Si, Al distribution in micas; analysis by high-resolution29Si NMR spectroscopy

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.