**Preface to "Modularity and Twinning in Mineral Crystal Structures"**

In 2004, the book *Crystallography of Modular Materials* (IUCr/Oxford University Press) by G. Ferraris, E. Makovicky and S. Merlino (2nd edition 2008) systematically reviewed theory and examples of mineral crystal structures that can be described as built up by periodic repetition at the atomic scale of either one (planar) module (polytypes) or more modules—m1, m2, . . . mn—with different compositions (polysomes). Whereas a series of polytypes, being based on the same module, essentially show a constant chemical composition, the members of a polysomatic series have different chemical compositions that depend on the m1/m2. . . /mn ratio. In a polysomatic series, the physical properties are a function of the chemistry of the modules and of their piling; thus, tailoring of the properties is possible. A special class of polytypes is rationalized by the so-called OD (Order/Disorder) theory. Twinning, i.e., the oriented association of two or more individuals of the same crystalline compound, is considered a modular structure at the macroscopic scale. The members of a series of polytypes or of polysomes usually have in common a supercell that, according to the reticular theory of twinning, favours the formation of twins.

> **Giovanni Ferraris** *Editor*
