There are two natural parent isotopes of uranium, 238U and 235U. The half-lives are 4.49 × 109 and 7.13 × 108 yr respectively, so that they and their daughters survive as common trace elements in igneous rocks. Upon aqueous weathering, uranium is relatively soluble UO22+ as carbonate and phosphate complexes of the uranyl ( ) ion. These are transported in fresh and salt waters and may be precipitated in derived rocks; see Gascoyne (1982) for geochemical details.