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Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
¶ E-mail address: mschindl{at}lakeheadu.ca
The crystal structures, chemical compositions and occurrence of uranyl-oxide hydroxy-hydrate minerals are interpreted in terms of the combined binary-representation – bond-valence approach to oxysalt minerals developed by Schindler & Hawthorne (2001a, b, c). A generalized interstitial complex can be written [[m]Ma + [n]Mb 2+ [l]Mc 3+ (H2O)d (H2O)e [q](OH)f (H2O)g](a+2b+3c–f)+, where d is the number of transformer (H2O) groups, e is the number of non-transformer (H2O) groups, and g is the number of interstitial (H2O) groups not bonded to any interstitial cation. The Lewis acidity of this interstitial complex can be 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. 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. Detailed predictions of the compositions of other complexes are made for the uranyl-oxide hydroxy-hydrate minerals. There is fairly close agreement between the predicted ranges of interstitial complex and those observed in Nature. A connection is established between the crystal structures of uranyl-oxide hydroxyhydrate minerals, and their chemical composition [molar ratio (MO) + (H2O) : (UO3)]. The type of interstitial cations and the general classes of polymerization of P-, U- and D-type chains in the structural units change systematically with log [M2+]/[H+]2 and [(MO)] + H2O]/[UO3)]. Structural units may be formally related by a chemical reaction that consumes two H+ and one M2+ cation. Combining this equation with the law of mass action, an expression can be formulated that allows arrangement of the structural units in log [M2+]–H space and calculation of the slopes of the associated phase-boundaries. The result is an activity–activity diagram with the correct topology and a relative scale along each of the axes. The general classes of polymerization of P, U- and D-type chains in the structural units change systematically across this activity–activity diagram.
Keywords: uranyl-oxide minerals, binary representation, bond-valence theory, activity–activity diagrams.
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