Forsterite Dissolution Research Articles

The experimental determination of hydromagnesite precipitation rates at 22.5–75ºC

Natural hydromagnesite (Mg5(CO3)4(OH)2·4H2O) dissolution and precipitation experiments were performed in closed-system reactors as a function of temperature from 22.5 to 75ºC and at 8.6 < pH < 10.7. The equilibrium constants for the reaction Mg5(CO3)4(OH)2·4H2O + 6H+ = 5Mg2+ + 4HCO3– + 6H2O were determined by bracketing the final fluid compositions obtained from the dissolution and precipitation experiments. The resulting constants were found to be 1033.7±0.9, 1030.5±0.5 and 1026.5±0.5 at 22.5, 50 and 75ºC, respectively. Whereas dissolution rates were too fast to be determined from the experiments, precipitation rates were slower and quantified. The resulting BET surface areanormalized hydromagnesite precipitation rates increase by a factor of ~2 with pH decreasing from 10.7 to 8.6. Measured rates are approximately two orders of magnitude faster than corresponding forsterite dissolution rates, suggesting that the overall rates of the low-temperature carbonation of olivine are controlled by the relatively sluggish dissolution of the magnesium silicate mineral.

Do organic ligands affect forsterite dissolution rates?

Far-from equilibrium, steady state forsterite dissolution rates were measured at pH ∼3 and 25°C in aqueous solutions containing 0.1m/kg NaCl and up to 0.1mol/kg of 13 distinct dissolved organic ligands in mixed-flow reactors. The organic ligands considered in this study include those common in Earth surface environments and those considered as potential catalysts for use in CO2 sequestration efforts: acetate, oxalate, citrate, EDTA4−, glutamate, gluconate, malonate, aspartate, tartrate, malate, alginate, salycilate and humate. The presence of up to 0.1mol/kg of each organic ligand altered forsterite dissolution rates less than 0.2 log units, which is the estimated uncertainty of the measured rates. Results obtained in this study, therefore, suggest that the presence of aqueous organic anions negligibly affects forsterite far-from equilibrium dissolution rates in most natural environments, and indicate that forsterite carbonation may not be appreciably accelerated by organic ligand catalysis.

Systematic review of forsterite dissolution rate data

This paper demonstrates a method for systematic analysis of published mineral dissolution rate data using forsterite dissolution as an example. The steps of the method are: (1) identify the data sources, (2) select the data, (3) tabulate the data, (4) analyze the data to produce a model, and (5) report the results. This method allows for a combination of critical selection of data, based on expert knowledge of theoretical expectations and experimental pitfalls, and meta-analysis of the data using statistical methods.Application of this method to all currently available forsterite dissolution rates (0<pH<14, and 0<T<150°C) normalized to geometric surface area produced the following rate equations:For pH<5.6 and 0°<T<150°C, based on 519 datalogrgeo=6.05(0.22)-0.46(0.02)pH-3683.0(63.6)1/T(R2=0.88)For pH>5.6 and 0°<T<150°C, based on 125 datalogrgeo=4.07(0.38)-0.256(0.023)pH-3465(139)1/T(R2=0.92)The R2 values show that ∼10% of the variance in r is not explained by variation in 1/T and pH. Although the experimental error for rate measurements should be±∼30%, the observed error associated with the logr values is ∼0.5logunits (±300% relative error). The unexplained variance and the large error associated with the reported rates likely arises from the assumption that the rates are directly proportional to the mineral surface area (geometric or BET) when the rate is actually controlled by the concentration and relative reactivity of surface sites, which may be a function of duration of reaction. Related to these surface area terms are other likely sources of error that include composition and preparation of mineral starting material.Similar rate equations were produced from BET surface area normalized rates. Comparison of rate models based on geometric and BET normalized rates offers no support for choosing one normalization method over the other. However, practical considerations support the use of geometric surface area normalization. Comparison of Mg and Si release rates showed that they produced statistically indistinguishable dissolution rates because dissolution was stoichiometric in the experiments over the entire pH range even though the surface concentrations of Mg and Si are known to change with pH. Comparison of rates from experiments with added carbonate, either from CO2 partial pressures greater than atmospheric or added carbonate salts, showed that the existing data set is not sufficient to quantify any effect of dissolved carbonate species on forsterite dissolution rates.

