Abstract
The ∞ 2 [Si2O 5 2− ] frame in phyllosilicate minerals is distorted through the rotation and tilting of the silicate tetrahedra and interacts with octahedral cations through its apical oxygens. Qualitative perturbation theory and extended Huckel band structure calculations demonstrate that rotation and tilting distortions of the ∞ 2 [Si2O 5 2− ] frame have little influence on orbital interactions within the frame. The effects which are observed can be traced to next-nearestneighbor, oxygen-oxygen interactions. Analysis of band widths and crystal-orbital-overlap-populations demonstrate the importance of O(2s) orbitals in the silicate bond. Interactions between Si(3s, 3p) and O(2s) atomic orbitals account for about half of the bonding overlap in the Si-O bond. Crystal orbitals within the ∞ 2 [Si2O 5 2− ] frame are perturbed in kaolinite, lizardite, pyrophyllite and talc through interactions of the apical oxygens with octahedrally coordinated Al(III) and Mg(II). These interactions appear to involve states that are non-bonding in an isolated frame, having little effect on the Si-Oapical bond while significantly reducing the apical-oxygen atomic population.
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