High pressure high temperature elasticity study of sodium disilicate glass

Many deep earth geophysical phenomena observed are related to physical properties of silicate melts under extreme conditions. Understanding the structures and physical properties of silicate melts at a fundamental level is essential to help us understand the dynamics of mineral crystallization and fractionation, thermal transport, etc. inside the Earth. Sodium disilicate (Na2Si2O5) glass is a good analog for studying amorphous phase basalt since they have similar ratios of non-bridging oxygen to tetrahedrally coordinated cations (NBO/T). Besides, sodium disilicate is not only simple in chemistry but also has a low melting point, thus permitting detailed experimental studies using currently available techniques. Synthesized sodium disilicate glass was ground into powder and loaded into a Paris-Edinburgh (PE) cell. The PE high pressure apparatus was installed inside the 16-BM-B hutch at the Advanced Photon Source. In our experiment, the glass sample was pressed up to 2GPa and heated up to 1000oC. At various pressure and temperature conditions, ultrasonic elastic wave velocities of the amorphous sample were collected using the pulse reflection method with a 10o Y-cut LiNbO3 transducer mounted on one end of the PE anvil. White beam x-ray radiographs that covered the entire length of the sample were also collected to determine the elastic wave travel distance. Experimental results show that before the glass transition temperature (~700oC at ambient pressure), the transverse wave velocity remains nearly constant with increasing temperature, while the longitudinal wave velocity decreases monotonically. A sudden drop in transverse wave velocity is observed above the glass transition temperature at 800oC. Within the 2GPa pressure range, we did not observe clear pressure dependence of the elastic wave velocities in sodium disilicate glass.