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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

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1945-3027
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Volume 88, Issue 10

Issues

OH– in synthetic and natural coesite

Monika Koch-Müller / Przemyslaw Dera
  • Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Road N.W., Washington, D.C. 20015-1305, U.S.A.
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/ Yingwei Fei
  • Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Road N.W., Washington, D.C. 20015-1305, U.S.A.
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/ Barry Reno
  • Trinity University, Department of Geosciences, 1932-715 Stadium Drive, San Antonio, Texas 78212-7200, U.S.A.
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/ Nikolai Sobolev
  • Russian Academy of Science, Siberian Branch, Institute of Mineralogy and Petrography, 3 Koptyug Avenue, Novosibirsk 90, 630090,
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/ Erik Hauri
  • Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5251 Broad Branch Road N.W., Washington, D.C. 20015, U.S.A
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/ Richard Wysoczanski
  • Smithsonian Institution, Department of Mineral Sciences, National Museum of Natural History, Washington, D.C. 20560-0119, U.S.A.
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Published Online: 2015-03-31 | DOI: https://doi.org/10.2138/am-2003-1007

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, ν2a, ν2b, and ν3) in the energy range of 3450-3580 cm-1, consistent with the hydrogarnet substitution [Si4+(T2) + 4O2- = vaT2 + 4OH-], two weak sharp bands at 3537 and 3500 cm-1 (v6a and ν6b) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm-14 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-1710) and the ν13 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 ν710 OH bands is quite different from that of the ν13 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.

About the article

Received: 2002-09-06

Accepted: 2003-03-20

Published Online: 2015-03-31

Published in Print: 2003-10-01


Citation Information: American Mineralogist, Volume 88, Issue 10, Pages 1436–1445, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2003-1007.

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© 2015 by Walter de Gruyter Berlin/Boston.

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