ABSTRACT
Some igneous rocks contain phenocrysts, which are large crystals that grew during an initial period of slow crystallization, set in a comparatively much finer-grained matrix that crystallized in a later phase of more rapid crystallization. Metamorphic rocks have somewhat similar things, large crystals set in a finer-grained matrix, called porphyroblasts. These, however, grew in solid rock rather than a silicate liquid. Small numbers of large crystals can grow during metamorphism if a new mineral nucleates (starts to grow) in only a few places, under conditions where the chemical components needed for the new crystals can migrate quickly. This may be accomplished, for example, by diffusion through or flow of grain boundary fluid. Loss of those chemical components from the fluid to the growing porphyroblasts limits the activity (effective concentration) of those components, which limits the degree of supersaturation, which in turn inhibits nucleation of new grains. Ultimately, the small number of mineral nuclei grow into a small number of large crystals.
