Summary: This paper is primarily concerned with the interpretation of phenomena associated with phase transformations in single crystals, as observed by current experiments using electron-optical techniques. An analysis of permissible metastable behaviour in such single crystals, in terms of lattice theory, is provided. The dominant role of symmetry in defining the constraints on permissible be-haviour of this kind is explained. The resulting theory indicates that, at low temperatures, permissible thermal fluctuations in local chemical composition, or order, become strongly degenerate and lead to the development of non-Bragg diffraction effects. The theory and practical study of the phenomena by electron microscopy are also described.

The theory of metastable behaviour in a single crystal subject to symmetry constraints is then tested by re-examining the experimental data available for two systems that have been studied recently where such behaviour might be suspected. The first of these concerns the low-temperature behaviour of adularia, and the second deals with incipient exsolution phenomena in the alkali feldspar from a pantellerite. The data in each case appear to be compatible with the general theory proposed. Finally the theory is used to analyse the low-temperature behaviour of the intermediate plagioclase feldspars. In this case the theory indicates that both antiphase phenomena and schiller effects are associated with metastable behaviour in a system subject to strong symmetry constraints at low temperature. The implications of the analysis in relation to peristerite exsolution phenomena are discussed.