Hydrothermal synthesis and the crystal structure of borate cancrinite (Na,Ca)2[Na6(AlSiO4)6](BO3) · 2H2O

Transparent prismatic single crystals of borate cancrinite (Na,Ca)₂Na₆(AlSiO₄)₆](BO₃) · 2H₂O are prepared through hydrothermal crystallization. The parameters of the hexagonal unit cell and intensities of 10806 reflections are measured on an Enraf—Nonius CAD4 automated diffractometer. The compound crystallizes in the hexagonal crystal system with the unit cell parameters a = 12.745(4) Å, c = 5.180(2) Å, V = 728.6(4) ų, and space group P6₃. The structure is determined by direct methods and refined using the full-matrix least-squares procedure in the anisotropic approximation for the non-hydrogen atoms. The refinement of the structure is performed to the final discrepancy factor R ₁ = 0.027 for 2889 unique reflections with I > 2 σ (I). In the structure of the borate cancrinite, the AlO₄ and SiO₄ tetrahedra form a zeolite-like framework in which twelve-membered hexagonal channels are occupied by sodium atoms and BO₃ groups, whereas six-membered channels are filled with sodium and calcium atoms and water molecules. The mean interatomic distances are found to be as follows: (Si-O)_mean = 1.614 Å and (Al-O)_mean = 1.741 Å in the AlO₄ and SiO₄ tetrahedra, (Na-O)_mean = 2.542 Å in the seven-vertex sodium polyhedra, and (Na,Ca)-O]_mean = 2.589 Å in the ditrigonal bipyramids.