Quoting Tuttle and Bowen in 1958 (abstract, page 3): ″A classification of salic rocks based on the nature of the alkali feldspar is proposed.
The classification has two major divisions: (1) subsolidus, and (2) hypersolvus, depending on the whereabouts of the soda feldspar.
The two major divisions are further subdivided according to the nature of the alkali feldspar modification.″[1] Note that here the word "subsolidus" unfortunately looks like a misprint and probably has to be replaced by "subsolvus".
Because Na and K are extremely mobile by solid-state diffusion, cooling gives rise to multiple forms of unmixing products,[2] e.g. various kinds of perthite.
"Due to slow reaction kinetics, feldspars usually do not attain equilibrium states and thus their thermodynamic behaviour is not yet fully understood.