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1.
Carbonation of partially serpentinized and weathered peridotites was studied experimentally under hydrothermal conditions (T: 200 °C, PCO2: 130-180 bars). Experiments were performed in a closed system using whole rock drill core samples (height: 1 cm, diameter: 1 cm) as starting material. The initial samples were composed mainly of meshwork serpentine, relicts of primary olivine and an olivine weathering product (deweylite assemblage). Two types of solutions, each with a total salt content corresponding to that of average seawater (35 g/L dissolved salts), were used: (1) a Na-Ca-Cl solution (12.5 g/L CaCl2 + 22.5 g/L NaCl) and (2) a NaCl solution (35 g/L NaCl). After 15-25 days of experimental treatment, the samples were partly covered with carbonates. In addition, noticeable carbonation reactions had occurred below the sample surfaces within zones with thicknesses up to 250 μm. In the Na-Ca-Cl solution, both the olivine relicts and the deweylite assemblage were partly replaced by calcite along the surrounding serpentine veins. However, the extent of calcitization was found to be considerably larger for the deweylite assemblage than for the olivine. Bulk fluid analyses show an increase in the Mg and Si concentrations with reaction time. In the NaCl solution, the deweylite assemblage was partly dissolved resulting in large voids within the reaction zone. In contrast, the olivine was replaced by magnesite. Under the conditions of our experiments, the meshwork serpentine was not reactive, but aided fluid infiltration into the rock samples. The experimentally produced microtextures closely resemble those found in natural examples. Our study elucidates the mechanisms by which carbonates form in ultramafic rocks under relatively high PCO2-T conditions and particularly in the presence of Ca-bearing aqueous solutions. The existence of a serpentine meshtexture and the presence of weathering products formed from primary Mg-silicates may have significant beneficial effects on in situ CO2 mineral sequestration in ultramafic rocks.  相似文献   

2.
The global warming of Earth’s near-surface, air and oceans in recent decades is a direct consequence of anthropogenic emission of greenhouse gases into the atmosphere such as CO2, CH4, N2O and CFCs. The CO2 emissions contribute approximately 60% to this climate change. This study investigates experimentally the aqueous carbonation mechanisms of an alkaline paper mill waste containing about 55 wt% portlandite (Ca(OH)2) as a possible mineralogical CO2 sequestration process. The overall carbonation reaction includes the following steps: (1) Ca release from portlandite dissolution, (2) CO2 dissolution in water and (3) CaCO3 precipitation. This CO2 sequestration mechanism was supported by geochemical modelling of final solutions using PHREEQC software, and observations by scanning electron microscope and X-ray diffraction of final reaction products. According to the experimental protocol, the system proposed would favour the total capture of approx. 218 kg of CO2 into stable calcite/ton of paper waste, independently of initial CO2 pressure. The final product from the carbonation process is a calcite (ca. 100 wt%)-water dispersion. Indeed, the total captured CO2 mineralized as calcite could be stored in degraded soils or even used for diverse industrial applications. This result demonstrates the possibility of using the alkaline liquid–solid waste for CO2 mitigation and reduction of greenhouse effect gases into the atmosphere.  相似文献   

3.
A MgO-based binder developed to simultaneously solidify/stabilize contaminated sediment and store CO2 has been described previously. The objectives of the study presented here were to investigate the kinetics of the carbonation reactions of the binder and the extent to which carbonation occurred and to identify the optimal conditions for using the binder. The carbonation reaction was clearly faster and the degree of carbonation higher at CO2 concentrations of 50 and 100% than in the ambient atmosphere (which contains 0.04% CO2). A modified unreactive core model adequately described the kinetics. The rate constants were 0.0217–0.319 h?1 and were proportional to the degree of carbonation. A high degree of carbonation, 93.8%, was achieved at a CO2 concentration of 100%. The water to sediment ratio strongly affected carbonation, the optimal ratio being around 0.7. The relative humidity did not strongly affect the carbonation performance. The carbonation products were magnesite (MgCO3) and nesquehonite (MgCO3·3H2O). X-ray diffraction analysis showed that brucite (Mg(OH)2) was not present, suggesting that brucite was very quickly transformed into magnesium carbonates, the presence of which was confirmed by thermal gravimetric analysis. The results indicated that, in 7 d, 1 kg of binder could sequester up to 0.507 kg of CO2 in a 100% CO2 atmosphere. The results indicate that the MgO-based binder has great potential to be used to sequester CO2 under accelerated carbonation conditions.  相似文献   

