ELECTRON DIFFRACTION AND LATTICE IMAGE STUDY ON CEBAITE

Cebaite Ce 2 Ba 3 (CO 3 ) 5 F 2 is a new mineral discovered in China. Its electron diffraction spots can be divided into two groups according to the intensity. The stronger one results from the substructure which is similar to that of Huanghoite and can be described by a hexagonal subcell with parameters α 0 =5.06?, c 0 =9.63?, while the other group comes from the superstructure which belongs to monoclinic system. The parameters of the real unit cell are α=21.2? b=5.06? c=13.l? β=95°. The possible space groups are C2/m, Cm and C2. The basic vectors of the substructure and those of the superstructure in the real space are as follows :Some frings with spacing smaller than the original one and some broad black bands often appear in lattice images formed by h00 systematic reflexions. The cause of this is discussed. It is also discussed whether there is any superstructure with longer period in this mineral.