Single crystals of zircon (ZrSiO 4) cut into (010) thin toils were irradiated with 700 and 1500 keV Kr +, 1500 keV Xe + and 400 keV He + ions to investigate the role of electronic or ionization process vs nuclear collision during damage accumulation. The irradiations were conducted at room temperature with in situ transmission electron microscopy (TEM) using the HVEM Tandem Facility at Argonne National Laboratory. The level and ratio of electronic and nuclear energy loss in the sample vary with the mass and energy of the ions. Analysis of 700, 1500 keV Kr + and 1500 keV Xe + irradiations demonstrated that zircon samples were amorphized at ~0.9 dpa (2.0-4.8 × 10 14 ions/cm 2: assuming displacement energy equals to 15 eV) regardless of the level of total ionization energy deposited. The 400 He + irradiation, for which there was the highest electronic/nuclear energy loss ratio, was performed to 1× 10 16 ions/cm 2 at a much higher dose rate and gave no indication of amorphization. Therefore, displacement damage by nuclear collision must be the controlling mechanism in ion beam-induced amorphization of zircon.