Modular structures – The paradigmatic case of heterophyllosilicates

As an example of modular structures, the layered silicates named heterophyllosilicates are reviewed with emphasis on their polysomatic, merotype, plesiotype and topologic aspects. The opportunities offered by a modular analysis of complex inorganic structures are shown via the procedure followed to model the crystal structure of nafertisite and to build various types of series. The heuristic power of the modular crystallography is also proved by successful analyses of oriented intergrowths (and twinning) in members of the bafertisite mero-plesiotype series and the consequent appearance of disorder in the crystal structures of samples affected by syntaxy, e.g. epistolite. Finally, the hypotheses of using, by analogy with phyllosilicates, HOH layers of some heterophyllosilicates to obtain hybrid layered materials are reviewed. In fact, the heterogeneous HOH layers of the heterophyllosilicates can formally be derived by substituting rows of disilicate groups with X-polyhedra (X mainly Ti) in the T sheets of a TOT phyllosilicate layer.