Modularity and Twinning in Mineral Crystal Structures

The articles published in this Special Issue reprint offer a sound update of research fields
where the concepts of modular crystallography are important and provide unique keys to
understand and to solve problems of structural crystallography. Polytypism,
polysomatism, and twinning are fertile fields of research, and their basic principles-
o����en coupled with the OD (order-disorder) theory-are powerful tools to solve and
classify related crystal structures. Research on twinning and its consequences on
structure and properties of crystalline materials is cutting-edge, e.g., dealing with
relations between twin walls and piezoelectricity.