  We study metastable clusters in a colloidal system with competing
interactions. A short-ranged polymer-induced attraction drives clustering,
while a weak, long-ranged electrostatic repulsion prevents extensive
aggregation. We compare experimental yields of cluster structures expected from
theory, which assumes simple addition of the competing isotropic interactions.
For clusters of size $4\leq m\leq6$, the yield is significantly less than that
expected. We attribute this to an anisotropic self-organized surface charge
distribution linked to the cluster symmetry: non-additivity of electrostatic
repulsion and polymer-induced attraction. 7-membered clusters have a clear
optimal yield of the expected pentagonal bipyramid structure as a function of
strength of the attractive interaction.
