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Mixed-layer minerals are a remarkable example of one-dimensional order-disorder observed in crystals. They comprise layers of differing structure and compositions that alternate in variable proportions and with different distributions. Mixed-layer minerals may be described in terms of modular crystallography if the interstratified layers are considered as different modules having different structures and compositions and coexisting in crystals. In the following mixed-layering, interlayering and inter-stratification will be used as synonymous terms for minerals formed of two or more kinds of layers.

Interstratification effects have been found in structures of various natural and synthetic compounds: layer silicates, phyllomanganates, sulphides, carbonates, oxides and hydroxides including high temperature superconductors, intercalated graphites and other lamellar compounds. Interlayering in natural environments is especially widespread among clay minerals which differ in type of layers and in their alternation patterns. Very often a detailed study of apparently discrete minerals show evidence of at least small amount of a second component.

There are several reasons for special interest to mixed-layer minerals. In diffraction they have given a strong impulse to new theoretical and methodological developments since their unusual diffraction effects cannot be interpreted using conventional structural analysis techniques.

Studies of mixed-layer minerals with modern physical and physicoehemical methods have shown that there are different ways along which transformation of a mineral into another one may proceed, and gave a considerable contribution to our understanding of structure mechanism of phase transformations.

Interstratification effects have posed new problems for the thermodynamics, for example whether mixed-layer minerals are always metastable, whether they

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