The crystal structure of thornasite, Na12Th3[Si8O19](4)(H2O)(18): A novel interrupted silicate framework

The crystal structure of thornasite, Na12Th3[Si8O19](4)(H2O)(18), Z = 6, rh ombohedral, a = 29.124(1), c = 17.2602(9) Angstrom, V = 12,679.2(9) Angstro m(3), space group R (3) over bar c, has been solved by direct methods, and refined to an agreement index (R1) of 5.7% calculated, based on 2266 unique observed reflections collected using MoK alpha X-radiation and a CCD-based detector. The structure consists of a new interrupted silicate framework w ith composition [Si8O19](6-). The framework density is 15.4 atoms per 1000 Angstrom(3), which is typical for microporous aluminosilicate materials. Th e framework density, together with the presence of a three-dimensional netw ork of channels in the structure, permit thomasite to be designated as a ne w member of the zeolite structure family. The framework consists of cubic c ages and distorted six-membered rings linked via common edges. The circuit symbol is (4(3)14(3))(4(3)6(2)14)(3)(6.10.12)(3)(6(2)10)(6). Interruptions of the framework are caused by the presence of Th4+ cations that are coordi nated by eight O atoms. Na+ cations and H2O groups are located in channels through the structure. The theoretical parent framework for thornasite may be constructed by the insertion of two additional Si atoms in place of one Th atom. The theoretical framework density is 18.0 atoms per 1000 Angstrom( 3); the circuit symbol is (4(3)10(3))(4(3)6(2)10)(6)(4(3)6(2)8)(3)(4(3)6.8. 10)(3)(4(2)6(2)8.10)(3)(4(1)6(3)8.10)(3).