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Coupled boron and hydrogen incorporation in coesite

^* Department Chemie der Erde, GeoForschungsZentrum Potsdam, Telegrafenberg 14473, Potsdam, Germany

^* Corresponding author, e-mail: fdeon{at}gfz-potsdam.de

So far, synthetic and natural coesite is known to incorporate hydrogen mainly via the hydrogarnet substitution. In this study, we synthesized for the first time coesite which incorporates hydrogen solely via a boron-based defect. The syntheses were carried out in a multi-anvil press at relatively high pressures (P) (9–12 GPa) and high temperatures (T) (1000–2000 °C) using boron-doped starting material with water in excess. The Fourier transform infrared (FTIR) spectra of these coesite crystals taken in the OH stretching region consist of two sharp peaks at 3535 and 3500 cm^–1, which were assigned previously to two different boron-based point defects, _6b and _6a, respectively. The water and boron content was determined by FTIR (water) and secondary ion mass spectrometry (SIMS) (boron): coesite crystals grown at 9 GPa and 1400 °C incorporate about 920 H/10⁶ Si and 1600 B/10⁶ Si. Polarized single-crystal spectra revealed that both hydroxyl groups must point in the crystallographic a direction of the structure but with a high degree of disorder around their equilibrium positions. The crystals were further investigated by high-pressure IR spectroscopy. From the specific response of the OH bands to increasing pressure in combination with literature data on structural changes with pressure, we conclude that _6b arises either from vibrations of an O1–H6b. . . O4 group with B at the T1 site or an O4–H6b. . . O1 group with B at the T2 site. Due to its different pressure behavior compared to _6b, the _6a band can be assigned either to vibrations of an O4–H6a. . . O5 group with B at the T1 site or an O5–H6a. . . O4 group with B at the T2 site. Most likely, the relatively high pressures and temperatures used to synthesize coesite in this study, compared to previous studies, favor the formation of the B-based defect as such a defect is accompanied by a decrease in the size of the tetrahedral site. In contrast, through the hydrogarnet substitution, vacant tetrahedral sites are up to 20 % larger than SiO₄ tetrahedra.

Key-words: coesite, boron-based defect, polarized measurements, high-pressure FTIR spectroscopy.