  Impacts of individual slow highly charged ions on alkaline earth halide and
alkali halide surfaces create nano-scale surface modifications. For different
materials and impact energies a wide variety of topographic alterations have
been observed, ranging from regularly shaped pits to nano-hillocks. We present
experimental evidence for a second threshold for defect creation supported by
simulations involving the initial electronic heating and subsequent molecular
dynamics. From our findings a unifying phase diagram underlying these diverse
observations can be derived. By chemically etching of CaF$_2$ samples after
irradiation with slow highly charged ions both above and below the potential
energy threshold for hillock formation another threshold exists above which
triangular pits are observed after etching. This threshold depends on both the
potential and kinetic energies of the incident ion. Simulations indicate that
this second threshold is associated with the formation of defect aggregates in
the topmost layers of CaF$_2$.
