Silicon and aluminium site distributions in 2:1 layered silicate clays

Although the layer charge in 2:1 phyllosilicate minerals is known to result from the replacement of tetrahedral Si or octahedral Al, Fe and Mg by ions of lower charge, there is only limited information concerning the distribution of layer charge from X-ray crystallographic data 1–3 . Here we use 29 Si and 27 Al magic angle spinning (MAS) NMR spectroscopy to examine the site distribution of tetrahedral Si and Al in a series of synthetic trioctahedral clays with (Si/Al) tetr values in the range 2.74–7.69. Analysis of the 29 Si spectra shows that Loewenstein's rule for Al occupancy of the tetrahedral sheet is obeyed and that there is some short-range ordering for Al sites. The nature of the ordering is explained in part by the results of electrostatic potential energy calculations. In general, strong 27 Al resonances are observed for Al in tetrahedral sites, but the resonances due to Al in octahedral sites are considerably weaker than would be expected on the basis of chemical analysis. Consequently, no quantitative analysis of the 27 Al spectra is possible.