Step-edge: Looking at the 240°-outer-corner process [OC0 ↔ OC1, Ea for = 0.29 eV, Ea rev = 0.22 eV], we notice a thermodynamically favored OC0 site, strengthening the hypothesis of growth of triangular islands with only <1 ! 1 ! 20>-steps on Mg(0001) . Alternatively, hexagonal islands with alternating <112 ! 0>-and <1 ! 1 ! 20>-steps would be possible at higher temperatures due to vibrational and configurational entropy effects, as assumed for fcc Al(111). The competing exchange process at the 240°-outer-corner has a higher activation energy and seems