Olivine Dissolution Rates Research Articles

On progress and rate of the peritectic reaction Fo + SiO2 → En in natural andesitic arc magmas

Using a high resolution ion microprobe with SCAPS imaging, the peritectic reaction of forsterite+silica to enstatite was studied down to submicron level in a natural andesitic tephra from the Central Plateau of North Island, New Zealand. The fayalitic component of natural olivines is stable in high-silica melts, and therefore the reaction is in fact a two-step progress: 1. Dissolution of Mg-rich olivine, rate-limited by Fe–Mg interdiffusion at the crystal rim, results in enrichment of Fe in the crystal rim and of Mg in the c. 1μm wide melt boundary layer around the crystal. 2. Magnesian pyroxenes preferentially but not exclusively nucleate in the melt boundary layer and grow; as soon as these microlites touch the rim of the dissolving olivine, they shield the crystals from the silica-rich melt, thereby preventing further olivine dissolution. At this point, Fe–Mg interdiffusion begins to destroy the Fe-enrichment of the olivine rim. The reaction is completed when the dissolving olivine crystal is completely mantled by magnesian pyroxene microlites. Thick pyroxene mantles are likely the result of pyroxene overgrowth rather than due to peritectic transformation. The morphology of the olivine rim preserves information about the reaction history of the grain. Modeling of Fe–Mg interdiffusion in the olivine rim following its shielding from the melt by pyroxene overgrowth may yield the rates of olivine dissolution and the rates of pyroxene growth if temperature is known. For the tephra we have studied, microlite thermometry yields a temperature of 1137 (±41)°C, indicating an olivine dissolution rate of 3–6×10−11ms−1 and an initially volumetric pyroxene growth rate of 2.2–5.6×10−21m3s−1. This points to timescales between olivine crystal uptake into the SiO2-rich melt and explosive eruption at the surface of a few hours to at most a day.

The dissolution of olivine added to soil: Implications for enhanced weathering

Chemical weathering of silicate minerals consumes atmospheric CO2 and is a fundamental component of geochemical cycles and of the climate system on long timescales. Artificial acceleration of such weathering (“enhanced weathering”) has recently been proposed as a method of mitigating anthropogenic climate change, by adding fine-grained silicate materials to continental surfaces. The efficacy of such intervention in the carbon cycle strongly depends on the mineral dissolution rates that occur, but these rates remain uncertain. Dissolution rates determined from catchment scale investigations are generally several orders of magnitude slower than those predicted from kinetic information derived from laboratory studies. Here we present results from laboratory flow-through dissolution experiments which seek to bridge this observational discrepancy by using columns of soil returned to the laboratory from a field site. We constrain the dissolution rate of olivine added to the top of one of these columns, while maintaining much of the complexity inherent in the soil environment. Continual addition of water to the top of the soil columns, and analysis of elemental composition of waters exiting at the base was conducted for a period of five months, and the solid and leachable composition of the soils was also assessed before and after the experiments. Chemical results indicate clear release of Mg2+ from the dissolution of olivine and, by comparison with a control case, allow the rate of olivine dissolution to be estimated between 10−16.4 and 10−15.5 moles(Mg)cm−2s−1. Measurements also allow secondary mineral formation in the soil to be assessed, and suggest that no significant secondary uptake of Mg2+ has occurred. The olivine dissolution rates are intermediate between those of pure laboratory and field studies and provide a useful constraint on weathering processes in natural environments, such as during soil profile deepening or the addition of mineral dust or volcanic ash to soils surfaces. The dissolution rates also provide critical information for the assessment of enhanced weathering including the expected surface-area and energy requirements.

