Polytypism and microstructures of the mixed-layer member B_2S, CaCe_3(CO_3)_4F_3 in the bastnaesite-(Ce)-synchysite-(Ce) series

摘要

The crystal structures and polytypism of bastnaesite-(Ce) (CeCO_3F, B layer) and synchysite-(Ce) (CeCO_3F·CaCO_3, S layer) mineral series are very complicated in Sichuan Province, Southwest China. Four new polytypes: 6R, 2H_2, 4H and 12H of the mixed-layer member CaCe_3(CO_3)_4F_3 (B_2S) 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 (dededgfgl…). They reveal that the mixed-layer member B_2S 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 [CO_3] groups between the unit layers of B_2S. We have observed the {0001} microtwin of the 6R polytype and the syntactic intergrowths among the different polytypes formed by stacking faults in B_2S.