Geological system

E. P. Katsanis, P. H. Kmmrine, and J. S. Ealcone, Jr., "Chemistry of Precipitation and Scale Formation in Geological Systems," SPE preprint 11802, National Symposium on Oil Field and Geothermal Chemisty, Denver, Colo., June 1, Society of Petroleum Engineers, 1983. 

Occurrence. Iodine [7553-56-2] is widely distributed in the Hthosphere at low concentrations (about 0.3 ppm) (32). It is present in seawater at a concentration of 0.05 ppm (33). Certain marine plants concentrate iodine to higher levels than occur in the sea brine these plants have been used for their iodine content. A significant source of iodine is caUche deposits of the Atacama Desert, Chile. About 40% of the free world s iodine was produced in Japan from natural gas wells (34), but production from Atacama Desert caUche deposits is relatively inexpensive and on the increase. By 1992, Chile was the primary world producer. In the United States, underground brine is the sole commercial source of iodine (35). Such brine can be found in the northern Oklahoma oil fields originating in the Mississippian geological system (see Iodine and iodine compounds). 

For plutonium in the tri- and tetravalent state, when hydrolysis would dominate the solution chemistry, most sorption phenomena in geologic systems can be looked upon largely as physical adsorption processes. Ion exchange processes, as defined above, would be... 

Bird, D.K. and Helgeson, H.C. (1981) Chemical interaction of aqueous solution with epidote-feldspar mineral assemblages in geologic systems II, Equilibrium constraints in metamorphic/geothermal processes. Am. J. Set, 281, 576-614. 

Volcanoes are complex geological systems which perhaps played an active role in biogenesis, although we do not know how. 

The variety of life forms to be found near hydrothermal vents does not, of course, mean that life itself originated there these geological systems are much too unstable for that. The dynamics of tectonic plates cause the vents to disappear after some decades, or at most after a few hundred years. According to Nils Holm from the Department of Geology and Geochemistry at the University of Stockholm, the discovery of the hydrothermal vents led to intense, and in some cases controversial, discussions of the question as to whether hydrothermal systems were the birthplaces of life around four billion years ago. Many geologists believe that hydrothermal activity on the primeval Earth was probably stronger than it is today, as the thick... 

Let us first introduce some important definitions with the help of some simple mathematical concepts. Critical aspects of the evolution of a geological system, e.g., the mantle, the ocean, the Phanerozoic clastic sediments,..., can often be adequately described with a limited set of geochemical variables. These variables, which are typically concentrations, concentration ratios and isotope compositions, evolve in response to change in some parameters, such as the volume of continental crust or the release of carbon dioxide in the atmosphere. We assume that one such variable, which we label/ is a function of time and other geochemical parameters. The rate of change in / per unit time can be written... 

In general, the formulation of the problem of vapor-liquid equilibria in these systems is not difficult. One has the mass balances, dissociation equilibria in the solution, the equation of electroneutrality and the expressions for the vapor-liquid equilibrium of each molecular species (equality of activities). The result is a system of non-linear equations which must be solved. The main thermodynamic problem is the relation of the activities of the species to be measurable properties, such as pressure and composition. In order to do this a model is needed and the parameters in the model are usually obtained from experimental data on the mixtures involved. Calculations of this type are well-known in geological systems O) where the vapor-liquid equilibria are usually neglected. 

Bird D. K. and Helgeson H. C. (1980). Chemical interaction of aqueous solution with epidolite-feldspar mineral assemblage in geologic systems, I Thermodynamic analysis of phase relations in the system Ca0-Fe0-Fe203-Al203-Si02-H20-C02. Amer. Jour. Set, 280 907-941. 

Helgeson H. C., Brown T. H., and Leeper R. H. (1976). Handbook of Theoretical Activity Diagrams Depicting Chemical Equilibria in Geologic Systems Involving an Aqueous Phase at 1 atm and O to 300 °C. Freeman, Cooper, San Francisco. 

It is commonplace to assume a form of the Gibbs energy function which excludes the pressure variable for solid-state phase transformations, as the magnitude of the PAV term is small at atmospheric pressures. This is of course not the case in geological systems, or if laboratory experiments are deliberately geared to high-pressure environments. Klement and Jayaraman (1966) provide a good review of the data available at the time when some of the earliest CALPHAD-type calculations were made (Kaufman and Bernstein 1970, Kaufman 1974). Much work was also carried out on specific alloy systems such as Fe-C (Hilliard 1963) and the Tl-In system (Meyerhoff and Smith 1963). 

In theory, one can use statistical thermod3mamics to calculate the partition functions of all the species from first principles, AS, AH, and hence k. For simple systems, the calculation results are in good agreement with experimental data (e.g.. Chapter 3 in Laidler, 1987). For complicated geological systems, however, it is not possible to calculate k from first principles, but the concept of activated complexes is very useful for a microscopic understanding of the reaction... 

As long as care is taken so that effective binary diffusivity obtained from experiments under the same set of conditions is applied to a given problem, the approach works well. Although the limitations mean additional work, because of its simplicity and because of the unavailability of the diffusion matrices, the effective binary diffusion approach is the most often used in geological systems. Nonetheless, it is hoped that effort will be made in the future so that multi-component diffusion can be handled more accurately. 

Thus the sufficient condition to reach secular equilibrium in an undisturbed geological system is enough isolated time for the system. The time to reach secular equilibrium is normally five half-lives of the daughter nuclides. For h, the half-life for is 75,000 years, and after 375,000 years, or five half lives of °Th, the system reaches equilibrium. Similarly, the half-life for Ra is 1,600 years, and Th-Ra secular equilibrium can be reached after five half-lives of a, or 8,000 years. 

This is the well-known Rayleigh fractionation law it is applicable to geological systems only when the bulk partition coefficients remain constant throughout fractional crystallization. 

Natural fission reactors in the Franceville basin, Gabon A review of the conditions and results of critical event in a geological system. Geochimica Cosmochimica Acta, 60, 4831-4852. 

Tnorganic geochemistry is dominated by equilibrium processes. Most reactions are rapid, and thermodynamic equilibria are established within geologically short time spans. Many reactions which appear slow in the laboratory still proceed sufficiently fast to influence the composition of geological systems. Noteworthy exceptions involve certain metastable ions like CO32", S042-, and P043-, which may persist even in unfavorable environments for millions if not billions of years. 

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