Читать книгу Geochemistry - William M. White - Страница 92

Solutions are defined as homogeneous phases produced by dissolving one or more substances in another substance. In geochemistry we are often confronted by solutions: as gases, liquids, and solids. Free energy depends not only on T and P, but also on composition. In thermodynamics it is generally most convenient to express compositions in terms of mole fractions, X_i, the number of moles of i divided by the total moles in the substance (moles are weight divided by atomic or molecular weight). The sum of all the X_i fractions must, of course, total to 1.

Solutions are distinct from purely mechanical mixtures. For example, salad dressing (oil and vinegar) is not a solution. Similarly, we can grind anorthite (CaAl₂Si₂O₈) and albite (NaAlSi₃O₈) crystals into a fine powder and mix them, but the result is not a plagioclase solid solution. The Gibbs free energy of mechanical mixtures is simply the sum of the free energy of the components. If, however, we heated the anorthite–albite mixture to a sufficiently high temperature that the kinetic barriers were overcome, there would be a reordering of atoms and the creation of a true solution. Because this reordering is a spontaneous chemical reaction, there must be a decrease in the Gibbs free energy associated with it. This solution would be stable at 1 atm and 25°C. Thus, we can conclude that the solution has a lower Gibbs free energy than the mechanical mixture. On the other hand, vinegar will never dissolve in oil at 1 atm and 25°C because the Gibbs free energy of that solution is greater than that of the mechanical mixture.