The accurate lattice parameters of Fe I boracite, Fe3B7O13I, in the low-temp. range between -40 and -120 Deg were successfully detd. by using a special x-ray diffraction technique. To investigate the symmetry change occurring in this temp. range, Weissenberg diagrams were also studied. Ferroelec. Fe3B7O13I exhibits 2 kinds of monoclinic C2s and rhombohedral C36v phases, in addn. to the well-known orthorhombic C52v phase. The 2 kinds of monoclinic modifications, which are stable in the narrow temp. regions between the orthorhombic and rhombohedral phases, appear sep. depending on the history of temp. change of the crystal: one phase arises between -70 and -82 Deg with decreasing temp., the other between -55 and -68 Deg with increasing temp. They have almost similar crystal structures but possess slightly different lattice parameters, thermal expansion coeffs., and optical birefringences. The deviation of the axial angle b from 90 Deg in the monoclinic phases is less than half a minute. The deviation of the rhombohedral axial angle g is also extremely small; the difference from 60 Deg is 48 at -120 Deg. A simple model is proposed to explain the reason why the 2 monoclinic phases must bridge the orthorhombic and rhombohedral phases.