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Jiann-Shing Lee, Shu-Cheng Yu, Shu-Fang Tung, Wen-Ji Bai, Jing-Sui Yang, Qing-Song Fang, Zeming Zhang
The crystal structure of natural 33R moissanite from Tibet
Keywords: Moissanite, Polytype, Si-C double layer, Single crystal structure analysis, X-ray diffraction
The crystal structure of the natural 33R moissanite, recovered as inclusions in chromitites from Luobusa harzburgite in Tibet, was determined from X-ray diffraction data collection by a Nonius CAD-4 four-circle diffractometer. The crystal is trigonal in symmetry with a space group of R3m. Identical to the synthetic specimen, the cell parameters are a = 3.076, c = 83.12 Å for the natural sample. Full-matrix least-squares refinement of the structure, including anisotropic displacement parameters, gave the final R factor of 4.80%.
The bond angles and tetrahedral angle variance of the present crystal structure show that the tetrahedra are quite regular in geometry. The distance between two adjacent c double layers, 2.517 ± 0.001 Å, is slightly shorter than the average h–c distance of 2.523 ± 0.001 Å. With 36% hexagonal double layers, the present 33R moissanite is inferred to crystallize at high temperature condition and thus come from a depth beneath Earth’s crust. The structure refinements of this work indicate that the present moissanite exhibits smaller thermal displacement parameters. Most of the Si—C bond distances parallel to the c axis are longer than other distances. This result as well as longer bond distances of Si—C along c axis may be indicative of a weaker covalent bonding force along this direction.
Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag
Print ISSN: 0044-2968
Volume: 221, 03/2006
Pages: 213 - 217