In order to compare the irradiation-induced behaviour of monazite and ThSiO4, a large single crystal of monazite from Norway (Arendal monazite) containing several ThSiO4 inclusions was investigated. The estimated theoretical self-irradiation dose received by monazite near those inclusions was approximately 3 x 1019 α-decay/g. Transmission electron microscopy (TEM) analyses were performed on both host and inclusion. The TEM samples were prepared by the focused ion beam (FIB) technique. Monazite and ThSiO4 were found to have very different textures, including the presence of an amorphous zone between them. Crystalline monazite showed mottled diffraction contrast, which is characteristic of irradiation damage in this mineral. Observations suggest that there is a maximum defect density in monazite which cannot be exceeded. It is unlikely that some natural monazite can surpass this defect density. In contrast, ThSiO4 is completely amorphous and exhibits an unusual spherical, bubble-like texture. The size of these spheres is in the range of 10 to 200 nm. It is proposed that ThSiO4 is composed of aggregated spheres very similar to a gel. The zone between these two phases was certainly amorphized due to irradiation induced by α-decay of Th and U. It could be a preferential alteration zone and act as high-diffusive pathway for elements.