Harold C. Helgeson

Evaluation of irreversible reactions in geochemical processes involving minerals and aqueous solutions—I. Thermodynamic relations

Abstract Application of the thermodynamic principles of chemical petrology and solution chemistry to the study of geochemical processes permits prediction of the consequences of reaction between aqueous solutions of electrolytes and typical igneous, metamorphic, and sedimentary mineral assemblages. A given geochemical process can be represented by a set of reversible and irreversible chemical reactions that corresponds to an array of linear differential equations relating partial equilibrium and nonequilibrium in thermodynamic systems. Simultaneous evaluation of these equations defines the nature and extent of the compositional change and redistribution of species in the aqueous phase, the order of appearance of stable and metastable phases, and the mass transfer resulting from irreversible reactions between the minerals and the aqueous solution.

Evaluation of irreversible reactions in geochemical processes involving minerals and aqueous solutions—II. Applications

Abstract Equilibrium relations among common rock-forming minerals and aqueous solutions over a range of temperatures and pressures are known experimentally for a number of systems and can be calculated for others. This information permits prediction of the mass transfer involved in chemical reactions characteristic of geochemical processes. Calculations of this kind are used to examine various chemical and geologic implications of irreversibility in idealized models of weathering, evaporative concentration, diagenesis, hydrothermal rock alteration, and ore deposition.

Calculation of mass transfer in geochemical processes involving aqueous solutions

Abstract Differential equations providing for simultaneous dissolution of multiple reactant minerals, precipitation of mineral assemblages, variable activity of H 2 O, oxidation-reduction reactions, binary solid solution, and changes in activity coefficients in both open and closed systems are incorporated in a grand matrix equation describing mass transfer in geochemical processes. Computer evaluation of this equation affords quantitative prediction of the extent to which minerals are produced and/or destroyed as well as changes in the composition of phases and distribution of species in geologic systems in which an irreversible reaction takes place between a given mineral or mineral assemblage and an aqueous solution at constant temperature and pressure. Computers and thermodynamic data currently available permit mass transfer calculations to be carried out for systems involving more than 60 components, phases, and chemical species at temperatures up to 300°C.

Activity/composition relations among silicates and aqueous solutions; II, Chemical and thermodynamic consequences of ideal mixing of atoms on homological sites in montmorillonites, illites, and mixed-layer clays

The activities of thermodynamic components of clay minerals corresponding in composition to pyrophyllite, muscovite, paragonite, and margarite were computed from chemical analyses reported in the literature assuming ideal mixing of atoms on homological sites in the minerals. These activities were then used to generate stability fields for smectites, illites, and mixed-layer clays on logarithmic activity diagrams representing equilibrium among minerals and aqueous solutions at 25°C and 1 bar. Comparative analysis indicates that the approach affords close approximation of both mineral and water compositions in geologic systems.РезюмеРассаитывались активности термодинамических компонентов глинистых минералов, соответствующих по составу пирофиллиту, мусковиту, парагониту и Маргариту. Расчет был проведен на основании опубликованных данных химических анализов, предполагая идеальную смесь атомов в гомологических местах минералов. Полученные величины активностей использовались для определения полей стабильности смектитов, иллитов, и переслаивающихся глин на логаритмических диаграммах активностей, представляющих равновесие между минералами и водным раствором при температуре 25°С и давлении 1 бар. Сравнительный анализ указывает на то, что этот подход хорошо описывает состав минералов и воды в геологических системах. [E.G.]ResümeeDie Aktivitäten der thermodynamischen Komponenten von Tonmineralen, die in ihrer Zusammensetzung Pyrophyllit, Muskovit, Paragonit, und Margarit entsprechen, wurden aus chemischen Analysen, die in der Literatur angegeben sind, mittels Computer berechnet, wobei eine ideale Mischung von Atomen auf homologen Plätzen in den Mineralen angenommen wird. Diese Aktivitäten wurden dann verwendet, um die Stabilitätsbereiche von Smektiten, Illiten und Wechsellagerungstonen in logarithmischen Diagrammen aufzustellen, die Gleichgewicht zwischen den Mineralen und den wässrigen Lösungen bei 25°C und 1 Bar darstellen. Vergleichende Analysen deuten darauf hin, daß dieses Vorgehen zu einer guten Annäherung an die Mineral- und Wasserzusammensetzung in geologischen Systemen führt. [U.W.]RésuméLes activités des composés thermodynamiques de minéraux argileux correspondant en composition à la pyrophyllite, muscovite, paragonite, et margarite ont été computés à partir d’analyses chimiques rapportées dans la littérature, supposant un mélange idéal d’atomes sur des sites homologues dans les minéraux. Ces activités ont alors été utilisées pour générer des champs d’équilibre pour des smectites, illites et argiles à couches mélangées sur des diagrammes d’activité logarithmique représentant l’équilibre entre les minéraux et des solutions aqueuses à 25°C et 1 bar. L’analyse comparative indique que cette approche permet une approximation proche des compositions minérales et aqueuses dans des systèmes géologiques. [D.J.]

COMPOSITIONAL END MEMBERS AND THERMODYNAMIC COMPONENTS OF ILLITE AND DIOCTAHEDRAL ALUMINOUS SMECTITE SOLID SOLUTIONS

Consideration of XRD, TEM, AEM, and analytical data reported in the literature indicates that dioctahedral aluminous smectite and illite form two separate solid solutions that differ chemically from one another primarily by the extent of Al substitution for Si, the amount of interlayer K, and the presence of interlayer H2O. The data indicate that limited dioctahedral-trioctahedral and dioctahedral-vacancy compositional variations occur in both minerals. Excluding interlayer H2O and based on a half unit cell [i.e., O10(OH)2], natural dioctahedral smectite and illite solid solutions fall within the compositional limits represented by A0.3

On the correlation of expandability with mineralogy and layering in mixed-layer clays

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Summary and critique of the thermodynamic properties of rock forming minerals

Si4O10(OH)2-AR2+ R3+ Si4O10(OH)2-A0.25

Thermodynamics of hydrothermal systems at elevated temperatures and pressures

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Theoretical prediction of the thermodynamic behavior of aqueous electrolytes by high pressures and temperatures; IV, Calculation of activity coefficients, osmotic coefficients, and apparent molal and standard and relative partial molal properties to 600 degrees C and 5kb

Al0.25Si3.75O10(OH)2 for smectites and A0.8

Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions; I, Theoretical considerations

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