Laboratory Modeling of Northern Great Plains Salt Efflorescence Mineralogy

Abstract

AbstractRecent studies of salt efflorescences in North Dakota and from several additional sites in the surrounding region indicated that the mineralogy of the efflorescences is predominantly sulfate salts of Na and Mg: mirabilite, thenardite, epsomite, hexahydrite, konyaite, and bloedite. Experiments consisting of isothermal evaporation of solutions with fixed Na2SO4/MgSO4 ratios were used to construct crystallization isotherms for the Na2SO4‐MgSO4‐H2O system at 28 and 40°C in order to model the salt efflorescence mineralogy. The first precipitates at 28°C were mirabilite, konyaite, and epsomite. At 40°C, thenardite replaced mirabilite as a first precipitate. The results indicated that the mineralogy is best described as a metastable system in which konyaite plays a dominant role. Konyaite is metastable when in contact with its saturated solution, but persists for long periods of time in dry efflorescence. The presence of konyaite in the system also allows mirabilite, thenardite, and epsomite to form supersaturated solutions with respect to the equilibrium phase assemblages. Reference to the liquidus‐solidus‐subsolidus portions of the experimental isotherms enables one to predict >90% of the observed natural assemblages that precipitate from sulfatic saline waters in North Dakota. This information allows us to determine which solid phases are controlling the shallow saline groundwater chemistry.