SULFATE MINERALS: A BOND-VALENCE APPROACH TO THE STRUCTURE, CHEMISTRY AND OCCURRENCE OF COMPLEX OXYSALT MINERALS

Minerals can be divided into (1) the structural unit, which is defined by strong bonds between atoms, and (2) the interstitial complex, which links structural units into a continuous structure. The interstitial complex is usually cationic and is characterized by its Lewis acidity or electrophilic strength. The structural unit is usually anionic and is characterized by its Lewis basicity. The valence-matching principle examines the interaction between these structural components, and a stable structure forms when the Lewis acidity of the interstitial complex closely matches the Lewis basicity of the structural unit. We may express the Lewis acidity of a generalized interstitial complex {[m]M1+a [n]M2+b [l]M3+c (H₂O)d (H₂O)e (OH)f (H₂O)d})a+2b+3c-f)+ graphically in terms of the coordination numbers [m], [n] and [l] of the cations M of amounts a, b and c, the amounts of transformer, d, non-transformer, e, and hydrogen-bonded, g, (H₂O) groups, and the amount of (OH), f. The intersection of this function with the range of Lewis basicity of a specific structural unit defines the values of the coefficients m, n, l, a, b, c, d, e, f that are possible for a stable structure.

The range in Lewis basicity for a structural unit corresponds to the range of pH over which a mineral is stable. Thus we can correlate directly the change in polymerization of structural units as a function of pH. There is a reasonable correlation between the average basicity of the structural unit and the pH of the parent aqueous solution. Thus we have established a connection between the details of a crystal structure and their conditions of formation.