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1.
黄土地层中的CaCO3与环境 总被引:20,自引:1,他引:20
本文根据大量观察分析,对黄土地层中的CaCO3含量、存在形式、淀积深度进行了研究,并进行了分类。结果表明,不仅CaCO3含量能够反映气候变化,而且其存在形式和淀积深度同样能反映气候变化;其淀积深度很少受时间影响,比含量更能可靠地用于气候研究。 相似文献
2.
Peter Deines 《Geochimica et cosmochimica acta》2009,73(24):7256-348
Carbon isotopic exchange between graphite and three polymorphs of CaCO3 was investigated at temperatures of 600-1400 °C and at pressures from 1.4 to 2.3 GPa. Fractionation factors at all temperatures were determined by the partial exchange treatment of Northrop and Clayton (1966).Graphite starting material for the majority of the experiments was milled in water for 20-25 h, producing aggregates of nanosheets. The sheets range in width from 50 to 1000 nm and in thickness from 20 to 30 nm, and they retain hexagonal symmetry.Isotopic exchange appears to be the sum of surface exchange and interior exchange. At 1100-1400 °C, interior exchange exceeded surface exchange, probably by a combination of grain growth, as determined by increase in crystallite size, recrystallization, as observed in FESEM images, and diffusion. In some runs at 1200 and 1400 °C with an isotopic contrast between the initial graphite and calcite of close to 50‰, equilibrium fractionation was actually overstepped due to a kinetic effect. A weighted regression of fractionation factors from the high-temperature runs yields the line of equilibrium interior exchange:
3.
Lurdes Fernández-Díaz Ángeles Fernández-González 《Geochimica et cosmochimica acta》2010,74(21):6064-6076
The nucleation and growth of CaCO3 phases from aqueous solutions with SO42−:CO32− ratios from 0 to 1.62 and a pH of ∼10.9 were studied experimentally in batch reactors at 25 °C. The mineralogy, morphology and composition of the precipitates were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and microanalyses. The solids recovered after short reaction times (5 min to 1 h) consisted of a mixture of calcite and vaterite, with a S content that linearly correlates with the SO42−:CO32− ratio in the aqueous solution. The solvent-mediated transformation of vaterite to calcite subsequently occurred. After 24 h of equilibration, calcite was the only phase present in the precipitate for aqueous solutions with SO42−:CO32− ? 1. For SO42−:CO32− > 1, vaterite persisted as a major phase for a longer time (>250 h for SO42−:CO32− = 1.62). To study the role of sulfate in stabilizing vaterite, we performed a molecular simulation of the substitution of sulfate for carbonate groups into the crystal structure of vaterite, aragonite and calcite. The results obtained show that the incorporation of small amounts (<3 mole%) of sulfate is energetically favorable in the vaterite structure, unfavorable in calcite and very unfavorable in aragonite. The computer modeling provided thermodynamic information, which, combined with kinetic arguments, allowed us to put forward a plausible explanation for the observed crystallization behavior. 相似文献
4.
Subsolidus and melting relations for the CaCO3-MgCO3 join at 30 kb have been determined using piston-cylinder apparatus. Data are also presented for the melting curve of CaCO3 to 30 kb, the decomposition and melting curves of MgCO3 to 36 kb, and the calcite-aragonite transition at 800°C, 950°C and 1100°C. At 30kb, the melting loop for the CaCO3-MgCO3 join extends from 1610°C (CaCO3) to 1585°C (MgCO3) through a liquidus minimum at 1290°C (near 42 mole% MgCO3). The dolomite-magnesite solvus barely intersects the 30 kb melting loop to produce a peritectic reaction at 1385°C. Integration of the new experimental data with other published data permits construction of a complete P-T projection and a sequence of isobars for the CaCO3-MgCO3 join for pressures between 5 and 30 kb. The phase relations for this join provide part of the essential framework of the model peridotite system CaO-MgO-SiO8-CO2-H2O, which has particular application to the origin of carbonatitic and kimberlitic magmas. In light of the accumulating evidence for CO2 in various forms within the upper mantle and of its effect on magmatic processes, analysis of the melting relations in this system is of considerable importance. 相似文献
5.
