Exsolution
Exsolution is the separation of one mineral out of another as a crystal cools and can no longer hold its mixed chemistry.
Exsolution happens when a crystal that formed at high temperature as a single mixture becomes unstable as it cools. The atoms slowly sort themselves into two separate minerals, usually as thin parallel layers called lamellae or as small blebs inside the host crystal.
Two meteorite examples stand out. In pyroxenes from eucrites and other slowly cooled igneous rocks, calcium-rich augite separates from calcium-poor pigeonite or orthopyroxene, leaving fine stripes visible under the microscope. In iron meteorites, the Widmanstätten pattern is a form of exsolution in metal: as nickel-rich taenite cools, low-nickel kamacite grows within it in plates along four directions.
The thickness of exsolution lamellae depends on how long the crystal spent at high temperature. Thin lamellae mean faster cooling; wide ones mean slower. That makes exsolution a natural stopwatch for the thermal history of asteroids, revealing whether a rock cooled near the surface or deep underground.
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