Understanding the weakly bonded constituents in oxysalt minerals

Abstract

The crystal structure of a mineral may be divided into two parts: (1) the structural unit , an array of high-bond-valence polyhedra that is usually anionic in character, and (2) the interstitial complex , an array of large low-valence cations, simple anions and (H 2 O) groups that is usually cationic in character. Interstitial complexes link the structural units with weak cation-anion and hydrogen bonds into a continuous structure, and the breakdown of a structure is usually controlled by the strengths of the weak bonds that link the structural units together. The interstitial complex is (usually) a complex cation, and can be characterized by its Lewis acidity , a measure of the electrophilic character of the complex. The structural unit is (usually) a complex oxyanion, and can be characterized by its Lewis basicity . The interaction between the structural unit and the interstitial complex can be examined using the principle of correspondence of Lewis acidity-basicity . If one examines a series of structures with the same structural unit, it is evident that the average coordination of the O atoms of the structural unit varies slightly from one structure to another, producing a range of Lewis basicity for this specific structural unit. In this way, a specific structural unit can be stable over a range of Lewis basicity ( i.e., over a specific pH range). The formula of an interstitial complex may be written in the following way: { [m] M + a [n] M 2+ b · [l] M 3+ c (H 2 O) d (H 2 O) e (OH) f (H 2 O) g } (a+2b+3c–f)+ , where [ m ], [ n ] and [ l ] are coordination numbers, a, b and c are the numbers of monovalent, divalent and trivalent cations, d is the number of transformer (H 2 O) groups, e is the number of (H 2 O) groups bonded to two interstitial cations or one interstitial cation and one hydrogen bond, f is the number of interstitial (OH) groups, and g is the number of (H 2 O) groups not bonded to any cation. The number of transformer (H 2 O) groups strongly affects the Lewis acidity of the interstitial complex, and the variation in Lewis acidity of a generalized interstitial complex can be graphically represented as a function of the number of transformer (H 2 O) groups. Where the Lewis acidity of a generalized interstitial complex overlaps the range of Lewis basicity of a specific structural unit, the principle of correspondence of Lewis acidity-basicity is satisfied and a stable structural arrangement is possible. Detailed predictions of the compositions of interstitial complexes are made for the borate, sulfate and uranyl-oxide-hydroxy-hydrate minerals. There is fairly close agreement between the predicted ranges of interstitial complex and those observed in Nature.