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[PCG20-08] Phase transition of glassy forsterite in contact with liquid water
Keywords:forsterite glass, water, hydrous mineral
Introduction
Forsterite (Mg2SiO4) in glassy state is a major component of interstellar silicate dust grains [1]. The Interstellar dust undergoes phase transitions through various reactions with adsorbed water molecules which called mineral evolution [2]. Various studies have been performed to investigate the reactions between forsterite and water.
Yamamoto and Tachibana [3] investigated the effects of water vapor on crystallization of glassy forsterite, and found that the activation energy of the crystallization decreases as the water vapor pressure increases. Furthermore, Yamamoto et al. [4] showed that the serpentine (Mg3Si2O5(OH)4) and brucite (Mg(OH)2) are formed with the serpentinization reaction of glassy forsterite and water vapor in high pressure conditions by the following reaction:
2 Mg2SiO4 + 3H2O = Mg3Si2O5(OH)4 + Mg(OH)2
Kubo et al. [5] performed molecular dynamics (MD) calculations of forsterite-water systems and found that brucite is formed from dissolved MgOxHy in liquid water. The MD result suggests that the interstitial water can be a liquid state in glassy forsterite at temperatures above the boiling point without pressurization, because the density of the interstitial water is higher than that of ordinary water.
To understand the effects of liquid water on phase transition of glassy forsterite, we investigated the structural changes of forsterite-water systems with hydrothermal reaction. The structural analyses were mainly performed using the X-ray diffraction (XRD) method.
Experiments
The nanoparticles of glassy forsterite (~100 nm) were synthesized by an induced thermal plasma method from Mg(OH)2 and SiO2 [3,6]. The hydrothermal reactions were performed using an autoclave. The glassy forsterite and distilled water were introduced in a titanium cell. The starting samples were heated from the room temperature to a reaction temperature (473-573K) at 1.67K･min-1 in heating rate. Then, the temperature was kept at the reaction temperature for 0-48 hours. The XRD measurements were performed after cooling and drying the samples. The pressures in the cell were the saturated vapor pressures of each temperature (e.g., 8.7 MPa at 573 K). Run products from crystalline forsterite (~2 µm) with water were used for comparison with the results of glassy forsterite with water.
Results and discussion
The result shows that brucite and serpentine are formed from glassy forsterite at 573 K, whereas no diffraction peaks are observed for the XRD pattern of the sample from crystalline forsterite after the hydrothermal reaction at 573 K for 20 hours. The results suggest that the serpentinization reaction are promoted in the interface between the glassy forsterite and water in liquid state.
For the samples from glassy forsterite, the intensities of the diffraction peaks assigned to brucite and serpentine depend on reacted time and temperature. From the changing rate of the intensities of the diffraction peaks, it was found that the formation of brucite begins at lower temperature and shorter reaction time in comparison with that of serpentine. For the reaction of glassy forsterite with water in liquid state, brucite is formed from dissolved Mg components in liquid water [5], although the formation of serpentine proceeds in the forsterite-water interface. This suggests that the serpentine formed in the early stages at the interface is a glassy state, while the crystalline brucite grows in the liquid water phase.
We discuss the mechanisms of the phase transition in the water-forsterite system.
[1] T. Henning, Annu. Rev. Astron. Astrophys. 48, 21-46 (2010).
[2] J.M. Greenberg, J.I. Hage, Astrophys. 361, 260-274 (1990).
[3] D. Yamamoto, S. Tachibana, ACS Earth Space Chem. 2, 778-786 (2018).
[4] D. Yamamoto, S. Tachibana, H. Nagahara, K. Ozawa, A. Tsuchiyama, J. Geosci. Union Meeting (2014).
[5] A. Kubo, J. Nishizawa, T. Ikeda-Fukazawa, Chem. Phys. Lett. 805, 139932 (2022).
[6] Y. Imai, Experimental study of circumstellar silicate dust evolution by crystallization processes using laboratory infrared spectroscopy, PhD Thesis, Osaka Univ. (2012).
