Chemical Bonds and Distribution of Cations in Silicates

Abstract

It is shown on theoretical grounds that the electronegativity of oxygen in silicates depends upon the Si-0 structure as well as upon the Al/Si ratio in the (Si/Al)O₄ tetrahedra. The more Si-O-Si links in a silicate structure, the larger the electronegativity of the oxygen atoms attached to metallic ions. That is, the effective electronegativity value of oxygen increases stepwise through the sequence: orthosilicate → pyrosilicate ($$Si_{2}O_{7}^{6-}$$) → metasilicate ($$SiO_{3}^{2-}$$) → double-chain silicate ($$Si_{8}O_{22}^{12-}$$) → phyllosilicate ($$Si_{5}O_{10}^{6-}$$) → tectosilicate. This means that the corresponding hypothetical acids increase in strength through the sequence and that the metal-oxygen bonds in the corresponding salts become more ionic throughout the series. It is demonstrated that the "base exchange" of cations between two oxysalts commonly takes place in such a way that the most electropositive metal combines with the most acidic anion, and the less electropositive metal with the less acidic anion. This combination generally represents the lowest total energy and the highest bond energy. It is shown how the different metallic cations distribute themselves among the different types of silicates, depending on the variable electronegativity of oxygen in the structures and on the position of the metals in the electronegativity scale of elements. The rock-making minerals are in good agreement with the theory.