La24Sr14-7x[Si36N72] (O1-xFx)(14) with x = 0.489 was obtained as a microcrystalline product by metathesis at 1500 degrees C in a radio-frequency furnace starting from Si(NH)(2), La(NH2)(3), SrH2, LaF3, and CeF3. The structure of the new nitridosilicate oxide fluoride was determined by combining transmission electron microscopy (TEM) and single-crystal X-ray diffraction using a microfocused synchrotron beam. The structure model with pronounced disorder [P6(3)/mmc, Z = 1, a = 16.2065(3), c = 9.4165(1) angstrom, R-1(obs) = 0.0436] was confirmed by electron diffraction and aberration-corrected Z-contrast scanning TEM. The highly symmetric AB(2) framework, which was theoretically predicted but not yet realized, consists of all-side vertex-sharing SiN4 tetrahedra that form channels along [001] filled with La, Sr, O, and F atoms. The connectivity pattern is related to that of tridymite. X-ray spectroscopy and bond-valence-sum calculations were further taken into account for assignment of the N, O, and F atoms.