Crystal structure of lisetite, CaNa₂Al₄Si₄O₁₆

The crystal structure of lisetite CaNa₂Al₄Si₄O₁₆, a = 8.260, b = 17.086, c = 9.654 Å, space group Pbc2₁ Z = 4, has been determined through direct methods using 1445 intensities measured with an automated X-ray diffractometer. The anisotropic least-squares refinement led to a final discrepancy index R = 0.026 for the 1450 observed reflections. Lisetite is a tectosilicate with a framework based upon the connection of four-membered rings of tetrahedra whose apical oxygens point alternately up (U) and down (D) with respect to the (100) plane (i.e., UDUD rings). The ordered distribution of Al and Si among the tetrahedra results in the lack of the symmetry center. Ca and Na are surrounded by seven and six oxygen atoms, respectively, at average distances of 2.53 and 2.48 Å and occur in cavities sandwiched between layers of four- and eight-membered rings of tetrahedra. The structure of lisetite is related to that of feldspars, which is also based on four-membered rings of tetrahedra; however, the rings in feldspars are of the UUDD type, and lisetite contains 1.5 more extra-framework cations. Whereas lisetite shares the same Al-Si framework with banalsite BaNa₂Al₄Si₄O_l6, the latter has a different arrangement of the large cations. Other combinations of extra-framework cations should be possible in a tectosilicate family based upon UDUD rings of tetrahedra.