Gekoppelte Substitutionen im Melilith- und Sodalith-Strukturtyp
Since its formulation 50 years ago, Loewenstein’s rule has become a standard argument in the crystal chemistry of, notably, layer and framework silicates and zeolites. The validity of the rule is usually taken for granted, and it is used rather schematically. The rule is normally used as an argument to account for two experimental observations: i) In alumosilicates, AlO4-tetrahedra tend to avoid each other, leading to alternating AlO4- and SiO4-tetrahedra for a Si : Al ratio of 1:1; ii) Except in rare cases, alumosilicates have an Al : Si ratio smaller than or equal to 1. While observation i) could be rationalized theoretically, observation ii) is not easily explained, in fact, it does not seem that fully convincing arguments have been put forward for its explanation. We have prepared series of compounds where Loewenstein’s rule is breached systematically, and continuously. These belong either to the melilite, or to the sodalite structure type. Crystallographic, structural, chemical and physical features and properties have been determined and are reported.