[S-CG62] Rheology, fracture and friction in Earth and planetary sciences

[SCG62-09] Molecular-scale structure of SiO₂ melt and glass under tension revealed by MD simulation

*Ryota Mamizuka¹, Satoshi Okumura¹, Hiroshi Sakuma² (1.Department of Earth Science, Tohoku University, 2.National Institute for Materials Science)

Magma fragmentation is a key process controlling explosive eruptions. The brittle fragmentation occurs when silicate melt deforms at high rates. Additionally, the viscosity of silicate melt decreases with an increase in a deformation rate (shear thinning). Thus, silicate melt simultaneously shows solid-like and liquid-like behavior under high deformation rates. Okumura et al. (2023) experimentally demonstrated that, under tensional deformation, the position of the First Sharp Diffraction Peak (FSDP) in the X-ray diffraction (XRD) pattern of silicate melt shifts when the deformation rate is high and the melt shows shear thinning and brittle failure. FSDP is the peak in the XRD pattern, indicating the presence of the medium-range order. The ring structure formed by the SiO₄ tetrahedra network corresponds to the medium-range order of silicate melt. Therefore, the results obtained by the previous experiments imply that the ring structure of silicate melt changes under high deformation rate and may be the origin of shear thinning and brittle failure. In this study, the molecular dynamics (MD) simulation was conducted on SiO₂ melt/glass to clarify the evolution of ring structure under deformation. We focused on the changes of ring size distribution (the numbers of three- to fifteen-membered rings) and FSDP under tensional deformation. The simulations were performed using LAMMPS with the potential reported by Vashishta et al. (1990), in which the cycles of tension deformation and relaxation were imposed. The structure factor and ring size distribution were analyzed using the RINGS software. The results of our simulations are summarized below. (1) Under tensional deformation, the number of five- and six-membered rings decreases, and the number of large rings increases with strain at temperatures <2000 K (slow relaxation), while no systematic changes in the ring size distribution were observed at temperatures >2500 K (fast relaxation). (2) The FSDP position was found to shift to lower Q values at temperatures <2000 K, while no systematic changes in the FSDP position were observed at temperatures >2500 K. Our simulation indicates that the ring size distribution changes when the deformation is fast compared to the relaxation of silicate structure and the FSDP position shifts via the change of the ring size distribution.