12:00 〜 12:15
[SIT26-12] Raman spectroscopic investigation of α-β quartz phase transition in hydrothermal diamond-anvil cell and acquisition of equation of states of aqueous solutions
キーワード：α-β quartz phase transition, Raman spectroscopy, Equation of states of aqueous solutions, Hydrothermal diamond-anvil cell, Isochore
The α-β quartz phase transition temperatures (Ttr, qtz’s; up to ~781 ºC) were determined at various pressures (P’s; up to ~802 MPa) in a hydrothermal diamond-anvil cell (HDAC; Li et al., 2016; Rev. Sci. Instrum., 87, p. 053108-1) by monitoring the Raman shift of the α-quartz band near 128 cm-1 (at 24.2 ºC). When compared with the commonly used α-quartz band near 465 cm-1 (e.g., Schmidt and Ziemann, 2000; Am. Mineral. 85, p. 1725), the rate of reduction in wavenumber during H2O isochoric heating is about five times higher. In addition, the sudden change of the rate of reduction in wavenumber at Ttr, qtz is much more sharp and clear, making the α-quartz band near 128 cm-1 a much better choice for detecting the phase transition.
Our experimental procedures were similar to those of Shen et al. (1993; Am. Mineral. 78, p. 694), except Raman spectroscopic method instead of laser interferometry was used to determine Ttr, qtz. A quartz wafer (prepared from a natural crystal from Asikaerte Be pegmatite in Xinjiang, China) together with or without H2O were loaded in the sample chamber, which was a hole in a Re gasket between two diamond anvils; H2O pressure medium was not needed for experiments at 0.1 MPa total pressure.
Raman spectra were acquired during a heating cycle of the experiments after the sample was kept at a constant T for more than 3 minutes. We used a JY/Horiba LabRam HR Evolution Raman system, with 532.06 nm (frequency doubled Nd:YAG) laser excitation, a SLWD 50x Olympus objective having 0.35 numerical aperture, a 1800-groove/mm grating with a spectral resolution of about 0.2 cm-1, and ~14 mW laser light was focused on the sample during the measurement. Spectra were collected in one spectrographic window (from 77 to 593 cm-1) for either 30 s (below 700 ºC) or 60 s (above 700 ºC) with two accumulations per spectrum.
The bulk density of H2O in the sample chamber for the observed Ttr, qtz was determined by measuring the homogenization T (Th) after the liquid-vapor phase separation during isochoric cooling. The two K-type thermocouples in HDAC were calibrated with the melting points of NaNO3 (306.8 ºC) and NaCl (800.5 ºC), and the uncertainties in T measurements are ±1.5 ºC. The associated pressures at Th (Ph) and Ttr, qtz (Ptr, qtz) were calculated based on the equation of state (EOS) of H2O (Wagner and Pruβ, 2002; J. Phys. Chem. Ref. Data 31, p. 387). The straight line connecting (Th, Ph) and (Ttr, qtz, Ptr, qtz) in a P-T space is near the isochore of that bulk density of H2O. Similar approach was successfully applied to obtain isochores of 2 m ZnCl2 solution (Bassett et al., 2000; Zeitsch. Kristallogr. 215, p. 711), and will be extended to other geologically important aqueous solutions at T’s up to 1000 ºC using HDAC and Raman spectroscopy.
The α-β quartz phase boundary obtained in this study can be represented by: (Ptr, qtz) (±8.8 MPa) = 0.0015 (Ttr, qtz)2 + 1.8268 (Ttr, qtz) – 1544.5, where (Ttr, qtz) is between 574 and 781 ºC; with R2 = 0.9998. Our results agree, within experimental uncertainties, with those reported by Mirwald and Massonne (1980; J. Geophys. Res. 85, p. 6983), but with some deviations from other previous data.
Our experimental procedures were similar to those of Shen et al. (1993; Am. Mineral. 78, p. 694), except Raman spectroscopic method instead of laser interferometry was used to determine Ttr, qtz. A quartz wafer (prepared from a natural crystal from Asikaerte Be pegmatite in Xinjiang, China) together with or without H2O were loaded in the sample chamber, which was a hole in a Re gasket between two diamond anvils; H2O pressure medium was not needed for experiments at 0.1 MPa total pressure.
Raman spectra were acquired during a heating cycle of the experiments after the sample was kept at a constant T for more than 3 minutes. We used a JY/Horiba LabRam HR Evolution Raman system, with 532.06 nm (frequency doubled Nd:YAG) laser excitation, a SLWD 50x Olympus objective having 0.35 numerical aperture, a 1800-groove/mm grating with a spectral resolution of about 0.2 cm-1, and ~14 mW laser light was focused on the sample during the measurement. Spectra were collected in one spectrographic window (from 77 to 593 cm-1) for either 30 s (below 700 ºC) or 60 s (above 700 ºC) with two accumulations per spectrum.
The bulk density of H2O in the sample chamber for the observed Ttr, qtz was determined by measuring the homogenization T (Th) after the liquid-vapor phase separation during isochoric cooling. The two K-type thermocouples in HDAC were calibrated with the melting points of NaNO3 (306.8 ºC) and NaCl (800.5 ºC), and the uncertainties in T measurements are ±1.5 ºC. The associated pressures at Th (Ph) and Ttr, qtz (Ptr, qtz) were calculated based on the equation of state (EOS) of H2O (Wagner and Pruβ, 2002; J. Phys. Chem. Ref. Data 31, p. 387). The straight line connecting (Th, Ph) and (Ttr, qtz, Ptr, qtz) in a P-T space is near the isochore of that bulk density of H2O. Similar approach was successfully applied to obtain isochores of 2 m ZnCl2 solution (Bassett et al., 2000; Zeitsch. Kristallogr. 215, p. 711), and will be extended to other geologically important aqueous solutions at T’s up to 1000 ºC using HDAC and Raman spectroscopy.
The α-β quartz phase boundary obtained in this study can be represented by: (Ptr, qtz) (±8.8 MPa) = 0.0015 (Ttr, qtz)2 + 1.8268 (Ttr, qtz) – 1544.5, where (Ttr, qtz) is between 574 and 781 ºC; with R2 = 0.9998. Our results agree, within experimental uncertainties, with those reported by Mirwald and Massonne (1980; J. Geophys. Res. 85, p. 6983), but with some deviations from other previous data.