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OH^– in synthetic and natural coesite

¹ GeoForschungsZentrum Potsdam, Sektion 4.1, Telegrafenberg, Potsdam D-14473, Germany
² Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Road N.W., Washington, D.C. 20015-1305, U.S.A.
³ Trinity University, Department of Geosciences, 1932-715 Stadium Drive, San Antonio, Texas 78212-7200, U.S.A.
⁴ Russian Academy of Science, Siberian Branch, Institute of Mineralogy and Petrography, 3 Koptyug Avenue, Novosibirsk 90, 630090, Russia
⁵ Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5251 Broad Branch Road N.W., Washington, D.C. 20015, U.S.A
⁶ Smithsonian Institution, Department of Mineral Sciences, National Museum of Natural History, Washington, D.C. 20560-0119, U.S.A.

Correspondence: ^* E-mail: mkoch{at}gfz-potsdam.de

The incorporation of hydrogen into the coesite structure was investigated at pressures ranging from 4.0–9.0 GPa and temperatures from 750–1300 °C using Al and B doped SiO₂ starting materials. The spectra show four sharp bands (₁, _2a, _2b, and ₃) in the energy range of 3450–3580 cm^–1, consistent with the hydrogarnet substitution [Si^4+(T2) + 4O^2– = va^T2 + 4OH^–], two weak sharp bands at 3537 and 3500 cm^–1 (_6a and _6b) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm^–1 (₄ and ₅) attributed to substitution of Si⁴⁺ by Al³⁺ + H. More than 80% of the dissolved water is incorporated via the hydrogarnet substitution mechanism. The hydrogen solubility in coesite increases with pressure and temperature. At 7.5 GPa and 1100 °C, 1335 H/10⁶ Si is incorporated into the coesite structure. At 8.5 GPa and 1200 °C, the incorporation mechanism changes: in the IR spectra four new sharp bands appear in the energy range of 3380–3460 cm^–1 (₇–₁₀) and the ₁–₃ bands disappear. Single crystal X-ray diffraction, Raman spectroscopy, polarized single-crystal and in situ high-pressure FTIR spectroscopy confirm that the new bands are due to OH^– in coesite. The polarization and high-pressure behavior of the ₇-₁₀ OH bands is quite different from that of the ₁–₃ bands, indicating that the H incorporation in coesite changes dramatically at these P and T conditions. Quantitative determination of hydrogen solubility in synthetic coesite as a function of pressure, temperature, and chemical impurity allow us to interpret observations in natural coesite. Hydrogen has not previously been detected in natural coesite samples from ultra high-pressure metamorphic rocks. In this study, we report the first FTIR spectrum of a natural OH-bearing coesite. The dominant substitution mechanism in this sample is the hydrogarnet substitution and the calculated hydrogen content is about 900 ± 300 H/10⁶ Si. The coesite occurs as an inclusion in diamond together with an OH-bearing omphacite. The shift of the OH-bands of coesite and omphacite to lower energies indicates that the minerals are still under confining pressure.

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