aAl(OH)+−aH+13 (1 − exp (− A3RT)) where k+ stands for a rate constant equal to 1.7 × 10−17 mol/cm2/s, aH+ and aAl(OH)4− designate the activities of H+ and Al(OH)4− , respectively, A refers to the chemical affinity of the overall reaction, R signifies the gas constant and T denotes the temperature in K. Corresponding experiments performed in a batch-type reactor illustrate the consistency between dissolution rates generated in open and closed systems.

Dissolution rate of quartz in lead and sodium electrolyte solutions between 25 and 300°C: Effect of the nature of surface complexes and reaction affinity

The dissolution rate of quartz has been measured at 25°C in batch reactors and at 200 and 300°C in mixed flow reactors. These experiments have been carried out in both pure H2O and solutions containing Na or Pb at various ionic strengths and pH. The measured rates were found to increase significantly with the addition of either Na or Pb. In an attempt to determine the mechanism of these effects, the degree of adsorption of Na and Pb were measured on amorphous silica at 25 and 150°C. At 25°C, Na is found to adsorb on the quartz surface as an outer-sphere complex, and the corresponding dissolution rate increase is explained by an increase of the ionic strength. By contrast, at 25°C, lead, which forms inner-sphere complexes, increases the quartz dissolution rate specifically. At high temperature, quartz dissolution is promoted in the presence of both Na and Pb by a pH-dependent formation of surface inner-sphere complexes. This effect tends to vanish when the degree of saturation of the solution increases, as a result of the competition between electrolyte and aqueous silica adsorption on quartz surface. These results show that the electrolytes which adsorb as inner-sphere complexes dominate the overall reaction at conditions far from equilibrium only. Consequently, for a large range of chemical affinity quartz dissolution in Na and Pb electrolyte solutions can be modeled within the framework of the Transition State Theory by simply taking into account the protonated surface species and the ionic strength of the solution.

Dissolution kinetics of strained calcite

Abstract Interaface-limited dissolution of minerals occurs non-uniformly with preferential attack at sites of excess surface energy such as dislocations, edges, point defects, microfractures, etc. Strained crystals are predicted to show higher dissolution rates due to the increased internal energy associated with dislocations and due to enhanced nucleation of dissolution pits at dislocation outcrops on the surface. Using calcite strained to different degrees, we have observed a measurable rate enhancement of two to three times relative to unstrained crystals at temperatures from 3 to 80°C. This rate enhancement is large compared to that predicted from the calculated increase in crystal activity due to strain energy, but small compared to the three orders of magnitude difference in dislocation densities for the crystals tested (106–109 cm−2). Measurements over a range of pH (4.5–8.3) and temperature (3–80°C) showed that the rate enhancement increased with increasing pH and decreasing temperature. Calculations based on the excess free energy of screw dislocations suggest that dissolution rate enhancement should become significant above a critical defect density of roughly 107 cm−2, in apparent agreement with our observations. Crystal dissolution comprises several parallel processes operating in parallel at active sites. The small relative enhancement of dissolution rate with defect density reflects the greater quantity of dissolved material delivered to solution from receding edges and ledges relative to material coming from point defects and dislocations. Our data, coupled with existing information on other minerals, suggest that generally applicable kinetic measurements can be made on low-strain, macroscopic mineral specimens. However, kinetic data on highly strained minerals should include measurement of defect density because of the rate vs. strain correlation. Selective dissolution can be expected to occur in naturally-deformed rocks, where heterogeneity in dislocation distribution could cause solution transfer and deformation.