Intermediate sulfates in barite-celestite isomorphic series: Composition and mode of occurrence

Intermediate (mixed) sulfates of the BaSO₄-SrSO₄ isomorphic series are relatively rare in nature. Nevertheless, large resources of such sulfates have been revealed in Russia at carbonatite-type deposits (Karasug in Tuva and Khalyuta in the western Transbaikal region) and at the Oktyabr’skoe skarn magnetite deposit in the Angara region. The compiled database of chemical compositions of mixed Ba-Sr sulfates (876 items) comprises data related to the above deposits and other worldwide occurrences. Ba-Sr sulfates are formed in a wide range of conditions from sedimentary basins to high-temperature magmatic systems, and their generalized compositional intervals completely overlap. At the same time, sulfates of various natural assemblages and generations occupy discrete intervals separated from one another in isomorphic series. The statistical boundaries of these intervals correspond to 15, 36, and 66 (±1) mol % SrSO₄. Many Ba-Sr sulfates contain up to 7–11 mol % CaSO₄.