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1.
Variations in bulk Mg/Si ratios in the various groups of chondritic meteorites indicate that Mg/Si fractionation occurred in the primitive solar nebula. Enstatite (MgSiO3) evaporates incongruently forming forsterite (Mg2SiO4) as an evaporation residue; therefore, evaporation of enstatite produces Mg/Si variations in solid (Mg-rich) and gas (Si-rich) and must be considered as a probable process responsible for Mg/Si fractionation recorded in chondrites. To understand the evaporation kinetics of enstatite, incongruent evaporation experiments on enstatite single crystals have been carried out in vacuum and in hydrogen gas at temperatures of 1300 to 1500°C. A polycrystalline forsterite layer is formed on the surface of enstatite by preferential evaporation of the SiO2 component, both in vacuum and in hydrogen gas. The thickness of the forsterite layer in vacuum increases with time in the early stage of evaporation and later the thickness of the forsterite layer remains constant (several microns). This is due to the change in the rate limiting process from surface reaction plus nucleation and growth to diffusion in the surface forsterite layer. The activation energy of the diffusion-controlled evaporation rate constant of enstatite is 457 (±58) kJ/mol. A thinner forsterite layer is formed on the surface of enstatite in hydrogen gas than in vacuum. Evaporation of enstatite in hydrogen gas is also considered to be controlled by diffusion of ions through the forsterite layer. The thin forsterite layer formed in hydrogen gas is ascribed to the enhanced evaporation rate of forsterite in the presence of hydrogen gas.The results are applied to incongruent evaporation under the solar nebular conditions. The steady thickness of the forsterite of nebular pressure-temperature conditions is estimated to be submicron because of the enhanced evaporation rate of forsterite under hydrogen-rich nebular conditions if evaporated gases are taken away immediately and no back reaction occurs (an open system). Because enstatite grains in the solar nebula would be comparable to the estimated steady thickness of forsterite, evaporation of such enstatite grains under kinetic conditions could play an important role in producing variations in Mg/Si ratios between solid and gas in the solar nebula. 相似文献
2.
M. Bellotto A. Gualtieri G. Artioli S. M. Clark 《Physics and Chemistry of Minerals》1995,22(4):207-217
The decomposition reaction of kaolinite has been investigated as a function of the defectivity of the starting material and the temperature of reaction. Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation, both under a constant heating rate (heating rates from 10 to 100° C/min) and in isothermal conditions (in the temperature range 500 to 700° C). The apparent activation energy of the dehydroxylation process is different for kaolinites exhibiting a different degree of stacking fault density. The results of the analysis of the kinetic data indicate that the starting reaction mechanism is controlled by diffusion in the kaolinite particle. The diffusion process is dependent on the defective nature of both kaolinite and metakaolinite. At high temperatures, and at higher heating rates, the reaction mechanism changes and the resistance in the boundary layer outside the crystallites becomes the rate-limiting factor, and nucleation begins within the reacting particle. During the final stage of the dehydroxylation process the reaction is limited by heat or mass transfer, and this might be interpreted by the limited diffusion between the unreacted kaolinite domains and the metakaolinite matrix. 相似文献
3.
The replacement of magnetite by hematite was studied through a series of experiments under mild hydrothermal conditions(140 -220℃, vapour saturated pressures) to quantify the kinetics of the transformation and the relative effects of redox and non-redox processes on the transformation. The results indicate that oxygen is not an essential factor in the replacement reaction of magnetite by hematite, but the addition of excess oxidant does trigger the oxidation reaction, and increases the kinetics of the transformation. However, even under high O_2(aq) environments, some of the replacement still occurred via Fe2+ leaching from magnetite. The kinetics of the replacement reaction depends upon temperature and solution parameters such as pH and the concentrations of ligands, all of which are factors that control the solubility of magnetite and affect the transport of Fe2+ (and the oxidant) to and from the reaction front. Reaction rates are fast at ~200℃, and in nature transport properties of Fe and,in the case of the redox-controlled replacement, the oxidant will be the rate-limiting control on the reaction progress. Using an Avrami treatment of the kinetic data and the Arrhenius equation, the activation energy for the transformation under non-redox conditions was calculated to be 26 ± 6 kJ mol-1.This value is in agreement with the reported activation energy for the dissolution of magnetite, which is the rate-limiting process for the transformation under non-redox conditions. 相似文献
4.