4.
Mineral aerosols play a significant role in gas–solid interfacial and atmospheric chemistry. Carbonation of olivine aerosol, which takes place in a multiphase reaction processes, can be an effective means to reduce the concentration of atmospheric carbon dioxide. Due to the presence of a huge reserve of silicate minerals in nature, olivine aerosol could be an ideal potential raw material for mineral carbonation for its higher reactivity with H2O and CO2. However, quantitative information about the carbonation process on the surface of natural olivine aerosol is not available. In this paper, calculations on the carbonation reaction processes with and without a H2O molecule using a periodic olivine model has been carried out via the density functional theory. The pathways and their corresponding energies and structures in the carbonation reactions have been established, and the effect of water as means to reduce the energy barriers and stabilize the carbonated structures by forming hydrogen bonds has been confirmed.  相似文献   

5.
 The carbonation reaction CaMg(CO3)2 (dolomite)+2SiO2 (coesite)=CaMgSi2O6 (diopside)+2 CO2 (vapor) has been determined experimentally between 3.5 and 6 GPa in a multiple-anvil, solid-media apparatus. This reaction, a candidate for carbonation of eclogites (garnet+clinopyroxene) in the Earth’s mantle, lies at higher pressure for a given temperature than do the carbonation reactions for peridotites (olivine+orthopyroxene±clinopyroxene). A depth interval may exist within the Earth’s mantle under either ‘normal’ or ‘subduction’ thermal regimes where carbonated peridotite could coexist with carbonate-free, CO2-bearing eclogite. Received: 25 May 1994/Accepted: 13 June 1995  相似文献   

6.
The global rise in atmospheric greenhouse gas concentrations calls for practicable solutions to capture CO2. In this study, a mineral carbonation process was applied in which CO2 reacts with alkaline lignite ash and forms stable carbonate solids. In comparison to previous studies, the assays were conducted at low temperatures and pressures and under semi-dry reaction conditions in an 8 L laboratory mixing device. In order to find optimum process conditions the pCO2 (10-20%), stirring rate (500-3000 rpm) and the liquid to solid ratio (L/S = 0.03-0.36 L kg−1) were varied. In all experiments a considerable CO2 uptake from the gas phase was observed. Concurrently the solid phase contents of Ca and Mg (hydr)oxides decreased and CaCO3 and MgCO3 fractions increased throughout the experiments, showing that CO2 was stabilized as a solid carbonate. The carbonation reaction depends on three factors: Dissolution of CO2 in the liquid phase, mobilization of Ca and Mg from the mineral surface and precipitation of the carbonate solids. Those limitations were found to depend strongly on the variation of the process parameters. Optimum reaction conditions could be found for L/S ratios between 0.12 and 0.18, medium stirring velocities and pCO2 between 10% and 20%.Maximum CO2 uptake by the solid phase was 4.8 mmol g−1 after 120 min, corresponding to a carbonation efficiency for the alkaline material of 53% of the theoretical CO2 binding capacity. In comparison to previous studies both CO2 uptake and carbonation efficiencies were in a similar range, but the reaction times in the semi-dry process were considerably shorter. The proposed method additionally allows for a more simple carbonation setup due to low T and P, and produces an easier to handle product with low water content.  相似文献   

