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[PCG22-03] Effects of water states on phase transition of amorphous forsterite
Keywords:amorphous forsterite, water, hydrous mineral
Introduction
Inclusions of liquid water have been found in the samples collected from the carbonaceous asteroid Ryugu [1]. This suggests that water can be a key material for the mineral evolutions. Forsterite (Mg2SiO4) in crystalline state exists in protoplanetary disks[2], whereas amorphous forsterite is a main component of dust grains in interstellar molecular clouds [3]. In interstellar molecular clouds, the dust grains are covered with amorphous ice. Although most water molecules in amorphous ice desorb from the mineral grains with heating during the formation of protoplanetary disks, residues can be confined in the grain boundaries as "interstitial water" [4].
Yamamoto and Tachibana [5] found that water vapor has effects to accelerate the transition of forsterite from amorphous to crystalline states due to decrease in the activation energies for breaking the covalent bonds of Si–O and Mg–O. On the other hand, Yamamoto et al. [6] showed that serpentine (Mg3Si2O5(OH)4) and brucite (Mg(OH)2) are formed with the serpentinization reaction of amorphous forsterite and water vapor in high pressure conditions.Kubo et al [7] performed molecular dynamics calculations of forsterite-water system and found that brucite forms from dissolved MgOxHyin liquid water. The results suggest that the transition process from amorphous forsterite in liquid water differs from that in water vapor.
To understand the effects of liquid water on the phase transition from amorphous forsterite, we analyzed the structural changes of amorphous forsterite during hydrothermal reactions with water in liquid and vapor states. The methods of X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), and scanning electron microscopy (SEM) were used for the analyses.
Experiments
The nanoparticles of amorphous forsterite were produced using an induced thermal plasma method with Mg(OH)2and SiO2[5]. The amorphous forsterite and distilled water were put in a titanium cell in an autoclave for the hydrothermal reaction. The state (i.e., liquid or vapor) and vapor pressure of water were controlled with the initial amount of water in the titanium cell. If the water amount was sufficient, liquid water coexisted, and the pressure in the cell were the saturated vapor pressures of each temperature (e.g., 8.7 MPa at 573 K). The samples were heated at 1.67 K min-1from room temperature (RT) to a reaction temperature (473-573 K) and kept for 0-96 hours. The XRD, TG-DTA, and SEM measurements were performed after cooling the samples to RT and drying for 2 days in a vacuum desiccator at RT. The XRD measurements were performed at RT. The TG-DTA measurements were performed at 10 K min-1from RT to 1273 K.
Results and discussion
From the XRD results, it was found that crystalline brucite, serpentine, and magnesium silicate hydrate (MSH) are formed from amorphous forsterite by reaction with liquid water [8], whereas only MSH is formed by reaction with water vapor under the condition of 1.15 MPa at 573 K for 96 h.The hexagonal crystals of brucite and string-like fragments of MSH were observed in the SEM images. The shifts of broad bands of MSH in the XRD patterns suggest that MSH transforms into the serpentine owing to a dehydration of the interstitial water between the layered structures.
From the TG-DTA results, the relative amounts of the products from the hydrothermal reaction were estimated. The brucite and serpentine dehydrate as follows at around 620 and 873 K, respectively.
Mg(OH)2®MgO + H2O (1)
Mg3Si2O5(OH)4®Mg2SiO4+ MgSiO3+ 2H2O (2)
The relative amounts of brucite and serpentine were determined from the mass decreases around 620 and 873 K, respectively. After the reactions (1) and (2), MgO and MgSiO3reacted to be the forsterite crystal at around 1100 K. On the other hand, MSH formed by the reaction with water vapor transformed into forsterite crystal without formation of an intermediate product.
We discuss the mechanisms of the transition process in the water-forsterite system.
[1] T. Nakamura et al., Science379, 8671 (2023).
[2] F.J. Molster, L.B.F.M. Waters, A.G.G.M. Tielens, C. Koike, H. Chihara,Astron. Astrophys.382, 241-255 (2002).
[3] T. Henning, Annu. Rev. Astron. Astrophys.48, 21-46 (2010).
[4] T. Kobayashi, T. Ikeda-Fukazawa, ACS Earth Space Chem.8, 129-136 (2024).
[5] D. Yamamoto, S. Tachibana, ACS Earth Space Chem.2, 778-786 (2018).
[6] D. Yamamoto, S. Tachibana, H. Nagahara, K. Ozawa, A. Tsuchiyama, J. Geosci. Union Meeting (2014).
[7] A. Kubo, J. Nishizawa, T. Ikeda-Fukazawa, Chem. Phys. Lett. 805, 139932 (2022).
[8] Y. Igami, A. Tsuchiyama, T. Yamazaki, M. Matsumoto, Y. Kimura,Geochimica et Cosmochimica Acta 293, 86-102 (2021).
