The transformation kinetics and microstructures of glass-ceramics, which deviate from those of a stoichiometric cordierite compound, are greatly dependent on the compositions of the starting glasses. Compositions richer in (MgO,SiO2) than the stoichiometric cordierite compound suppress the formation of p-cordierite, yet enhance the crystallization of a-cordierite, resulting in a higher content of alpha -cordierite, In contrast, compositions richer in Al2O3 than the stoichiometric cordierite compound have no effect on the crystallization of ol-cordierite. Thus, most of the glass crystallizes to mu -cordierite in the initial stage, followed by the slow transformation of mu -cordierite into an alpha -phase, which results in a low content of alpha -cordierite.