AbstractThe precipitation of nonstoichiometric crystals (of different composition from the parent glass) motivated us to study the stoichiometric Na₂O·2CaO·3SiO₂ ( N₂CS₃) glass‐forming system comprehensively. The concentration profiles of Na and Ca in the diffusion zone around Na_4+2xCa_4−x[Si₆O₁₈], , crystals in an N2C3S glass were characterized by energy‐dispersive spectroscopy and were then theoretically described. We show that the diffusion zone is formed because of the rejection of part of calcium by the growing crystal. The radial variation of the Ca concentrations and the crystal nucleation rate through the diffusion zone were analyzed simultaneously. We found that the nucleation rate in the diffusion zone near the crystal–glass interface drastically decreases due to the increase in the work of critical cluster formation resulting from the composition change. In contrast, the diffusion activation barrier remains practically constant. Moreover, the Ca diffusion coefficients, D_Ca, in Na₂O·2CaO·3SiO₂ and CaO·SiO₂ glasses show the same Arrhenius dependence, demonstrating that (surprisingly) D_Ca does not depend on the glass composition in the CaO·SiO₂–Na₂O·SiO₂ metasilicate joint. This work provides the quantitative spatial distribution of chemical components in a partially crystallized glass, shedding light on solid solution crystallization in a glass‐forming liquid and its consequences.