Transmission Electron Microscopic Study of New, Regular, Mixed-Layer Structures in Calcium-Rare-Earth Fluorocarbonate Minerals

Xiuling Wu, Dawei Meng, Zhaolu Pan, Guangming Yang and Douxing Li
Department of Physics, China University of Geosciences, Wuhan 430074, P.R. China
Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Academia Sinica, Shenyang 110015, P.R. China

Abstract: Structures of calcium-rare-earth (Ca-RE) fluorocarbonate minerals from southwest China have been investigated using selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). They are described as combinations of layers of bastnäsite-(Ce), CeFCO3 (B layers), and synchysite-(Ce), CeFCO3·CaCO3 (S layers) (Donnay and Donnay, 1953). We report here the discovery of six new, regular, mixed-layer structures in parisite-(Ce) observed using SAED and HRTEM. The symmetry, cell parameters, chemical formulae and stacking of the structural unit layers, etc., were determined for each. The regular, mixed-layer structures are formed by stacking unit layers of bastnäsite-(Ce) and synchysite-(Ce) in varying proportions along the c axis. HRTEM shows that there are different distribution modes for the Ce-F ion layers, the CO3 ion groups between the Ce-F ion layers and the CO3 ion groups between the Ce-F and Ca ion layers. The minerals may be regarded as polymorphs with the same chemical composition and the same spacing of the unit layers, but different arrangements of ion layers in the structural unit layers and different stacking sequences of the structural unit layers.

Keywords: Ca-RE fluorocarbonate mineral • mixed-layer structure • high-resolution transmission electron microscopy

Mineralogical Magazine; February 1998 v. 62; no. 1; p. 55-64; DOI: 10.1180/002646198547468
© 1998, The Mineralogical Society
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