Presentation Information
[PPS04-P18]Effects of iron on surface structure of amorphous olivine
*Haruumi Sue1, Tomoko Fukazawa1 (1.Meiji University)
Keywords:
Amorphous forsterite,Iron,Surface structure,Interface with liquid water
Introduction
Olivine is one of the most abundant rock-forming minerals in the upper mantle of the Earth [1]. Olivine forms a solid-solution series between the Mg end-member forsterite (Mg2SiO4) and the Fe end-member fayalite (Fe2SiO4). Natural olivine is in the mantle of Earth is Mg-rich, with iron number (Fe / (Mg + Fe)) of 4 - 13 % [2]. While olivine predominantly occurs in a crystalline state in the Earth and other planetary bodies, it has also been identified in an amorphous state in interstellar molecular clouds and protoplanetary disks [3]. Although the aqueous alteration of crystalline forsterite has been extensively studied, there are few studies for the aqueous alteration of amorphous forsterite [4]. In particular, there are no reports on aqueous alteration for Fe-bearing forsterite in the amorphous state, despite its widespread occurrence in meteorites and planetary materials. To understand the alteration mechanisms of amorphous forsterite, it is essential to clarify the structure and dynamics of the surface and its interface with water. The surface structure of amorphous forsterite has unique effects on the structure and diffusivity of coexisting water [5, 6]. In this study, we performed molecular dynamics (MD) simulations to investigate the structures of surface of Fe-bearing amorphous forsterite and interface between Fe-bearing amorphous forsterite and liquid water.
Experiments
The MD simulations were performed using the MXDORTO program developed by Kawamura [7]. An initial structure of Fe-bearing forsterite crystal was prepared by partially substituting 10% of Mg with Fe in a forsterite crystal consisting of 2400 Mg2SiO4. The Fe-bearing crystalline forsterite was heated from 10 K to 3100 K at a heating rate of 2.0 K fs-1 under the NTV ensemble with three-dimensional periodic boundary conditions, followed by rapid quenching to 10 K over 0.25 ns at the same rate using the NTP ensemble to obtain the amorphous structure. Amorphous ice phase composed of 2880 H2O molecules was introduced into the vacuum layer of the Fe-bearing amorphous forsterite. The system was then heated from 10 K to 600 K under the NTP ensemble. For comparison, MD simulations were performed on a system in contact with a vacuum layer in the absence of water.
Results and discussion
The results show that the melting point of Fe-bearing amorphous forsterite in contact with a vacuum layer is about 284 K lower than that of pure amorphous forsterite. This results from the larger amplitude of the thermal vibration of Fe in the surface layer than that of Mg. The thermal vibrations of Fe are enhanced due to lower coordination number of Fe in the surface. For the interface with water, the density of the interface layer of the Fe-bearing amorphous forsterite is slightly larger than that of the surface in the absence of water due to the hydration interactions between Fe and surrounding water. Furthermore, it was found that the density and mobility of atoms on the surface layer depends on the composition of the surface layer owing to the difference in the coordination states. This suggests that iron not only stabilizes the local structure of amorphous forsterite but also promotes its reaction with water. The results may provide an important insight into mineral-water interactions and aqueous alteration mechanism of amorphous forsterite.
[1] S. Demouchy, S. Mackwell, Phys Chem Minerals 30 (2003) 486-494.
[2] C. Bollinger, S. Merkel, P. Cordier, P. Raterron, Phys. Earth Planet. Inter. 240 (2015) 95-104.
[3] T. Henning, Annu. Rev. Astron. Astrophys. 48 (2010) 21-46.
[4] Y. Igami, A. Tsuchiyama, T. Yamazaki, M. Matsumoto, Y. Kimura, Geochim. Cosmochim. Acta 293 (2021) 86.
[5] A. Kubo, J. Nishizawa, T. Ikeda-Fukazawa, Chem. Phys. Lett. 805 (2022) 139932.
[6] Y. Kobayashi, T. Ikeda-Fukazawa, ACS Earth. Space. Chem. 8 (2024) 129.