7.
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8.
Hydrated Portland cement was reacted with CO2 in supercritical, gaseous and aqueous phases to understand the potential cement alteration processes along the length of a wellbore, extending from a deep CO2 storage reservoir to the shallow subsurface during geologic carbon sequestration. The 3-D X-ray microtomography (XMT) images showed that the cement alteration was significantly more extensive with CO2-saturated synthetic groundwater than dry or wet supercritical CO2 at high P (10 MPa)-T (50 °C) conditions. Scanning electron microscopy with energy dispersive spectroscopy (SEM–EDS) analysis also exhibited a systematic Ca depletion and C enrichment in cement matrix exposed to CO2-saturated groundwater. Integrated XMT, XRD and SEM–EDS analyses identified the formation of an extensive carbonated zone filled with CaCO3(s), as well as a porous degradation front and an outermost silica-rich zone in cement after exposure to CO2-saturated groundwater. Cement alteration by CO2-saturated groundwater for 2–8 months overall decreased the porosity from 31% to 22% and the permeability by an order of magnitude. Cement alteration by dry or wet supercritical CO2 was slow and minor compared to CO2-saturated groundwater. A thin single carbonation zone was formed in cement after exposure to wet supercritical CO2 for 8 months or dry supercritical CO2 for 15 months. An extensive calcite coating was formed on the outside surface of a cement sample after exposure to wet gaseous CO2 for 1–3 months. The chemical–physical characterization of hydrated Portland cement after exposure to various phases of CO2 indicates that the extent of cement carbonation can be significantly heterogeneous depending on the CO2 phase present in the wellbore environment. Both experimental and geochemical modeling results suggest that wellbore cement exposure to supercritical, gaseous and aqueous phases of CO2 during geologic C sequestration is unlikely to damage the wellbore integrity because cement alteration by all phases of CO2 is dominated by carbonation reactions. This is consistent with previous field studies of wellbore cement with extensive carbonation after exposure to CO2 for three decades. However, XMT imaging indicates that preferential cement alteration by supercritical CO2 or CO2-saturated groundwater can occur along the cement–steel or cement–rock interfaces. This highlights the importance of further investigation of cement degradation along the interfaces of wellbore materials to ensure permanent geologic carbon storage.  相似文献   

9.
In this paper, we demonstrated a new approach to CO2 mineral sequestration using wollastonite carbonation assisted by sulfuric acid and ammonia. Samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and 29Si nuclear magnetic resonance. The change in Gibbs free energy from ?223 kJ/mol for the leaching reaction of wollastonite to ?101 kJ/mol for the carbonation reaction indicated that these two reactions can proceed spontaneously. The leached and carbonated wollastonite showed fibrous bassanite and granular calcium carbonate, respectively, while the crystal structure of pristine wollastonite was destroyed and the majority of the Ca2+ in pristine wollastonite leached. The chemical changes in the phases were monitored during the whole process. A high carbonation rate of 91.1 % could be obtained under the action of sulfuric acid and ammonia at 30 °C at normal atmospheric pressure, indicating its potential use for CO2 sequestration.  相似文献   

10.
A detailed study of low-pressure gas-solid carbonation of chrysotile in dry and humid environments has been carried out. The evolving structure of chrysotile and its reactivity as a function of temperature (300-1200 °C), humidity (0-10 mol %) and CO2 partial pressure (20-67 mol %), thermal preconditioning, and alkali metal doping (Li, Na, K, Cs) have been monitored through in-situ X-ray photoelectron spectroscopy, isothermal thermogravimetry/mass spectrometry, ex-situ X-ray powder diffraction, and water and nitrogen adsorption/desorption. Based on chrysotile crystalline structure and its nanofibrilar orderliness, a multistep carbonation mechanism was elaborated to explain the role of water during chrysotile partial amorphisation, formation of periclase, brucite, and hydromagnesite crystalline phases, and surface passivation thereof, during humid carbonation. The weak carbonation reactivity was rationalized in terms of incongruent CO2 van der Waals molecular diameters with the octahedral-tetrahedral lattice constants of chrysotile. This lack of reactivity appeared to be relatively indifferent to the facilitated water crisscrossing during chrysotile core dehydroxylation/pseudo-amorphisation and surface hydroxylation induced product stabilization during humid carbonation. Thermodynamic stability domains of the species observed at low pressure have been thoroughly discussed on the basis of X-ray powder diffraction patterns and X-ray photoelectron spectroscopy evidence. The highest carbon dioxide uptake occurred at 375 °C in moist atmospheres. On the basis of chrysotile fresh N2 BET area, nearly 15 atoms out of 100 of the surface chrysotile brucitic Mg moiety have been carbonated at this temperature which was tantamount to the carbonation of about 2.5 at. % of the total brucitic Mg moiety in chrysotile. The carbonation of brucite (Mg(OH)2) impurities coexisting in chrysotile was minor and estimated to contribute by less than 17.6 at. % of the total converted magnesium. The presence of cesium traces (3 Cs atoms per 100 Mg atoms) was found to boost chrysotile carbonation capacity by a factor 2.7.  相似文献   

