The authors report measurements of the silicon X-ray K emission bands of the crystalline polymorphs of silica (SiO₂): alpha -quartz, beta -cristobalite, beta -tridymite, coesite (all having 4:2 coordination), stishovite (6:3 coordination) and of the vitreous forms Infrasil and Suprasil (a-SiO₂). For alpha -quartz and coesite the Si L_2,3-emission bands are also presented. The K spectra of the various polymorphs of 4:2-coordinated silica exhibit small but distinct differences; the L spectra are virtually identical. The spectra are discussed along with available calculations of the electronic structure of alpha -quartz, beta -cristobalite and a-SiO₂. While for alpha -quartz and a-SiO₂ reasonable overall agreement with the theoretical results is found, the calculated width of the gap between the bonding and non-bonding valence bands in beta -cristobalite is incompatible with experiment.