The crystal structure of natural 33R moissanite from Tibet

Abstract

Abstract 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.