Forsterite Dissolution in Saline Water at Elevated Temperature and High CO2 Pressure

The rates and mechanisms of magnesium silicate dissolution can control the aqueous chemistry in ways that influence carbonate mineral precipitation during geologic carbon sequestration (GCS). A series of batch experiments was performed with forsterite (Mg(1.81)Fe(0.19)SiO(4)) powder to determine the effects of pressure (10-100 bar CO(2)), temperature (25-100 °C), and salinity (0-50,000 mg/L NaCl) on its dissolution rate at conditions relevant to GCS. Dissolution rates and products were determined by analysis of the aqueous phase, equilibrium and reaction path modeling, and solid phase characterization by scanning electron microscopy and X-ray diffraction. After an initially rapid dissolution period, the dissolution rate declined significantly, an effect that is attributed to the formation of a silica-rich layer at the forsterite surface. The initial dissolution rate increased with increasing temperature and increasing CO(2) pressure; the effect of CO(2) was through its influence on the pH. The dissolution rate was enhanced by NaCl, which may have been due to its inhibition of the formation of a silica-rich surface layer. The experimental results provide information about magnesium silicate dissolution at conditions that will be encountered during GCS that can be used to predict the fate of CO(2) and the evolution of subsurface geochemistry following CO(2) injection.

An experimental study of magnesite precipitation rates at neutral to alkaline conditions and 100–200 °C as a function of pH, aqueous solution composition and chemical affinity

Magnesite precipitation rates were measured at temperatures from 100 to 200°C as a function of saturation state and reactive fluid composition in mixed flow reactors. Measured rates were found to increase systematically with increasing saturation state but to decrease with increasing reactive fluid aqueous CO32- activity and pH. Measured rates are interpreted through a combination of surface complexation models and transition state theory. In accord with this formalism, constant saturation state BET surface area normalized magnesite precipitation rates (rMg) are a function of the concentration of protonated Mg sites at the surface (>MgOH2+) and can be described using: rMg=kMg-KCO3KOHKCO3KOH+KOHaCO32-+KCO3aOH-n1-ΩMgn where kMg- represents a rate constant, KOH and KCO3 stand for equilibrium constants, ai designates the activity of the subscripted aqueous species, n refers to a reaction order equal to 2, and ΩMg denotes the saturation state of the reactive solution with respect to magnesite. Retrieved values of n are consistent with magnesite precipitation control by a spiral growth mechanism. The temperature variation of the rate constant can be described using kMg-=Aaexp(-Ea/RT), where Aa represents a pre-exponential factor equal to 5.9×10−5mol/cm2/s, Ea designates an activation energy equal to 80.2kJ/mol, R denotes the gas constant, and T corresponds to the absolute temperature. Comparison of measured magnesite precipitation rates with corresponding forsterite dissolution rates suggest that the relatively slow rates of magnesite precipitation may be the rate limiting step in mineral carbonation efforts in ultramafic rocks.

Oxalate-promoted forsterite dissolution at low pH

We ran a series of 124 semi-batch reactor experiments to measure the dissolution rate of forsterite in solutions of nitric and oxalic acid solutions over a pH range of 0–7 and total oxalate concentrations between 0 and 0.35 m at 25 °C. We found that the empirical rate law for the dissolution of forsterite in these solutions is r = 10 - 7.03 a H + 0.46 + 10 - 5.44 m ox 2 - 0.40 a H + 0.47 where a H + is the activity of hydrogen ions, m ox is the concentration of oxalate in molal, and the rate is given in (mol/m 2 s). The first term of the rate law expresses the dissolution rate for solutions with no oxalate and the second term expresses the additional rate produced by the presence of oxalate ions. Our experiments show that oxalate-promoted dissolution rates depend upon both oxalate concentration and pH. Based on this, we propose a reaction mechanism in which a hydrogen ion and an oxalate ion are simultaneously present in the activated complex for the reaction that releases H 4SiO 4 into solution. By analogy, we propose that water acts as a ligand in the absence of oxalate, so the first term of our rate law has a H 2 O m (=1) as a hidden term. Thus, our results suggest that both a proton and a ligand (either an anion or a water molecule) must be present in the activated complex for forsterite dissolution, suggesting that proton-promoted and ligand-promoted dissolution are simply two aspects of the same process, and that the distinction between the two is artificial. The increased dissolution rate of forsterite produced by oxalate ions at a concentration of 0.001 m, a concentration comparable to the amount of organic acids in typical soils, translates into an approximately a 6-fold increase in forsterite dissolution rates at pH > 4.2. This suggests that organic acids have a measurable, but not profound, effect on chemical weathering.

