Chemical Composition and Spectral Characteristic of Colour-Change Bastnäsite(Ce)

Bastnäsite(Ce), a cerium-dominant rare-earth element fluorocarbonate, is a rare gemstone which could display alexandrite effect. In this paper, the authors studied the chemical compositions and spectral characteristics of bastnäsite samples by means of electron microprobe, Fourier transform infrared spectroscopy, Raman spectroscopy and ultraviolet-visible spectroscopy. The results of electron microprobe indicated that Ce、La、Nd、Pr and Sm are the main rare-earth elements in bastnäsite. The structural formulas calculated by the electron microprobe data of two samples are (Ce_0.50La_0.27Nd_0.16Pr_0.05Sm_0.02)(CO₃)F and (Ce_0.47La_0.24Nd_0.20Pr_0.05Sm_0.03)(CO₃)F respectively. The middle-infrared bands are assigned to the different vibrations of CO₃^2- and the near-infrared bands are associated with CO₃^2-, Pr³⁺, Nd³⁺or Sm³⁺. Raman bands are associated with various CO₃^2- vibrations and lattice modes involving F^-. The results of UV-Vis absorption spectroscopy showed that most of the bands are induced by 4f-4f electronic transition of Nd³⁺. Based on the results of electron microprobe analysis, it is considered that the colour of bastnäsite is produced by Nd³⁺. Since the two strong absorption peaks near 576 nm and 740 nm have approximately equal transmission rate, it is inferred that the alexandrite effect of bastnäsite is caused by Nd³⁺.