The microstructure of the MBE-grown Si/CaF2/Si (111) heterostructures is studied by high resolution transmission electron microscopy (HRTEM). Two distinct interface structure models have been identified from comparison with image simulations. In the first model interfacial Ca atoms bond to T sites. In the second model Ca bonds to T-4 sites. No one proposed model alone can describe the microstructure. Imaging conditions have been identified by analysis of simulated images which enable unambiguous discrimination between alternative models. No differences have been found between the microstructures of the top and bottom interfaces. It has been shown by simulations that intermixing effects at the interface can be detected. However, such effects have not been observed experimentally Stacking faults and twins have been observed in the top Si layer. Sensitivity of the microstructure to 200 kV electron irradiation is quantified to determine the electron dose budget for unobtrusive HRTEM observation.