A granite which has crystallized above the solvus temperature and hence contains only one alkali feldspar type. The solvus temperature is that temperature below which two alkali feldspars of contrasting composition are in equilibrium. Granites which crystallize below the solvus temperature (subsolvus granites) immediately crystallize two alkali feldspar types, one K-rich, the other Na-rich. The feldspars from either hyper- or subsolvus granites may suffer sub-solidus exsolution if cooling is slow enough. This is an equilibrium process which takes place when the rock is entirely solid and involves splitting of the feldspars into K- and Na-rich lamellae by internal diffusion of ions, to give perthitic (see perthite) or antiperthitic feldspars.
Subjects: Earth Sciences and Geography.