Relaxation of Libyan desert glass: Evidence for negative viscosity–pressure dependence in silica?

Libyan desert glass (LDG) with silica content >99 mol% was examined to obtain evidence for negative viscosity–pressure dependence in silica. Calorimetric scanning experiments under ambient pressure revealed a shift of ∼22 K in glass transition temperature (Tg) from pristine to relaxed LDG, respectively. While the endothermic overshot in the isobaric heat capacity at Tg remains practically unaffected, the shift occurs due to a decrease in the onset of relaxation. Because in silicate glasses, caloric and kinetic glass transition are strongly coupled, this finding indicates that kinetic freezing of LDG originally occurred at lower temperature than it does in glasses of equivalent composition under normal conditions. Considering the most probable origin of LDG – a meteoritic impact – and assuming that at least some compression is preserved in natural LDG samples, this observation is interpreted as evidence for decreasing viscosity with increasing pressure, and is related to decreasing Si–O–Si bond angle in the pressure-regime below 1 GPa.