11.
《地学前缘(英文版)》2018,9(6):1945-1955
CO2 mineral sequestration (in ultrabasic or basaltic rocks) has been considered as a promising long-term and stable approach to reduce CO2 in the atmosphere and would counteract the effect of global warming. Meanwhile, clays are widely found in ultrabasic reservoirs. In our study, clays were observed in natural olivine samples, which were used for laboratory experiments in a supercritical CO2 system at 140 °C and 15 MPa. Initial olivine samples were crushed into two sizes which were large grains of ∼850–1000 μm and powder particles of ∼75–150 μm, with the durations of 400 and 1000 h for the powder and grains, respectively. The results showed amorphous silica was newly formed and this passivating layer could mitigate the water-rock interaction to some extent, but it would not play a long-term prohibited effect on secondary mineral carbonate formation as it is a Fe(III) free silica coating. More interestingly, the secondary carbonates were observed to form near the surface sites where locates more clays. Our findings provide insights into the reaction mechanisms of olivine-scCO2-water interaction process in natural ultrabasic rocks.  相似文献   

12.
Carbonation and decarbonation of eclogites: the role of garnet   总被引：3，自引：0，他引：3  
Carbonates are potentially significant hosts for primordial and subducted carbon in the Earth's mantle. In addition, the coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation state of the mantle. Carbonate-silicate-vapor reactions control how carbonate + silicate assemblages may form from carbon-bearing vapor + silicate assemblages with increasing pressure. In olivine-bearing rocks such as peridotite, considered the dominant rock type in the upper mantle, the lowest-pressure carbonate-forming reactions involve olivine (±clinopyroxene) reacting with CO2 (e.g., Wyllie et al. 1983). In eclogitic rocks, the essential mineral assemblage is omphacitic clinopyroxene + garnet, without olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction dolomite + 2 coesite = diopside + 2 CO2 was studied experimentally by Luth (1995). The alternative possibility that garnet reacts with CO2 is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope + 3 CO2 between 5 and 11 GPa in multi-anvil apparatus. At the temperatures ≥1200 °C, carbonation of eclogitic rocks with increasing pressure will proceed initially by reaction with clinopyroxene, because the pyrope-carbonation reaction lies at higher pressures for a given temperature than does the diopside-carbonation reaction. Diluting the pyrope component of garnet and the diopside component of clinopyroxene to levels appropriate for mantle eclogites does not change this conclusion. At lower temperatures, appropriate for “cold” slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via reaction with garnet, but this possibility awaits experimental confirmation. Decarbonation of an eclogite under “normal mantle” geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection cell, would be governed by the formation of clinopyroxene + CO2. At higher pressure than this reaction, any CO2 produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite. Release of CO2 from eclogite into mantle peridotite would form carbonate at sub-solidus conditions and produce a dolomitic carbonate melt if temperatures are above the peridotite-CO2 solidus. Received: 4 May 1998 / Accepted: 23 December 1998  相似文献   

13.
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 CO2-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.  相似文献   

14.
We use a reactive diffusion model to investigate what happens to CO2 injected into a subsurface sandstone reservoir capped by a chlorite- and illite-containing shale seal. The calculations simulate reaction and transport of supercritical (SC) CO2 at 348.15 K and 30 MPa up to 20,000 a. Given the low shale porosity (5%), chemical reactions mostly occurred in the sandstone for the first 2000 a with some precipitation at the ss/sh interface. From 2000 to 4000 a, ankerite, dolomite and illite began replacing Mg–Fe chlorite at the sandstone/shale interface. Transformation of chlorite to ankerite is the dominant reaction occluding the shale porosity in most simulations: from 4000 to 7500 a, this carbonation seals the reservoir and terminates reaction. Overall, the carbonates (calcite, ankerite, dolomite), chlorite and goethite all remain close to local chemical equilibrium with brine. Quartz is almost inert from the point of its dissolution/precipitation. However, the rate of quartz reaction controls the long-term decline in aqueous silica activity and its evolution toward equilibrium. The reactions of feldspars and clays depend strongly on their reaction rate constants (microcline is closer to local equilibrium than albite). The timing of porosity occlusion mostly therefore depends on the kinetic constants of kaolinite and illite. For example, an increase in the kaolinite kinetic constant by 0.25 logarithmic units hastened porosity closure by 4300 a. The earliest simulated closure of porosity occurred at approximately 108 a for simulations designed as sensitivity tests for the rate constants.These simulations also emphasize that the rate of CO2 immobilization as aqueous bicarbonate (solubility trapping) or as carbonate minerals (mineral trapping) in sandstone reservoirs depends upon reaction kinetics – but the relative fraction of each trapped CO2 species only depends upon the initial chemical composition of the host sandstone. For example, at the point of porosity occlusion the fraction of bicarbonate remaining in solution depends upon the initial Na and K content in the host rock but the fraction of carbonate mineralization depends only on the Ca, Mg, Fe content. Since ankerite is the dominant mineral that occludes porosity, the dissolved concentration of ferrous iron is also an important parameter. Future efforts should focus on cross-comparisons and ground-truthing of simulations made for standard case studies as well as laboratory measurements of the reactivities of clay minerals.  相似文献   

