Oxidation pattern of the magic cluster Sip was studied by first-principles calculations of Si7On (n = 1-14) clusters. The lowest-energy structures of the Si-rich clusters consist of a pure Si fragment and an oxide fragment. The oxidation is found to extend from one edge throughout the whole cluster as the number of the oxygen atoms increases. Fragmentation energy analysis shows that the Si-rich clusters can often dissociate into a small pure Si-n (n = 2-6) cluster and a silicon oxide fragment. The O-rich clusters tend to separate into a SiO molecule and a silicon oxide fragment, except for Si7O14 which can easily lose an oxygen molecule in the fragmentation. The ionic clusters Si7On +/- (n = 1-14) were also studied. (c) 2006 Elsevier B.V. All rights reserved.