Gregg Marland 《Geochimica et cosmochimica acta》1975,39(1):83-91
Experimental runs were made in cold-seal pressure vessels using synthetic CaCO8·6H2O, calcite and aragonite as starting materials. The reaction CaCO3·6H2O (ikaite) ? CaCO3 (calcite I) + 6H2O was reversed across its metastable extension into the aragonite stability field and the phase boundary is defined by brackets at 4.14kb, 14.3°C and 2.96 kb, ?3.0°C. An invariant point for CaCO3·6H2O, calcite I, aragonite and water thus occurs at about 3.02 kb and ?2.0°C. No other reaction could be reversed. Calculations based on the equilibrium phase boundary between calcite and ikaite and the available thermochemical data for calcite and water yield the stadard free energy of formation, standard enthalpy of formation and third law entropy of CaCO3·6H2O at 25°C and 1 bar total pressure; ?607.3 kcal/mole, ?705.8 kcal/mole, and 88.4 cal/deg mole, respectively. 相似文献
6.
Dissolution rates of limestone covered by a water film open to a CO2-containing atmosphere are controlled by the chemical composition of the CaCO3-H2O-CO2 solution at the water-mineral interface. This composition is determined by the Ca2+-concentration at this boundary, conversion of CO2 into H+ and in the solution, and by diffusional mass transport of the dissolved species from and towards the water-limestone interface. A system of coupled diffusion-reaction equations for Ca2+, , and CO2 is derived. The Ca2+ flux rates at the surface of the mineral are defined by the PWP-empirical rate law. These flux rates by the rules of stoichiometry must be equal to the flux rates of CO2 across the air-water interface. In the solution, CO2 is converted into H+ and . At low water-film thickness this reaction becomes rate limiting. The time dependent diffusion-reaction equations are solved for free drift dissolution by a finite-difference scheme, to obtain the dissolution rate of calcite as a function of the average calcium concentration in the water film. Dissolution rates are obtained for high undersaturation. The results reveal two regimes of linear dissolution kinetics, which can be described by a rate law F = αi(miceq − c), where c is the calcium concentration in the water film, ceq the equilibrium concentration with respect to calcite. For index i = 0, a fast rate law, which here is reported for the first time, is found with α0 = 3 × 10−6 m s−1 and m0 = 0.3. For c > m0ceq, a slow rate law is valid with α1 = 3 × 10−7 m s−1 and m1 = 1, which confirms earlier work. The numbers given above are valid for film thickness of several tenths of a millimetre and at 20 °C. These rates are proven experimentally, using a flat inclined limestone plate covered by a laminar flowing water film injected at an input point with known flow rate Q and calcium concentration. From the concentration measured after flow distance x the dissolution rates are determined. These experiments have been performed at a carbon-dioxide pressure of 0.00035 atm and also of 0.01 atm. The results are in good agreement to the theoretical predictions. 相似文献
7.
Melanoidin syntheses, which are carried out by preparing aqueous solutions of glucose and glycine, and glucose and alanine are examined with respect to variation of concentration of reactants, hydrogen ion concentration, molecular weights, functional groups and elemental compositions with increasing heating time at constant temperature (90°C). The experiments reveal that “melanoidins” consist of a mixture of fulvic acid-, humic acid- and kerogen-like polymers, with the initial formation of fulvic acid-, followed by humic acid- and kerogen-like polymers, but showing a period of coexistence of humic acid- and kerogen-like polymers. The kerogen-like polymers have elemental compositions similar to type II or II/III kerogens. The rate of melanoidin formation is much higher in the presence of montmorillonite. The CaCO3 association reactions of the same experiments, however, show extremely low rates of melanoidin formation. Laboratory simulations seem to be consistent with the observation that carbonate sediments usually contain small amounts of kerogens compared with clayey sediments, both Recent and ancient. 相似文献
8.