Forsterite dissolution and magnesite precipitation at conditions relevant for deep saline aquifer storage and sequestration of carbon dioxide

The products of forsterite dissolution and the conditions favorable for magnesite precipitation have been investigated in experiments conducted at temperature and pressure conditions relevant to geologic carbon sequestration in deep saline aquifers. Although forsterite is not a common mineral in deep saline aquifers, the experiments offer insights into the effects of relevant temperatures and P CO 2 levels on silicate mineral dissolution and subsequent carbonate precipitation. Mineral suspensions and aqueous solutions were reacted at 30 °C and 95 °C in batch reactors, and at each temperature experiments were conducted with headspaces containing fixed P CO 2 values of 1 and 100 bar. Reaction products and progress were determined by elemental analysis of the dissolved phase, geochemical modeling, and analysis of the solid phase using scanning electron microscopy, infrared spectroscopy, and X-ray diffraction. The extent of forsterite dissolution increased with both increasing temperature and P CO 2 . The release of Mg and Si from forsterite was stoichiometric, but the Si concentration was ultimately controlled by the solubility of amorphous silica. During forsterite dissolution initiated in deionized water, the aqueous solution reached supersaturated conditions with respect to magnesite; however, magnesite precipitation was not observed for reaction times of nearly four weeks. Magnesite precipitation was observed in a series of experiments with initial solution compositions that simulated extensive forsterite dissolution. The precipitation of magnesite appears to be limited by the process of nucleation, and nucleation requires a critical saturation index between 0.25 and 1.14 at 95 °C and 100 bar P CO 2 . Magnesite precipitation is fastest in the presence of an initial magnesite seed. Although magnesite precipitates do form on the surfaces of forsterite particles, the presence of the forsterite surface does not significantly accelerate magnesite precipitation relative to solid-free systems.

An experimental study of forsterite dissolution rates as a function of temperature and aqueous Mg and Si concentrations

Steady state forsterite dissolution rates, at far from equilibrium conditions and pH=2±0.04, were measured as a function of aqueous magnesium and silica concentrations, and temperatures from 25°C to 65°C. All rates were measured in mixed flow reactors and exhibited stoichiometric dissolution. Measured rates are found to be independent of both aqueous magnesium and silica concentrations. The temperature dependence of the pH 2 forsterite dissolution rates obtained in this study is consistent with an Arrhenius equation of the form r=A A exp(−E A /RT) where r signifies the overall forsterite steady state dissolution rate, A A refers to a pre-exponential factor equal to 0.190 mol cm −2 s −1, E A designates an activation energy equal to 63.8 kJ/mol, R represents the gas constant, and T denotes absolute temperature. The observed variation of forsterite dissolution rates with aqueous composition is interpreted to originate from its dissolution mechanism. The forsterite structure consists of isolated silica tetrahedra that are branched together by magnesium octahedra chains. MgO bonds apparently break more rapidly than SiO bonds in this structure. The breaking of octahedra chain linking MgO bonds, which is apparently catalyzed by hydrogen ion adsorption at acidic conditions, leads directly to the destruction of the mineral. As the rate-controlling precursor complex for this mineral is formed by a hydrogen adsorption reaction, forsterite dissolution rates are unaffected by aqueous Mg and Si activities.

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

The forward dissolution rate of San Carlos forsterite Fo 91 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), ΣCO 2 (0 to 0.05 M), aqueous magnesium (10 −6 to 0.05 M) and silica (10 −6 to 0.001 M) concentrations. In CO 2-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 < [Mg 2+] tot < 0.04 M. Aqueous carbonate ions strongly inhibit dissolution in alkaline solutions when aCO 3 2− > 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 SiO 4 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/cm 2/s)=2.38×10 −11 {>Si 2O-H +}+1.62×10 −10 {>MgOH 2 +} where {> i} stands for surface species concentration (mol/m 2). 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 >MgOH 2 + 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 CO 2 consumption, i.e., unlike for feldspars, there is a negative feedback between pCO 2 and forsterite weathering rate. This should be taken into account when modeling the effect of mafic mineral weathering on CO 2 global balance.

