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Bastnäsite [REE(CO 3 )F] is the main mineral of REE ore deposits in carbonatites. Synthetic bastnäsite-like compounds were precipitated from aqueous solutions by many different methods, but previous attempts to model magmatic crystallization of bastnäsite from hydrous calciocarbonatite melts were unsuccessful. Here we present the first experimental evidence that bastnäsite and two other REE carbonates, burbankite, and lukechangite, can crystallize from carbonatite melt in the synthetic system La(CO 3 )F-CaCO 3 -Na 2 CO 3  at temperatures between 580 and 850 °C and a pressure 100 MPa. The experiments on starting mixtures of reagent-grade CaCO 3 , Na 2 CO 3 , La 2 (CO 3 ) 3 , and LaF 3  were carried out in cold-seal rapid-quench pressure vessels. The studied system is an isobaric pseudoternary join of a quinary system where CO 2  and fluorides act as independent components. Liquidus phases in the run products are calcite, nyerereite, Na carbonate, bastnäsite, burbankite solid solution (Na,Ca) 3 (Ca,La) 3 (CO 3 ) 5 , and lukechangite Na 3 La 2 (CO 3 ) 4 F. Calcite and bastnäsite form a eutectic in the boundary join La(CO 3 )F-CaCO 3  at 780 ± 20 °C and 58 wt% La(CO 3 )F. Phase equilibria in the boundary join La(CO 3 )F-Na 2 CO 3  are complicated by peritectic reaction between Ca-free end-member of burbankite solid solution petersenite (Pet) and lukechangite (Luk) with liquid (L):

Crystallization of bastnäsite and burbankite from carbonatite melt in the system La(CO 3 )F-CaCO 3 -Na 2 CO 3 at 100 MPa