Thermodynamic, kinetic, and density-driven pathways in the selective transformation of amorphous silica to α-quartz

Silica, valued for its unique properties and abundance, is widely studied and applied in various fields. Quartz, the most stable polymorph at ambient conditions, can be obtained from amorphous silica through calcination. However, achieving this transformation without mineralizers such as alkaline salts is extremely challenging, since metastable phases like cristobalite often form even at temperatures where quartz is the thermodynamically stable phase. In this work, an optimized calcination strategy is proposed to selectively obtain polycrystalline α-quartz from amorphous silica pellets by varying process parameters such as target temperature, dwell time, and cooling rates. Quantitative analyses by X-ray powder diffraction and scanning electron microscopy reveal that the initial density of the precursors significantly influences both the thermodynamics and kinetics of the structural phase transformation. A phenomenological explanation of these findings is proposed, considering initial density and morphology of amorphous silica as key driving factors in the process. Additional experiments performed under high-pressure/high-temperature conditions underline the complementary roles of thermodynamics and kinetics in the formation of the target phase.