This letter reports the growth of spatially separated hexagonal and cubic phases of GaN on a patterned Si͑001͒ substrate by metalorganic vapor-phase epitaxy. The substrate surface was patterned with grooves having a 355 nm period. Each groove consisted of two opposed Si͕111͖ facets that were separated by Si͑001͒ surfaces. Epitaxial growth of GaN on this substrate began selectively on the Si͕111͖ facets and yielded the GaN hexagonal phase. With further growth, the two hexagonal phase regions separately grown on the opposed Si͕111͖ facets coalesced, with strongly misaligned c axes ͑ϳ110°͒. The GaN grown after coalescence was subsequently confirmed, by transmission electron microscopy and photoluminescence, to be of cubic phase.