Silica is known as the archetypal strong liquid, exhibiting an Arrhenius
viscosity curve with a high glass transition temperature and constant
activation energy. However, given the ideally isostatic nature of the silica
network, the presence of even a small concentration of defects can lead to a
significant decrease in both the glass transition temperature and activation
energy for viscous flow. To understand the impact of trace level dopants on the
viscosity of silica, we measure the viscosity-temperature curves for seven
silica glass samples having different impurities, including four natural and
three synthetic samples. Depending on the type of dopant, the glass transition
temperature can vary by nearly 300 K. A common crossover is found for all
viscosity curves around ~2200-2500 K, which we attribute to a change of the
transport mechanism in the melt from being dominated by intrinsic defects at
high temperature to dopant-induced defects at low temperatures