MAIN STRUCTURAL SUBDIVISIONS AND STRUCTURAL FORMULAS OF SULFATE MINERALS

Main structural subdivisions of sulphate minerals and synthetic compou nds were considered in the frame of a new structural classification of minerals, based on the strength distribution and directional characte r of the bonds. The specific features of their crystal chemistry were expressed by the corresponding structural formulas. The recommendation s expressed by the Subcommittee on the Nomenclature of the Inorganic S tructure Types of the IUCr Commission on the Crystallographic Nomencla ture 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) framew ork. Sulphate atomic structures occur mostly when individual atoms for m either closest packing or any other close packing. In contrast to th e present study this subdivision was not considered in all previous sy stematics of sulphate minerals. The next categories comprise the struc tures formed either by isolated SO4 tetrahedra and MOn polyhedra with the M-O bond strengths comparable with those of the S-O bonds, or by t he complex structural units, where SO4 tetrahedra are linked to MOn po lyhedra. 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 reveal ed as a result of the structural systematics.