Metastable equilibrium density of hydroxyl-free synthetic vitreous silica

It is shown that the volume behaviour of hydroxyl-free vitreous silica (type IV silica glass) is similar to that of the so-called natural vitreous silica (fused quartz, type I and II silica glasses) with low hydroxyl content. Thus, in view of that, a distinction may be made between two groups, types I, II and IV on the one hand and type III, the hydroxyl-rich silica glasses, on the other hand, although a distinction may also be made between the group of “natural” (type I and II) and “synthetic” silica glasses (type III and IV) in view of production method and starting material (quartz or SiCl₄). It is further shown that the type III silica glasses give an example for a change from the anomalous volume behaviour of “pure” silica glasses (thermal expansion, high-temperature minimum of volume) to the common behaviour of silicate glasses.