Diffraction patterns from many different icosahedral quasicrystals exhibit localized diffuse scattering that typically takes the form of partial spherical shells in reciprocal space. Although many theoretical models for these shells have been proposed, none of these models adequately explains all the experimental observations. A new model based upon local α-phase coordination of bonded Mackay clusters in the icosahedral phase is presented. Diffraction simulations successfully predict most of the observed diffuse scattering features.