ausblenden:
Schlagwörter:
coesite; crystal-structure; framework-silicates; FTIR-spectra; high-pressure; high-temperature; infrared-spectra; pressure; silica-minerals; silicates; solubility; spectra; temperature
DDC:
550 - Earth sciences
Zusammenfassung:
The incorporation of hydrogen into the coesite structure was investigated at pressures ranging from 4.0-9.0 GPa and temperatures from 750-1300 degrees C using Al and B doped SiO (sub 2) starting materials. The spectra show four sharp bands (v (sub 1) , v (sub 2a) , v (sub 2b) , and v (sub 3) ) in the energy range of 3450-3580 cm (super -1) , consistent with the hydrogarnet substitution [Si (super 4+(T2)) +4O (super 2-) = va (super T2) +4OH (super -) ], two weak sharp bands at 3537 and 3500 cm (super -1) (v (sub 6a) and v (sub 6b) ) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm (super -1) (v (sub 4) and v (sub 5) ) attributed to substitution of Si (super 4+) by Al (super 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 degrees C, 1335 H/10 (super 6) Si is incorporated into the coesite structure. At 8.5 GPa and 1200 degrees C, the incorporation mechanism changes: in the IR spectra four new sharp bands appear in the energy range of 3380-3460 cm (super -1) (v (sub 7) -v (sub 10) ) and the v (sub 1) -v (sub 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 (super -) in coesite. The polarization and high-pressure behavior of the v (sub 7) -v (sub 10) OH bands is quite different from that of the v (sub 1) -v (sub 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+ or -300 H/10 (super 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
