The structural evolution of magnesium difluoride from an amorphous deposit has been investigated by in situ powder X-ray diffraction (XRPD) and transmission electron microscopy (TEM). Crystalline MgF2 was evaporated at different temperatures, which define the vapor pressures in the range from 4.7 x 10(-3) to 1.6 x 10(-1) mbar, and deposited onto various substrates. The temperature of the substrate was systematically varied from -228 to 25 degrees C. Magnesium difluoride was obtained as an amorphous sample when deposited on a substrate kept at a temperature below -100 degrees C. Upon warming, the deposit transforms via the CaCl2 type of structure (beta-MgF2, at 70 degrees C) into the stable rutile type (alpha-MgF2, 250 degrees C) by a displacive order-disorder phase transition. The new beta-MgF2 polymorph was refined assuming the orthorhombic CaCl2 type of structure (Pnnm, No. 58) with the lattice constants a = 4.592(1) angstrom, b = 4.938(3) angstrom, and c = 2.959(3) angstrom. When deposited above -50 degrees C, samples crystallize directly in the rutile structure.