Crystal Structure of Natural Non-metamict Ti- and Fe2+-rich Chevkinite-（Ce）

作     者：LI Guowu1,YANG Guangming2,MA Zhengsheng1,SHI Nicheng1,XIONG Ming1,FAN Haifu3 and SHENG Ganfu4 1 China University of Geosciences(Beijing),Beijing 100083 2 China University of Geosciences(Wuhan),Wuhan,Hubei 430074 3 Institute of Physics,Chinese Academy of Sciences,Beijing 100080 4 Chengdu Institute of Geology and Mineral Resources,Chengdu,Sichuan 610082 

出 版 物：《Acta Geologica Sinica(English Edition)》 (地质学报(英文版))

年 卷 期：2005年第3期

页      面：325-331页

核心收录：

学科分类：081803[工学-地质工程] 08[工学] 0818[工学-地质资源与地质工程] 

主　　题：non-metamict Ti- and Fe2+-rich chevkinite-(Ce) crystal structure space group superstructure 

摘      要：The crystal structure of non-metamict Ti- and Fe2+-rich chevkinite-(Ce) has been redetermined with the single -crystal sample collected from Bayan Obo, Inner Mongolia, China. The chemical formula of the sample is Ce4Fe2Ti3Si4O22. The crystals are monoclinic with the unit cell parameters a = 13.4656(15) ?, b = 5.7356(6) ?, c = 11.0977(12) ?, β= 100.636(2)o, V = 842.39 (16) ?3 and Z = 2. The structures of Ti- and Fe2+-rich chevkinite-(Ce) were refined with space groups P21/a and C2/m. Least-squares refinement results show that both structural models of Ti- and Fe2+-rich chevkinite-(Ce) are very good, R[F22σ(F2)] =0.027 with P21/a and R[F22σ(F2)] =0.021 with C2/m. In order to illustrate the relationship between the two space groups P21/a and C2/m, the distribution of diffraction intensities was inspected. Pseudo extinction was found, i.e., reflections with h+k=2n are systematically strong, while those with h+k=2n+1 are weak. By neglecting the systematically weak (h+k=2n+1) reflections the space group becomes C2/m. There is a mirror plane in the C2/m perpendicular to the b axis. However, oxygen atoms in the P21/a model are of a symmetrical relationship with the corresponding pseudo mirror plane. It is concluded that the crystal structure of non-metamict Ti- and Fe2+-rich chevkinite-(Ce) is a superstructure with the space group of P21/a, which is of pseudo symmetry corresponding to the space group C2/m.