Olivine dissolution and carbonation under conditions relevant for in situ carbon storage

In order to evaluate the chemistry and kinetics of mineral carbonation reactions under conditions relevant to subsurface injection and storage of CO2, olivine alteration was studied at 60°C and 100bar CO2 pressure, including olivine dissolution and the formation of carbonate minerals. Batch experiments were performed with olivine (Fo92), water, CO2, and NaCl inside gold cells contained within rocking autoclaves. Two reproducible experiments yielded an initial (1h) dissolution rate of 9.50±0.10×10−11 and a long-term (10–70days) rate of 1.69±0.23×10−12molcm−2s−1. The long-term rate is consistent with previously published rate laws at 4.5<pH<5.5. The dissolution rates presented here are constant with increasing pH in the same range, suggesting a pH-independent dissolution mechanism at elevated CO2(aq) and SiO2(aq) at 60°C. A Si-rich surface layer forms on olivine grains within 2days of reaction and appears to slow dissolution by passivating the surface. The olivine dissolution rate decreases by 2 orders of magnitude over 4days as the system approaches amorphous silica saturation but remains constant thereafter based on a linear increase in Mg concentrations. Secondary phases consist of amorphous silica and magnesite, with up to 7mol% olivine converted to Mg-carbonate over 94days of reaction. Magnesite precipitation rates could not be precisely quantified due to experimental limitations. However, our minimum estimate of 1.40×10−13molcm−2s−1 suggests that the precipitation rate is several orders of magnitude greater than predicted by previous studies. Finally, the presence of 0.5M NaCl resulted in a decrease in olivine dissolution rate at reaction times of less than 4days, but a significant enhancement of the reaction rate at reaction times greater than 4days relative to electrolyte-free experiments. Our results suggest that geochemical models developed to predict the behavior of subsurface CO2 storage systems in mafic and ultramafic rocks should incorporate the effects of dissolved species, including SiO2 and NaCl.

Quantifying the kinetics of olivine dissolution in partially closed and closed batch reactor systems

This study quantified the rates of olivine dissolution under constant air injection, in partially closed batch reactors and in control closed batch reactors (in the absence of air flow), in the presence of 4g of olivine, and 0, 0.001 and 0.01M NaNO3. Experiments were performed for 40,440 and 36,048min in partially closed and closed reactors respectively. Under constant air injection, the solution pH ranged from 6.8 and 7.3 for all conditions at the start of the experiments, to 7.5 and 7.8 at 40,440min. In the absence of air flow, an alkaline pH of 9.6 to 9.3 was observed for 36,048min. This pH difference is attributed to the constant buffering the solution through the formation of dissolved carbonate species (H2CO3/HCO3−). The alkaline pH, measured in the presence of olivine, implied a mechanism of pH control caused by the presence of Si(OH)4 species from the mineral surface. This effect was suggested by a drop to near neutral pH of the same aliquots, upon filtration of the olivine through a N2 degassed filter. A decrease in total alkalinity was measured for experiments under constant air injection, as a function of increasing NaNO3, suggesting a mechanism of Na+/CO32− interaction is involved in the enhancement of olivine dissolution rates through proton/Mg exchange. In the absence of air flow, the total alkalinity reached a maximum of 0.25mM, compared to 0.9mM with air flow at 0.01M NaNO3. Magnesium concentration increased in the presence of NaNO3 and permanently dissolving CO2 in partially closed batch reactors. This strongly suggests a key role of Na+ (coupled to CO2 dissolution) for enhancing olivine dissolution. Mg/Si ratios for all conditions showed incongruent olivine dissolution, suggesting a combination of two dissolution mechanisms at alkaline and near neutral pH values, in closed and partially closed batch reactors. The rate of olivine dissolution (ROL) was calculated as a function of mineral surface area and forsterite equilibrium solution parameters. In the presence of constant air injection, highest log ROL values range from −11 to −10.7 at the acidic to near neutral conditions (pH=6.8 to 7.2). Slower dissolution rates, with log ROL values of −11.5 to −11.4 were determined for experiments in the absence of air flow at alkaline pH values (9.6 to 9.8). Constant air injection (i.e. constant dissolution of CO2 at atmospheric pressure) in the presence of Na+ has proven effective for enhancing olivine dissolution.

