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Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
The sluggishness of nucleation, one of the most poorly understood and least-discussed processes involved in mineral reactions, is an important cause of disequilibrium during metamorphism. Both field-based and experimental petrological studies show that metastable phases can persist far from equilibrium on long time scales because the stable phases are unable to nucleate. This behaviour applies not only to solid-solid reactions but also to some dehydration and melting reactions. Although the degree of overstepping required for nucleation during polymorphic phase transformations is relatively small (2–4 kbar on an experimental time scale), nucleation can be much more sluggish for reactions involving two or more product phases. When reactions occur at conditions of large overstepping due to delayed nucleation, reaction rates can be very fast because of the effect of the large free energy change on growth kinetics. Under these circumstances, new metastable phases may form instead of the stable mineral assemblage, as predicted by the Ostwald step rule. Such metastable phases can persist during metamorphism, even during high-strain ductile deformation and fluid influx, but without any obvious texture evidence to indicate that they are metastable.
