Fused quartz and silica are excellent electrical insulators. The large band gap inherent in the electronic structure of the silicon-oxygen bond results in electrical conduction being limited to current carried by mobile ionic impurities. Since the level of these impurities is very low, the electrical resistivity is correspondingly high.
Since ionic conduction is related to the diffusion coefficient of the ionic carriers, the resistivity also has a strong exponential temperature dependence. Hence, unlike typical conductors such as metals, the resistivity decreases with increasing temperature.
The dielectric constant of quartz glass has a value of about 4 which is significantly lower than that of other glasses. This value changes little over a wide range o frequencies. The reason for the low dielectric constant is, once again, the lack of highly charged mobile ions but it also results from the stiffness of the silicon-oxygen network which imparts a very low polarizability to the structure.
