  Muscovite mica sheets with a thickness of 25 {\mu}m were irradiated by
various kinds of swift heavy ions (Sn, Xe and Bi) in HIRFL. The fluences ranged
from 1$\times$10^{10} ions/cm^2 to 8$\times$10^{11} ions/cm^2. The electronic
energy loss (dE/dx)_e was increased from 14.7 keV/nm to 31.2 keV/nm. The band
gap and Urbach energy of pristine and irradiated mica were analyzed by
ultraviolet- visible spectroscopy. Periodic fringes in long wave length of the
absorption spectra caused by interference phenomenon, were disturbed as the
(dE/dx)_e increased. It was suggested that the chemical bonds between
Tetrahedral-Octohedral-Tetrahedral (TOT) layers of mica were destroyed. Thus
the smooth surface was cleaved after irradiation. The band gap was narrowed
down with the increasing (dE/dx)_e and fluences. The values of Urbach energy
were increased as the (dE/dx)_e and fluences gradually increased. It was
indicated that the amount of defects and the proportion of amorphous structure
were increased in mica irradiated under increased (dE/dx)_e and fluences.
Fluences took a distinctly important role in optical properties of mica.