Victor L. Vinograd Benjamin P. Burton Neil L. Allan 《Geochimica et cosmochimica acta》2007,71(4):974-983
Thermodynamic mixing properties and subsolidus phase relations of the rhombohedral carbonate system, (1 − x) · CaCO3 − x · MgCO3, were modelled in the temperature range of 623-2023 K with static structure energy calculations based on well-parameterised empirical interatomic potentials. Relaxed static structure energies of a large set of randomly varied structures in a 4 × 4 × 1 supercell of calcite (a = 19.952 Å, c = 17.061 Å) were calculated with the General Utility Lattice Program (GULP). These energies were cluster expanded in a basis set of 12 pair-wise effective interactions. Temperature-dependent enthalpies of mixing were calculated by the Monte Carlo method. Free energies of mixing were obtained by thermodynamic integration of the Monte Carlo results. The calculated phase diagram is in good agreement with experimental phase boundaries. 相似文献
9.
Carbonate melts are important active metasomatic agents and efficient transport agents; their thermodynamic properties at high temperatures and pressures are therefore of considerable interest for various geochemica applications. However, due to the extreme challenges in relevant experiments, current knowledge of even the density of carbonate melts is limited. In this study, we provide high quality volumetric data of Ca CO3-melt from firs principles at high temperatures and pressures(up to3,500 K and 60 GPa). The accuracy of these data is demonstrated through comprehensive comparison with available experimental data and a thorough discussion of the predictability of the re-scaling method proposed in this study. Based on the simulations, an equation of state has been established that is critical to relevant highly disputed questions such as the decomposition and solidification boundaries of Ca CO3 melts, the latter of which is briefly discussed in this study with a newly derived ab initio melting curve to high pressures. 相似文献
10.
11.
深海沉积物剖面中CaCO3含量变化与气候变迁密切相关,冰期时的高,间冰期时的低。若大陆上黄土地层中CaCO3含量变化也有类似的性质,就更有利于气候长期变迁的海—陆对比以及探讨气候变迁的原因。本文根据陕西洛川黄土地层剖面中110多块样品的分析资料,用相关分析和方差分析,考察CaCO3含量与黄土、古土壤形成环境的关系,并用频谱分析初步探讨约70万年来气候变迁的周期以及气候变迁的海—陆对比等问题。 相似文献
12.
Pablo Cubillas Stephan Köhler Carole Causserand 《Geochimica et cosmochimica acta》2005,69(23):5459-5476
Coupled CaCO3 dissolution-otavite (CdCO3) precipitation experiments have been performed to 1) quantify the effect of mineral coatings on dissolution rates, and 2) to explore the possible application of this coupled process to the remediation of polluted waters. All experiments were performed at 25°C in mixed-flow reactors. Various CaCO3 solids were used in the experiments including calcite, aragonite, and ground clam, mussel, and cockle shells. Precipitation was induced by the presence of Cd(NO3)2 in the inlet solution, which combined with aqueous carbonate liberated by CaCO3 dissolution to supersaturate otavite. The precipitation of an otavite layer of less than 0.01 μm in thickness on calcite surfaces decreases its dissolution rate by close to two orders of magnitude. This decrease in calcite dissolution rates lowers aqueous carbonate concentrations in the reactor such that the mixed-flow reactor experiments attain a steady-state where the reactive fluid is approximately in equilibrium with otavite, arresting its precipitation. In contrast, otavite coatings are far less efficient in lowering aragonite, and ground clam, mussel, and cockle shell dissolution rates, which are comprised primarily of aragonite. A steady-state is only attained after the precipitation of an otavite layer of 3-10 μm thick; the steady state CaCO3 dissolution rate is 1-2 orders of magnitude lower than that in the absence of otavite coatings. The difference in behavior is interpreted to stem from the relative crystallographic structures of the dissolving and precipitating minerals. As otavite is isostructural with respect to calcite, it precipitates by epitaxial growth directly on the calcite, efficiently slowing dissolution. In contrast, otavite’s structure is appreciably different from that of aragonite. Thus, it will precipitate by random three dimensional heterogeneous nucleation, leaving some pore space at the otavite-aragonite interface. This pore space allows aragonite dissolution to continue relatively unaffected by thin layers of precipitated otavite. Due to the inefficiency of otavite coatings to slow aragonite and ground aragonite shell dissolution, aragonite appears to be a far better Cd scavenging material for cleaning polluted waste waters. 相似文献
13.
