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1.
A tetradeuteriated cholestane was heated with kerogen isolated from the Messel oil shale at 350 °C for 25 different time periods (ranging from 0.5 to 80 h) in borosilicate vessels. Three isomers of tetradeuteriated dimethylperhydrophenanthrene were formed and one of these was the major saturated hydrocarbon product under most conditions. The stereochemistry for these isomers was unknown but if the stable 5α,8β,9α(H),10β(CH3) sterane configuration is inherited from the cholestane then there are four possible isomers namely: I and II [13β(H) and 13α(H) diastereomers, respectively, of 10,13-dimethylperhydrophenanthrene] as well as III and IV [14β(H) and 14α(H) diastereomers, respectively, of 10,14-dimethylperhydrophenanthrene]. Molecular mechanics revealed that I is the most stable form and therefore is the component that dominated the saturated hydrocarbon degradation products. The identification of only three isomers of deuteriated dimethylperhydrophenanthrene in the pyrolysis experiments is supported by the calculated stabilities (ΔΔG) as isomer IV is considerably more unstable (by 1.7 kcal/mol) than II which is the least stable of I, II and III. If these compounds can be detected in crude oils their distributions may be useful maturity indicators in oils that have been generated at high thermal maturity. 相似文献
2.
The study of geochemical transformations undergone by ‘biological markers’ after their incorporation into sediments is an important field of organic geochemistry. Combined with laboratory simulation experiments, molecular mechanics calculations have been shown to be very useful to establish the reaction pathways, and to predict intermediate components and stable reaction end products, especially in the case of the acid-catalyzed isomerization reactions of steroid and terpenoid hydrocarbons. Many commercially available softwares are able to optimize (minimize) the geometries of molecules and compute some of their thermodynamical data with either molecular mechanics (MM) or semi-empirical methods of quantum chemistry. In order to verify the reliability of these methods, we have computed the relative thermodynamic stabilities of a large number of steradiene isomers with MM3 (Tripos Inc.), MM+ (HYPERCHEMTM) and MM2 (Chem3D, CambridgeSoft Corp.) empirical force fields, and with AM1 and PM3 (HYPERCHEMTM) semi-empirical methods. The calculation results of thermodynamic stabilities of steradiene isomers are used to explain the compounds produced by the rearrangement of cholesta-3,5-diene when treated with p-toluenesulfonic acid in acetic acid at 70°C. The end products, namely the spirosteradienes 7–8, obtained by this treatment are the most stable steradiene isomers according to all computational methods. The relative thermodynamic stabilities of cholestadienes are also consistent with the mechanism postulated for the spirosteradiene formation proceeding through a pathway including cholestadienes 2–6 as intermediates. 相似文献
3.
Doubts have been expressed as to whether solar matter is sufficiently reducing to explain the minerals occurring in the enstatite chondrites and in the enstatite achondrites. Thermodynamic calculations on the stabilities of TiN, Si2N2O, CaS and silicon-bearing iron metal show that these substances can form under equilibrium conditions from a nebula of solar composition provided that the total pressure exceeds ~ 1 atm and that thermodynamic equilibria are frozen in at near-formation temperatures. 相似文献
4.
视剖面图温压计研究进展评述 总被引：3，自引：1，他引：2
视剖面图温压计是目前定量估算变质温度和压力的主要方法之一。其理论基础为热力学平衡原理与质量守恒定律。利用内恰的热力学数据库和相应的成分-活度模型对特定成分体系进行视剖面图计算,可以正演模拟给定温度和压力条件下的矿物组合、矿物丰度及其成分,与实测岩石对比,可以准确获取岩石的形成温度和压力。在应用视剖面图温压计时,应选择受全岩成分影响小、且受退变质作用影响微弱的矿物成分;对于有成分变化的体系,应结合多种方法恢复有效全岩成分。文中通过视剖面图对KFMASH(K2O-Fe O-Mg O-Al2O3-Si O2-H2O)体系下不同矿物组合内多硅白云母硅含量及其等值线斜率的研究,认为多硅白云母硅含量及其等值线斜率主要由不同矿物组合内多硅白云母参与的契尔马克替换及纯转换端元反应中的主导反应决定,进一步深化了对变质过程中控制矿物成分变化的内部缓冲反应机制的理解,从而也可为选择和应用矿物温压计提供指导。因此,视剖面图方法是目前研究变质岩石形成条件及变质作用精细过程的最佳方法之一。 相似文献
5.
