The intermediate plagioclase feldspars: an example of a structural resonance

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Abstract
K -space symmetry rules are used to elucidate the nature of possible ordering behaviour in a single crystal. These rules are then applied to the observed behaviour in the intermediate plagioclase feldspars. Group theoretical analysis of the possible ordering behaviour in a solid solution indicates that, at arbitrary points in k -space it is possible to have an interaction between two different ordering schemes associated with the development of the same superlattice. This interaction corresponds to a structural resonance and leads to the lowering of the energy for the ordered states at the point of accidental degeneracy in the Brillouin zone of the ordering single crystal. The effect is only likely in the case of a non-equilibrium cooling cycle. The structural and other implications of the group-theoretical model are tested against the available experimental and structural data for the intermediate plagioclase feldspars, and shown to be satisfactory.