A model is a simplified version of reality that is useful as a tool. A successful model strikes a balance between realism and practicality. Properly constructed, a model is neither so simplified that it is unrealistic nor so detailed that it cannot be readily evaluated and applied to the problem of interest. Geologic maps constitute a familiar class of models. To map a sedimentary section, a geologist collects data at certain outcrops. He casts his observations in terms of the local stratigraphy, which is itself a model that simplifies reality by allowing groups of sediments to be lumped together into formations. He then interpolates among his data points (and projects beneath them) to infer positions for formation contacts, faults, and so on across his field area. The final map is detailed enough to show the general arrangement of formations and major structures, but simplified enough, when drawn to scale, that small details do not obscure the overall picture. The map, despite its simplicity, is without argument a useful tool for understanding the area's geology. To be successful, a geochemical model should also portray the important features of the problem of interest without necessarily attempting to reproduce each chemical or mineralogical detail. The first and most critical step in developing a geochemical model is conceptualizing the system or process of interest in a useful manner. By system, we simply mean the portion of the universe that we decide is relevant. The composition of a closed system is fixed, but mass can enter and leave an open system. A system has an extent, which the modeler defines when he sets the amounts of fluid and mineral considered in the calculation. A system's extent might be a droplet of rainfall, the groundwater and sediments contained in a unit volume of an aquifer, or the world's oceans. The “art” of geochemical modeling is conceptualizing the model in a useful way. Figure 2.1 shows schematically the basis for constructing a geochemical model. The heart of the model is the equilibrium system, which remains in some form of chemical equilibrium, as described below, throughout the calculation.
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