Modular structures inherited from high-temperature disordered parent ones: an example of Ca2SiO4, Na2SO4–K2SO4 sulfates and related minerals

Abstract

Abstract Crystal structures of Ca2SiO4, Na2SO4–K2SO4 sulfates and related minerals bubnovaite K2Na8Ca(SO4)6 and dobrovolskyite Na4Ca(SO4)3 were described as modular ones for the first time. A microblock, or a module [M(TO4)6] that is consisted of an octahedron interlinked by six vertices with six adjacent tetrahedra was considered as a structural unit inherited upon cooling from a high-temperature disordered parent one. A relationship between the parent and inherited microblocks was established. As a result, fifteen possible types of modules maintaining a trigonal symmetry of the microblocks were derived based on this relationship. It was shown, that the minerals and compounds structurally-related with α-Na2SO4 derived superstructures were formed as a result of a cooling of the high-temperature phases containing the disordered parent microblock. Here, a driving force of the inheritance is a tendency of the structure to order upon cooling. The reasons of a formation of a microblock from the parent one are mainly determined by ionic radius and type of a cation occupying the octahedral site. A finding of minerals with the described structural features could be a promising tool for a synthesis of novel compounds with useful properties.