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Raman spectroscopy of the 10-Å phase at simultaneously HP-HT

¹ Dipartimento di Scienze della Terra, Università di Perugia, 06100 Perugia, Italy
² Bayerisches Geoinstitut, Universitat Bayreuth, 95440 Bayreuth, Germany

^* Corresponding author, e-mail: comodip{at}unipg.it

Raman and X-ray diffraction data demonstrated that the 10-Å phase is compressed metastably up to 42 GPa at room conditions and that water molecules remain between the sheets, forming hydrogen bonds (Comodi et al., 2006). The effect of temperature on the pressure evolution of the 10-Å phase, Mg₃Si₄O₁₀(OH)₂H₂O, was studied by collecting Raman spectra simultaneously at high pressure (up to 27 GPa) and high temperature (up to 776 K) in electrically heated diamond-anvil cells.

The Raman frequencies of all low-frequency modes (between 100 and 1200 cm^–1) increased monotonically with pressure and slightly decreased with temperature along different isobars. The shift of the stretching bands of H₂O was opposite with P and T increase: the frequency of the 3597 cm^–1 mode decreased with pressure (d/dP = –0.81 cm^–1 GPa^–1) and increased with temperature (d/dT = 0.025 cm^–1 K^–1), whereas the frequency of the 3673 cm^–1 mode increased with P (d/dP = –1.38 cm^–1 GPa^–1) and decreased with T (d/dT = –0.011 cm^–1 K^–1). The hydroxyl stretching frequency at 3623 cm^–1 increased with both P and T.

The FWHM of the low-frequency modes, was independent from temperature and increased linearly with pressure up to 22 GPa, where Raman signals disappeared. Reversible behaviour indicated that no major reconstructive transition, such as amorphization occurred in the pressure-temperature range investigated. Temperature strongly affected the FWHM of the 3597 cm^–1 mode, which disappeared at 570 K and 9 GPa, whereas at room temperature it was followed to about 12 GPa.

The broadening of the OH-stretching was modelled with a progressive pressure-induced H disordering as observed in other hydrous minerals. Thus the 10-Å phase should undergo to an order-disorder phase transition at 12 GPa and room temperature. The increase of temperature reduced the pressure at which the transition occurred.

The measured mode shifts were used to determine isothermal and isobaric Gruneisen mode parameters, which were significantly higher than those of talc. The low values of the average calculated intrinsic anharmonic parameters, a_i, of 0.52x10^–5 K^–1, indicated that a quasi-harmonic vibrational model may be used to describe the thermochemical properties of the 10-Å phase.

HP-HT Raman spectra indicate that the 10-Å phase can host water at high P/T gradients and that it is a good candidate to transfer water in the mantle beyond the antigorite stability field.

Key-words: 10-Å phase, Raman spectroscopy, high pressure, high temperature, diamond-anvil cell, Grüneisen parameters, water molecule vibrations.