Microbial Acceleration of Olivine Dissolution via Siderophore Production

Silicate minerals represent an important reservoir of essential nutrients at Earth's surface. Due to the slow kinetics of primary silicate mineral dissolution and the potential for nutrient sequestration by secondary mineral precipitation, the bioavailability of many silicate-bound nutrients may be limited by the ability of microorganisms to actively scavenge these nutrients via organic ligand production. In this study, the effectiveness of ligand production as a means to scavenge Fe from Fe-silicates is addressed through targeted laboratory experiments using olivine as a model mineral.Preliminary results show that microbial Fe-binding ligands (i.e. siderophores) can accelerate olivine dissolution rates stoichiometrically by almost an order of magnitude in experiments buffered at circumneutral pH. In addition to higher reaction rates, organic Fe-binding ligands fostered the accumulation of dissolved Fe in solution, which was below detection in the abiotic experiments due to the precipitation of secondary Fe minerals in the presence of O2. Accelerated olivine dissolution rates in the presence of microbial Fe-binding ligands is somewhat unexpected because these ligands are known to be highly selective towards Fe3+ whereas olivine dominantly contains Fe2+. Spectrophotometric analysis of the ligand complexes produced during reaction with olivine reveals the dominance for Fe3+ -ligand complexes in solution.

The deleterious effect of secondary phases on olivine carbonation yield: Insight from time-resolved aqueous-fluid sampling and FIB-TEM characterization

Geological storage of CO2 in mafic and ultramafic rocks relies on the dissolution of their silicate components, followed by the precipitation of carbonates, the overall process being commonly referred to as carbonation. To gain a better understanding of the rate- and yield-controlling factors of mineral carbonation, three batch experiments were conducted in CO2-saturated water, at (a) 90°C, (b) 120°C and (c) 170°C, respectively (with citrate ligands added to the 90°C experiment) in a Ti-reactor at pCO2=280bar, using San Carlos olivine (Fo88) as a model mineral of (ultra)mafic environments. Those physicochemical conditions were purposely chosen so as to promote the dissolution rate of olivine. (a) At 90°C, ~40wt.% of olivine were dissolved within 3weeks, a result which suggests that the passivation barrier evidenced in previous studies carried out in citrate-free media had been overcome. However, while saturation with respect to magnesite was overstepped in this experiment, the carbonation yield remained below ~1wt.%, which we attributed to the slow kinetics of magnesite precipitation. (b) At 120°C, the concentration of Mg and Si monitored by aqueous fluid sampling reached an apparent plateau for over 4weeks, at conditions close to saturation with respect to amorphous silica, and with Fe concentration being below the detection limit. A resumption of the dissolution process was subsequently observed while the concentration of Fe suddenly increased in solution. The concentration plateau was attributed either to the formation of a protective Si–Fe(III) layer, or to the occurrence of Fe(III) phases clogging the porosity of a secondary silica layer. The resumption of olivine dissolution was ascribed to the increasingly reducing conditions that led to the breakdown of the protective phases. This late resumption led to a carbonation yield slightly above 2wt.%. (c) At 170°C, Fe was incorporated into a presumably permeable Fe-rich phyllosilicate layer, so that the dissolution process was likely less affected by such an interfacial layer. Nevertheless, carbonate minerals still formed in low quantity (~6wt.%), while other amorphous phases represented the main sink for Mg cations. Overall, this study emphasizes that achieving carbonation reactions in reducing environments and circum-neutral aqueous solutions may represent a necessary requirement for making olivine carbonation a viable process in these temperature ranges. It also insists on the importance of transitory phases, which form both at the fluid-silicate interface and from the bulk solution. While they may not appear in overall carbonation equations or in thermodynamic databases, arguably they can drive the global kinetics of olivine dissolution and its transformation towards magnesite.

Peridotite dissolution and carbonation rates at fracture surfaces under conditions relevant for in situ mineralization of CO2