Branched and isoprenoidal glycerol dialkyl glycerol tetraether (GDGT) membrane lipids have been widely used to reconstruct past climate and environmental change. They are not, however, widely applied to peat deposits and the controls on their distributions in peats remain unclear. Here, we present a high resolution record of branched and isoprenoid GDGT concentrations and distributions from a peat core from the Tibetan Plateau that spans the last 13 kyr, a period characterised by distinct dry and wet periods in the region. The lowest concentrations of total branched glycerol dialkyl glycerol tetraethers (brGDGTs) occurred during a presumably dry interval in the mid-Holocene, suggesting that brGDGTs-producing bacteria are less productive under such conditions, perhaps reflecting their putative anaerobic ecology. The mean annual air temperature (MAT) estimates derived from the methylation index of brGDGTs and cyclisation ratio of brGDGTs (MBT′/CBT) are higher than present mean annual temperature in the region and closer to summer temperatures, perhaps due to seasonal production of brGDGTs. The downcore distributions of isoprenoidal and branched GDGTs are dominated by GDGT-0 and brGDGT II, respectively. The high fractional abundances of GDGT-0 in warm and especially wet intervals suggest that these conditions are favourable for some groups of methanogenic archaea. The mid-Holocene dry interval is associated with an increase in the fractional and absolute abundance of crenarchaeol, which could be indicative of enhanced ammonia-oxidising archaeal-mediated nitrogen cycling under these conditions. Taken together, variations of GDGT concentrations in peats appear to document the response of microbial processes to climate change and variations in the biogeochemical environment. 相似文献
5.
6.
Reductive dissolution of arsenic-bearing ferrihydrite 总被引：2，自引：0，他引：2
Jasmine J. Erbs Brian C. Reinsch Gregory V. Lowry Subir K. Banerjee 《Geochimica et cosmochimica acta》2010,74(12):3382-8833
Ferrihydrites were prepared by coprecipitation (COP) or adsorption (ADS) of arsenate, and the products were characterized using solid-state methods. In addition, the kinetics of reductive dissolution by hydroquinone of these well-characterized materials were quantified. Characterization and magnetism results indicate that the 10 wt% As COP ferrihydrite is less crystalline and possibly has smaller crystallite size than the other ferrihydrites, which all have similar crystallinity and particle size. The results from reductive dissolution experiments show similar reaction rates, reaction mechanism, and activation energy for ferrihydrite precipitated with or without added arsenate. However, a marked decrease in reactivity was observed for 10 wt% As ADS ferrihydrite. The decrease is not attributed to differences in activation energy but rather the preferential blocking of active sites on the ferrihydrite surface. Results demonstrate that arsenic may be released by the reductive dissolution of arsenic-bearing ferrihydrite regardless of whether the arsenic is coprecipitated with or adsorbed onto the ferrihydrite. However, under these reaction conditions, release from materials with adsorbed arsenate greatly exceeds that from materials with coprecipitated arsenate. In fact, a considerable amount of arsenic was released from the 10 wt% ADS ferrihydrite before reductive dissolution was initiated. Therefore, the characterization of arsenate-bearing iron oxide materials to determine the method of arsenate incorporation into structures—perhaps by quantification of Fe-Fe coordination with EXAFS spectroscopy—may lead to improved predictions of the large-scale release of arsenic within aquifer systems under reducing conditions. 相似文献
7.
Oxidation of the relatively iron sulfide-poor Dugald River zinc-lead lode in northwest Queensland and reaction of the acid solutions with carbonate has resulted in an undifferentiated gossan profile. The gossan is composed predominantly of quartz, goethite, hematite, barite, adularia, plumbian jarosite, plumbogummite and minor mica, chlorite, kaolinite and montmorillonite. Barite and adularia are formed by the breakdown of the barium feldspar hyalophane (K, Na, Ba)[(Al, Si)4O8] which occurs in the lode.Lead in the gossan is contained within the secondary minerals plumbogummite and plumbian jarosite, and in traces of anglesite and cerussite, whereas Zn occurs in the barite, secondary lead minerals and coronadite structures, and is adsorbed by iron oxides, phyllosilicates and carbonaceous matter. Only traces of zinc minerals smithsonite, goslarite and hemimorphite were detected.Use of Gresens' general metasomatic equation has enabled quantitative determination of compositional changes resulting from the oxidation of the ore. Silicon, Al, Ti and Ba are essentially immobile under the mildly acidic oxidizing conditions. In decreasing order of mobility Cd, Ca, S, Na, K, Mn, Mg, Zn, Ni and Cu, together with CO2 and Tl, have been leached from the gossan profile, while Ag, Sb, Se, As, Fe and Pb appear to have been added to the gossan, notably in a zone of solution-deposited secondary minerals where they have been concentrated, possibly as a result of leaching from the surface gossan. 相似文献
8.
