  There has long been a discrepancy between the size distributions of Ar$_n^+$
clusters measured by different groups regarding whether or not magic numbers
appear at sizes corresponding to the closure of icosahedral (sub-)shells. We
show that the previously observed magic cluster size distributions are likely
the result of an unresolved Ar$_n$H$^+$ component, that is, from protonated
argon clusters. We find that the proton impurity gives cluster geometries that
are much closer to those for neutral rare gas clusters, which are known to form
icosahedral structures, than the pure cationic clusters, explaining why the
mass spectra from protonated argon clusters better matches these structural
models. Our results thus show that even small impurities, e.g.\ a single
proton, can significantly influence the properties of clusters.