Steranes and terpanes biodegrade at a slower rate than isoprenoids and survive moderate biodegradation. Heavy biodegradation results in destruction of regular steranes, survival of diasteranes (20R better than 20S) and tricyclic terpanes and transformation of hopanes to Ring A/B demethylated hopanes. These survivors can be used as source fingerprints in biodegraded crudes. The structure of predominant steranes in undegraded to moderately degraded fossil fuels was proven to be 14β, 17β(H) (20R + S) by molecular spectroscopy. These compounds plus the 20S epimers of regular 5α-steranes (20R) were identified as major constituents and their 5β-counterparts as minor components in a cholestane isomerizate (300°C, Pt on C), allowing assessment of relative thermodynamic stabilities. An observed increase of optical activity in heavily degraded crudes from three different basins is interpreted to be the result of bacterial transformation of terpanes and steranes to new optically active species plus enrichment of the latter by n + isoparaffin depletion rather than total bacterial synthesis. Diagnostic ion profiling by GC-MS-C is a convenient tool for surveying the relative abundance of individual diasteranes and regular steranes plus distinguishing epimeric and ring skeletal isomeric series in complex fossil fuel mixtures. A new practical method of determining the absolute quantities of individual steranes by spiking with 5β-cholane and integration of mass chromatograms is described. 相似文献
6.
Ewa Kolaczkowska Nacer-Eddine Slougui David S. Watt Robert E. Maruca J. Michael Moldowan 《Organic Geochemistry》1990,16(4-6)
The thermodynamic stability of selected alkylated, dealkylated and rearranged 17α- and 17β-hopane isomers in the C27, C28, C29, C30 and C31 families were calculated using molecular mechanics (MM2) methods and, where possible, calculated equilibrium ratios of certain isomers were compared with observed ratios of isomers in thermally mature crude oil samples. Those calculated and observed ratios having similar values include: (1) the relative distributions among 17β(H)/17α(H) and 21β(H)/21α(H)-hopanes including the absence of the 17β(H),21β(H)- and 17α(H),21α(H)-hopanes; (2) the 22R/22S ratios in 30-methyl-17α-hopane and 30-methyl-17β-moretane; (3) the relative distributions among 17α(H)/17β(H)- and 21α(H)/21β(H)-28,30-bisnorhopanes and among 25,28,30-trisnorhopanes, including the relatively greater stability of 17β(H) isomers in contrast to the regular hopane series; and (4) the ratios of 28(18−17S)abeo hopanes with respect to their unrearranged counterparts including the C27 compounds, Ts/Tm. 相似文献
7.
自然界中矿物多以固溶体形式存在,据其晶体化学特征计算热力学性质是开展矿物成因理论研究的基础。本文引入描述二元矿物固溶体热力学性质的假三元模型,计算得到了透辉石-硬玉固溶体系列的热力学性质。该模型通过构造一种高度有序的中间相,同时考虑长程和短程有序效应,基于热力学平衡态矿物固溶体自由能最低的规律,可以计算特定组分下矿物的平衡自由能、焓和熵等热力学参数。本文针对透辉石-硬玉固溶体体系,取绿辉石为其中间有序态,计算了其活度-成分关系和温度-组分相图等,发现绿辉石随温度升高的有序无序相变为一级相变,相变温度为1 148±25 K,与实验研究结果一致。本文获得的透辉石-绿辉石-硬玉体系的热力学参数可用于视剖面图方法研究MORB成分的岩石的榴辉岩相变质作用过程。 相似文献
8.
广西拉么Zn—Cu矿床中夕卡岩的主要矿物特点与Ca—Fe—Si—C—O体系T—lgfO2图解 总被引：1，自引：0，他引：1
用电子探针数据研究了该矿床中主要夕卡岩矿物特点及其变化规律。采用理想结晶溶液固溶体位置混合模式,计算了石榴子石和辉石矿物对中钙铁榴石和钙铁辉石的摩尔分数。在计算纯固相和流体相参加的反应抵达平衡时的边界方程中,加上理想结晶固溶体中端元组分摩尔分数的修正项后,我们便可以计算出有纯固相、流体相和理想结晶溶液固溶体参加反应的在给定T、P条件下的lgfo_2值。计算结果表明:无变度点4周围的Hed+And+Wol组合对本矿区具有重要的地质地球化学意义。 相似文献
9.