Whereas the dissolution of pure single phases (e.g. olivine and other mafic minerals) has been the focus of many studies, no investigation has been reported on the progress of reactions at and within polymineralic, transgranular fracture surfaces cutting peridotites. We document experiments that address the evolution of dissolving peridotite surfaces, and the rates of dissolution and carbonation reactions that occur at these surfaces, under both open- and closed-system conditions relevant for in situ CO2-sequestration. The results of experiments, conducted under quasi-open system conditions, on solid samples of peridotite whose surfaces were taken as an analogue of free fracture surfaces, demonstrate apparent rates of olivine dissolution at the free surfaces that are equivalent to or upwards of 100 times greater than those determined for pure olivine under similar conditions. This increase in apparent olivine dissolution rate is ascribed to fluid penetration along grain boundaries and veins, which resulted in increased accessibility of reactive olivine surface area. Apparent dissolution rates under closed system conditions, where serial olivine dissolution and magnesite precipitation take place, are ∼1 order of magnitude slower owing to changes in fluid composition and pH. In both of these reaction environments, it is apparent that serpentinized veins and mineral grain boundaries within the peridotite allowed fluids to penetrate the rock and to promote the release of divalent metals and silica from within the sample or simulated fracture walls. These results indicate that the microstructure of free (fracture) surfaces exerts a dominant control on peridotite dissolution and carbonation rates, relative to the proportion of highly reactive minerals that compose such ultramafic rock. The rapid dissolution we observe under open system conditions implies that CO2-mineralization may be viable, in open systems, if the composition of injected fluids can be maintained at or near the initial composition. Fracture clogging that results from precipitation and fluid phase pH changes in closed systems, however, can drastically limit peridotite carbonation.

Dissolution rates of alkaline rocks by carbonic acid: Influence of solid/liquid ratio, temperature, and CO2 pressure

Dissolution rates of alkaline rocks, including wollastonite (CaSiO3), olivine (Mg2SiO4), and phlogopite (KMg3AlSi3O10(OH)2), with high pressure aqueous CO2 solution were measured to examine the feasibility of CO2 fixation via carbonation. Influence of solid/liquid ratio (1.0–10g/250mL), temperature (303–353K), and CO2 pressure (1.0–3.0MPa) on the extraction rates of calcium or magnesium ions was investigated. Under the experimental conditions studied, the calcium ion extraction rate from wollastonite was the highest among the three rock samples studied. The calcium concentration reached about 120mg/L, and about 12% of the calcium in wollastonite sample was extracted after 60min at 353K with 1.0MPa CO2. The calcium and magnesium extraction ratios from the alkaline rocks were much lower than those from waste concrete powder. Increasing the extraction time and temperature would be an effective way to promote calcium extraction from wollastonite.

Relationships between Fe redistribution and Po2 during mineral dissolution under low O2 conditions

Iron-bearing olivine was dissolved at different partial pressures of atmospheric oxygen (Po2) in a glove box to clarify the relationships between Fe redistribution and Po2 during mineral dissolution and to provide a deeper understanding of weathering under low oxygen conditions. The olivine was first dissolved at pH 4.5–6.0 and 15–55°C under the ambient air using a modified flow-through system. Some portion of Fe2+ released from olivine was oxidized and precipitated as Fe3+ oxides/hydroxides, and remained in the system, while the other portion flowed out of the system as Fe2+. Then, a kinetic model, considering mineral dissolution, water flow and Fe2+ oxidation rates, was made to estimate Fe behavior at different pHs, temperatures and dissolved O2 concentrations during dissolution. The kinetic model was applied to the experimental results under the ambient air; the validity of the model was confirmed by good agreement between the observed and calculated Ф values where Ф is a ratio of the amount of Fe3+ precipitated to the total amount of Fe2+ actually dissolved from olivine, an indicator of the oxidation state of a mineral dissolution system. Then, by using the kinetic model, we analyzed the results of the olivine dissolution experiments at 0.20 atm to 8.7×10–5atm of Po2 and at constant pH and temperature. While the dissolution rates of olivine were not affected significantly by Po2, the olivine dissolution has indicated that Ф decreases with decrease in Po2 and the Ф variation at low Po2 cannot be explained by the classic rate law of Fe2+ oxidation. The calculations of Ф based on the above kinetic model have revealed that the observed Ф variation is well explained by the decrease in the power of [O2] (x) with decrease in [O2] for the Fe2+ oxidation rate, –d[Fe2+]/dt=k[Fe2+][O2]x[OH–]y (k: the rate constant, 0⩽x⩽1, and y∼2); the x value decreased from 1.0 to 0.76 at 0.20 to 8.7×10–5atm of Po2, respectively. The Φ−Po2 relationships imply that the Φ value in a paleosol (fossil weathering profile) increases with increase in Po2 in the Paleoproterozoic.

