Exsolution is a process by which a solid solution phase unmixes into two separate phases in the solid state. Exsolution occurs only in minerals whose compositions vary between two or more pure endmember compositions. In some minerals intermediate compositions become unstable compared with a mixture of two separate compositions of the mineral and unmixing will occur. Usually exsolution occurs by the growth of lamellae within the original host crystal during cooling. Exsolution is impeded by rapid cooling and is thus not usually observed in crystals within volcanic rocks formed close to the surface.