Forsterite (Mg2SiO4) in glassy state is a major component of interstellar silicate dust grains [1]. The Interstellar dust undergoes phase transitions through various reactions with adsorbed water molecules which called mineral evolution [2]. Various studies have been performed to investigate the reactions between forsterite and water.
Yamamoto and Tachibana [3] investigated the effects of water vapor on crystallization of glassy forsterite, and found that the activation energy of the crystallization decreases as the water vapor pressure increases. Furthermore, Yamamoto et al. [4] showed that the serpentine (Mg3Si2O5(OH)4) and brucite (Mg(OH)2) are formed with the serpentinization reaction of glassy forsterite and water vapor in high pressure conditions by the following reaction:
2 Mg2SiO4 + 3H2O = Mg3Si2O5(OH)4 + Mg(OH)2
Kubo et al. [5] performed molecular dynamics (MD) calculations of forsterite-water systems and found that brucite is formed from dissolved MgOxHy in liquid water. The MD result suggests that the interstitial water can be a liquid state in glassy forsterite at temperatures above the boiling point without pressurization, because the density of the interstitial water is higher than that of ordinary water.
To understand the effects of liquid water on phase transition of glassy forsterite, we investigated the structural changes of forsterite-water systems with hydrothermal reaction. The structural analyses were mainly performed using the X-ray diffraction (XRD) method.
Experiments
The nanoparticles of glassy forsterite (~100 nm) were synthesized by an induced thermal plasma method from Mg(OH)2 and SiO2 [3,6]. The hydrothermal reactions were performed using an autoclave. The glassy forsterite and distilled water were introduced in a titanium cell. The starting samples were heated from the room temperature to a reaction temperature (473-573K) at 1.67K･min-1 in heating rate. Then, the temperature was kept at the reaction temperature for 0-48 hours. The XRD measurements were performed after cooling and drying the samples. The pressures in the cell were the saturated vapor pressures of each temperature (e.g., 8.7 MPa at 573 K). Run products from crystalline forsterite (~2 µm) with water were used for comparison with the results of glassy forsterite with water.
Results and discussion
The result shows that brucite and serpentine are formed from glassy forsterite at 573 K, whereas no diffraction peaks are observed for the XRD pattern of the sample from crystalline forsterite after the hydrothermal reaction at 573 K for 20 hours. The results suggest that the serpentinization reaction are promoted in the interface between the glassy forsterite and water in liquid state.
For the samples from glassy forsterite, the intensities of the diffraction peaks assigned to brucite and serpentine depend on reacted time and temperature. From the changing rate of the intensities of the diffraction peaks, it was found that the formation of brucite begins at lower temperature and shorter reaction time in comparison with that of serpentine. For the reaction of glassy forsterite with water in liquid state, brucite is formed from dissolved Mg components in liquid water [5], although the formation of serpentine proceeds in the forsterite-water interface. This suggests that the serpentine formed in the early stages at the interface is a glassy state, while the crystalline brucite grows in the liquid water phase.
We discuss the mechanisms of the phase transition in the water-forsterite system.
[1] T. Henning, Annu. Rev. Astron. Astrophys. 48, 21-46 (2010).
[2] J.M. Greenberg, J.I. Hage, Astrophys. 361, 260-274 (1990).
[3] D. Yamamoto, S. Tachibana, ACS Earth Space Chem. 2, 778-786 (2018).
[4] D. Yamamoto, S. Tachibana, H. Nagahara, K. Ozawa, A. Tsuchiyama, J. Geosci. Union Meeting (2014).
[5] A. Kubo, J. Nishizawa, T. Ikeda-Fukazawa, Chem. Phys. Lett. 805, 139932 (2022).
[6] Y. Imai, Experimental study of circumstellar silicate dust evolution by crystallization processes using laboratory infrared spectroscopy, PhD Thesis, Osaka Univ. (2012).