The geochemistry of uranium in apatite and phosphorite

Apatite contains only traces of uranium, yet as apatite is a minor constituent in most rocks and the major constituent of a few very large deposits, it accounts, paradoxically, for both dispersal and concentration of uranium in nature. Uranium typically makes up 0.001 to 0.01 percent of primary igneous apatite and 0.005 to 0.02 percent of sedimentary marine apatite. Marine reworked apatite becomes enriched in uranium to as much as 0.1 percent. This is demonstrated by the greater uranium content of the texturally more complex phases within a single deposit. Uranium can be secondarily leached from or introduced into apatite by ground water. These secondary changes are indicated by pronounced concentration gradients within single pebbles of apatite as well as by the redistribution of uranium among different mineral hosts in leached and altered deposits of phosphorite. The postdepositional enrichment of uranium in phosphorite may be entirely residual as in the Tennessee "brown-rock" deposits, or greatly enhanced by ground-water additions as in the South Carolina phosphates. Moreover, the pattern of enrichment reflects the conditions and intensities of weathering. Highly acid lateritic weathering has caused supergene enrichment of uranium in the aluminum phosphate zone of the Bone Valley formation. In contrast, surficial enrichment characterizes the moderately weathered Cooper Marl of South Carolina. Isolated fossil bones or phosphate pebbles may contain almost one percent of uranium as a result of ground water enrichment. Such enrichment is comparable to the postdepositional uptake of fluorine by bone or insular phosphorite, and in some places the processes are synchronous and show good mutual correlations as they are codependent on the some ground water source. It is proposed that uranium replaces calcium in the apatite structure. This is indicated by several lines of investigation. Uranium and calcium contents are parallel in sections of leached and altered phosphorite. Ionic radii of tetravalent uranium (0.97 A) and divalent calcium (O.99 A) are virtually identical, and much of the uranium in igneous, sedimentary, and bone apatite is found to be tetravalent. Petrographic and chemical analyses and nuclear emulsion studies have shown that uranium in apatite is disseminated rather than locally concentrated. In addition, phosphate deposits are essentially devoid of uranium minerals. The igneous apatites contain from 10 to 66 percent of their uranium as U(IV). In apatite from a suite of related igneous rocks both the total uranium and the U(IV)/U(VI) ratio vary as the total uranium in the rock. Thus, the U(IV)/U(VI) ratio in igneous apatite reflects the prevalent equilibrium conditions in the crystallizing magma. In marine phosphorite thus far investigated the tetravalent uranium ranges from a few percent to more than 90 percent of the total uranium. Taken alone such statistics suggest a great variation in the initial U(IV)/U(VI) ratio of uranium emplaced in apatite. Thus, experimental evidence that bone and apatite pellets can remove uranyl uranium from solution suggests that much of the U(VI) found in natural apatite may be of primary origin, fixed as chemically adsorbed uranyl anions. It is proposed, however, that uranium in marine apatite is emplaced primarily as U(IV), structurally fixed. This follows from the fact that the higher U(IV)/U(VI) ratios are found in the younger unweathered marine apatites, and in apatites recently reworked by marine transgression. U(IV) is readily oxidized to U(VI), and postdepositional weathering, facilitated by radioactive decay, has most probably lowered the initially high U(IV)/U(VI) ratio in many older phosphorites. Apatite, by effectively removing the small amounts of U(IV) produced in sea water by reduction of (UOa)*s , causes more U(IV) to be produced for its own uptake. As the marine apatite is far from saturated with respect to uranium, it thus interferes with the attainment of equilibrium while fixing an unusual quantity of U(IV). The name regenerative capture is proposed for this type of concentration in which the fixation of an insignificant valence species interferes with the equilibrium producing the ion and thus generates a continuing supply for further uptake and results, ultimately, in unexpectedly large build-ups of the insignificant trace element in the host mineral.