Abstract
The electrostatic lattice energies of expanded and unexpanded micas are calculated starting from a “generic” structure the ionic charges of which are varied. The mode of expansion is to move the layers apart perpendicular to (001), the K+ ions remaining midway between the layers. The energy required for expansion is a quadratic function of the layer charge. It is larger when the layer charge is in the octahedral sites (K x Al2−x Mg x Si4O10(OH)2) than when it is in the tetrahedral sites (K x Mg3Si4−x Al x O10(OH)2).
Fluormicas have a slightly larger expansion energy than OH-micas. With the tetrahedral layer charge, dioctahedral micas have a slightly larger expansion energy than trioctahedral micas. This mode of expansion is less favourable than the mode usually adopted, viz. an expansion whereby the K ions divide themselves between the layers. The energy difference increases with the separation distance and is about 60 kJ mol−1 at 2.5 Å expansion. An intercalated water layer would be necessary to stabilize the K ions in positions midway between the layers.
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Donald, H., Jenkins, B. & Hartman, P. Application of a new approach to the calculation of electrostatic energies of expanded Di- and trioctahedral micas. Phys Chem Minerals 6, 313–325 (1980). https://doi.org/10.1007/BF00307621
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DOI: https://doi.org/10.1007/BF00307621