Reactions of aluminum-carbon cluster anions with oxygen were investigated under well-defined thermal conditions in a fast flow reactor. The resistance of particular species to the etching of oxygen is observed and is explained through the predicted shell closings of the Jellium model. The presence of Al7C-, in particular, is compatible with this cluster being a composite Jellium system stabilized by ion-polarizability interactions. This would be the first species of which we are aware that exits as a magic cluster in this context.