Solutions, Mixtures, and Models

Let us not grace loose thinking with the word model . 10.1. SOLUTIONS, MIXTURES, AND MODELS 

Thus if A is albite and B is anorthite, then (A,B) is plagioclase, and we say that the plagioclase solid solution is more stable than a mechanical mixture of grains of albite and anorthite. On the other hand if A is diopside and B is anorthite, little or no mutual solution takes place because in this case 

It will be helpful to recall our discussion of rnetastable systems in Chapter 3, where we explained that constituents that are together but do not react or interact in any way, such as those separated by a partition, or those having an activation energy barrier, are examples of metastable systems. Metastable systems have an extra constraint that prevents their constituents from reacting to achieve their lowest potential. In the case of the mechanical mixture albite-anorthite it is the activation energy for water-halite it could be a partition in a composite system. In this chapter, for lack of a better term, we use mechanical mixture for this kind of metastable state. 

In the following sections we will quantify some of the thermodynamic properties of mechanical mixtures and ideal and non-ideal solutions. As we detail the properties of ideal solutions, it will become clear that they are strictly hypothetical another thermodynamic concept, like true equilibrium , which is a limiting state for real systems. Ideal solutions, in other words, are another part of the thermodynamic model, not of reality. It is a useful concept, because real solutions can be compared to the hypothetical ideal solution and any differences described by using correction factors (activity coefficients) in the equations describing ideal behavior. These correction factors can either be estimated theoretically or determined by actually measuring the difference between the predicted (ideal) and actual behavior of real solutions. 

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