Polytypism and microstructures of the mixed-layer member B2S, CaCe3(CO3)4F3 in the bastnaesite-(Ce)–synchysite-(Ce) series

The crystal structures and polytypism of bastnaesite-(Ce) (CeCO3F, B layer) and synchysite-(Ce) (CeCO3F·CaCO3, S layer) mineral series are very complicated in Sichuan Province, Southwest China. Four new polytypes: 6R, 2H2, 4H and 12H of the mixed-layer member CaCe3(CO3)4F3 (B2S) of this series were discovered by X-ray diffraction analyses and high resolution transmission electron microscopy, structural symmetry, cell and subcell parameters, possible space group, and structural stacking mode (dededgfg/…). They reveal that the mixed-layer member B2S is formed by the ordered stacking of two structural unit layers of bastnaesite-(Ce) (B layer) and one unit layer of synchysite-(Ce) (S layer) along the c direction. The forming of the new polytypes may depend on distributions and the periodical variation of orientation of the [CO3] groups between the unit layers of B2S. We have observed the {000l} microtwin of the 6R polytype and the syntactic intergrowths among the different polytypes formed by stacking faults in B2S.