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Mechanics----When in the upper crust P-T-etc. conditions are favorable for a metamorphic mineral to grow, nucleation can start. The small nuclei have a relatively high surface energy, which forms an energy barrier for their growth. The number of nuclei and their survival rate determines whether many small or a few large porphyroblasts form. This number depends on:the availability of favorable nucleation sites;the driving force for the metamorphic reaction (overshoot of PT-conditions)transport rate of elements that form new mineral and elements that have to be removed to make space available[1].