The microstructure of a muscovite mica exposed to a rare gas ion beam has been studied by transmission electron microscopy. The investigation of damage without implantation was carried out using argon and helium ions of sufficient energy to traverse the 100-150 nm mica specimens. For 340 keV Ar++ irradiation, amorphization of mica occurred at a fluence as low as 3.5 × 1014 ions · cm-2, which corresponds to 0.29 dpa. Muscovite can be amorphized using 80 keV helium ions, but this requires a much higher fluence and damage production of 4.6 × 1016 ions · cm-2 and 0.60 dpa, respectively. Since helium irradiation results principally in ionization energy loss, it indicates that amorphization of muscovite results mainly from nuclear interactions. Complete amorphization of muscovite mica is found to take place for all ions at approximately the same amount of nuclear energy transfer to energetic primary knock-on atoms, assuming a recoil energy greater than 500 eV. This suggests that amorphization occurs directly in dense displacement cascades. A significant amount of helium, 100 ppm, can be implanted into muscovite mica without destroying the crystal structure.