Experimental XANES and ELNES spectra at the silicon K and edges and oxygen K edges in three polymorphs of (-quartz, coesite, and stishovite) are compared to calculated multiple-scattering spectra using different cluster sizes and potentials ( and complex Hedin - Lundqvist). The comparison demonstrates that relative energies and intensities of the experimental spectra are well reproduced in the energy range up to above the edge by cluster sizes of at least 50 atoms and using the potential. For the tetrahedral structures of -quartz and coesite further improvement can be obtained by setting the exchange parameter to 0.828.