Abstract The several-hundred-hour mechanicalmortar dry grinding on talc was examined by X-ray diffraction method, differential thermal analysis, electron microscopy and density measurements, and they were compared with those of kaolin minerals. In the dry grinding on talc, some characteristics in the structural changes by dry grinding were found. As the grinding progresses, the original crystals cleave and fracture and then split into fine crystals. Disordering in structure of these fine crystals is observable before the talc structure disintegrates completely. In talc, a new substance is produced by dry grinding, which is somewhat related to but different from the original talc structure. The appearance of this new phase is most remarkable in the stage of grinding where the crystalline pattern of talc entirely disappears in the X-ray diagram. This new phase has a non-crystalline structure, but the strurcture is not completely amorphous. It crystallizes into a new crystal at about 800°C. As the grinding progresses, the structure of this new phase gradually disintegrates to a completely amorphous structure similar to that of a silica-magnesia mixed gel. The formation of a zeolitic structure is not so remarkable; also, even if this structure is formed, the period of its stage is very short. It is an important characteristic that talc wholly changes into a new non-crystalline substance by dry grinding. It is generally supposed that talc does not easily suffer the effects of dry grinding because the structure is stronger than that of kaolin minerals.