根据黄土高原第四纪古土壤和风化带的广泛调查,发现了CaCO3 等化学成分的不连续淀积、厚层及多层淀积等特殊地质现象,结合CaCO3 含量分析与入渗实验资料,建立了风化淋滤带CaCO3 淀积深度新理论。该理论表明,CaCO3 迁移到淀积深度所需时间很短,可以忽略时间因素对它的影响,能够作为研究风化淋滤带的许多地质问题的较可靠依据。当CaCO3 淀积深度小于古土壤发育带厚度时,可确定土壤已向风化壳转变 当Ca CO3 淀积深度大于古土壤层厚度时,可确定土壤为淋溶型、中酸性土壤 当同一风化剖面中或同一层古土壤下部出现两层、三层或厚度异常大的CaCO3 淀积层时,指示当时出现了两个或两个以上成壤期和相应的气候变化。 相似文献
14.
The results of experiments on the hydrothermal dolomitization of calcite (between 252 and 295°C) and aragonite (at 252°C) by a 2 M CaCl2-MgCl2 aqueous solution are reported and discussed. Dolomitization of calcite proceeds via an intermediate high (ca. 35 mole %) magnesian calcite, whereas that of aragonite is carried out through the conversion of the reactant into a low (5.6 mole %) magnesian calcite which in turn transforms into a high (39.6 mole %) magnesian calcite. Both the intermediate phases and dolomite crystallize through a dissolution-precipitation reaction. The intermediate phases form under local equilibrium within a reaction zone surrounding the dissolving reactant grains. The volume of the reaction zone solution can be estimated from Sr2+ and Mg2+ partitioning equations. In the case of low magnesian calcite growing at the expense of aragonite at 252°C, the total volume of these zones is in the range of 2 × 10?5 to 2 × 10?4 1., out of 5 × 10?3 1., the volume of the bulk solution.The apparent activation energies for the initial crystallization of high magnesian calcite and dolomite are 48 and 49 kcal/mole, respectively.Calcite transforms completely into dolomite within 100 hr at 252°C. The overall reaction time is reduced to approximately 4 hr at 295°C. The transformation of aragonite to dolomite at 252°C occurs within 24 hr. The nature of the reactant dictates the relative rates of crystallization of the intermediate phases and dolomite. With calcite as reactant, dolomite growth is faster than that of magnesian calcite; this situation is reversed when aragonite is dolomitized.Coprecipitation of Sr2+ with dolomite is independent of temperature (within analytical error) between 252 and 295°C. Its partitioning, with respect to calcium, between dolomite and solution results in distribution coefficients in the range of 2.31 × 10?2 to 2.78 × 10?2. 相似文献
15.
Calculations based on approximately 350 new measurements (CaT-PCO2) of the solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C indicate the following values for the log of the equilibrium constants KC, KA, and KV respectively, for the reaction CaCO3(s) = Ca2+ + CO2?3: where T is in oK. At 25°C the logarithms of the equilibrium constants are ?8.480 ± 0.020, ?8.336 ± 0.020 and ?7.913 ± 0.020 for calcite, aragonite and vaterite, respectively.The equilibrium constants are internally consistent with an aqueous model that includes the CaHCO+3 and CaCO03 ion pairs, revised analytical expressions for CO2-H2O equilibria, and extended Debye-Hückel individual ion activity coefficients. Using this aqueous model, the equilibrium constant of aragonite shows no PCO2-dependence if the CaHCO+3 association constant is between 0 and 90°C, corresponding to the value logKCahco+3 = 1.11 ± 0.07 at 25°C. The CaCO03 association constant was measured potentiometrically to be between 5 and 80°C, yielding logKCaCO03 = 3.22 ± 0.14 at 25°C.The CO2-H2O equilibria have been critically evaluated and new empirical expressions for the temperature dependence of KH, K1 and K2 are , and logK2 = ?107.8871 ? 0.03252849T + 5151.79/T + 38.92561 logT ? 563713.9/T2 which may be used to at least 250°C. These expressions hold for 1 atm. total pressure between 0 and 100°C and follow the vapor pressure curve of water at higher temperatures.Extensive measurements of the pH of Ca-HCO3 solutions at 25°C and 0.956 atm PCO2 using different compositions of the reference electrode filling solution show that measured differences in pH are closely approximated by differences in liquid-junction potential as calculated by the Henderson equation. Liquid-junction corrected pH measurements agree with the calculated pH within 0.003-0.011 pH.Earlier arguments suggesting that the CaHCO+3 ion pair should not be included in the CaCO3-CO2-H2O aqueous model were based on less accurate calcite solubility data. The CaHCO+3 ion pair must be included in the aqueous model to account for the observed PCO2-dependence of aragonite solubility between 317 ppm CO2 and 100% CO2.Previous literature on the solubility of CaCO3 polymorphs have been critically evaluated using the aqueous model and the results are compared. 相似文献
16.
