9:30 AM - 9:45 AM
[SMP29-02] Hydrostatic effect on the pressure-induced amorphization of natrolite
Keywords:natrolite, pressure-induced amorphization, pressure-transmitting medium, nitrogen
Zeolites are hydrous aluminosilicates with a microporous structure, which can incorporate water and various gas molecules due to their characteristic framework structure. Naturally occurring in the surface layers of the Earth and Mars, they also have very important industrial applications as ion exchange materials, catalysts and adsorbents. Numerous zeolites have been synthesized and classified according to their skeletal structure. Studies have been reported on the pressure response of zeolites, such as penetration and adsorption of pressure-transmitting media and pressure-induced amorphization, depending on the size of the pores.
Natrolite has the chemical formula Na2(Si3Al2)O10·2H2O and has four- and eight-membered ring (MR) structures in its crystal structure composed of SiO6 and AlO6 linkages. The size of these rings is larger than water, methanol, ethanol and nitrogen molecules. Sodium ions and H2O molecules are arranged in the 8MR and other cations can be exchanged for the Na+ cation part. In previous studies, natrolite is known to incorporate more water while undergoing reversible pressure-induced phase transitions up to around 1.5 GPa (Liu et al. 2010). At higher pressures, amorphization has been reported with water and methanol-ethanol 4 : 1 mixture (ME) as a pressure-transmitting medium (PTM), but there are discrepancies in their reported amorphization pressures. In this study, therefore, the influence of the PTM on the pressure-induced amorphization in natural natrolite is examined. High-pressure experiments were carried out in a diamond anvil cell (DAC) using water, ME or nitrogen as a PTM. In situ observations at room temperature and high pressure via Raman spectroscopy and synchrotron X-ray diffraction (XRD) were performed to clarify the pressure behavior of natrolite.
As a result of the Raman spectroscopy measurements in aqueous condition, the Raman bands derived from the MR structure and the OH stretching vibration of water in natrolite shifted to higher wavenumber with pressure. These Raman shifts broadened significantly at 17-24 GPa, confirming amorphization. Even in ME medium, amorphization occurred at 16-19 GPa. This pressure value was higher than the 7.7 GPa reported in Goryainov et al. (2005), and the timing of the disappearance of the OH band was also different. The Bragg reflections disappeared at 13 GPa and did not recover at ambient pressure. In nitrogen, a different behavior was observed: at 6-8 GPa, the Raman band, which originates from the 4MR structure, split and the OH band disappeared at the same time. Subsequent amorphization occurred around 12 GPa; XRD measurements showed a splitting of the 111 and 220 Bragg reflections around 9-10 GPa. Although the disappearance of the Bragg reflection was also observed by 14.2 GPa, the crystalline phase totaly recovered at ambient pressure.
These results show that in nitrogen, even though the pressure at which broadening occurred was lower than in the other PTMs, complete amorphization did not occur at 15 GPa and the natrolite structure was retained. It is considered that nitrogen, the largest molecular among the PTM used, caused distortion of the crystal structure, straining water molecules adsorbed within the MR structure and accelerating the phase transition and associated amorphization. It is therefore suggested that the nature of the PTM and the molecular size influenced the structural changes in natrolite. The interaction of natrolite with nitrogen in the Martian surface may also affect the ability of natrolite to retain water in the subsurface environment. Future work is required to elucidate the behavior with other gases such as methane and carbon dioxide.
Natrolite has the chemical formula Na2(Si3Al2)O10·2H2O and has four- and eight-membered ring (MR) structures in its crystal structure composed of SiO6 and AlO6 linkages. The size of these rings is larger than water, methanol, ethanol and nitrogen molecules. Sodium ions and H2O molecules are arranged in the 8MR and other cations can be exchanged for the Na+ cation part. In previous studies, natrolite is known to incorporate more water while undergoing reversible pressure-induced phase transitions up to around 1.5 GPa (Liu et al. 2010). At higher pressures, amorphization has been reported with water and methanol-ethanol 4 : 1 mixture (ME) as a pressure-transmitting medium (PTM), but there are discrepancies in their reported amorphization pressures. In this study, therefore, the influence of the PTM on the pressure-induced amorphization in natural natrolite is examined. High-pressure experiments were carried out in a diamond anvil cell (DAC) using water, ME or nitrogen as a PTM. In situ observations at room temperature and high pressure via Raman spectroscopy and synchrotron X-ray diffraction (XRD) were performed to clarify the pressure behavior of natrolite.
As a result of the Raman spectroscopy measurements in aqueous condition, the Raman bands derived from the MR structure and the OH stretching vibration of water in natrolite shifted to higher wavenumber with pressure. These Raman shifts broadened significantly at 17-24 GPa, confirming amorphization. Even in ME medium, amorphization occurred at 16-19 GPa. This pressure value was higher than the 7.7 GPa reported in Goryainov et al. (2005), and the timing of the disappearance of the OH band was also different. The Bragg reflections disappeared at 13 GPa and did not recover at ambient pressure. In nitrogen, a different behavior was observed: at 6-8 GPa, the Raman band, which originates from the 4MR structure, split and the OH band disappeared at the same time. Subsequent amorphization occurred around 12 GPa; XRD measurements showed a splitting of the 111 and 220 Bragg reflections around 9-10 GPa. Although the disappearance of the Bragg reflection was also observed by 14.2 GPa, the crystalline phase totaly recovered at ambient pressure.
These results show that in nitrogen, even though the pressure at which broadening occurred was lower than in the other PTMs, complete amorphization did not occur at 15 GPa and the natrolite structure was retained. It is considered that nitrogen, the largest molecular among the PTM used, caused distortion of the crystal structure, straining water molecules adsorbed within the MR structure and accelerating the phase transition and associated amorphization. It is therefore suggested that the nature of the PTM and the molecular size influenced the structural changes in natrolite. The interaction of natrolite with nitrogen in the Martian surface may also affect the ability of natrolite to retain water in the subsurface environment. Future work is required to elucidate the behavior with other gases such as methane and carbon dioxide.