The relevance of the concepts of fragility and of the glass transition temperature for the understanding of crystal nucleation and growth in glass-forming liquids is explored. It is shown that classical fragility can be relevant for the understanding of the crystallization behaviour only if several severe conditions are fulfilled that are rarely met. By this reason, a new definition of liquid fragility is introduced, shown to be able to reflect appropriately the maxima of crystal nucleation, growth and overall crystallization processes is specified. Further, interpreted in terms of classical nucleation theory, for a variety of oxide glass forming liquids, the thermodynamic barrier for homogenous crystal nucleation exhibits at high undercooling an unusual increase with a decrease in temperature.
