Strain-free solvus

The compositional relations of the sodic and potassic phases in perthites depend on whether the phases are coherent or not. If the perthitic phases are non-coherent (or if the rock consists of separate grains of sodic and potassic feldspar) their equihbrium compositions are given by the strain-free solvus. This is the solvus which has been studied extensively in the past. These sentences. 

The outer contours of the subsolidus curves delineating the metastable immis-cibility regions in Figure 3.20 correspond to the strain-free solvus or binodal (soUd lines), the inner contours (dashed lines) to the spinodal. A brief introduction to the theory of spinodal demixing of crystals is provided in Appendix D. 

Figure 3.16 Solvus and spinodal decomposition fields as a function of elastic strain, (a) Strain-free or chemical solvus (b) strain-free spinodal (c) coherent solvus (d) coherent spinodal. From Ganguly and Saxena (1992). Reprinted with permission of Springer-Verlag, New York.

The energy of elastic strain modifies the Gibbs free energy curve of the mixture, and the general result is that, in the presence of elastic strain, both solvus and spinodal decomposition fields are translated, pressure and composition being equal, to a lower temperature, as shown in figure 3.16. 

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