Exsolution is the term applied to the processes of the separation of an initially homogeneous solution phase into two compositionally different immiscible phases. Exsolution occurs when a solution phase becomes less stable than its component phases. The instability of the solution phase may be triggered by a change in temperature (T), pressure (P), or chemical composition (X).
Both of T and P are geologically important system parameters. Most geologic systems are mainly composed of solid phases with minor amounts of fluids. Exsolution of fluid solutions is dependent on both T and P. On the other hand, exsolution of solid solutions significantly depends only on T, because solid phases usually show little volume change with pressure variation.
Exsolution begins on an atomic scale (see Mineral defects). As exsolution continues, the exsolved phases usually form subparallel lamellae, films or irregular patches. Such intergrowths may be observable by eye, but are