A comparative study on pressure-induced structural transformations in a basaltic glass and melt from Ab initio molecular dynamics calculations

Glasses are often used as models for understanding the corresponding liquid melts because they are generally assumed to share similar short and intermediate-range structural characteristics. First-principles molecular dynamics calculations have been performed to investigate the structural changes in a model basaltic (Ca 22 Mg 14 Al 16 Si 44 O 148 ) glass at 300 K and a corresponding basaltic melt at 2500 K as a function of pressure up to 25 GPa. The results show that the local structures of the melt and glass are similar over the investigated pressure range, yet there are subtle but distinct differences. The pressure trends on the average Si–O, Ca–O, Mg–O, Al–O, O–O and Si–Si distances for the glass and melt are found to be very close. At ambient pressure, both are composed primarily of Si–O and Al-O tetrahedra. As expected, the Si–O coordination increases from four to fivefold and subsequently to sixfold. However, changes in the nearest neighbor Si–O and O–O are found to behave quite differently between the glass and melt. The most significant differences are in the distributions of the Si–O–Si and O-Si–O angles, which lead to different local structures and packing of the polyhedra. However, the differences become smaller with increased pressures indicating that caution should be exercised in the use of glasses as models for probing pressure-induced structural changes in the mantle. The calculated viscosity-pressure relationship of the basaltic melt between 0 and 25 GPa is presented and compared with available experimental data. In addition, other intrinsic properties such as the bulk modulus and sound velocity are broadly similar but not identical between the basaltic glass and melt in the broad pressure range of 0–80 GPa.