The hydroxy acid suites extracted from the Murchison (MN), GRA 95229 (GRA) and LAP 02342 (LAP) meteorites have been investigated for their molecular, chiral and isotopic composition. Substantial amounts of the compounds have been detected in all three meteorites, with a total abundance that is lower than that of the amino acids in the same stones. Overall, their molecular distributions mirror closely that of the corresponding amino acids and most evidently so for the LAP meteorite. A surprising l-lactic acid enantiomeric excess was found present in all three stones, which cannot be easily accounted by terrestrial contamination; all other compounds of the three hydroxy acid suites were found racemic. The branched-chain five carbon and the diastereomer six-carbon hydroxy acids were also studied vis-a-vis the corresponding amino acids and calculated ab initio thermodynamic data, with the comparison allowing the suggestion that meteoritic hydroxyacid at these chain lengths formed under thermodynamic control and, possibly, at a later stage than the corresponding amino acids. 13C and D isotopic enrichments were detected for many of the meteoritic hydroxy acids and found to vary between molecular species with trends that also appear to correlate to those of amino acids; the highest δD value (+3450‰) was displayed by GRA 2-OH-2-methylbutyric acid. The data suggest that, while the amino- and hydroxy acids likely relate to common presolar precursor, their final distribution in meteorites was determined to large extent by the overall composition of the environments that saw their formation, with ammonia being the determining factor in their final abundance ratios. 相似文献
10.
J. Nicholls 《Contributions to Mineralogy and Petrology》2013,166(3):691-701
Perfect fractionation models help infer the conditions under which molten rock material travels from the planet’s interior to the Earth’s surface where it cools and crystallizes. Quantitative models of melt crystallization, perfect fractionation paths through P–T-composition space, return calculated values of predicted quantities that can be compared to glass, rock, and mineral compositions measured in lava flows. Perfect fractionation models are based on thermodynamics and material balance constraints. Linear combinations of chemical potentials define equilibrium positions. The composition of the melt follows a path directly away from the composition of the phases at saturation, a material balance criterion. The number of adjustable parameters in a perfect fraction model is limited to two by Duhem’s theorem. Perfect fractionation models with one or two phases at saturation require only one mole fraction and one adjustable parameter be specified to calculate the fractionation path. The temperature and compositions of the saturating phases are determined by the equilibrium equations for one- or two-phase saturation in a melt of known compositions. With three or more phases at saturation, the composition of the melt has to be adjusted in the general case. Fractionation paths can also be inferred from a sequence of thermodynamic states calculated by minimizing a thermodynamic potential. The minimization procedure produces a fraction path that is conceptually different from the perfect fractionation path. Perfect fractionation models can be constructed to conserve a particular thermodynamic potential or variable such as enthalpy or density. 相似文献
11.
Thermodynamic model for arsenic speciation in sulfidic waters: A novel use of ab initio computations
Recent discoveries demonstrate that the chemistry of arsenic in sulfidic waters is much more complex that previously believed. One implication is that all earlier thermodynamic data on stabilities of As thioanions require revision. Previously used experimental approaches for determining As thioanion stabilities may be inadequate to deal with the full range of complexity. Here we use computational as well as empirical information to construct a provisional model for equilibrium As thioanion distributions in sulfidic waters. Whereas previous authors have argued for either As(III) or As(V) thioanions, the new model predicts that both are important and can occur simultaneously under commonly encountered pH and ΣS−II conditions. At the order of magnitude level, the model reasonably predicts the solubility of As2S3 in sulfidic solutions, provides tentative peak assignments for published Raman spectroscopic data and plausibly accounts for how sulfide modifies the bacterial toxicity of As. The model yields a thermodynamic justification for how sulfide, which is usually regarded as a reducing agent, can counter-intuitively drive oxidation of As(III) to As(V), as has been observed both in the laboratory and in the field. Despite its uncertain accuracy, the model serves as a useful source of new, testable hypotheses about As geochemistry and highlights crucial experimental data needs. 相似文献
12.
