Crystallization of amorphous silicates far from equilibrium part II: Experimental insight into the key role of decoupled cation mobilities

The crystallization of amorphous and porous Ca–Mg-silicates prepared by a sol–gel method is studied in the glass transition range during isothermal and continuous heating experiments. A starting material of diopside (CaMgSi2O6) composition was primarily studied because it is a reference system to study crystal nucleation and growth. The annealed products were characterized by X-ray diffraction and transmission electron microscopy. In the glass transition range, the crystallization is not congruent. It follows a systematic sequence in which the most Ca-rich silicates present in the phase diagram crystallize first. This trend does not obey equilibrium thermodynamics predictions. Instead, this sequence is the result of the decoupled mobilities of network-modifying and network-forming cations. The high surface/volume ratio of gels likely exacerbates this effect compared to compositionally comparable glasses. Altogether, the study shows that local dynamics controls kinetic phase transitions in the glass transition range.