In this paper the control effects of lanthanide ion (Sm^3＋) on the crystallization of calcium ox-alate (CaC₂O₄) in pure H₂O system were investigated. By microcalorimetry, the effects of Sm^3＋on the crystallization kinetics of CaC₂O₄ were studied. Moreover, the control effects of Sm^3＋on the crystal structure, morphology, and size were investigated by X-ray diffraction (XRD) and scanning elec-tromicroscopy (SEM). The microcalorimetric results showed that the effects of Sm^3＋were not only on the initial nucleation of CaC₂O₄ but also on the crystal growth, and the optimal inhibition effects can be obtained only at appropriate concentration of Sm^3＋ (～2.50×10^－5 mol•L^－1). The XRD results showed that under given condi-tions, the addition of Sm^3＋ favors the formation of dihydrate (COD). The SEM results showed that the addition of Sm^3＋ causes significant controls on the crystal shape and size of CaC₂O₄, namely crystals with different morphologies and different sizes will be obtained as the concentration of Sm^3＋ varies.

Key words: lanthanide ion, calcium oxalate, microcalorimetry, crystallization kinetics, dihydrate, control effect