Determination of crystal structure of omphacite

The chemical formula of omphacite was expressed with (MII MI) (Si, Al)2O6. Cations that occupied the MII site were large cations such as Ca or Na, and Na/Na+Ca ratio ranged from 0.2 to 0.8; the 6-coordinated MI site accommodated cations such as Mg, Fe2+, Al, Fe3+, and Al/Al+Fe3+ ratio was more than 0.5. Omphacite space groups reported were C2/ c, P2, P2/n, P2/c, cell parameters area 0 = 0.9600–0.9630 nm,b{on0} = 0.8750–0.8830 nm,c0 = 0.5230–0.5290 nm,β = 106°40′–107°10′. The sample was picked up from the eclogites in Zhucheng County, Shangdong Province. The intensity data were collected with the RIGAKU RASA II-S four-circle single crystal diffractometer. The correct structure model was obtained by using the Patterson method and Fourier synthesis, SHELX-76 program, structure refinement with 905 independent diffraction points (|F0| > 3σ|F 0|). After the structure parameter refinement, the R-factor reduced to 0.0515. The crystal structure analysis indicates that omphacite has a new space group—Pn space group.