Crystal chemistry of orthosilicates and their analogs: the classification by topological types of suprapolyhedral structural units

A method is developed for the analysis and classification of orthosilicates and their analogs M-x (TO4)(y) containing M cations and tetrahedral TO4 anions. The method uses the concepts of coordination sequence and crystal structure 'reduced' graphs and is optimized for orthostructures of any complexity. First, the suprapolyhedral level of crystal structure organization was studied, where T tetrahedra were considered as templates for condensing M polyhedra, constructing as a result T polyhedral microensembles. Using this methodology, the crystal structures of 54 orthosilicates and orthogermanates were analyzed within the first 12 coordination spheres of T nodes and were arranged into 21 topological types. The topological types were expanded with the analogs found within the orthostructures of phosphates, sulfates etc. T polyhedral microensembles were used for the topological classification of reconstruction mechanisms of thermal and baric phase transitions of orthosilicates.