Modularity and Twinning in Mineral Crystal Structures

Dear Colleagues,

In 2004, the book Crystallography of Modular Materials (IUCr/Oxford University Press) by G. Ferraris, E. Makovicky and S. Merlino (2^nd edition 2008) systematically reviewed theory and examples of mineral crystal structures that can be described as built up by periodic repetition at atomic scale of either one (planar) module (polytypes) or more - m₁, m₂, … m_n - modules with different composition (polysomes). Whereas a series of polytypes, being based on a same module, shows essentially a constant chemical composition, the members of a polysomatic series have different chemical compositions that depends on the ratio m₁/m₂… /m_n. 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 macroscopic scale. The members of a series of polytypes or of polysomes have usually in common a supercell that, according to the reticular theory of twinning, favours the formation of twins. The nature, nomenclature, and bibliography of twinning are well summarized in the following web site http://www.crystallography.fr/mathcryst/twins.htm

Authors are invited to submit both experimental and theoretical articles dealing with the purposes of the Special Issue.

Prof. Dr. Giovanni Ferraris
Guest Editor

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