[7] K. Kawamura, MXDORTO, Japan Chemistry Program Exchange. 029 (1996).
Olivine is one of the most abundant rock-forming minerals in the upper mantle of the Earth [1]. Olivine forms a solid-solution series between the Mg end-member forsterite (Mg2SiO4) and the Fe end-member fayalite (Fe2SiO4). Natural olivine is in the mantle of Earth is Mg-rich, with iron number (Fe / (Mg + Fe)) of 4 - 13 % [2]. While olivine predominantly occurs in a crystalline state in the Earth and other planetary bodies, it has also been identified in an amorphous state in interstellar molecular clouds and protoplanetary disks [3]. Although the aqueous alteration of crystalline forsterite has been extensively studied, there are few studies for the aqueous alteration of amorphous forsterite [4]. In particular, there are no reports on aqueous alteration for Fe-bearing forsterite in the amorphous state, despite its widespread occurrence in meteorites and planetary materials. To understand the alteration mechanisms of amorphous forsterite, it is essential to clarify the structure and dynamics of the surface and its interface with water. The surface structure of amorphous forsterite has unique effects on the structure and diffusivity of coexisting water [5, 6]. In this study, we performed molecular dynamics (MD) simulations to investigate the structures of surface of Fe-bearing amorphous forsterite and interface between Fe-bearing amorphous forsterite and liquid water.
Experiments
The MD simulations were performed using the MXDORTO program developed by Kawamura [7]. An initial structure of Fe-bearing forsterite crystal was prepared by partially substituting 10% of Mg with Fe in a forsterite crystal consisting of 2400 Mg2SiO4. The Fe-bearing crystalline forsterite was heated from 10 K to 3100 K at a heating rate of 2.0 K fs-1 under the NTV ensemble with three-dimensional periodic boundary conditions, followed by rapid quenching to 10 K over 0.25 ns at the same rate using the NTP ensemble to obtain the amorphous structure. Amorphous ice phase composed of 2880 H2O molecules was introduced into the vacuum layer of the Fe-bearing amorphous forsterite. The system was then heated from 10 K to 600 K under the NTP ensemble. For comparison, MD simulations were performed on a system in contact with a vacuum layer in the absence of water.
Results and discussion
The results show that the melting point of Fe-bearing amorphous forsterite in contact with a vacuum layer is about 284 K lower than that of pure amorphous forsterite. This results from the larger amplitude of the thermal vibration of Fe in the surface layer than that of Mg. The thermal vibrations of Fe are enhanced due to lower coordination number of Fe in the surface. For the interface with water, the density of the interface layer of the Fe-bearing amorphous forsterite is slightly larger than that of the surface in the absence of water due to the hydration interactions between Fe and surrounding water. Furthermore, it was found that the density and mobility of atoms on the surface layer depends on the composition of the surface layer owing to the difference in the coordination states. This suggests that iron not only stabilizes the local structure of amorphous forsterite but also promotes its reaction with water. The results may provide an important insight into mineral-water interactions and aqueous alteration mechanism of amorphous forsterite.
[1] S. Demouchy, S. Mackwell, Phys Chem Minerals 30 (2003) 486-494.
[2] C. Bollinger, S. Merkel, P. Cordier, P. Raterron, Phys. Earth Planet. Inter. 240 (2015) 95-104.
[3] T. Henning, Annu. Rev. Astron. Astrophys. 48 (2010) 21-46.
[4] Y. Igami, A. Tsuchiyama, T. Yamazaki, M. Matsumoto, Y. Kimura, Geochim. Cosmochim. Acta 293 (2021) 86.
[5] A. Kubo, J. Nishizawa, T. Ikeda-Fukazawa, Chem. Phys. Lett. 805 (2022) 139932.
[6] Y. Kobayashi, T. Ikeda-Fukazawa, ACS Earth. Space. Chem. 8 (2024) 129.
[7] K. Kawamura, MXDORTO, Japan Chemistry Program Exchange. 029 (1996).