What is exsolution in mineralogy ?

Exsolution : definition

Exsolution is the result of the transformation by solid state cooling of a single phase stable at high temperature into a polymineral assembly at low temperature.

The classic example is that of perthites in feldspars. A solid solution of formula (K, Na) AlSi3O8 crystallizing at high temperature splits into two mineral phases on cooling : albite (NaAlSi3O8) and orthosis (KAlSi3O8). Minority albite appears as fine oriented inclusions, exsolutions are called perthites in this specific case.

Pyroxenes such as enstatite (orthopyroxene) and augite (clinopyroxene) regularly show exsolutions.

On the right, a green amazonite (microcline) with white albite exsolution.