Formation, oxidation and alteration of ianthinite

Ianthinite, UO2.86·1.5H2O, is a weathering product of uraninite, UO2+x, and has been reported as a corrosion product of UO2. Ianthinite alters to schoepite, UO3·2H2O, or studtite, UO4·2-4H2O, in water. The structures of ianthinite, shoepite and studtite are unknown, but their unit cell parameters differ significantly, and transformations are likely reconstructive. XRD powder data for ianthinite and becquerelite are similar, and they are probably isostructural; U4+, as U(OH)22+, replaces Ca2+ in the interlayer. The proposed formula is U(OH)22+[(UO2)6O4(OH)6]·6H2O. Oxidation in air, has been reported to form shoepite; however, in the absence of water, the oxidation of ianthinite, U(OH)22+[(UO2)6O4(OH)6]·6H2O + 1/2 O2 → (UO2)2+[(UO2)6O4(OH)6]·7H2O, shows that epi-ianthinite is structurally distinct from shoepite. The oxidation of fine-grained material is rapid, and the color change -- from dark purple to yellow -- is dramatic. The acicular habit of ianthinite is preserved during oxidation, and acicular open-quotes shoepiteclose quotes is observed in nature, as well as in leaching experiments on spent fuel. Possible substitution of Pu4+ for U4+ suggests that Pu-ianthinite may control Pu solubility in oxidizing, U-saturated solutions