TY - JOUR
T1 - Water Vapor Pressure Dependence of Crystallization Kinetics of Amorphous Forsterite
AU - Yamamoto, Daiki
AU - Tachibana, Shogo
N1 - Funding Information:
The authors thank Akira Tsuchiyama for providing the amorphous forsterite powder, Junji Yamamoto, Kohei Takahata and Yuuki Hagiwara for Raman spectroscopy. The authors also thank anonymous reviewers for their constructive reviews. The TEM analysis was carried out at the “Joint-use Facilities: Laboratory of Nano-Micro Material Analysis”, Hokkaido University, supported by “Material Analysis and Structure Analysis Open Unit (MASAOU)”. This work was supported by Ministry of Education, Sports, Science and Technology KAKENHI Grant.
Publisher Copyright:
© Copyright 2018 American Chemical Society.
PY - 2018/8/16
Y1 - 2018/8/16
N2 - Amorphous silicate dust grains, dominant solid components in the interstellar medium, are converted into crystalline silicate dust through thermal annealing in protoplanetary disks. Water vapor is a major reactive gas species in the protoplanetary disk, and it may affect the crystallization behavior of amorphous silicates. In this study, the water vapor pressure dependence of the crystallization kinetics of amorphous silicate with forsterite composition was investigated under controlled water vapor pressures ranging from ∼1 × 10-9 to 5 × 10-3 bar at 923-1023 K. We found that the crystallization rate depends on the water vapor pressure and becomes faster at higher water vapor pressures. We also found that the activation energy and the pre-exponential factor for crystallization rate decreases with increasing water vapor pressure. Water molecules dissolving into amorphous forsterite cut atomic bonds such as Si-O-Si and Mg-O-Mg through a hydroxyl (-OH) formation reaction. Rearrangement of structural units cut by hydroxyls occurs with a smaller energetic barrier, and thus water vapor can act as a catalyst to promote crystallization of amorphous forsterite. Based on the experimental data, we conclude that the temperature required for crystallization of amorphous forsterite within the lifetime of protoplanetary disks is ∼620-700 K irrespective of the water vapor pressure in the disk and that the observed crystalline forsterite dust in protoplanetary disks indicates the presence of dust annealed at temperatures above ∼620-700 K. Extraterrestrial materials record various thermal events in the early Solar System (e.g., chondrule formation). Considering that meteoritic evidence indicates that the H2O/H2 ratio was enhanced over the canonical ratio in the early Solar System, the thermal evolution of amorphous forsterite dust during various thermal events in the early Solar System should be discussed taking the effect of water vapor pressure into account.
AB - Amorphous silicate dust grains, dominant solid components in the interstellar medium, are converted into crystalline silicate dust through thermal annealing in protoplanetary disks. Water vapor is a major reactive gas species in the protoplanetary disk, and it may affect the crystallization behavior of amorphous silicates. In this study, the water vapor pressure dependence of the crystallization kinetics of amorphous silicate with forsterite composition was investigated under controlled water vapor pressures ranging from ∼1 × 10-9 to 5 × 10-3 bar at 923-1023 K. We found that the crystallization rate depends on the water vapor pressure and becomes faster at higher water vapor pressures. We also found that the activation energy and the pre-exponential factor for crystallization rate decreases with increasing water vapor pressure. Water molecules dissolving into amorphous forsterite cut atomic bonds such as Si-O-Si and Mg-O-Mg through a hydroxyl (-OH) formation reaction. Rearrangement of structural units cut by hydroxyls occurs with a smaller energetic barrier, and thus water vapor can act as a catalyst to promote crystallization of amorphous forsterite. Based on the experimental data, we conclude that the temperature required for crystallization of amorphous forsterite within the lifetime of protoplanetary disks is ∼620-700 K irrespective of the water vapor pressure in the disk and that the observed crystalline forsterite dust in protoplanetary disks indicates the presence of dust annealed at temperatures above ∼620-700 K. Extraterrestrial materials record various thermal events in the early Solar System (e.g., chondrule formation). Considering that meteoritic evidence indicates that the H2O/H2 ratio was enhanced over the canonical ratio in the early Solar System, the thermal evolution of amorphous forsterite dust during various thermal events in the early Solar System should be discussed taking the effect of water vapor pressure into account.
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U2 - 10.1021/acsearthspacechem.8b00047
DO - 10.1021/acsearthspacechem.8b00047
M3 - Article
AN - SCOPUS:85049250829
SN - 2472-3452
VL - 2
SP - 778
EP - 786
JO - ACS Earth and Space Chemistry
JF - ACS Earth and Space Chemistry
IS - 8
ER -