Recrystallisation
Recrystallisation is the slow regrowth of crystals in a solid rock when it is heated without melting.
Recrystallisation happens when a rock is heated enough for atoms to move through the solid, but not enough to melt it. Small grains merge into larger ones, glass turns into crystals, and boundaries become smoother and straighter.
In chondrites, recrystallisation is the hallmark of thermal metamorphism. Petrologic type 3 chondrites have sharp, distinct chondrules in a fine matrix. In types 4 and 5, the matrix coarsens and chondrule edges begin to blur. By type 6, recrystallisation is so advanced that chondrules are hard to see, and the rock has a granular texture of grains that meet at roughly 120° angles. Beyond type 6, partial melting begins.
Iron meteorites can recrystallise too. If an iron is reheated after its Widmanstätten pattern formed, for example by a later impact or atmospheric heating near the surface, the metal may form new granular kamacite grains that blur or erase the pattern. This explains the heat-affected zone just beneath the fusion crust of many irons.
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