From hydroxylbastnäsite to bastnäsite: A crystal chemistry perspective of the incomplete solid solution

Abstract Bastnäsite (REECO3[F, OH]) is a critical mineral in carbonatitic rare earth element (REE) deposits and serves as a primary global source of REEs. Understanding its crystal chemistry and thermodynamic properties is essential for modeling REE mobilization and deposition in various geological processes. Despite the complexity in natural bastnäsite—arising from cationic substitutions among REE species and coupled F-OH substitution—the impacts of F-OH substitution on its crystal structure and thermodynamic properties remain poorly constrained. In this study, we systematically investigate a synthetic bastnäsite-hydroxylbastnäsite (Bsn-Ce – Hbsn-Ce; CeCO3F-CeCO3OH) solid solution series using single-crystal X-ray diffraction and Raman spectroscopy. Notably, this is the first study to reveal: (1) the positions of hydrogen atoms in bastnäsite and hydroxylbastnäsite, as well as the order-disorder transition of hydrogen initiated by F-OH substitution in hydroxylbastnäsite, which explains the discrepancies in Raman spectroscopy between synthetic end-members and natural hydroxylbastnäsite; (2) the phase boundary between bastnäsite and hydroxylbastnäsite, with crystal structural data indicating a phase boundary at F = 0.41–0.52 apfu; and (3) the effect of F-OH substitution on the volume of the Bsn-Ce – Hbsn-Ce solid solution. This study is also the first attempt to establish a quantitative framework for describing the volume-composition behavior of incomplete solid solutions involving F-OH substitution. Furthermore, this study provides a foundational understanding of the crystal chemistry necessary for further investigations into the thermodynamic properties of Bsn-Ce – Hbsn-Ce solid solutions.