The Potential for Low-Temperature Abiotic Hydrogen Generation and a Hydrogen-Driven Deep Biosphere

The release and oxidation of ferrous iron during aqueous alteration of the mineral olivine is known to reduce aqueous solutions to such extent that molecular hydrogen, H2, forms. H2 is an efficient energy carrier and is considered basal to the deep subsurface biosphere. Knowledge of the potential for H2 generation is therefore vital to understanding the deep biosphere on Earth and on extraterrestrial bodies. Here, we provide a review of factors that may reduce the potential for H2 generation with a focus on systems in the core temperature region for thermophilic to hyperthermophilic microbial life. We show that aqueous sulfate may inhibit the formation of H2, whereas redox-sensitive compounds of carbon and nitrogen are unlikely to have significant effect at low temperatures. In addition, we suggest that the rate of H2 generation is proportional to the dissolution rate of olivine and, hence, limited by factors such as reactive surface areas and the access of water to fresh surfaces. We furthermore suggest that the availability of water and pore/fracture space are the most important factors that limit the generation of H2. Our study implies that, because of large heat flows, abundant olivine-bearing rocks, large thermodynamic gradients, and reduced atmospheres, young Earth and Mars probably offered abundant systems where microbial life could possibly have emerged.

An experimental study of crystalline basalt dissolution from 2 ⩽ pH ⩽ 11 and temperatures from 5 to 75 °C

Steady-state element release rates from crystalline basalt dissolution at far-from-equilibrium were measured at pH from 2 to 11 and temperatures from 5 to 75 °C in mixed-flow reactors. Steady-state Si and Ca release rates exhibit a U-shaped variation with pH where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions. Silicon release rates from crystalline basalt are comparable to Si release rates from basaltic glass of the same chemical composition at low pH and temperatures ⩾25 °C but slower at alkaline pH and temperatures ⩾50 °C. In contrast, Mg and Fe release rates decrease continuously with increasing pH at all temperatures. This behaviour is interpreted to stem from the contrasting dissolution behaviours of the three major minerals comprising the basalt: plagioclase, pyroxene, and olivine. Calcium is primarily present in plagioclase, which exhibits a U-shaped dissolution rate dependence on pH. In contrast, Mg and Fe are contained in pyroxene and olivine, minerals whose dissolution rates decrease monotonically with pH. As a result, crystalline basalt preferentially releases Mg and Fe relative to Ca at acidic conditions. The injection of acidic CO 2-charged fluids into crystalline basaltic terrain may, therefore, favour the formation of Mg and Fe carbonates rather than calcite. Element release rates estimated from the sum of the volume fraction normalized dissolution rates of plagioclase, pyroxene, and olivine are within one order of magnitude of those measured in this study.

Reply to Schuiling et al.: Different processes at work

Schuiling et al. (1) question our conclusion (2) that the annual dissolution rate of olivine is limited by the saturation of waters with silicic acid (H4SiO4), which is one product of the dissolution reaction of olivine. In support of this point they discuss findings of CO2 sequestration in a mine in Yukon, Canada, claiming that a minimum of 1,700 g C m−2 y−1 was sequestered between 1978 and 2004 by silicate weathering and precipitation of (mainly) magnesium carbonates (3). This value is approximately …

Influence of amorphous silica layer formation on the dissolution rate of olivine at 90 °C and elevated pCO 2

