Structural changes in silica gel after compression to 5 GPa and 100 °C for one hour were investigated using Raman and IR spectroscopies and X-ray diffraction measurements. The non-powdered silica gel sample was entirely crystallized to coesite under these experimental conditions, showing that the crystallization of silica gel to coesite can be readily achieved in spite of the low temperature conditions and short duration. This behavior may result because the average structure of silica gel is dominated by four-membered rings of SiO4 tetrahedra, there is a less-polymerized network structure due to the presence of silanol (Si-OH), and larger ring structures, such as 6- and 8-membered rings, of SiO4 tetrahedra are disrupted by water molecules under pressure. These properties may lead to successive polymerization and reconstruction of the SiO4 tetrahedra in the bulk silica-gel sample, enabling the easy formation of the coesite-like network structure. This shows the importance of the contribution of silanol and water molecules for coesite formation.

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