15.
Carbonation of magnesium seems to be an interesting option for long term storage of captured CO2. This paper provides an approach to sequestration of carbon dioxide in magnesium silicates using ultramafic rocks from the mountain of Vourinos, in Western Macedonia, Greece. For the experimental procedure five samples were used, consisted of dunite, hartzburgite and pyroxenite. The carbonation method chosen is the aqueous scheme. The results showed low (only about 10% of the stoichiometrically possible amount) transformation into magnesium carbonates for the majority of the samples. Insufficient reaction time, the particle size, or improper choice of reaction conditions are may be some of the reasons for the small amounts of carbonation observed. Further studies are needed in order to identify the various issues that were responsible.  相似文献   

16.
Understanding mechanisms and kinetics of mineral carbonation reactions relevant to sequestering carbon dioxide as a supercritical fluid (scCO2) in geologic formations is crucial to accurately predicting long-term storage risks. Most attention so far has been focused on reactions occurring between silicate minerals and rocks in the aqueous dominated CO2-bearing fluid. However, water-bearing scCO2 also comprises a reactive fluid, and in this situation mineral carbonation mechanisms are poorly understood. Using in situ high-pressure X-ray diffraction, the carbonation of brucite [Mg(OH)2] in wet scCO2 was examined at pressure (82 bar) as a function of water concentration and temperature (50 and 75 °C). Exposing brucite to anhydrous scCO2 at either temperature resulted in little or no detectable reaction over three days. However, addition of trace amounts of water resulted in partial carbonation of brucite into nesquehonite [MgCO3·3H2O] within a few hours at 50 °C. By increasing water content to well above the saturation level of the scCO2, complete conversion of brucite into nesquehonite was observed. Tests conducted at 75 °C resulted in the conversion of brucite into magnesite [MgCO3] instead, apparently through an intermediate nesquehonite step. Raman spectroscopy applied to brucite reacted with 18O-labeled water in scCO2 show it was incorporated into carbonate at a relatively high concentration. This supports a carbonation mechanism with at least one step involving a direct reaction between the mineral and water molecules without mediation by a condensed aqueous layer.  相似文献   

17.
《Applied Geochemistry》2006,21(9):1522-1538
Factors controlling the chemical composition of water interacting with finely-crushed kimberlite have been investigated by sampling pore waters from processed kimberlite fines stored in a containment facility. Discharge water from the diamond recovery plant and surface water from the containment facility, which acts as plant intake water, were also sampled. All waters sampled are pH-neutral, enriched in SO4, Mg, Ca, and K, and low in Fe. Pore-water samples, representing the most concentrated waters, are characterized by the highest SO4 (up to 4080 mg l−1), Mg (up to 870 mg l−1), and Ca (up to 473 mg l−1). The water discharged from the processing plant has higher concentrations of all major dissolved constituents than the intake water. The dominant minerals present in the processed fines and the kimberlite ore are serpentine and olivine, with small amounts of Ca sulphate and Fe sulphide restricted to mud xenoclasts. Reaction and inverse modeling suggest that much of the water-rock interaction takes place within the plant and involves the dissolution of chrysotile and Ca sulphate, and precipitation of silica and Mg carbonate. Evapoconcentration also appears to be a significant process affecting pore water composition in the containment facility. The reaction proposed to be occurring during ore processing involves the dissolution of CO2(g) and may represent an opportunity to sequester atmospheric CO2 through mineral carbonation.  相似文献   