For mitigating against rising levels of atmospheric CO 2, carbonation of M 2+-bearing silicates has been proposed as a possible option for sequestering CO 2 over long time spans. Due to its rapid far-from-equilibrium dissolution rate and its widespread occurrence in mafic and ultramafic rocks, olivine has been suggested as a potentially good candidate for achieving this goal, although the efficacy of the carbonation reaction still needs to be assessed. With this as a goal, the present study aims at measuring the carbonation rate of San Carlos olivine in batch experiments at 90 °C and pCO 2 of 20 and 25 MPa. When the reaction was initiated in pure water, the kinetics of olivine dissolution was controlled by the degree of saturation of the bulk solution with respect to amorphous silica. This yet unrecognized effect for olivine was responsible for a decrease of the dissolution rate by over two orders of magnitude. In long-term (45 days) carbonation experiments with a high surface area to solution volume ratio ( SA/ V = 24,600 m −1), the final composition of the solution was close to equilibrium with respect to SiO 2(am), independent of the initial concentration of dissolved salts (NaCl and NaClO 4, ranging between 0 and 1 m), and with an aqueous Mg/Si ratio close to that of olivine. No secondary phase other than a ubiquitous thin (≤ 40 nm), Si-rich amorphous layer was observed. These results are at odds with classic kinetic modeling of the process. Due to experimental uncertainties, it was not possible to determine precisely the dissolution rate of olivine after 45 days, but the long term alteration of olivine was indirectly estimated to be at least 4 orders of magnitude slower than predicted. Taken together, these results suggest that the formation of amorphous silica layers plays an important role in controlling the rate of olivine dissolution by passivating the surface of olivine, an effect which has yet to be quantified and incorporated into standard reactive-transport codes.

Importance of dissolution and precipitation kinetics for mineral carbonation

Studies of olivine carbonation are relevant for ex-situ CO2 storage, and the main results can be transferrable to geological storage of CO2. The total reaction rate of mineral and CO2 is always dependent on the slowest partial reaction, and with olivine the obtained conversion values were not correlated to dissolution rates. These direct carbonation results indicate that the precipitation rate of magnesite and not the dissolution rate of olivine was rate limiting for the reaction between olivine, CO2 and H2O at 185 °C and 115 bar. In industrial carbonation scenarios where the dissolution rate is not limiting, process optimization has to focus on the maximizing the precipitation kinetics. In industrial processes were precipitation is rate limiting, a conventional ball mill is probably the best alternative due to the low energy consumption, while in processes where the dissolution is rate limiting, more energy intensive pre-treatment methods may have potential. In the context of geological CO2 storage, the precipitation rate of carbonate will influence CO2 mineral trapping, and it is possible that precipitation kinetics is not equal to dissolution kinetics. Injection of CO2 into reservoir will lower the pH and thereby dissolve several minerals. The balance between dissolution and precipitation is dependent on the kinetics and solubility of the present minerals and possible products, where silica and carbonates probably are the most important. Mineral precipitation is unwanted in the storage formation due to lowering of porosity and possibly the permeability, but with time carbonate precipitation is positive due to mineral trapping of CO2.

Investigating dissolution of mechanically activated olivine for carbonation purposes

Mineral carbonation is one of several alternatives for CO2 sequestration and storage. The reaction rates of appropriate minerals with CO2, for instance olivine and serpentine with vast resources, are relatively slow in a CO2 sequestration context and the rates have to be increased to make mineral carbonation a good storage alternative. Increasing the dissolution rate of olivine has been the focus of this paper. Olivine was milled with very high energy intensity using a laboratory planetary mill to investigate the effect of mechanical activation on the Mg extraction potential of olivine in 0.01 M HCl solution at room temperature and pressure. Approximately 30–40% of each sample was dissolved and water samples were taken at the end of each experiment. The pH change was used to calculate time series of the Mg concentrations, which also were compared to the final Mg concentrations in the water samples. Percentage dissolved and the specific reaction rates were estimated from the Mg concentration time series. The measured particle size distributions could not explain the rate constants found, but the specific surface area gave a good trend versus dissolution for samples milled wet and the samples milled with a small addition of water. The samples milled dry had the lowest measured specific surface areas (<4 m2/g), but had the highest rate constants. The crystallinity calculated from X-ray diffractograms, was the material parameter with the best fit for the observed differences in the rate constants. Geochemical modelling of mechanically activated materials indicated that factors describing the changes in the material properties related to the activation must be included. The mechanically activated samples in general reacted faster than predicted by the theoretical models. Mechanical activation as a pre-treatment method was found to enhance the initial specific reaction rates by approximately three orders of magnitude for a sample milled dry for 60 min in a planetary mono mill compared to an unactivated sample. Wet milling in the planetary mill did not produce samples with the same maximum reaction rate as dry milling, but wet milling in general might be easier to implement into a wet carbonation process. Mechanical activation in a planetary mill is likely to consume too much energy for CO2 sequestration purposes, but the increase in obtained olivine rate constants illustrates a potential for using milling as a pre-treatment method.

