Geochemistry of uranium and thorium
Certain chemical characteristics of uranium and thorium govern their behavior in geologic processes. Their isotopic characteristics are less related to their roles in such processes and are not considered here.
Uranium (Katz and Rabinowitch, 1951; Kirk and Othmer, 1955a)—atomic number 92 and weight 238—has an ion size of 1.05 A and six valence electrons. Quadrivalent and sexivalent occurrences are common in nature. Uranium metal has a density of 19 and a melting point of about 1,133 C (Katz and Rabinowitch, 1951, p. 152).
The ion radius is very close to that of calcium (1.06 A) and trivalent rare earths (yttrium, 1.06 A), so uranium is preferentially captured by minerals of these elements (Goldschmidt, 1954, p. 562). Fluorine captures both quadrivalent uranium and thorium.
Sexivalent uranium compounds, because of their greater ionic potential, are chemically more mobile than quadrivalent uranium (Goldschmidt, 1954, p. 565).
Uranium reacts readily with all the nonmetallic elements and many metals including Hg, Pb, Cu, Fe, Ni, Mn, Co, Zn, and Be. Reactions with elements considered to be volatile mineralizers in magmatic and mantle emanations are important. UF6 forms at room temperature and U F4 forms at 200 to 400°C. UF4 is soluble in HC1 solutions at room temperature, remaining in the quadrivalent state. It also dissolves in a mixture of dilute sulfuric acid and silica, remaining as UF4. Uranium tetrachlorides are easily formed in gas-phase reactions at elevated temperatures or by liquid-phase reactions at moderate temperatures.