Recently, Evans (1963) presented an excellent review of the known crystal structures containing the uranyl ion, and drew certain general conclusions regarding the stability of the several types of uranyl ion coordination from a study of these structures. As an important feature of his paper, Evans (1963) discusses the possible structures of the natural uranium oxide hydrates and their salts, and presents compelling arguments for considering that the uranyl groups in these compounds have pentagonal coordination. These minerals include becquerelite, CaO·6UO3·11H2O; billietite, BaO·6UO3·11H2O; fourmarierite, PbO·4UO3·4H2O; masuyite, formula uncertain; schoepite, UO3·2H2O (phases I, II, and III); and vandendriesscheite, formula uncertain (phases I and II); the detailed crystallography of these has been given by Christ and Clark (1960). Evans considers that the very large crystal unit cells of these pseudohexagonal minerals, containing from 12 to 432 uranium atoms, result essentially from the problem of packing pentagonal units into a layer arrangement that is consistent with crystal symmetry. A proposed structure is illustrated for billietite and becquerelite (Evans’ Fig. 4); the structure leads to the corresponding chemical formulas Ba[(UO2)6O4 (OH)6]·8H2O and Ca[(UO2)6O4(OH)6]·8H2O. This structure, with a planar layer of charge —2, is most attractive in explaining the stability, as well as the complexity, of this kind of salt; it seems much more likely than the one resulting from a puckered-hexagon OH- coordination, with a neutral UO2(OH)2 layer, that was originally proposed by Christ and Clark (1960).

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