A new uranyl silicate, K2Ca4[(UO2)-(Si2O7)(2)], with a ID chain structure has been synthesized from a solution of mixed alkali- and alkaline-earth-metal cations under hydrothermal conditions at 550 degrees C and 1400 bar and characterized by single-crystal X-ray diffraction and photoluminescence spectroscopy. It crystallizes in the triclinic space group P (1) over bar (No. 2) with a = 6.6354(2) angstrom, b = 6.6791(2) angstrom, c = 9.6987(3) angstrom, alpha = 98.324(2)degrees, beta = 93.624(2)degrees, gamma = 112.310(2)degrees, and Z = 1. Its crystal structure consists of a ID chain of uranyl disilicate formed of corner-sharing UO6 tetragonal bipyramids and Si2O7 double groups. The adjacent chains are separated by K+ and Ca2+ cations. It is the first example of uranyl silicate with a 1D chain structure.