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The Si, Al distribution in tetrahedral sheets of micas has been analysed by the use of Monte Carlo simulation and by the calculation of electrostatic energies. A new method for the calculation of electrostatic energies for short-range-ordered distributions has been developed. The results of the analysis demonstrate that the Si, Al distribution is mainly controlled by the electrostatic requirements of homogeneous dispersion of Al and local balance of inter-layer cations. Furthermore, electrostatic interactions between contiguous layers also constitute a significant factor controlling the tetrahedral cation distribution. Electrostatic energies of long-range-ordered models based in the para or in the meta disposition of Al are very similar, indicating that there is no preference for a particular long-range ordered distribution. This result justifies the distribution model found by NMR analysis, which is long-range-disordered, with the meta and para dispositions of Al equally favoured.

The analysis of 29Si MAR-NMR spectra of micas and vermiculite shows that the two main factors that govern the Si, Al distribution in the tetrahedral sheet of these minerals are: (i) Loewenstein's rule (avoidance of Al-O-Al linkages), and (ii) local balance of charges. In muscovite, phlogopite and vermiculite, Al is randomly distributed in hexagonal ring sites (meta and para dispositions equally favoured), but complying with the two above restrictions. In margarite Si and Al are observed to alternate strictly, in agreement with previous X-ray diffraction analysis.