Sulfate minerals; I, Bond topology and chemical composition

The crystal structure of an oxysalt mineral may be considered in two parts: 1) the structural unit, an array of high-bond-valence polyhedra that is usually anionic, and 2) the interstitial complex, an array of large low-valence cations, simple anions, OH and H (sub 2) O groups that is usually cationic in character. The chemical composition of interstitial complexes in sulphate minerals is explained and predicted using intrinsic properties such as polarity, Lewis acidity, coodination numbers and the average charge of oxygen atoms in the structural unit (average basicity). The interstitial complex may be characterized by its Lewis acidity, a measure of the electrophilic character of the complex, and the structural unit can be characterized by its range in Lewis basicity. Any complex structural unit can be divided into an acidic component of (M (super 2+) phi n) polyhedra and a basic component of (SO (sub 4) ) groups. Further details are given and there is a useful appendix wth definitions of the terms used..