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Crystal Structure of Natural Non‐metamict Ti‐ and Fe 2+ ‐rich Chevkinite‐(Ce)
Author(s) -
Guowu LI,
Guangming YANG,
Zhengsheng MA,
Nicheng SHI,
Ming XIONG,
Haifu FAN,
Ganfu SHENG
Publication year - 2005
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/j.1755-6724.2005.tb00896.x
Subject(s) - metamictization , monoclinic crystal system , crystallography , crystal structure , space group , crystal (programming language) , materials science , chemistry , diffraction , physics , x ray crystallography , optics , programming language , computer science , nuclear physics , zircon
The crystal structure of non‐metamict Ti‐ and Fe 2+ ‐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 Ce 4 Fe 2 Ti 3 Si 4 O 22 . The crystals are monoclinic with the unit cell parameters a = 13.4656(15) Å, b = 5.7356(6) Å, c = 11.0977(12) Å, β 100.636(2)°, V = 842.39(16) Å 3 and Z = 2. The structures of Ti‐ and Fe 2+ ‐rich chevkinite‐(Ce) were refined with space groups P 2 1 / a and C 2/ m . Least‐squares refinement results show that both structural models of Ti‐ and Fe 2+ ‐rich chevkinite‐(Ce) are very good, R [ F 2 >2σ( F 2 )] =0.027 with P 2 1 / a and [ F 2 >2σ( F 2 )] =0.021 with C 2/ m . In order to illustrate the relationship between the two space groups P 2 1 / a and C 2/ m , the distribution of diffraction intensities was inspected. Pseudo extinction was found, i.e., reflections with h+k =2 n are systematically strong, while those with h+k =2 n +1 are weak. By neglecting the systematically weak ( h+k =2 n +1) reflections the space group becomes C 2/ m . There is a mirror plane in the C 2/ m perpendicular to the b axis. However, oxygen atoms in the P 2 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 Fe 2+ ‐rich chevkinite‐(Ce) is a superstructure with the space group of P 2 1 / a , which is of pseudo symmetry corresponding to the space group C 2/ m .