The cationic ordering in the octahedral and tetrahedral sheets of Al-rich synthetic phlogopites with composition K(Mg3−xAlx)[Al1+xSi3−xO10](OH)2 (0.0 < x < 1.0) has been investigated using a combined approach of Monte Carlo simulations based on the “J formalism” (Bosenick et al. 2001; Warren et al. 2001) and 1H, 29Si MAS and {1H} → 29Si CPMAS/HETCOR solid-state NMR spectroscopic experiments. Our results are compatible with the well-established Loewenstein’s rule of Al-Al avoidance in the tetrahedral sheets, but the Si/IVAl and Mg/VIAl distributions show a tendency to segregation of the excess Al atoms, and we observe a preference for VIAl and IVAl to occupy directly neighbored octahedral and tetrahedral sites. As a result the structure is separated into clusters of original phlogopite composition {K(Mg3)[AlSi3O10](OH)2} and clusters of “eastonite” composition {K(Mg2Al)[Al2Si2O10](OH)2} that encompass a whole T-O-T layer package, although Al is solved in the phlogopite structure homogeneously on a macroscopic level.

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