A new uranium(VI) silicate, K-5(UO2)(2)[Si4O12(OH)], has been synthesized by a high-temperature, high-pressure hydrothermal method. It crystallizes in the orthorhombic space group Pbcm (No. 57) with a = 13.1274(7) Angstrom, b = 12.2635(7) Angstrom, c = 22