SULFATE MINERALS. I. BOND TOPOLOGY AND CHEMICAL COMPOSITION

Abstract

The crystal structure an oxysalt may be divided into two parts: (1) the structural unit, an array high-bond-valance polyhedra that is usually anionic in character, and (2) the interstitial complex, an array of large low-valence cations, simple anions, (OH) and (H2O) groups that is usually cationic in character. The chemical compositions of interstitial complexes in sulfate minerals are explained and predicted using intrinsic properties such as polarity, Lewis acidity, coordination numbers and the average charge of oxygen atoms in the structural unit (average basicity). The interstitial complex can 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 [Mz+(H2O)n (OH)m(SO4)k] can be divided into an acidic component of (MZ+φn) polyhedra and a basic component of (SO4) groups. The ligands of the acidic component are primarily bond-valence donors, and the O atoms of the basic component are bond-valence acceptors. Neutral structural units must arrange themselves such that their acidic and basic parts match each other in order to allow linkage via hydrogen bonds. Additional (H2O) groups between structural units are required where hydrogen bonds cannot be accepted directly by a donor atom of the basic part of the structural unit. The Lewis acidities of interstitial complexes in sulfate minerals range from 0.10 to 0.25 vu (valence units), with frequency maxima at 0.13, 0.17, 0.20-0.21 and 0.25 vu. These maxima correspond to average coordination-numbers of oxygen atoms in the basic component of the structural unit. Using the characteristic range in Lewis basicity of a structural unit and the maximum frequencies of Lewis acidities, the most probable number of bonds from the interstitial complex to the structural unit may be predicted using the valence-matching principle. This number allows prediction of the types of interstitial complexes for a given structural unit.