15:30 〜 15:45
[SIT17-01] Pressure-induced structural change of basaltic glass up to 18 GPa
キーワード：珪酸塩ガラス、酸素原子充填率、X線回折、中性子回折
The structures of cold-compressed basaltic glass were investigated at pressures of up to 18 GPa using in situ X-ray and neutron diffraction to acquire insights into the physicochemical properties of deep magmas. On compression, basaltic glass changes its compression behavior: the mean O-O coordination number (CNOO) starts to rise with maintaining the mean O-O distance (rOO) above about 2–4 GPa, and then CNOO stops increasing and rOO begins to shrink along with the increase in the mean coordination number of Al (CNAlO) above ~9 GPa. The change around 9 GPa is interpreted by the change in the contraction mechanism from bending tetrahedral networks of glass to increasing oxygen packing ratio via the increase in CNAlO. The analysis of the oxygen packing fraction (ηO) under high pressure revealed that ηO exceeds the value for dense random packing, suggesting that the oxygen-packing hypothesis recently proposed cannot account for the pressure-induced structural transformations of silica and silicate glasses. The rise of the CNOO at 2–4 GPa reflects the elastic softening of fourfold-coordinated silicate glass, which may be the origin of anomalies of elastic moduli in basaltic glass at ~2 GPa previously reported by Liu and Lin (2014).
The widths of both the first sharp diffraction peak and the principal peak show contrastive compression behaviors between modified silicate and silica glasses. This result suggests that modified silicate glasses represent different pressure evolutions in the ranges of the intermediate- and the extended-range order structures from those of silica glass, likely due to the presence of modifier cations and the resultant formations of smaller rings and cavity volume.
Reference
Liu, J., and Lin, J.-F. (2014) Abnormal acoustic wave velocities in basaltic and (Fe,Al)-bearing silicate glasses at high pressures. Geophysical Research Letters, 41, 8832–8839.
The widths of both the first sharp diffraction peak and the principal peak show contrastive compression behaviors between modified silicate and silica glasses. This result suggests that modified silicate glasses represent different pressure evolutions in the ranges of the intermediate- and the extended-range order structures from those of silica glass, likely due to the presence of modifier cations and the resultant formations of smaller rings and cavity volume.
Reference
Liu, J., and Lin, J.-F. (2014) Abnormal acoustic wave velocities in basaltic and (Fe,Al)-bearing silicate glasses at high pressures. Geophysical Research Letters, 41, 8832–8839.