A new family of mixed anion cesium rare earth silicates exhibiting intense scintillation in several ranges of the visible spectrum are reported. Cs3RESi4O10F2 (RE = Y, Eu-Lu) have been synthesized using an enhanced flux growth method for the targeted growth of mixed anion systems. This is the first example of this method being used for the growth of oxyhalides. The compounds crystallize in the triclinic space group P (1) over bar with the lattice parameters a = 7.0832(2) angstrom, b = 7.1346(2) angstrom, c = 16.2121(5) angstrom, alpha = 95.8090(10), beta = 90.0580(10), gamma = 119.7650(10)degrees for the Tb analogue. The structure consists of REO6 and REO2F4 polyhedra connected by Si4O10 sheets with a previously unobserved silicate sheet topology that contains the uncommon cyclic Si3O9 trimers. The synthesis, structure, magnetic and optical properties are reported including, notably, intense X-ray scintillation in Cs3TbSi4O10F2.