The coalescence of self-catalyzed GaN nanowires grown on Si substrates has been examined in order to identify the cause of coalescence and to characterize the defect structures resulting from it. Coalescence is found to occur due to both nanowire crystallographic misalignment and initial nanowire proximity independent of the growth conditions. Defects occasionally result from coalesced nanowires, and the root cause of defect creation is found to be crystallographic misorientation between the nanowires. Several different defect structures are identified, and the types of defects that may form are found to depend on the degree of that misorientation. In the most extreme cases "zipper"-like arrays of dislocations and stacking faults were observed. (C) 2013 Elsevier B.V. All rights reserved.