Crystallization in a Ba‐containing magnesium aluminosilicate (Ba‐MAS) glass‐ceramic was studied using transmission electron microscopy. Ba‐MAS slabs were hot‐pressed at 1000°C to form dense glassy bodies, which were crystallized by further heating in air or argon between 850° and 1300°C. Heterogeneous nucleation of a metastable highquartz solid solution (μ‐cordierite) occurred at crystallization temperatures below 1100°C, followed by dendritic growth; the interdendrite phase was Ba‐rich silicate glass. The high‐quartz solid solution generally acted as a precursor for the nucleation of the stableβ‐phase, which was the final crystallization product. Surprisingly, slower crystallization kinetics were observed in argon than in air. Further morphological changes at the higher temperatures and longer annealing, times resulted from a Rayleigh type instability, which led to breakup of the interdendrite silicate rods, and simultaneous polygo