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Abstract:

The structures and phases developed in a variety of polysomatic series (e.g. the biopyriboles (M.A. 78-4032) and the sheet silicates) are shown to be similar to those predicted by a simple spin model, the axial next-nearest-neighbour Ising (ANNNI) model, in a magnetic field. The authors argue that the different polysomatic structures can be considered as thermodynamically stable phases composed of ordered sequences of chemically distinct structural modules. It is suggested that the key factors determining their stability are: 1) the chemical potential, which controls the proportion of the different modules and 2) the competing interactions between first and second neighbour modules within the structures.-R.A.H.