MODULAR FEATURES IN THE SYSTEMATICS OF CRYSTAL STRUCTURES. B. B. Zvyagin (Inst. of Ore Mineralogy RAS, Moscow, Russia), S. Merlino (University of Pisa, Italy), G. Ferraris (Univ. of Torino, Italy)

Modular features are extremely useful for any rational and natural approach to the systematics of crystal structures. In fact several wide structural families may be clearly and neatly described as characterized by the occurrence of few distinct modules, the various members of each family differing in relative amounts, sequence and stacking order of the constituting modules. Two modular systems displaying these features will be presented. One is formed by the modules of pyroxenes and spinel and includes mixed-modules sapphirines, ferrites and surinamites, single-module spinelloids, condensed-module biopyriboles, and some additional modular sets of structures not yet found in nature. This system is outstanding not only for its various polysomatic features but also for the distinct OD and polytypic features displayed by several members of this family.

The other is formed by two distinct kinds of silicate layers alternating with octahedral layers. Different layer combinations characterize the structures of reyerite, gyrolite, tungusite, fedorite, K- and Z-phases, and some modified structures of minehillite and baratovite. The various sets of module combination present polytypic and OD modifications differing in layer stacking, which is defined by the symmetry operations of translation, rotation, reflection and inversion relating successive modules, presenting unit cells in multiplicity relationships. Both modular systems provide an useful and powerful way for understanding the relationships between polymorphs, polytypes, OD structures, and polysomes. They also show the efficiency of the modular approach in description, analysis and derivation of both existing and hypothetical crystal structures.