Abstract
The stability and the behavior of hydrous minerals at high pressure and high temperature is of great interest, because the existence of water (actually, hydrogen) in the Earth's mantle could affect physical properties such as mantle rheology and processes such as magma generation and deep seismicity in subduction zones. Our final goal is to discuss this issue from the crystallographic comprehension of hydrogen bonding and compression mechanism of hydrous minerals. Nakamoto et al. (1955) and Novak (1974) reports systematics between hydrogen bonded O...O distance and O-H infrared stretching frequencies. If this systematics can apply to a relationship between pressure variation of O...O distance and O-H infrared stretching frequencies, it is expected that hydrogen bonding strengthens with increasing pressure. We have already reported the compression behavior of some hydrous minerals with weak (brucite and portlandite), intermediate (goethite) and strong hydrogen bondings (kalicinite). In this paper, we will report the compression behavior of nahcolite (NaHCO3) with strong hydrogen bondings and discuss systematics between pressure variations of hydrogen bonded O...O distances and strength of hydrogen bonding. We used synthetic powder sample of NaHCO3 and performed angle-dispersive x-ray diffraction experiments at high pressure at BL-18C in the Photon Factory. No observation of significant change in diffraction patterns recorded indicates that there is no structural transition occurs from 0.2 to 9 GPa. Obtained diffraction data were analyzed by the Rietveld structure refinement method under nonlinear constraints for selected interatomic distances and bond angles. We fixed shape of a triangular CO32-. This constrain could be reasonable because it was reported a triangular CO32- ion in several carbonates is almost uncompressed below 10 GPa (Ross and Reeder, 1992, Ross, 1997). The pressure-volume data were fit to a second-order Birch-Murnaghan equation of state. The bulk modulus K=36GPa (K'=4) was obtained. In the crystal structure of NaHCO3, there are one-dimensional infinite chains of hydrogen bonded bicarbonate ions almost parallel to the [101] direction. The compressibility of the b-axis is the hardest and this indicates that the hydrogen bonded O...O distance should shorten significantly. Indeed, although the hydrogen bonded O...O distance is 2.59 A at 0.2 GPa, the distance shortens to 2.40 A; at 9 GPa. When hydrogen bonded O...O distance becomes about 2.4 A, symmetrization of hydrogen bonding was observed in ice-VI (Hemley et al., 1987, Aoki et al., 1996). The hydrogen bonding in NaHCO3 at around 9 GPa should be greatly interesting. Since we are now performing IR measurements and neutron diffraction experiments under high pressure, detailed results will come up at the meeting.
