The crystal structure of naujakasite, a double sheet silicate

Abstract

The crystal structure of naujakasite from Tuperssuatsiait has been determined by X-ray data from four-circle computer controlled Philips PW 1100 diffractometer (Mo Kα radiation). Naujakasite crystallizes in the space group C2/m with cell dimensions a = 15.025(1), b = 7.991(2), c = 10.486(1) Å, β = 113°40'(5'); the structural formula is Na₆FeAl₄Si₈O₂₆, Z = 2. The structure has been solved by Patterson methods and refined by full matrix least-squares method to R_w = 0.027. Naujakasite is a sheet silicate with double layers of tetrahedra perpendicular to the c direction. All the oxygen atoms of the structure are shared between two tetrahedra except one that interrupts the three-dimensional linking. Each single layer is formed by rings of six tetrahedra and the linkage between two single layers gives rise to rings of four and six tetrahedra. Out of the three independent tetrahedral sites present in the structure one is occupied by silicon and the other two by (0.5 Si + 0.5 AI). The Na-O polyhedra (quite irregular) and the Fe-O polyhedron (a tetragonal bipyramid) are all linked together forming a three-dimensional framework in which all the oxygen atoms of the structure are involved.