The oxonitridosilicate chloride La6Ba3[Si17N29O2]Cl was synthesized by a high-temperature reaction in a radiofrequency furnace starting from LaCl3, BaH2, and the ammonolysis product of Si2Cl6. Diffraction data of a micrometer-sized single crystal were obtained using microfocused synchrotron radiation at beamline ID11 of the ESRF. EDX measurements on the same crystal confirm the chemical composition. The crystal structure [space group P6(3)/m (no. 176), a = 9.8117(14), c = 19.286(6) angstrom, Z = 2] contains an unprecedented interrupted three-dimensional network of vertex-sharing SiN4 and SiN3O tetrahedra. The SiN4 tetrahedra form dreier rings. Twenty of the latter condense in a way that the Si atoms form icosahedra. Each icosahedron is connected to others via six SiN4 tetrahedra that are part of dreier rings and via six Q(3)-type SiN3O tetrahedra. Rietveld refinements confirm that the final product contains only a small amount of impurities. Lattice energy (MAPLE) and bond-valence sum (BVS) calculations show that the structure is electrostatically well balanced. Infrared spectroscopy confirms the absence of N-H bonds.