Crystal Structure of Ca₂Na₂(CO₃)₃ (Shortite)

Ca₂Na₂(CO₃)₃ crystallizes in the orthorhombic unit cell a = 4.947(1) Å, b = 11.032(2) Å, and c = 7.108(1) Å at 25 °C with two formula weights in space-group Amm2. The structure has been redetermined, corrected, and refined to R = 0.025, R = 0.020 using 684 "observed" x-ray reflections from a single crystal. Corrections were made for absorption and isotropic extinction. In the extinction refinements, r refined to 0.00017(1) cm. The structure consists of Ca₂NaCO₃ layers interleaved with Na(CO₃)₂ layers. The Ca ion is coordinated strongly to nine oxygens, including three CO₃ edges, with Ca … O distances varying from 2.401(2) Å to 2.576(2) Å. One Na ion is coordinated strongly to eight oxygens, including two CO₃ edges, with Na … O distances from 2.429(2) Å to 2.605(1) Å. The other Na ion is coordinated strongly to six oxygens, including one CO₃ edge, at 2.296(1) Å to 2.414 Å, and weakly to a seventh at 3.050(3) Å. One CO₃ group is coordinated to seven cations, the other is coordinated to eight. The CO₃ groups have seemingly maximized their edge sharing with Ca ions rather than Na ions.