[S-MP29] Physics and Chemistry of Minerals

[SMP29-11] Rapid increase in sound velocity of calcium aluminosilicate glasses near 10 GPa

*Itaru Ohira¹, Steeve Gréaux², Yoshio Kono³, Jiejun Jing², Yoshihiro Inoue², Sho Kakizawa⁴, Noriyoshi Tsujino⁴, Yuji Higo⁴ (1.Department of Chemistry, Faculty of Science, Gakushuin University, 2.Geodynamics Research Center (GRC), Ehime University, 3.Department of Physics and Astronomy, Kwansei Gakuin University, 4.Japan Synchrotron Radiation Research Institute (JASRI))

Keywords:Aluminosilicate glass, High pressure, Sound velocity, Synchrotron X-ray

Aluminum ion is considered to play a key role in densification of aluminosilicate glasses, structural and compositional analogs of magmas. Al–O coordination number (CN) starts to increase at lower pressure condition than Si–O CN (e.g., [1]), suggesting that Al-rich aluminosilicate glasses may undergoes densification at lower pressures than Al-poor glasses. In addition to this change in short-range order, the recent study on CaAl₂O₄ glass found the quick densification by shrinking the voids between Ca and O at a narrow pressure range of 8–10 GPa [2]. The intermediate-range order change and the subsequent short-range order change may be a general process of the rapid densification of aluminosilicate glasses and melts. To clarify the possible existence of this densification process in more natural and complex systems, we conducted the combined in situ X-ray and ultrasonic measurements on CaO–Al₂O₃–SiO₂ (CAS) glasses in a wide range of Al/Si molar ratio (Al/Si = 1–4).
The sample glasses are CaAl₂Si₂O₈ (anorthite, Al/Si = 1), CaAl₂SiO₆ (Al/Si = 2), Ca₂Al₆Si₂O₁₅ (Al/Si = 3), and CaAl₄SiO₉ (Al/Si = 4) glasses, synthesized by the container-less aerodynamic heating system at GRC, Ehime University [2]. The ultrasonic measurements were carried out at the beamline BL04B1, SPring-8, and we obtained the V_P and V_S of the four glass samples up to 25 GPa. Under compression, all the samples showed the modest decrease in V_P and V_S, followed by an abrupt increase in both values over ~7–10 GPa. Above 10 GPa, the V_P and V_S continue to increase gradually and linearly with increasing pressure. In addition to the rapid increase in velocity, ~10% reduction of the sample lengths was also observed at ~7–10 GPa. The abrupt changes in elastic behavior are consistent with CaAl₂O₄ glass [2]. The present results indicate that the rapid densification at ~7–10 GPa might be a common process in CAS glass, and its pressure condition and the degree of velocity increase are independent of the Al/Si ratio.

References
[1] B. Mysen & P. Richet. Silicate glasses and melts: second edition. Elsevier (2019).
[2] I. Ohira et al. Phys. Rev. B 110, 054115 (2024).