ABSTRACT Geological fluids play important roles in many processes, such as mineral deposits, volcanic eruptions, magmatic activities, metamorphism, hydrothermal venting, geothermal evolution, petroleum and natural gas formation and migration, and waste disposal. The knowledge of the physico-chemical properties (activity, fugacity, phase equilibria, PVT properties, enthalpy, solubilities) is essential in the understanding of the geochemical behavior of the various processes associated with fluids. The knowledge is usually obtained from experimental observation. However, the experimental data are generally limited to a small range of natural conditions (temperature, pressure and composition). In order to understand geochemical behavior of the fluids quantitatively under conditions which experimental data do not cover, geochemists often use semi- theoretical models, such as equation of state and specific interaction models, because these models, if well established, should not only reproduce experimental data, but also extrapolate well beyond data range. In this article, examples of such models are presented. With computers becoming increasingly more powerful, molecular dynamics (MD) simulation has emerged as a very promising tool in the study of geological systems. This new method not only shows its power as computer experiment (to produce experi- mental data by computer simulation based on inter-particle interactions), but also accomplishes experimentally difficult tasks. It can also provide structural information and microscopic properties which are difficult to observe in lab. However, much remains to be done on aqueous systems.
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