Localized rheological weakening by grain-size reduction during lithospheric extension 总被引：1，自引：0，他引：1
Grain-size reduction may be a possible mechanism for the origin of localized deformation in the ductile regime. I investigated the effects of grain-size reduction due to dynamic recrystallization, cataclasis, and syntectonic metamorphic reaction on the stress envelope in the lithospheric mantle during extension by using a simple one-dimensional model. In this model, the lithosphere extends uniformly with a constant strain rate, and a fall in rock strength appears as a decrease in stress. Because grain-size distribution at the onset of extension is unknown, I regarded the steady state grain-size due to dynamic recrystallization as the initial size. Then, I evaluated the maximum effects of grain-size reduction by dynamic recrystallization during extension, and consequently examined the effects of grain-size reduction by cataclasis and metamorphic reaction under conditions when dynamic recrystallization occurs significantly. I find that it is difficult to bring about localized rheological weakening by grain-size reduction owing to dynamic recrystallization. In contrast, grain-size reduction by cataclasis can cause localized weakening during extension. There is a wide-ranging rate of grain-size reduction by means of cataclasis that causes localized weakening just below the Moho. I specified the reaction from spinel-lherzolite to plagioclase-lherzolite that plays a role in grain-size reduction by syntectonic metamorphism. The reaction occurs at depths less than 35 km, which is independent of the initial thermal state of the lithosphere. Localized rheological weakening can occur if the following conditions are satisfied: (1) grain-size before the reaction is greater than 0.7 mm under dry conditions and greater than 0.5 mm under wet conditions, and it decreases down to those values by the reaction; (2) grain-size decreases down to less than initial grain-size, when the dominant deformation mechanism is GSS creep at the onset of extension. It is also noted that dry conditions are more favourable for localized weakening. 相似文献
9.
A new process was provided for decomposition of ilmenite by concentrated KOH solution under atmospheric pressure. The significant effects of reaction temperature, KOH concentration, stirring speed, particle size, and alkali-to-ilmenite mass ratios on titanium extraction were studied. The temperature and initial particle size showed significant influence on titanium extraction. The experimental data of the extraction rates under the relevant operating variables were well interpreted with the shrinking core model under chemically controlled process. The apparent activation energy of the reaction was evaluated using the Arrhenius expression. Approximately 80–85% of the titanium could be leached from the ilmenite ore under the optimal conditions. 相似文献
10.
Near the outskirts of the Albanian port of Durres on the Adriatic Coast, at a place called Porto Romano, an old chemical complex
has been producing chromate salts for a period of ∼20 years (1972–1993). As a result large quantities of chromite ore processing
residue (COPR) waste were released mainly as suspended particles and/or dissolved species in effluents, which settled in a
decantation pond area, 200 × 250 m in size, east of the industrial plant. Unfortunately, pollutants were spread out with time.
Knowing that chromium mobility and toxicity in natural settings is species dependent, the species characterization has been
carried out in different sampling media (pond sediments, lagoonal sediments and water samples). Five pond profiles were sampled
within the decantation area up to a depth of ∼1 m, till the underlying Quaternary lagoonal silty-clays were reached. Total
Cr content is highly variable with values between 1,130 and 24,409 mg/kg which is clearly higher than the defined local background
values for lagoonal silty-clays (198 mg/kg of Cr). Leaching of Cr(VI) especially occurs in the pond sediments, which are low
in OC, CEC and clay content, under acidic and neutral pH conditions (e.g. up to ∼2,230 mg/kg Cr(VI) leached from a sample
containing a total concentration of Cr 12,200 mg/kg). Moreover, leaching of the Cr(III) occurs only under strictly acidic
conditions (maximum 1,144 mg/kg leached from a sample containing a total Cr-concentration of 17,608 mg/kg). In this study
also a number of natural attenuation conditions (i.e. reaction with lagoonal clays rich in organic matter and iron as well
as isomorphous substitution) have been recognized. 相似文献
11.
