Abstract

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.

Original languageEnglish (US)
Pages (from-to)2180-2189
Number of pages10
JournalAmerican Mineralogist
Volume102
Issue number11
DOIs
StatePublished - Nov 1 2017

Fingerprint

stishovite
Raman spectroscopy
Silicon Dioxide
silica
silicon dioxide
Water
water
Substitution reactions
substitution
substitutes
Stretching
rutile
Protons
crystal structure
Crystal structure
protons
Polymorphism
pressure reduction
lower mantle
decompression

Keywords

  • Free-Al hydrous stishovite
  • High-pressure studies
  • Hydrogen incorporation mechanism; Water in Nominally Hydrous and Anhydrous Minerals
  • Phase transition
  • Raman spectroscopy

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

Cite this

Raman spectroscopy of water-rich stishovite and dense high-pressure silica up to 55 GPa. / Nisr, Carole; Shim, Sang-Heon; Leinenweber, Kurt; Chizmeshya, Andrew.

In: American Mineralogist, Vol. 102, No. 11, 01.11.2017, p. 2180-2189.

Research output: Contribution to journalArticle

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abstract = "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.",
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AU - Nisr, Carole

AU - Shim, Sang-Heon

AU - Leinenweber, Kurt

AU - Chizmeshya, Andrew

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