Читать книгу Geochemistry - William M. White - Страница 84

In the previous chapter, we introduced thermodynamic tools that allow us to predict the equilibrium mineral assemblage under a given set of conditions. For example, having specified temperature, we were able to determine the pressure at which the assemblage anorthite + forsterite is in equilibrium with the assemblage diopside + spinel + enstatite. In that reaction the minerals had unique and invariant compositions. In the Earth, things are not quite so simple: these minerals are present as solid solutions*, with substitutions of Fe²⁺ for Mg, Na for Ca, and Cr and Fe³⁺ for Al, among others. Indeed, most natural substances are solutions; that is, their compositions vary. Water, which is certainly the most interesting substance at the surface of the Earth and perhaps the most important, inevitably has a variety of substances dissolved in it. These dissolved substances are often of primary geochemical interest. More to the point, they affect the chemical behavior of water. For example, the freezing temperature of an aqueous NaCl solution is lower than that of pure water. You may have taken advantage of this phenomenon by spreading salt to de-ice sidewalks and roads.

In a similar way, the equilibrium temperature and pressure of the plagioclase + olivine ⇌ clinopyroxene + spinel + orthopyroxene reaction depends on the composition of these minerals. To deal with this compositional dependence, we need to develop some additional thermodynamic tools, which is the objective of this chapter. This may seem burdensome at first: if it were not for the variable composition of substances, we would already know most of the thermodynamics we need. However, as we will see in Chapter 4, we can use this compositional dependence to advantage in reconstructing conditions under which a mineral assemblage or a hydrothermal fluid formed.

A final difficulty is that the valance state of many elements can vary. Iron, for example, may change from its Fe²⁺ state to Fe³⁺ when an igneous rock weathers. The two forms of iron have very different chemical properties; for example, Fe²⁺ is considerably more soluble in water than is Fe³⁺. Another example of this kind of reaction is photosynthesis, the process by which CO₂ is converted to organic carbon. These kinds of reactions are called oxidation–reduction, or redox reactions. The energy your brain uses to process the information you are now reading comes from oxidation of organic carbon – carbon originally reduced by photosynthesis in plants. To fully specify the state of a system, we must specify its “redox” state. We treat redox reactions in the final section of this chapter.

Though Chapter 4 will add a few more tools to our geochemical toolbox, and treat a number of advanced topics in thermodynamics, it is designed to be optional. With completion of this chapter, you will have a sufficient thermodynamic background to deal with a wide range of phenomena in the Earth, and most of the topics in the remainder of this book.