The source rock sample of the Shahejie Formation（upper Es4） in Jiyang Sag was pyrolyzed under open system with the Rock-Eval-Ⅱapparatus,and then kinetic model parameters were calibrated for investigating the effect of uncertainty of pre-exponential factors on kinetic parameters and geological applications,where the parallel first-order reaction rate model with an average pre-exponential factor and discrete distribution activity energies was used.The results indicate that when the pre-exponential factor changes from low to high,an extreme value for residual errors occurs.And with the increasing pre-exponential factor,the distribution shape of activation energies are nearly the same,but the values of activation energies move higher integrally,and the average activation energy increases about 12 kJ/mol for every 10-fold of the pre-exponential factors.Extrapolating the geological heating rate of 3.3℃/Ma,the results show that with the increases in pre-exponential factor, the geological temperature corresponding to TR0.5（transforming ratio of hydrocarbon generation is 50%） increases gradually,and the additional temperature gradually decreases.Combined with geochemical data of source rock,the kinetic parameters with different pre-exponential factors are used to calculate the transformation ratio of hydrocarbon generation,and the result indicates that kinetic parameters corresponding to the better optimized pre-exponential factor are more suitable.It is suggested that the risk assessment of hydrocarbon generation kinetic parameters should be enhanced when using the kinetic method to appraise the oil-gas resources.Meantime,the application result of different kinetic parameters should be verified with geological and geochemical data of source rock in the target area;therefore,the most suitable kinetic parameters for target can be obtained. 相似文献
12.
A. Gualtieri M. Bellotto G. Artioli S. M. Clark 《Physics and Chemistry of Minerals》1995,22(4):215-222
The present work is a follow-up of the investigation on the decomposition reaction of kaolinite as a function of the defectivity of the starting material and the temperature of reaction. In the present work we study the high temperature reaction of mullite synthesis from kaolinite, from the starting point of the results obtained in the first part.Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation in isothermal conditions. The apparent activation energy for mullite nucleation and growth is found to be related to the defective structure of the starting kaolinite, which thus must have an influence on the chemical homogeneity of the amorphous intermediate phase. 相似文献
13.
14.
Laurent Truche Gilles Berger Damien Guillaume 《Geochimica et cosmochimica acta》2010,74(10):2894-2914
The kinetics of abiotic redox reactions induced by hydrogen are poorly documented although it represents a growing area of interest in terms of both nuclear waste storage assessment and the comprehensive study of hydrogen-rich fluid in mid-ocean ridge hydrothermal systems. We present an experimental kinetics study of pyrite reduction into pyrrhotite under significant H2 pressure and mid-hydrothermal conditions. We describe the mechanism and kinetic behavior of this reaction by combining textural and solution analyses under various conditions of temperature, pyrite particles size, H2 pressure and pH. When pH is controlled by calcite, the reaction presents all the characteristics of a coupled dissolution-precipitation mechanism occurring at the pyrite-pyrrhotite interface. By considering the chemical affinity of the coupled reaction as a function of reaction extent, we demonstrate that the spatial coupling is induced both by pyrite as a substrate for pyrrhotite nucleation and by the role of fluid chemistry at the reaction front. Far from equilibrium with respect to pyrite, the kinetics of sulfide production associated with the reaction are linearly related to the square root of time with an activation energy of 53 kJ/mol. This value is higher than what is expected for a diffusion-controlled kinetic regime. We suggest that the reaction rate is controlled both by pyrite reductive dissolution and by sulfide diffusion through the porous pyrrhotite microstructure. We provide a simple sulfide production-rate expression on the basis of our measured rate constants that can be used in geochemical modeling to further evaluate the impact of hydrogen on pyrite under nuclear waste disposal conditions. 相似文献
15.
It has been widely accepted that a chemically altered, protective surface layer regulates the dissolution, and hence the weathering, of plagioclase feldspars under Earth's surface conditions. In this study, we examine this hypothesis in detail with the aid of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Using these techniques, we have been unable to find any direct evidence suggesting the presence of a chemically altered coating on feldspar surfaces which have been weathered in the lab. Instead, our results suggest that the mechanism controlling feldspar weathering is a surface controlled reaction.Based on SEM observations and measurements of rate of release of silica, we postulate that the process of feldspar dissolution proceeds in two stages in the lab. Initially, one observes the dissolution of ultrafine (? 1 μm diameter) particles which are produced during grinding of the sample, and which adhere tenaciously to the surfaces of larger grains. This is the stage which results in the non-linear rates of dissolution which are commonly observed in the lab (parabolic kinetics). Secondly, the dissolution occurs at sites of excess surface energy such as at dislocations or similar crystal defects. This process yields linear rates of dissolution, and it dominates the weathering of feldspars in the field. 相似文献
16.