The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt

This study presents lithium (Li) and magnesium (Mg) isotope data from experiments designed to assess the effects of dissolution of primary phases and the formation of secondary minerals during the weathering of basalt. Basalt glass and olivine dissolution experiments were performed in mixed through-flow reactors under controlled equilibrium conditions, at low pH (2–4) in order to keep solutions undersaturated (i.e. far-from equilibrium) and inhibit the formation of secondary minerals. Combined dissolution–precipitation experiments were performed at high pH (10 and 11) increasing the saturation state of the solutions (moving the system closer to equilibrium) and thereby promoting the formation of secondary minerals. At conditions far from equilibrium saturation state modelling and solution stoichiometry suggest that little secondary mineral formation has occurred. This is supported by the similarity of the dissolution rates of basalt glass and olivine obtained here compared to those of previous experiments. The δ 7Li isotope composition of the experimental solution is indistinguishable from that of the initial basalt glass or olivine indicating that little fractionation has occurred. In contrast, the same experimental solutions have light Mg isotope compositions relative to the primary phases, and the solution becomes progressively lighter with time. In the absence of any evidence for secondary mineral formation the most likely explanation for these light Mg isotope compositions is that there has been preferential loss of light Mg during primary phase dissolution. For the experiments undertaken at close to equilibrium conditions the results of saturation state modelling and changes in solution chemistry suggest that secondary mineral formation has occurred. X-ray diffraction (XRD) measurements of the reacted mineral products from these experiments confirm that the principal secondary phase that has formed is chrysotile. Lithium isotope ratios of the experimental fluid become increasingly heavy with time, consistent with previous experimental work and natural data indicating that 6Li is preferentially incorporated into secondary minerals, leaving the solution enriched in 7Li. The behaviour of Mg isotopes is different from that anticipated or observed in natural systems. Similar to the far from equilibrium experiments initially light Mg is lost during olivine dissolution, but with time the δ 26Mg value of the solution becomes increasingly heavy. This suggests either preferential loss of light, and then heavy Mg from olivine, or that the secondary phase preferentially incorporates light Mg from solution. Assuming that the secondary phase is chrysotile, a Mg-silicate, the sense of Mg fractionation is opposite to that previously associated with silicate soils and implies that the fractionation of Mg isotopes during silicate precipitation may be mineral specific. If secondary silicates do preferentially remove light Mg from solution then this could be a possible mechanism for the relatively heavy δ 26Mg value of seawater. This study highlights the utility of experimental studies to quantify the effects of natural weathering reactions on the Li and Mg geochemical cycles.

Coastal spreading of olivine to control atmospheric CO 2 concentrations: A critical analysis of viability

Qualitative proposals to control atmospheric CO 2 concentrations by spreading crushed olivine rock along the Earth's coastlines, thereby accelerating weathering reactions, are presently attracting considerable attention. This paper provides a critical evaluation of the concept, demonstrating quantitatively whether or not it can contribute significantly to CO 2 sequestration. The feasibility of the concept depends on the rate of olivine dissolution, the sequestration capacity of the dominant reaction, and its CO 2 footprint. Kinetics calculations show that offsetting 30% of worldwide 1990 CO 2 emissions by beach weathering means distributing of 5.0 Gt of olivine per year. For mean seawater temperatures of 15–25 °C, olivine sand (300 μm grain size) takes 700–2100 years to reach the necessary steady state sequestration rate and is therefore of little practical value. To obtain useful, steady state CO 2 uptake rates within 15–20 years requires grain sizes <10 μm. However, the preparation and movement of the required material poses major economic, infrastructural and public health questions. We conclude that coastal spreading of olivine is not a viable method of CO 2 sequestration on the scale needed. The method certainly cannot replace CCS technologies as a means of controlling atmospheric CO 2 concentrations.