As geochemical appraisals of mineral regions of commercial prospectivity evolve, the organic matter associated with metal rich ores has attracted greater attention. Petroleum basin and modern seafloor hydrothermal vent studies have suggested that organic matter can have a significant influence on the behaviour of mineralising fluids. There have been many isolated reports of certain organic compositional or morphological (e.g. pyrobitumen) features showing an apparent relationships with hydrothermal fluids or minerals, raising expectations that organic based parameters might be useful to mineral exploration. However, the understanding of organic–inorganic relationships in Earth systems is far from complete. For example, the detailed mechanics of the interaction of organics with hydrothermal fluids over geological time remain largely undefined. Organic geochemistry studies have traditionally involved the measurement and interpretation of the hydrocarbon composition of sedimentary rocks. Here we review the types of aliphatic hydrocarbons, aromatic hydrocarbons and metalloporphyrins often detected from organic geochemical investigations in mineral-rich regions. Such molecular data can be particularly diagnostic of biochemical sources and the palaeo-environments at the time mineral associated organic matter was deposited. Sub-surface trends of hydrocarbon alteration may also reflect major biogeochemical processes such as thermal maturity and biodegradation. Organic geochemistry data can also occasionally provide information about the nature (e.g., origin, composition, temperatures) and migration pathways of hydrothermal fluids and can make a contribution to holistic ore genesis models. The well preserved organic matter associated with the economic “Here's Your Chance” Pb–Zn–Ag Mine (Paleoproterozoic Barney Creek Formation, McArthur Basin, Australia) and the transition metal-rich Early Permian Kupferschiefer Formation (Germany–Poland) have attracted significant attention. A more detailed summary of the organic character of these deposits is provided to highlight the contribution organic geochemistry can make to understanding mineralisation processes. Most organic geochemical studies of highly mineralised regions, however, have not adequately addressed the significance of organic matter to mineralisation. A slightly different analytical focus than traditionally used for exploration appraisal of petroleum hydrocarbons may be required to properly evaluate the significance of organic species to the mobilisation, transport and deposition of ore metals. The characterisation and subsequent thermodynamic modeling of organic substances and complexes within metalliferous hydrothermal systems will contribute to a better understanding of the nature and role of organic–inorganic fluids or other affiliated organics in ore systems. 相似文献
13.
露天地下开采隔离层稳定性研究 总被引：2，自引：0，他引：2
露天地下开采隔离层稳定性分析是矿山中经常遇到的问题。以大宝山矿露天地下开采的工程实例,采用数值分析方法,对隔离层稳定性进行分析计算,发现空区顶板的拉应力是关系隔离层稳定性的关键因素。通过对隔离层的安全厚度数值计算,与5种理论计算方法结果进行了比较,综合求和归一法数据处理和多项式数值逼近得到了不同空区跨度与隔离层安全厚度关系。结果为露天地下开采隔离层稳定性分析的方法进行了验证和补充,也为矿山设计隔离层厚度提供了参考,对指导露天地下开采安全生产施工具有重要意义。 相似文献
14.
Fougerite is a new iron oxide, a mixed M(II)–M(III) hydroxide, a member of the green rust group. Its structure consists of a brucitic layer of Fe(III)–Fe(II)–Mg(II), where the excess of the positive charge due to Fe3+ is compensated in the interlayer by anions. The limits of composition are structurally and geochemically constrained, and the stabilities of the mineral and green rusts are obtained by a thermodynamic model of a regular solid solution, for different compensating anions and for any allowed composition of the brucitic layer. 相似文献
15.
利用量子化学半经验计算ＰＭ３方法，对硅酸盐矿物结构中重要的六元环体系的各种同分异构体进行了能量差值的计算研究。同实验结果相比，计算的能量差值△Ｅ具有较好的精度，本文用这些△Ｅ值具体讨论了六元环的各种形式中Ａｌ避免原则的正确性和多样性。 相似文献
16.
