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THE STEREOCHEMISTRY AND CHEMICAL COMPOSITION OF INTERSTITIAL COMPLEXES IN URANYL-OXYSALT MINERALS

Michael Schindler and Frank C. Hawthorne

Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T2N2, Canada

The crystal structures and chemical compositions of uranyl-oxysalt minerals (primarily with sheet structural units) are interpreted in terms of the binary representation – bond-valence approach to the structure and chemistry of oxysalts. There is a coherent relation between the average basicity of the structural unit and [CN_in], the average number of bonds to oxygen atoms of the structural unit from the interstitial complex and adjacent structural units. This relation allows calculation of the range of Lewis basicity for specific structural units. The Lewis acidity of an interstitial complex is expressed graphically as a function of the amounts and coordination numbers of monovalent, divalent and trivalent interstitial cations and the amount of interstitial transformer (H₂O) groups. The range in Lewis basicity for a specific structural unit may also be expressed graphically, and where there is overlap of the Lewis acidity and Lewis basicity, the valence-matching principle is satisfied, and the details of the possible interstitial complexes can be derived. There are three distinct types of interstitial (H₂O) groups: transformer, non-transformer and inverse-transformer. Inverse-transformer (H₂O) groups accept three bonds from cations, other (H₂O) groups and (OH) groups of the structural unit. Their occurrence is rare and is limited to minerals with low bond-valence distribution factors. Detailed predictions of the number of transformer, non-transformer and inverse-transformer (H₂O) groups in the minerals of the meta-autunite, uranophane, phosphuranylite, carnotite, zippeite and uranyl-hydroxy-hydrate groups (and synthetic analogues) are in good agreement with the observed chemical compositions.

Keywords: H₂O groups, uranyl minerals, bond-valence theory, acidity, basicity, valence-matching principle, structural unit, interstitial complex.