Water Vapor Pressure Dependence of Crystallization Kinetics of Amorphous Forsterite

Daiki Yamamoto, Shogo Tachibana

Research output: Contribution to journalArticlepeer-review

19 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)778-786
Number of pages9
JournalACS Earth and Space Chemistry
Volume2
Issue number8
DOIs
Publication statusPublished - Aug 16 2018
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology
  • Atmospheric Science
  • Space and Planetary Science

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