More on Components and Phases

Before proceeding to a discussion of the applications of the Phase Rule in geology, we must unfortunately return to the question of the definition of components and phases. 

TVace Components are those components, usually chosen as elements, that occur at very low concentration in the minerals of the system (e.g., V, Cr, Co etc. in a granite), and are not an essential part of any phase in the sense that their concentrations could be varied over wide ranges without changing the number of phases. Each therefore could represent a degree of freedom, but it is usually one that doesn t interest us. 

Determinative Components are those left after subtracting all the components in the above categories. They are thus the set of independent components the abundances of which control the nature and proportions of the phases, other than those eliminated from consideration. The determinative components are those that are used in plotting diagrams of the phase relations. 

All these terms for various kinds of components are perhaps useful, and the discussions of Thompson, Weill and Fyfe, and Korzinskii are recommended reading for those particularly interested in this topic. We prefer however to offer the discussion on applications, which follows, unburdened by the details of the definitions and discussions by these authors. Our discussion could only get more difficult to follow if we introduced these terms, which are in fact not much used. Our point of view with respect to the disagreements expressed by these authors should be apparent from our discussion. 

We have already mentioned ( 3.3.5) some of the reasons that real equilibrium states are only approximations to the true equilibrium state required by the thermodynamic model. These had to do with imperfections in minerals, and by the same reasoning minerals themselves are not part of the model. That is, they are not phases in the thermodynamic sense, despite common usage. However, just as natural systems approach true equilibrium sufficiently closely that thermodynamics can be applied to them, so naturally occurring phases such as minerals approach thermodynamic phases sufficiently closely that thermodynamics can be applied to them e.g., they usually obey the Phase Rule. Obviously, some minerals approximate a thermodynamic phase more closely than do others e.g., almost any quartz crystal is more homogeneous than a zoned plagioclase. But on close examination, it will be found that no minerals, and perhaps no solids, satisfy the requirements of the thermodynamic model. Thermodynamic phases are just as much an abstraction as thermodynamic comjxjnents or thermodynamic equilibrium, or any other part of the thermodynamic model. 

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