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Main structural subdivisions of sulphate minerals and synthetic compounds were considered in the frame of a new structural classification of minerals, based on the strength distribution and directional character of the bonds. The specific features of their crystal chemistry were expressed by the corresponding structural formulas. The recommendations expressed by the Subcommittee on the Nomenclature of the Inorganic Structure Types of the IUCr Commission on the Crystallographic Nomenclature were also considered. Consequently the structural systematics of sulphates includes the following five main categories of structures: 1) atomic (or close-packed), 2) group, 3) chain, 4) sheet and 5) framework. Sulphate atomic structures occur mostly when individual atoms form either closest packing or any other close packing. In contrast to the present study this subdivision was not considered in all previous systematics of sulphate minerals. The next categories comprise the structures formed either by isolated SO 4  tetrahedra and MO n  polyhedra with the M–O bond strengths comparable with those of the S–O bonds, or by the complex structural units, where SO 4  tetrahedra are linked to MO n  polyhedra. The category of the structures, involving different types of structural units, corresponds to the structural unit with the highest dimensionality. Crystal chemical peculiarities of sulphates are revealed as a result of the structural systematics.

Main structural subdivisions and structural formulas of sulphate minerals