Natural Anhydrous Sulfates with Alkali Cations: Structural Features, Comparative Crystal Chemistry, and Genetic Mineralogy

The review summarizes the data on the crystal chemistry and genetic mineralogy of all known natural anhydrous sulfates with Na or/and K as species-defining alkali cations (61 mineral species, 48 structural types). Only two genetic types of mineralization, namely, volcanic fumaroles and evaporites, are significant for these minerals. Both these mineral-forming systems are characterized by a low pressure, but they strongly differ in the temperature and crystallization mechanisms. In terms of the structural classification, these minerals are divided into the following two large groups: minerals having anionic motifs consisting only of SO₄ tetrahedra (and also of additional F, Cl, and CO₃ anions) and minerals having heteropolyhedral anionic motifs consisting of SO₄ tetrahedra and polyhedra of cations with relatively high-strength characteristics (Mg, Mn, Zn, Cu, Fe, V, Al, Bi, Mo, As, Te, and sometimes Ca). This is consistent with the genetic types. Thus, representatives of the first group predominate among evaporitic minerals, whereas representatives of the second group prevail among fumarolic minerals. A clear positive correlation was revealed between the degree of polymerization of a heteropolyhedral anionic motif and the number of minerals, which have such motifs. Thus, the heteropolyhedral motifs can be described as frameworks or pseudoframeworks with different topologies in 18 minerals, as layers in 11 minerals, and as chains in 8 minerals. Only one mineral contains isolated heteropolyhedral complex. The review discusses different types of decay of solid solutions and other solid-state transformations in high-temperature sulfates with aphthitalite-like structures from volcanic fumaroles.