Olivine dissolution in basaltic melt

The main purpose of this work is to understand and quantify diffusive and convective olivine dissolution in basaltic melt. Crystal dissolution and growth in a magma chamber is often accompanied by the descent or ascent of the crystal in the chamber due to gravity. The motion induces convection that enhances mass transport. Such convective dissolution and growth rates have not been quantified before. MgO diffusivity in the melt ( D MgO), MgO concentration of the interface melt ( C 0) and the effective thickness of the compositional boundary layer (δ) are necessary parameters to model the convective dissolution. Experiments of non-convective olivine dissolution in a basaltic melt were conducted at 1271–1480 °C and 0.47–1.42 GPa in a piston–cylinder apparatus. At specific temperature and pressure conditions, multiple experiments of different durations show that the interface melt reaches near-saturation within 2 min. Therefore, diffusion, not interface reaction, is the rate-controlling step for non-convective olivine dissolution in basaltic melt. The compositional profile length and olivine dissolution distance are proportional to the square root of experimental duration, consistent with diffusive dissolution. D MgO and C 0 are obtained from the experimental results. D MgO displays Arrhenian dependence on temperature, but the pressure dependence is small and not resolved. C 0 increases with increasing temperature and decreases with increasing pressure. Comparison with literature data shows that D MgO depends strongly on the initial melt composition, while C 0 does not. δ is estimated from fluid dynamics. D MgO/δ, which characterizes the kinetic and dynamic aspects of convective crystal dissolution, is parameterized as a function of temperature, pressure, and olivine composition. Convective olivine dissolution rate in basaltic melt can be conveniently calculated from the model results. Application to convective crystal growth and xenolith digestion is discussed.

Predicting the neutralisation of acid mine drainage in anoxic olivine drains

The precipitation of gypsum within anoxic limestone drains subjected to very acidic and sulphate rich acid mine drainage is a problem that can be prevented by initial neutralisation of the drainage using magnesium-rich olivine. The applicability of this concept is limited to the low pH region (i.e. pH < 3) where gypsum precipitation is a potential problem and the olivine dissolution rate is sufficiently fast to be of practical importance. This paper presents a simple finite element model for predicting the performance of olivine drains with respect to the neutralisation of highly acidic solutions under anoxic conditions and low pH. The model is based on the kinetic rate expression for olivine dissolution and the sulphuric acid dissociation equilibrium. To test the model, a series of column experiments were conducted in which olivine sand (0.1–1 mm) was subjected to different solutions of dilute sulphuric acid (0.0025–0.01 M) that was pumped through the column at various flow rates. For practical purposes the synthetic solutions contained no iron. The steady-state effluent pH obtained from each experimental run was used to calculate the relative amount of neutralisation. The neutralisation process, as expressed by the relative neutralisation values, was adequately described by the simple prediction model.

Validation of a population balance model for olivine dissolution

The dissolution of silicate minerals, among them olivine, in water enables its subsequent reaction with carbon dioxide to form magnesium carbonate, a process called aqueous mineral carbonation. A general model for the dissolution of olivine, based on a population balance approach, has been developed. For this purpose, the dissolution rate of olivine has been measured as a function of varying particle size and pH at 120 ∘ C . Three separate particle populations in three different size ranges were used: a sub 90, a 90–180, and a 180– 355 μ m size fraction. The pH was varied between 2 and 4.75 using HCl. Experiments were carried out in a flow-through reactor under a nitrogen atmosphere of 20 bar. The dissolution extent varied from 12% up to complete dissolution, depending on experimental conditions. Particle size distributions of the different size fractions were measured with a Coulter Multisizer ® . Using the assumption of a surface controlled reaction, the solution to the population balance equation was coupled with a reactor model. Data were fitted to the model to obtain a shape modulated dissolution rate, k v 1 / 3 D , combining the volume shape factor k v with the dissolution rate D, which has the dimensions of a velocity. Including earlier published dissolution experiments an overall correlation for the dissolution rate was regressed. Using the general model, the limitations of the simplified model employed in an earlier publication are illustrated.