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.

Research highlights
► The crystallization of porous Ca–Mg-silicates prepared by a sol–gel is studied.
► In the glass transition range, the crystallization follows a systematic sequence.
► This trend does not obey equilibrium thermodynamics predictions.
► It is the result of decoupled mobilities between network-formers and -modifiers.
► Local dynamics controls kinetic phase transitions in the glass transition range.