Exsolution

Exsolution is a process in which a solid solution that formed at high temperature becomes unstable as conditions change, typically during cooling, and separates into two or more distinct solid phases that were originally part of a single homogeneous phase. The separation occurs by diffusion of solute elements and is driven by changes in the solubility limits or ordering energetics of the system. Exsolution often yields a characteristic intergrowth of the end-member phases rather than a uniform mixture.

In geology and mineralogy, exsolution commonly produces lamellae or intergrowth textures within crystals. The resulting microstructure can be lamellar, needlelike, or reticulate and reflects the cooling history and diffusion pathways of the host mineral. Exsolution is distinguished from simple precipitation because the separated phases are the end-members of the original solid solution, maintaining a coherent or semi-coherent relationship with the host lattice.

Notable examples include oxide and silicate solid solutions. In magnetite-titanomagnetite systems, exsolution during cooling can produce lamellae of ulvöspinel (Fe2TiO4) within a magnetite (Fe3O4) host, creating visible textures and influencing magnetic properties. In olivine, fayalite lamellae can exsolve within forsterite in the forsterite–fayalite system, a texture commonly observed in high-temperature igneous and metamorphic rocks. Exsolution textures are also observed in other mineral systems and can be influenced by pressure, composition, and thermal history.

In materials science, exsolution can occur in solid-solution alloys during aging or annealing, forming fine-scale precipitates or lamellae that modify mechanical, magnetic, or optical properties.