Dissolution process of forsterite coexisting with interstitial water
Y. Kobayashi, T. Ikeda-Fukazawa
Department of Applied Chemistry, Meiji University, JapanInclusions of liquid water have been found in the samples collected from the carbonaceous
asteroid Ryugu [1]. Though the origin of water is not conclusive, the deposited ice on mineral
dust grains in interstellar molecular clouds could be a potential cause. In interstellar molecularclouds, mineral dust grains are covered with amorphous ice mantles [2]. The major componentof the mineral dust grains is forsterite (Mg2SiO4) in glassy state [3], which is considered toundergo a mineral evolution through various processes to be a crystalline state in planets.
Although most water molecules desorb from the mineral grains with heating during theformation of protoplanetary disk, residues can be confined in the grain boundaries as aninterstitial water. The interstitial water of forsterite has unique properties. Kubo et al. [4] foundthat the interstitial water of glassy forsterite has higher density than that of ordinary water andexists as liquid state at high temperatures above the boiling point. However, the mechanisms ofthe structural change of the interstitial water have not yet been well understood. In this study,
molecular dynamics (MD) calculations were performed to investigate the effects of thicknessof water layer on structure of interstitial water and forsterite.
MD calculations were performed using the MXDORTO program developed by Kawamura [5].
Fundamental orthorhombic cells consisting of 484–3311 H2O and 1176 Mg2SiO4 (forsterite incrystalline or glassy states) were used as the initial structures. The thicknesses of water layer dbetween the forsterite phases were controlled with the number of H2O to be 1–6 nm at the initialstructure. The MD code was run with an NTP ensemble at 280–670 K. The pressure was kept
at 0.1 MPa. The structure was analyzed using the equilibrated states. For the comparison, thepure water system consisting of 360 H2O was also calculated.
Dissolutions of Mg and Si from the glassy forsterite toward interstitial water were observed attemperatures above 280 K for the system with d > 1 nm. The dissolved Mg exists as MgO6Hx
unit and forms Mg(OH)2 crystal by collisions of the MgO6Hx units in water layer [4]. Thedissolved Si maintained tetrahedral SiO4 structure due to the strong Si–O bond.
For crystalline forsterite, in contrast, no dissolution of Mg and Si was observed at temperatures
below 610 K. Only Mg dissolved at temperatures above 630 K for the system with d < 1.5 nm.
The dissolution of Mg generates a defect on the surface of crystalline forsterite. The distortionof structure around the defects might promote an additional dissolution of Mg. The dissolution
occurs for the systems with d < 1.5 nm, because the density of water increases as the thicknessof the water layer decreases. The result suggests that the dense structure of interstitial water has
an effect to enhance the decomposition of crystalline forsterite. Since the number of dissolved
Mg was extremely small, the formation of Mg(OH)2 crystal was not observed in the calculation
period of the present study (3.0 ns).
References
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[2]
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[4]
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