Observation of an anomalous density minimum in vitreous silica

Vitreous silica is known to have several anomalous variations in its physical properties including a temperature dependent density maximum. We report the results of high-resolution density relaxation experiments that conclusively show for the first time the existence of a density minimum for silica below its glass-transition temperature at similar to950 degreesC. This unusual density-temperature relationship is shown to be consistent with the existence of a polyamorphic phase transition in silica with a negative Clapeyron slope.