Anhydrous natural sulfates with alkali cations: structural features, comparative crystal chemistry, and genetic mineralogy

This review presents data on the crystal chemistry and genetic mineralogy of all known natural anhydrous sulfates with species-defining alkali cations—Na or/and K (61 mineral species, 48 structural types). These minerals are primarily associated with two genetic mineralization environments: volcanic fumaroles and evaporites. Both environments are characterized by low (atmospheric) pressure but differ significantly in temperature and crystallization mechanisms. Based on structural classification, the studied minerals are divided into two major groups: (1) those with an anionic motif consisting solely of SO₄ tetrahedra (sometimes including additional anions such as F, Cl, or CO₃), and (2) those with heteropolyhedral anionic motifs composed of SO₄ tetrahedra and polyhedra of cations with relatively high-strength characteristics (e.g., Mg, Mn, Zn, Cu, Fe, V, Al, Bi, Mo, As, Te, and occasionally Ca). This classification aligns with genetic types: the first group predominates in evaporitic environments, while the second group is more common in fumarolic settings. A clear positive correlation has been established between the degree of polymerization of the heteropolyhedral anionic motif and the number of minerals in which such motifs occur. Specifically, 18 minerals exhibit frameworks or pseudo-frameworks of varying topologies, 11 minerals feature layered motifs, eight minerals contain chain-like motifs, and only one mineral contains isolated heteropolyhedral complexes. Additionally, the review discusses various types of solid-solution breakdown and other solid-state transformations in high-temperature sulfates with aphthitalite-like structures from volcanic fumaroles.