During the synthesis of liebigite and andersonite, starting from tetrasodium uranylcarbonate and calcium ions, an unknown stable and crystalline intermediate phase is formed at a Ca2+/Na+ ratio lying between those necessary to form the above-cited compounds. Chemical analysis of this yellow-green intermediate phase results in the formula: Ca1.54Na0.63[UO2(CO3)(3)].xH(2)O (x = 5.38). The scanning electron micrographs show foliated lathlike crystals. From single-crystal data, the crystals are found to be orthorhombic, a 18.150(3), b 16.866(6), c 18.436(3) Angstrom, space group Pnnm. The basic structural units consist of three UO2(CO3)(3) units. Each U atom is -coordinated, resulting in a distorted hexagonal-bipyramidal polyhedron. A chemical analysis indicates that the structure is short of positive charges, which could be balanced by the presence of oxonium ions. Results of the crystal-structure analysis support this proposal, which ensures a tight fit of the water molecule that is not attached to either Na+ or Ca2+ ions. The powder-diffraction spectrum calculated from the single-crystal structure corresponds very well with the measured one. The fluorescence and phosphorescence spectra are given, as well as information on the thermal decomposition, as revealed by thermogravimetric and differential scanning calorimetric analyses.