Relationship between Optical Basicity and Thermochemistry of Silicates

The optical basicity treatment of oxidic network systems, for example silicates, has been successful in correlating a range of properties; since the method has the advantage of avoiding such problems as those associated with single ion" activities, it allows comparisons between many different systems. However, several of the properties dealt with are traditionally viewed in terms of thermodynamics. It is therefore of significance that a link has now been discovered between optical basicity and thermochemistry, and is established through the quantitative relationship between optical basicity and electronegativity. This link shows that thermodynamic stability increases with increasing negative charge borne by the oxygen atoms and, in principle, should allow heats of formation, DeltaH(f)degrees, of silicate compounds to be predicted from their optical basicity values which, in turn, are calculated from chemical composition. This possibility is examined by making comparisons with experimentally obtained values of DeltaH(f)degrees, and it is shown that there is good agreement for silicates composed of the oxides CaO, MgO. Al2O3, and SiO2.