A simple toy model is analyzed within the framework of multichemical equilibrium to explain the spontaneous formation of clusters of certain metal oxides (termed "polyoxometalates" in the literature) in acidified aqueous solutions. Remarkably, thermodynamically stable clusters do not always correspond to magic numbers, i.e., local minima of the (bonding) free energy as a function of size. It is shown that cluster formation in metal oxides closely resembles micelle formation in surfactant systems.