Inclusions of liquid water have been found in the samples collected from the carbonaceous asteroid Ryugu [1]. This suggests that water can be a key material for the mineral evolutions. Forsterite (Mg2SiO4) in crystalline state exists in protoplanetary disks[2], whereas amorphous forsterite is a main component of dust grains in interstellar molecular clouds [3]. In interstellar molecular clouds, the dust grains are covered with amorphous ice. Although most water molecules in amorphous ice desorb from the mineral grains with heating during the formation of protoplanetary disks, residues can be confined in the grain boundaries as "interstitial water" [4].
Yamamoto and Tachibana [5] found that water vapor has effects to accelerate the transition of forsterite from amorphous to crystalline states due to decrease in the activation energies for breaking the covalent bonds of Si–O and Mg–O. On the other hand, Yamamoto et al. [6] showed that serpentine (Mg3Si2O5(OH)4) and brucite (Mg(OH)2) are formed with the serpentinization reaction of amorphous forsterite and water vapor in high pressure conditions.Kubo et al [7] performed molecular dynamics calculations of forsterite-water system and found that brucite forms from dissolved MgOxHyin liquid water. The results suggest that the transition process from amorphous forsterite in liquid water differs from that in water vapor.
To understand the effects of liquid water on the phase transition from amorphous forsterite, we analyzed the structural changes of amorphous forsterite during hydrothermal reactions with water in liquid and vapor states. The methods of X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), and scanning electron microscopy (SEM) were used for the analyses.
Experiments
The nanoparticles of amorphous forsterite were produced using an induced thermal plasma method with Mg(OH)2and SiO2[5]. The amorphous forsterite and distilled water were put in a titanium cell in an autoclave for the hydrothermal reaction. The state (i.e., liquid or vapor) and vapor pressure of water were controlled with the initial amount of water in the titanium cell. If the water amount was sufficient, liquid water coexisted, and the pressure in the cell were the saturated vapor pressures of each temperature (e.g., 8.7 MPa at 573 K). The samples were heated at 1.67 K min-1from room temperature (RT) to a reaction temperature (473-573 K) and kept for 0-96 hours. The XRD, TG-DTA, and SEM measurements were performed after cooling the samples to RT and drying for 2 days in a vacuum desiccator at RT. The XRD measurements were performed at RT. The TG-DTA measurements were performed at 10 K min-1from RT to 1273 K.
Results and discussion
From the XRD results, it was found that crystalline brucite, serpentine, and magnesium silicate hydrate (MSH) are formed from amorphous forsterite by reaction with liquid water [8], whereas only MSH is formed by reaction with water vapor under the condition of 1.15 MPa at 573 K for 96 h.The hexagonal crystals of brucite and string-like fragments of MSH were observed in the SEM images. The shifts of broad bands of MSH in the XRD patterns suggest that MSH transforms into the serpentine owing to a dehydration of the interstitial water between the layered structures.
From the TG-DTA results, the relative amounts of the products from the hydrothermal reaction were estimated. The brucite and serpentine dehydrate as follows at around 620 and 873 K, respectively.
Mg(OH)2®MgO + H2O (1)
Mg3Si2O5(OH)4®Mg2SiO4+ MgSiO3+ 2H2O (2)
The relative amounts of brucite and serpentine were determined from the mass decreases around 620 and 873 K, respectively. After the reactions (1) and (2), MgO and MgSiO3reacted to be the forsterite crystal at around 1100 K. On the other hand, MSH formed by the reaction with water vapor transformed into forsterite crystal without formation of an intermediate product.
We discuss the mechanisms of the transition process in the water-forsterite system.
[1] T. Nakamura et al., Science379, 8671 (2023).
[2] F.J. Molster, L.B.F.M. Waters, A.G.G.M. Tielens, C. Koike, H. Chihara,Astron. Astrophys.382, 241-255 (2002).
[3] T. Henning, Annu. Rev. Astron. Astrophys.48, 21-46 (2010).
[4] T. Kobayashi, T. Ikeda-Fukazawa, ACS Earth Space Chem.8, 129-136 (2024).
[5] D. Yamamoto, S. Tachibana, ACS Earth Space Chem.2, 778-786 (2018).
[6] D. Yamamoto, S. Tachibana, H. Nagahara, K. Ozawa, A. Tsuchiyama, J. Geosci. Union Meeting (2014).
[7] A. Kubo, J. Nishizawa, T. Ikeda-Fukazawa, Chem. Phys. Lett. 805, 139932 (2022).
[8] Y. Igami, A. Tsuchiyama, T. Yamazaki, M. Matsumoto, Y. Kimura,Geochimica et Cosmochimica Acta 293, 86-102 (2021).