Formation of perovskite and calzirtite during zirconolite alteration

Synthetic zirconolites doped with rare earth elements (REEs) are corroded in a closed system at elevated temperature and pressure for various fluid compositions. Together with previous studies, the results indicate only a weak corrosion below 250°C at 50 MPa. Above that temperature and up to 500°C zirconolite (CaZrT2O7) displays more rapid rates of corrosion and may be covered by various secondary phases, depending on the fluid composition. Above 500°C in Na-rich fluids, zirconolite is replaced by perovskite (CaTiO3) and calzirtite (Ca2Zr5Ti2O16), but the REEs and Hf (as actinide analogues and/or neutron absorbers) are incorporated into the secondary phases. Perovskite and calzirtite exhibit an unusual crystal chemistry as determined by transmission electron microscopy and microanalysis.