Evangelos Moulas Stefan M. Schmalholz Yury Podladchikov Lucie Taj
manov Dimitrios Kostopoulos Lukas Baumgartner 《Journal of Metamorphic Geology》2019,37(1):1-14
Pressure is one of the most important parameters to be quantified in geological problems. However, in metamorphic systems the pressure is usually calculated with two different approaches. One pressure calculation is based on petrological phase equilibria and this pressure is often termed thermodynamic pressure. The other calculation is based on continuum mechanics, which provides a mean stress that is commonly used to estimate the thermodynamic pressure. Both thermodynamic pressure calculations can be justified by the accuracy and applicability of the results. Here, we consider systems with low‐differential stress (<1 kbar) and no irreversible volumetric deformation, and refer to them as conventional systems. We investigate the relationship between mean stress and thermodynamic pressure. We discuss the meaning of thermodynamic pressure and its calculation for irreversible processes such as viscous deformation and heat conduction, which exhibit entropy production. Moreover, it is demonstrated that the mean stress for incompressible viscous deformation is essentially equal to the mean stress for the corresponding viscous deformation with elastic compressibility, if the characteristic time of deformation is five times longer than the Maxwell viscoelastic relaxation time that is equal to the ratio of shear viscosity to bulk modulus. For typical lithospheric rocks, this Maxwell time is smaller than c. 10,000 years. Therefore, numerical simulations of long‐term (>10 kyr) geodynamic processes, employing incompressible deformation, provide mean stress values that are close to the mean‐stress value associated with elastic compressibility. Finally, we show that for conventional systems the mean stress is essentially equal to the thermodynamic pressure. However, mean stress and, hence, thermodynamic pressure can be significantly different from the lithostatic pressure. 相似文献
17.
18.
Based on the thermodynamic perturbation theory an equation of state (EOS) for molecular fluids has been formulated which can be used for many fluid species in geological systems. The EOS takes into account four substance specific parameters. These are the molecular dipole moment, the molar polarizability and the two parameters of the Lennard-Jones potential. For many fluids these parameters can be evaluated directly or indirectly from experimental measurements. In the absence of direct experimental determinations, as a first approximation, for a pure fluid the parameters of the Lennard-Jones potential can be evaluated using the critical temperature and the critical density if for polar molecules in addition the dipole moment is known with reasonable accuracy. The EOS with its model potential has the appropriate asymptotic behaviour at high pressures and temperatures and can be used to calculate both vapor-liquid equilibria and thermodynamic properties of single phase fluids up to at least 10 GPa and 2000 K. Currently, parameters for 98 inorganic and organic compounds are available. In this article the EOS for pure fluids is presented. In a further communication the EOS is extended to fluid mixtures (Churakov and Gottschalk, 2003). 相似文献
19.
A number of sources of uncertainty are involved in thermobarometric calculations, the most important of which are associated with analytical precision, activity–composition ( a – x ) relationships, and thermodynamic data. Statistical treatment of these uncertainties results in relatively large uncertainties on the calculated values of pressure and temperature. Little can be done, at least in the short term, about the magnitude of such uncertainties, and any thermobarometric calculations in which they are not taken into account should be treated with caution. Given that uncertainties associated with a–x models and thermodynamic data are systematic when applied to multiple samples with the same mineral assemblage, a solution to the problem of imprecise absolute thermobarometry can be obtained via a relative thermobarometric technique referred to as the Δ PT approach. The Δ PT approach offers a major improvement in the precision of thermobarometry if the calculations can be presented in a Δ PT context. 相似文献
20.
Capillary geochemistry in non-saturated zone of soils. Water content and geochemical signatures 总被引：1，自引：0，他引：1
The unsaturated zone (UZ) retains aqueous solutions against gravity by capillary forces. This suction state corresponds to a decreasing internal pressure of the water, which modifies its thermodynamic properties. Accordingly, the speciation of solutes and the solubility of solids and gases in such capillary solutions change. The volumetric capillary water content of the soil at high suction can be calculated extrapolating the water retention curves (WRC) with the Rossi–Nimmo model. Interestingly, several tens of liters per cubic meter of soil can be thus suctioned, a sufficiently large volume to support that: (1) capillary water is not restricted to nanosized pores, which means it disobeys the Young–Laplace law and is metastable with respect to vapor (superheating); and (2) the geochemistry of capillary solutions might significantly influence the subsurface mass transfer. Two field situations are here interpreted using the capillary thermodynamic properties: (1) the trapping of sand grains during the growth of desert roses (gypsum), and (2) the development of abnormal paragenetic sequences in some saprolites.The capillary approach is extended to the soil solids, so that the micro-mineralogy can be explicitly (though sketchily) integrated in the calculations. The key conclusion is that capillarity changes the saturation indexes (and so the reaction rates) at given solution composition, in a way consistent with the field observations. This perspective amounts to geochemically distinguishing the capillary and percolating solutions, which is interestingly analogous to the immobile and mobile water distinction already often integrated in UZ flow models. 相似文献