On the internal nucleation of melting

Crystals of quartz have been superheated by some 450°C and crystals of albite by some 185°C above their respective melting points. In all cases, melting took place by the nucleation of liquid at the external surfaces (and internal boundaries as well in the case of albite). No evidence for the internal nucleation of liquid was found at any superheat for either material. The results of quartz indicate an exceptionallu large barrier to the internal nucleation of liquid. It is suggested that this large nucleation barrier is associated with the strain energy of forming a liquid nucleus within the crystalline phase. It is also indicated that the nucleation of liquid at the external surfaces of crystals at negligible superheats suggests that the free surfaces of liquids do not per se serve as preferred nucleating sites for crystallization - and that the crystal nucleation often observed at external surfaces or internal surfaces is in fact associated with condensed second-phase impurities.