A case study of depolymerization in silicates: Melting of quartz and zircon crystallization at high pressure

The experimental study of depolymerization reaction SiO2 + ZrO2 → ZrSiO4 reveals that melting of quartz and crystallization of zircon in the presence of water increases at high pressure. The melting rate of quartz is identified as the rate-determining step of the reaction. In highly polymerized silicates, Si-O-Si bond angles are subject to alteration under the influence of pressure. This phenomenon is particularly noticeable within the three-dimensional lattice structure of quartz, where each corner of the tetrahedra is shared. Theoretical analysis of the Si-O-Si bond angle bending demonstrates a change in oxygen hybridization from sp to sp2, favoring larger Si-O distances. The sp2 hybridization in bridging oxygen promotes acid attack by hydrogen ions of water, resulting in SiO bond heterolysis in quartz. The depolymerization from tectosilicates to nesosilicates leads to an increase in oxygen “size” and polarizability, as well of closer packing of oxygen atoms. This change in behavior and packing of oxygens is further illustrated by the high-pressure-driven transition from tridymite to stishovite. Additionally, the impact of network-modifying elements on the structure of polymerized silicates is investigated; particularly those acting as electron donors, which lengthen Si-O distances in quartz. The findings are discussed in relation to key topics in Earth Sciences, including the crystal chemical basis of the Bowen's reaction series, the “metallization” in rocky planets and the structural controls of hydrous and dry melting in silicates.