Evaporite minerals succession

An explanation is given for the crystallization succession during sea‐water evaporation, namely for the differences between the equilibrium solubility diagram of the marine‐type system and the “solar diagram”: The magnesium ions are known to exist in sea‐type solutions predominantly as hexaaqua complexes. Highest crystallization rates and lowest supersaturations required for nucleation should be exhibited by the magnesium salts, crystal structures of which incorporate unchanged hexaaqua magnesium complexes. For the salts with crystal structures in which the water molecules from the magnesium hexaaqua ions are partially or completely replaced by other ligands, the nucleations require high supersaturations and for this reason they do not appear in the “solar diagram”. This model is used to explain also some genetic pecularities of evaporite deposits: primary evaporite minerals are those which occur according to the “solar diagram” of crystallization. The evaporite minerals featuring in the equilibrium diagram and absent in the “solar diagram” occur in nature most frequently as secondary minerals.