TY - JOUR
T1 - Raman spectroscopy of water-rich stishovite and dense high-pressure silica up to 55 GPa
AU - Nisr, Carole
AU - Shim, Sang-Heon
AU - Leinenweber, Kurt
AU - Chizmeshya, Andrew
N1 - Funding Information:
We thank Ulrich Haussermann for providing TGA traces and valuable discussions on the water contents of hydrous stishovite samples. We thank the two anonymous reviewers and the associate editor for the discussion that greatly improved this paper. This work was supported by an NSF grant to K.L., S.H.S., and C.N. (EAR1321976). The results reported herein also benefitted from collaborations and/or information exchange within NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate.
Publisher Copyright:
© 2017 Walter de Gruyter GmbH. All rights reserved.
PY - 2017/11
Y1 - 2017/11
N2 - Recent studies have shown that mineral end-member phases (δ-AlOOH phase, phase H, and stishovite) with rutile-type or modified rutile-type crystal structures and solid solutions between them in the MgO-Al2O3-SiO2 system can store large amounts of water and can be stable at high pressures and high temperatures relevant to the Earth's lower mantle. The Al-H charge-coupled substitution (Si4+ → Al3+ + H+) has been proposed to explain the storage capacity found in some of these phases. However, the amount of H+ found in some recent examples does not match the expected value if such substitution is dominant, and it is difficult to explain the larger water storage in stishovite with such a mechanism alone. An octahedral version of the hydrogarnet-like substitution (Si4+ → 4H+) has been proposed to explain the incorporation of protons in Al-free, water-rich stishovite. Yet, the high-pressure structural behavior of OH in this phase has not yet been measured. In this study, we report high-pressure Raman spectroscopy measurements on Al-free hydrous stishovite with 3.2 ± 0.5 wt% water up to 55 GPa. At ambient pressure, we find that the OH stretching modes in this phase have frequencies lying in between those in low-water aluminous stishovite and those in δ-AlOOH, suggesting a strength of the hydrogen bonding intermediate between these two cases. After decompression to 1 bar, we observe modes that are similar to the IR-active modes of anhydrous and hydrous stishovite, suggesting that the existence of Si defects in the crystal structure can activate the inactive modes. For both lattice and OH-stretching modes, our data show a series of changes at pressures between 24 and 28 GPa suggesting a phase transition (likely to CaCl2-type). While some of the lattice mode behaviors are similar to what was predicted for the AlOOH polymorphs, the OH mode of our hydrous stishovite shows a positive frequency shift with pressure, which is different from δ-AlOOH. All our spectral observations suggest that water-rich pure dense silica has a distinct proton incorporation mechanism from aluminous low-water stishovite and δ-AlOOH, supporting the proposed direct substitution.
AB - Recent studies have shown that mineral end-member phases (δ-AlOOH phase, phase H, and stishovite) with rutile-type or modified rutile-type crystal structures and solid solutions between them in the MgO-Al2O3-SiO2 system can store large amounts of water and can be stable at high pressures and high temperatures relevant to the Earth's lower mantle. The Al-H charge-coupled substitution (Si4+ → Al3+ + H+) has been proposed to explain the storage capacity found in some of these phases. However, the amount of H+ found in some recent examples does not match the expected value if such substitution is dominant, and it is difficult to explain the larger water storage in stishovite with such a mechanism alone. An octahedral version of the hydrogarnet-like substitution (Si4+ → 4H+) has been proposed to explain the incorporation of protons in Al-free, water-rich stishovite. Yet, the high-pressure structural behavior of OH in this phase has not yet been measured. In this study, we report high-pressure Raman spectroscopy measurements on Al-free hydrous stishovite with 3.2 ± 0.5 wt% water up to 55 GPa. At ambient pressure, we find that the OH stretching modes in this phase have frequencies lying in between those in low-water aluminous stishovite and those in δ-AlOOH, suggesting a strength of the hydrogen bonding intermediate between these two cases. After decompression to 1 bar, we observe modes that are similar to the IR-active modes of anhydrous and hydrous stishovite, suggesting that the existence of Si defects in the crystal structure can activate the inactive modes. For both lattice and OH-stretching modes, our data show a series of changes at pressures between 24 and 28 GPa suggesting a phase transition (likely to CaCl2-type). While some of the lattice mode behaviors are similar to what was predicted for the AlOOH polymorphs, the OH mode of our hydrous stishovite shows a positive frequency shift with pressure, which is different from δ-AlOOH. All our spectral observations suggest that water-rich pure dense silica has a distinct proton incorporation mechanism from aluminous low-water stishovite and δ-AlOOH, supporting the proposed direct substitution.
KW - Free-Al hydrous stishovite
KW - High-pressure studies
KW - Hydrogen incorporation mechanism; Water in Nominally Hydrous and Anhydrous Minerals
KW - Phase transition
KW - Raman spectroscopy
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U2 - 10.2138/am-2017-5944
DO - 10.2138/am-2017-5944
M3 - Article
AN - SCOPUS:85030972647
SN - 0003-004X
VL - 102
SP - 2180
EP - 2189
JO - American Mineralogist
JF - American Mineralogist
IS - 11
ER -