III.

Occurrence

Uranium never occurs naturally in the free state but is found as an oxide or complex salt in minerals such as pitchblende and carnotite. It has an average concentration in the crust of the Earth of about 2 parts per 1 million, and, among the elements, ranks about 48th in natural abundance in crustal rocks. Pure uranium consists of more than 99 percent of the isotope uranium-238, less than 1 percent of the fissile isotope uranium-235, and a trace of uranium-234, formed by radioactive decay of uranium-238. Among the artificially produced isotopes of uranium are uranium-233, uranium-237, and uranium-239. Isotopes ranging from mass number 222 to 242 are known.