"Programme of natural analogues studies" and identification of the chemical state of uranium in natural substances

The research program named "Programme of Natural Analogues Studies" concern s the chemical behavior of Th-230, U-234, and U-238 in the earth crust, whi ch is governed by a number of physicochemical factors, in particular, by th e chemical state of the parent U-238. In this article the problem of identi fication of the chemical state and mineral forms of uranium in natural subs tances of various origins and compositions is discussed, and processes givi ng rise to mixed U(IV)-U(VI) systems are analyzed. Possible coeistence in t he solid phase of three oxidation states of uranium, U(IV), U(V), and U(VI) , makes the traditional methods of solution chemistry inefficient for ident ification of these forms in natural formations. The state of the art of ins trumental methods for analysis of natural uranium compounds is discussed. T he magnetometrical and optical methods (IR and electronic spectroscopy) are of limited use for this purpose. Information furnished by X-ray Sf-photoel ectron spectroscopy or by the method of chemical shifts (CSs) of the uraniu m L-alpha 1 X-ray line (similarly to any other single line) is insufficient for calculations if an additional criterion for choosing an analytical fun ction and distinguishing U(V) from a U(IV)-U(VI) mixture is lacking. It is concluded that the model of the correlation diagram of the chemical shifts of the ULalpha 1 and ULbeta 1 X-ray lines eliminates the uncertainty in cal culation of the uranium valence forms in the case of uranium oxide systems. However, this procedure should be additionally substantiated experimentall y and theoretically when passing to a wide range of natural compounds.