Mutant crystals in Na2O · 2CaO · 3SiO2 glasses

This paper presents strong evidence for a continuous variation in glass and crystal compositions during crystallization, from the early stages of nucleation up to full transformation of a stoichiometric Na2O · 2CaO · 3SiO2 glass, as well as of similar glasses that are depleted or enriched in sodium. During crystallization, the crystal compositions approach those of the parent glasses, leading to the inference that the critical clusters were also sodium-enriched. The existence of such mutant crystals is reported for the first time. The crystals’ increase in sodium content diminishes the thermodynamic barrier for nucleation owing to a decrease in the critical nucleus/liquid interfacial energy. This conclusion is evident also from an analysis of the nucleation rate as a function of the glass composition. Our conclusions are based on the analysis of a plethora of data: overall crystallization kinetics, crystal growth velocity, changes in the crystal’s lattice parameters, variation of the reversible polymorphic transition temperature with the degree of crystallinity and, finally, quantitative measurements of glass and crystal composition by EDS. The deviation of the nucleus composition from the expected stoichiometry may significantly impact the analysis of nucleation theories.