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

Abstract

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) angstrom, b = 5.7356(6) angstrom, c = 11.0977(12) angstrom, beta= 100.636(2)degrees, V = 842.39 (16) angstrom(3) and Z = 2. The structures of Ti- and Fe2+-rich chevkinite-(Ce) were refined with space groups P2(I)/a and C2/m. Least-squares refinement results show that both structural models of Ti- and Fe2+-rich chevkinite-(Ce) are very good, R[F-2> 2 sigma(F-2)] =0.027 with P2(1)/a and R[F-2> 2 sigma(F-2)] =0.021 with C2/m. In order to illustrate the relationship between the two space groups P2(1)/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 P2(1)/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 P2(1)/a, which is of pseudo symmetry corresponding to the space group C2/m.