CaCO3Ca(OH)2CaS serves as a model system for sulfide solubility in carbonatite magmas. Experiments at 1 kbar delineate fields for primary crystallization of CaCO3, Ca(OH)2 and CaS. The three fields meet at a ternary eutectic at 652°C with liquid composition (wt%): CaCO3 = 46.1%, Ca(OH)2 = 51.9%, CaS = 2.0%. Two crystallization sequences are possible for liquids that precipitate calcite, depending upon whether the liquid is on the low-CaS side, or the high-CaS side of the line connecting CaCO3 to the eutectic liquid. Low-CaS liquids precipitate no sulfide until the eutectic temperature is reached leading to sulfide enrichment. The higher-CaS liquids precipitate some sulfide above the eutectic temperature, but the sulfide content of the melt is not greatly depleted as the eutectic temperature is approached. Theoretical considerations indicate that sulfide solubility in carbonate melts will be directly proportional to and inversely proportional to ; it also is likely to be directly proportional to melt basicity, defined here by . A strong similarity exists in the processes which control sulfide solubility in carbonate and in silicate melts. By analogy with silicates, ferrous iron, which was absent in our experiments, may also exert an important influence on sulfide solubility in natural carbonatite magmas. 相似文献
17.
Guy L. Hovis 《Geochimica et cosmochimica acta》1984,48(3):523-525
Glasses in the systems NaAlSi3O8-KAlSi3O8 and NaAlSi3O8-Si4O8 have been studied by means of hydrofluoric acid solution calorimetry at 50°C. Results indicate small negative enthalpies of mixing in the former system and small positive departures from ideality in the latter. 相似文献
18.
Enthalpies of solution in 2PbO· B2O3 at 712°C have been measured for glasses in the systems albite anorthite diopside, NaAlO2-SiO2, Ca0.5AlO2-SiO2 and albite-anorthite-quartz. The systems albite-anorthite and diopside-anorthite show substantial negative enthalpies of mixing, albite-diopside shows significant positive heats of mixing. For compositions up to NaAlO2 = 0.42 (which includes the subsystem albite-silica) the system NaAlO2-SiO2 shows essentially zero heats of mixing. A negative ternary excess heat of mixing is found in the plagioclase-rich portion of the albite-anorthite-diopside system. The join Si4O8-CaAl2Si2O8 shows small but significant heats of mixing. In albite-anorthite-quartz. ternary glasses, the ternary excess enthalpy of mixing is positive.Based on available heat capacity data and appropriate consideration of the glass transition, the enthalpy of the crystal-glass transition (vitrification) is a serious underestimate of the enthalpy of the crystal-liquid transition (fusion) especially when the melting point, Tf, is many hundreds of degrees higher than the glass transition temperature, Tg. On the other hand, the same heat capacity data suggest that the enthalpies of mixing in albite-anorthite-diopside liquids are calculated to be quite similar to those in the glasses. The enthalpies of mixing observed in general support the structural models proposed by Taylor and Brown (1979a, b) and others for the structure of aluminosilicate glasses. 相似文献
19.
S.K. Saxena 《Geochimica et cosmochimica acta》1981,45(6):821-825
Using the model of fictive ideal components, Gibbs free energies of formation of pyrope and Al2O3-enstatite have been determined from the experimental data on coexisting garnet and orthopyroxene and orthopyroxene and spinel in the temperature range of 1200–1600 K. The negative free energies in kJ/mol are:
TK | 1200 | 1300 | 1400 | 1500 | 1600 |
Pyrope | 4869.92 | 4747.05 | 4614.26 | 4462.63 | 4311.00 |
Al2O3-enstatite | 1257.25 | 1244.28 | 1191.93 | 1158.67 | 1125.64 |