SAED and HREM Study of Order/Disorder and Structural Heterogeneity in Layer Minerals

Abstract

Many minerals exhibit unusual diffraction properties. In addition to strong, sharply defined and widely spaced reflections, corresponding to the substructure, their diffraction patterns contain additional weaker and more closely spaced satellite spots corresponding to a superstructure or a modulated structure. If the spacings of these satellite spots are integral multiples of the substructure spacings, then we are dealing with a superstructure or with a commensurate modulated structure (Cowley et al. 1979). If the substructure spots are not at rational multiples of the satellite spot spacings, the structure is a modulated incommensurate one (Cowley et al. 1979). Satellite spots can result from various structural factors. They are commonly associated with structure modulations resulting from cation ordering and/or positional displacements of atoms. For example, an ordered distribution of isomorphous cations can lead to a superstructure and therefore to satellites in the corresponding diffraction patterns. Modulations may result from the presence of domains separated by out-of-phase boundaries. In this case the periodicity of modulations is determined by the average separation of the boundaries. A survey of modulated mineral structures is given in (Buseck and Cowley 1983).