OH– in synthetic and natural coesite

Abstract

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 (ν₁, ν_2a, ν_2b, and ν₃) 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 (v_6a and ν_6b) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm^-1 (ν₄ and ν₅) 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 (ν₇-ν₁₀) and the ν₁-ν₃ 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 ν₇-ν₁₀ OH bands is quite different from that of the ν₁-ν₃ 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 ζ ± 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.