Atomistic Observation of Phase Transitions in Calcium Sulfates under Electron Irradiation

The detailed study of phase transition processes among various calcium sulfates is still insufficient in spite of its importance in artificial and natural processes. Here, the phase transformation gamma-CaSO4 -> beta-CaSO4 -> CaO is induced by electron beam irradiation and directly observed inside the transmission electron microscope (TEM). Relying on the real-time atomic-scale observation, the topological transition from gamma-CaSO4 (hexagonal) to beta-CaSO4 (orthorhombic) is investigated in detail, and the orientation relationship is determined: [0001](H)//[001](O) and (10 (1) over bar0)(H)//(010)(O). Furthermore, three beta-CaSO4 variants related by 120 rotation are observed, which is theorized to occur during the transformation from hexagonal to orthorhombic structure. The possible transition mechanism is thus discussed. This work offers detailed information on the transition mechanism concerning the calcium sulfates, which might facilitate their industrial applications and possibly further understanding of rock deformation in the earth crust.