Single crystals of two new uranyl silicates were grown from mixed alkali fluoride/alkali chloride fluxes. Polymorphism is observed in Cs-2(UO2)Si2O6, where hydrothermal conditions lead to the alpha polymorph and flux growth favors the beta polymorph. Rb-2(UO2)Si2O6 crystallizes in a monoclinic distortion of the alpha-Cs-2(UO2)Si2O6 structure type with space group I2/a and lattice parameters a = 14.9932(5) angstrom, b = 14.8032(5) angstrom, c = 16.2377(9) angstrom, and beta = 90.7220(10)degrees. beta-Cs-2(UO2)Si2O6 constitutes a new polymorph of Cs2(UO2)Si206 and crystallizes in the monoclinic space group C2 with lattice parameters a = 12.136(2) angstrom, b = 10.0426(17) angstrom, c 7.7896(13) angstrom, and beta = 95.849(4). The two structures are compared, the driving force behind the observed polymorphism is discussed, and the luminescence properties are reported for each compound.