Nuclear Chemical Effects in the Paragenetic Mineral Association Based on Polycrase

Abstract

A natural polymineral compound in which the major uranium-containing mineral is polycrase (Ti- Ta-niobate) described by the formula АВ₂О₆ was chosen as a model for studying the behavior of recoil atoms produced by α-decay of actinides. Polycrase, despite its metamict structure, is characterized by the following features distinguishing it from Ti-Ta-niobates of the formula А₂В₂О₇ (pyrochlore and betafite): (1) 1/3 of uranium atoms preserve the initial valence state, U(IV); (2) the U(IV) fraction is characterized by the isotope activity ratio AR(²³⁴U/²³⁸U) close to that at secular equilibrium; (3) the uranium atoms that underwent oxidation “memorize” their radiogenic origin; as a result, the AR(²³⁴U/²³⁸U) ratio in the U(VI) fraction is 0.90, and in the most altered part of the mineral it decreases to 0.77; (4) the parent uranium is relatively stable in the metamict structure of polycrase: The half-leaching time for ²³⁸U atoms is 2 times longer than that in betafite, zircon, or sphene.