AbstractFrom experimental evidence it has previously been shown that heterogeneous nucleation is non-reciprocal and therefore that the balance of interfacial energies is a controlling factor. IfσNL>σNS+σSLthe nucleus S can grow from the liquid L on the substrate N. When nucleation takes place the contact angleθis small and there is epitaxy between the nucleant and the nucleus that reduces theσNSto very low values so that, as a first approximation, heterogeneous nucleation can be expected whenσNL>σSL. Interfacial energiesσSLare roughly proportional to the melting point, and high melting point phases usually nucleate lower melting phases. The contrary may happen if the low melting phase has a low symmetry crystal. Nucleation can take place on high index faces of this crystal because, although its averageσNLis lower than theσSLof a higher melting point, high symmetry crystal, theσNLof its high index faces may be higher than theσSL.MST/845