Behavior of hydrogarnet-type defects in hydrous stishovite at various temperatures and pressures

Abstract

Dense polymorphs of silica have been demonstrated experimentally to incorporate from 1.5% to as much as 11.6% weight percent H₂O as OH groups, with implications for the hydrogen budgets of Earth and other planets. This OH is thought to enter the SiO₂ structure via a charge-balanced substitution in which silicon vacancies (V_(Si)) are compensated by protonating four of the surrounding six oxygen atoms, often referred to as a hydrogarnet-type defect. There are many possible configurations for this defect structure in dense silica, but the nature of these configurations and whether they can be distinguished experimentally is unknown. We present here density functional theory (DFT) calculations that systematically assess the possible configurations of a hydrogarnet-type defect in stishovite (rutile-type SiO₂), with direct comparisons to experimental vibrational spectroscopy data. We predict that stishovite synthesized at 450K and 10 GPa quenched to room temperature is dominated by a single defect type with tetrahedral geometry. This leads to OH stretching modes (2500-3000 cm⁻¹) and SiOH bending modes (~1400 to 1450 cm⁻¹) largely consistent with experimentally observed modes. One remaining issue is that our calculations produce results compatible with experimental data on H to D exchange, but do not explain why a considerable fraction of the 1420 cm⁻¹ mode shifts by only 40 cm⁻¹ in deuterated samples. At elevated pressures and temperatures, we find that a second square planar defect configuration also becomes favorable, leading to modes that should allow differentiation from the tetrahedral configuration.

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