| Summary: | The thermal behaviours of fully Mg2+ cation exchanged LTA, FAU (X and Y species), GIS and SOD zeolites were investigated in the temperature range from ambient to 1400 °C. The initial zeolite frameworks collapsed into amorphous intermediate substances after air heating between 800–850 °C. Prolonged annealing of these products above 850 °C induced recrystallization into sapphirine-like phases (Mg–LTA and Mg–FAUx zeolites) and μ-cordierite (Mg–FAUy and Mg–GIS zeolites) phases. These phases were unstable during further temperature/time treatment and between 1000–1100 °C, they polymorphously transformed into γ-cordierite, a new Mg2Al4Si5O18 polymorph. γ-Cordierite was formed as a transformation product from the investigated zeolite precursors, except for the Mg–SOD zeolite. Structurally the γ-cordierite phase is a modulated, Mg-stuffed derivative of cristobalite with the approximate composition Mg2Al4Si5O18, which precedes the appearance of α-cordierite. The orthorhombic unit cell dimensions calculated from X-ray powder diffraction pattern are: a = 16.387(7) Å, b = 8.835(4) Å, c = 7.853(3) Å. γ-Cordierite has a narrow range of temperature stability (1100–1200 °C) and during prolonged temperature/time treatment, it is polymorphously transformed to the α-cordierite phase.
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