O. P. Sharma K. D. Bal H. S. Saini K. K. Nagpaul 《Physics and Chemistry of Minerals》1979,5(2):133-139
Fission track annealing experiments for vermiculite mineral have been performed under optimised etching conditions and a correction curve translating track length reduction to track density reduction has been constructed. The blocking/closing temperature of the fission track system in the mineral has been calculated to be 125°±30° C. The corrected fission track age of vermiculite from Kasipatnam (Visakhapatnam), South India, has been calculated as 544±14 Ma. The activation energy and average uranium concentration of the mineral are 1.7 eV and 9.9×10?8 gg?1 respectively. 相似文献
17.
以广西大厂镇鲁塘铅锌矿尾砂为研究对象,通过淋滤实验研究了不同氧化还原条件下尾砂中Cu、Cd、Zn、Pb和As等元素的活化和迁移规律。结果表明：经高浓度氧化、高浓度还原条件处理的尾矿表现为pH<7的酸性环境,经低浓度氧化环境条件处理的尾砂呈现pH>7的弱酸性至弱碱性环境;尾砂中Cu、Cd元素活化迁移受pH值的影响明显,即高浓度还原和高浓度氧化条件可以促进Cu和Cd元素的迁移,酸性条件对Cu和Cd元素的迁移起到促进作用;Zn与Cd元素存在竞争吸附关系,但二者仍有明显差别;Pb和As元素受到还原条件的影响,能有效促进Pb和As元素的释放迁移。在淋滤实验前期,铅锌矿的表面阻力较小,由于环境酸碱性的改变,初始尾砂对重金属元素的吸附位能发生变化,重金属元素初期迁移能力得到加强;淋滤后期,矿物颗粒表面由于发生氧化还原反应,促使颗粒表面的阻力增加,重金属元素的溶出量减少,迁移能力受到抑制。 相似文献
18.
Arsenic incorporation into FeS2 pyrite and its influence on dissolution: A DFT study 总被引：3，自引：0，他引：3
FeS2 pyrite can incorporate large amounts of arsenic (up to ca. 10 wt%) and hence has a strong impact on the mobility of this toxic metalloid. Focussing on the lowest arsenic concentrations for which the incorporation occurs in solid solution, the substitution mechanisms involved have been investigated by assuming simple incorporation reactions in both oxidising and reducing conditions. The solution energies were calculated by Density Functional Theory (DFT) calculations and we predict that the formation of AsS dianion groups is the most energetically favourable mechanism. The results also suggest that the presence of arsenic will accelerate the dissolution and thus the generation of acid drainage, when the crystal dissolves in oxidising conditions. 相似文献
19.
在矿山复杂水文地质条件下如何进行快速封堵及动水注浆是注浆领域的技术难题，开发新的化学注浆材料是一个有效的解决途径。通过分析聚氨酯合成反应温度、加料方式、各类助剂、添加剂对比试验，对聚氨酯注浆材料的生产配方进行优化，并确定材料最佳配方。结果表明，以磷酸作为缓凝剂，乙酸乙酯为溶剂，PM-33为催化剂，采用4%纳米二氧化硅改性制得的注浆材料性能较好、遇水膨胀率高、反应过程及产物环保无毒。该研究可为复杂水文地质条件下的注浆工程提供一种新型的单组分水下速凝膨胀聚氨酯注浆材料。 相似文献
20.
Experimental simulation of dolomite dissolution under burial diagenesis conditions and thermodynamic interpretation 总被引：3，自引：0，他引：3
Simulating experiments on dolomite dissolution by acetic acid were made under burial diagenesis conditions,at temperatures ranging from 75℃to 130℃ and pressures from 20 MPA to 30MPa,The results show that the dissolution rate of dolomite increased rapidly with increasing temperature and prssure,From 75℃/Mpa to 130℃/30 MPa,the total amount of released ca and Mg increased from 32.98mg/L to 337.9mg/L,over one order of magnitude in difference,Thermodynamic calculation indicates that the increment of Gibbs free energy (ΔG） of the chemical reaction decreases with increasing temperature and pressure.This thermodynamic result is consistent with the experimental result.Based on the Experimental results.it is suggested that secondary prosities formed by dolomite dissolution under conditions of deep burial diagenesis should be more common than those under epigenesis and shallow burial conditions,and therefore dolomite reservoires in the formations that have been deeply buried should be more abundant than in the formations that have only been shallowly buried. 相似文献