A high resolution study of forsterite dissolution rates

The rates of dissolution of olivine (Fo 92) were measured over the pH range of 1.8 to 3.8 and from 25 to 45°C using an externally recycled mixed flow reactor. The data were filtered to remove several sources of systematic errors including non-steady state measurements, solution concentrations that were less than 50% above the detection limit, and Mg/Si rate ratios 40% (∼3σ or 98% confidence limit) greater or smaller than 1.84, which is the stoichiometric ratio of these elements in the mineral used. The effect of random errors on calculated values of E a and n were minimized by collecting a large amount of data. Twenty-eight different runs produced 772 rate measurements of which 284 were rejected in order to remove systematic and large random errors, leaving 488 data. The errors associated with these remaining data are normally distributed. The data were regressed to the equation log r- log A− E a 2.303R 1 T −n pH to find that log A = 0.54 ± 0.14, E a = 42.6 ± 0.8 kJ/mol, and n = 0.50 ± 0.004 (R 2 = 0.97). Based on these results, we postulate that the activated complex for forsterite dissolution involves a hydronium ion adsorbed to the forsterite surface in such a way that two of the three hydrogen atoms are associated with two Si-O-Mg bridging oxygen atoms. In this configuration, the adsorbed hydronium ion dissociates to form H-O-Si bonds as water molecules move from the solution to hydrate the two Mg atoms, breaking the Mg-O-Si bonds. This dissociation of the hydronium ion to form two H-O-Si groups on the forsterite surface leads to the observed reaction order for hydrogen ions of 0.5. The rate of breakdown of this activated complex is proportional to the rate at which the water molecules move from the solution to hydrate the Mg ions. Several other silicate mineral dissolution reactions, which have reaction orders of 0.5, may involve similar activated complexes.

Forsterite dissolution in superheated basaltic, andesitic and rhyolitic melts

AbstractDissolution rates of small forsterite spheres in superheated melts of basalt, andesite and rhyolite composition have been measured at 1300°C, atmospheric pressure. The rate is constant (83 µm hr−1) in the basalt, regardless of run duration. In the andesite the initial dissolution rate is 200µm hr−1, followed by a decrease to a constant value of 16µmhr−1 in 2–3 hours. Dissolution rate in the rhyolite decreases from an initial value of 1.7 to &lt;0.1 µmhr−1 over 280 hours and never reaches a constant rate. Once the rate of dissolution has become constant, the film of contaminated melt that forms in melt about a crystal does not thicken with time, indicating attainment of a steady-state condition. Steady state is attributed to natural convection arising from the difference in density between the film of contaminated melt surrounding a crystal and that beyond. The density difference is approximately 2% of the density of the rock melt.

Behavior of Li, Rb and Cs during basalt glass and olivine dissolution and chlorite, smectite and zeolite precipitation from seawater: Experimental investigations and modelization between 50° and 300°C

This study investigates experimentally the behavior of Li, Rb and Cs during the hydrothermal alteration of a basalt glass and of forsterite. The behavior of trace alkalis during the dissolution of a basalt glass and forsterite has been followed between 150° and 300°C: Rb and Cs are preferentially leached from the glass while Li exhibits a stoichiometric release with respect to silica. On the other hand, incongruent loss of Li relative to Si during forsterite dissolution reflects a crystallographic (site energy) control of the release of trace elements from crystalline silicates. The partitioning of Li, Rb and Cs between hydrothermal solutions and chlorites, smectites and zeolites has been experimentally investigated between 50° and 260°C. The alteration phases were synthesized by altering silicate glasses in solutions containing the trace alkalis. The lower the temperature, the higher the exchange capacity of the mineral or the lower the ionic hydration energy, then the more the element is incorporated into the solid phases. Computer simulations generated with the EQ3/6 software package and our experimental data on distribution coefficients account for the behavior of alkali trace elements during the alteration of basalt glasses by seawater at mid-ocean ridges.

Changes in surface area and morphology and the mechanism of forsterite dissolution

Scanning electron microscope (SEM) photographs of olivine grains shown that dissolution of olivine may occur more rapidly on some surfaces than on others and that initial dissolution of freshly crushed grains occurs primarily along lattice imperfections such as dislocations or cleavage planes. The SEM photographs generally do not show the presence of thick or continuous residual or precipitated layers which might render the dissolution reaction diffusion controlled. The specific surface area of olivine grains increased greatly during initial dissolution due to formation of etch features. However, despite the increased surface area the rate of dissolution decreased during the experiments. These observations suggest that some assumptions underlying derivation of the diffusion controlled models are invalid for olivine dissolution and suggest that dissolution is controlled by rates of surface reactions.