Exsolutionsphasen

Exsolutionsphasen, or exsolution phases, are phases that form inside a single solid-solution phase when that solution becomes thermodynamically unstable and separates into two or more end-member compositions by exsolution. The process typically occurs during cooling or slow chemical change, and it often preserves a coherent or semi-coherent crystal lattice across the phase boundary.

Mechanism and microstructure: Exsolution requires a miscibility gap in the system’s phase diagram and sufficient atomic mobility for diffusion. The result is an intergrowth of two end-member phases with similar crystal structures but different compositions. The interfaces are often coherent, creating lamellae, plates, or other fine intergrowths that can be oriented according to crystallography. The overall texture reflects diffusion kinetics, cooling rate, and the lattice parameters of the end members.

Common examples: In geology, exsolution in plagioclase feldspar yields albite-rich and anorthite-rich lamellae after slow cooling from a high-temperature solid solution. In oxide systems such as titanomagnetite (an Fe-Ti oxide solid solution), exsolution produces alternating magnetite and ilmenite lamellae, a classic texture in basaltic rocks. Similar exsolution occurs in other minerals and in metal alloys, where coherent intergrowths form during aging or annealing.

Significance: The presence and morphology of exsolution phases record thermal histories and diffusion processes, influence magnetic, optical, and mechanical properties, and assist petrographists and materials scientists in interpreting cooling rates, annealing histories, and the evolution of solid-solution systems. Exsolutionsphasen thus serve as a diagnostic feature of solid-state decomposition within solid solutions.