Thin-film waveguides of LiNbO3 have been grown on Al2O3-c by off-axis rf magnetron sputtering. The films have been characterized optically by prism coupling measurements, crystallographically by x-ray diffraction, and morphologically by atomic force microscopy. We find that optical losses can be dominated by scattering from large outgrowths that litter the surface of the film. These outgrowths are c- grains imbedded in a c+ matrix. Although some grains nucleate c-, others have their polarity reversed from c+ to c- after nucleation. A model will be presented to explain the preferential nucleation of c+ grains on Al2O3-c. The c- grains grow much faster than the c+ ones because of attractive coulombic forces between the c- grains and the ionized Li and Nb species in the sputter plume.