18.
Rare dunite and 2-pyroxene gabbro xenoliths occur in banded trachyte at Puu Waawaa on Hualalai Volcano, Hawaii. Mineral compositions suggest that these xenoliths formed as cumulates of tholeiitic basalt at shallow depth in a subcaldera magma reservoir. Subsequently, the minerals in the xenoliths underwent subsolidus reequilibration that particularly affected chromite compositions by decreasing their Mg numbers. In addition, olivine lost CaO and plagioclase lost MgO and Fe2O3 during subsolidus reequilibration. The xenoliths also reacted with the host trachyte to form secondary mica, amphibole, and orthopyroxene, and to further modify the compositions of some olivine, clinopyroxene, and spinel grains. The reaction products indicate that the host trachyte melt was hydrous. Clinopyroxene in one dunite sample and olivine in most dunite samples have undergone partial melting, apparently in response to addition of water to the xenolith. These xenoliths do not contain CO2 fluid inclusions, so common in xenoliths from other localities on Hualalai, which suggests that CO2 was introduced from alkalic basalt magma between the time CO2-inclusion-free xenoliths erupted at 106±6 ka and the time CO2-inclusion-rich xenoliths erupted within the last 15 ka.  相似文献   

19.
Far from equilibrium enstatite dissolution rates both open to atmospheric CO2 and CO2 purged were measured as a function of solution pH from 8 to 13 in batch reactors at room temperature. Congruent dissolution was observed after an initial period of incongruent dissolution with preferential Si release from the enstatite. Steady-state dissolution rates in open to atmospheric CO2 conditions decrease with increase in solution pH from 8 to 12 similar to the behavior reported by other investigators. Judging from the pH 13 dissolution rate, rates increase with pH above pH 12. This is thought to occur because of the increase in overall negative surface charges on enstatite as Mg surface sites become negative above pH 12.4, the pH of zero surface charge of MgO.Steady-state dissolution rates of enstatite increase above pH 10 when CO2 was purged by performing the experiments in a N2 atmosphere. This suggests inhibition of dissolution rates above pH 10 when experiments were open to the atmosphere. The dissolved carbonate in these solutions becomes dominantly CO32− above pH 10.33. It is argued that CO32− forms a >Mg2-CO3 complex at positively charged Mg surface sites on enstatite, resulting in stabilization of the surface Si-O bonds. Therefore, removal of solution carbonate results in an increase in dissolution rates of enstatite above pH 10. The log rate of CO2-purged enstatite dissolution in moles per cm2 per s as a function of increasing pH above pH 10 is equal to 0.35. This is consistent with the model of silicate mineral dissolution in the absence of surface carbonation in alkaline solutions proposed earlier in the literature.  相似文献   

20.
Recent work on the weathering of high standing islands (HSI’s) of New Zealand (Goldsmith et al., 2008), Dominica (Goldsmith et al., 2010) Martinique and Guadeloupe (Rad et al., 2006) and portions of the Philippines (Schopka et al., 2011) shows weathering rates based on stream water chemistry for areas draining andesitic terrains are comparable to weathering rates determined for basaltic terrains, indicating that andesite weathering might be much more important in drawing down atmospheric CO2 than previously recognized. While an easily erodible parent material has been largely attributed to sustaining rates at these locations, little is known to known regarding its associated reaction kinetics. We conducted a series of batch dissolution experiments on andesitic material collected from ∼10,000 year old tephra deposits from Dominica to determine the dissolution rate of major and trace mineral phases to better understand geochemical processes controlling weathering flux from these areas. Dissolution experiments were conducted over a range of pH (4 and 7) on bulk samples and mineral separates.The dissolution rates based on Si release from the Dominica tephra bulk samples were similar, and ranged from 0.04 to 0.13 μmole Si/g-day in water, and ∼0.14 to 0.27 μmole Si/g-day in dilute acid (initial pH ∼4). Although the bulk of the ash is predominately composed of vesicular felsic (Na–Al–Si) volcanic glass, reaction rates and stoichiometry indicate ash dissolution is dominated by the reactivity of trace Mg or Ca-bearing silicate phases (olivine, pyroxene or amphiboles) and Ca–phosphate phases (apatite), especially under slightly acidic conditions. Analysis of reacted phases by SEM shows little evidence of alteration of glassy material, whereas surfaces of Ca–Mg inosilicates, olivine and apatite show etched features indicative of dissolution. Results of the dissolution experiments suggest that, although these phases are relatively minor components of the ash, they contribute disproportionately to the overall weathering flux, and their reactivity may be particularly important in areas where physical weathering and erosion are constantly exposing new fresh surfaces available for chemical reaction.  相似文献   

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