[en] The single phase compound Cs8Nb10O23□(Si3O9)2, prepared by a solid state reaction, crystallizes in the hexagonal system. The structure is built up from NbO6 octahedra sharing corners and three-membered Si3O9 single rings sharing corners with the octahedra to form a three-dimensional framework which can be described formally as a sequence along the c axis of pyrochlore- and benitoite-type blocks. The cesium atoms are situated in two types of large cavities