Monika Koch-Müller, Przemyslaw Dera, Yingwei Fei, Barry Reno, Nikolai Sobolev, Erik Hauri, Richard Wysoczanski
March 31, 2015
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 SiO 2 starting materials. The spectra show four sharp bands (ν 1 , ν 2a , ν 2b , and ν 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 (v 6a and ν 6b ) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm -1 (ν 4 and ν 5 ) attributed to substitution of Si 4+ by Al 3+ + 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/10 6 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 (ν 7 -ν 10 ) and the ν 1 -ν 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 ν 7 -ν 10 OH bands is quite different from that of the ν 1 -ν 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 ζ ± 300 H/10 6 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.