  We consider geometric constraints for the addition of carbon atoms to the rim
of a growing nanotube. The growth of a tube proceeds through the conversion of
dangling bonds from armchair to zigzag and vice versa. We find that the growth
rate depends on the rim structure (chirality), the energy barriers for dangling
bond conversion, and the growth temperature. A calculated chirality
distribution derived from this minimalistic theory shows surprisingly good
agreement with experiment. Our ideas imply that the chirality distribution of
carbon nanotubes can be influenced by external parameters.
