Cryogenic brines
Cryogenic brines and associated salts require temperatures at or below the freezing point of the liquid phase. These salts crystallise from a cold, near-freezing, residual brine as it concentrates via the loss of its liquid phase, which is converting/solidifying to ice. As brine concentration increases, the freezing temperature decreases and minerals such as ikaite, hydrohalite, mirabilite, epsomite, potash bitterns and antarcticite can crystallise from the freezing brine (Figure A, B; Warren, 2016; Chapter 12). Brine freezing ends when the phase chemistry attains the eutectic point. This is the point when all compounds (including H2O) pass to the solid-state. Depending on the initial mineralisation and composition of the brine, the eutectic point is reached between -21 and -54 °C (Marion et al., 1999; Strakhov, 1970). The cryogenic concentration of seawater precipitates mirabilite at four times seawater salinity and hydrohalite at eight times. In contrast, evaporating seawater precipitates gypsum at 4-5 times the original concentration and halite and 10-11 times.
Cryogenesis. A) Brine freezing temperature falls as the NaCl concentration in the brine increase. B) Evaporation series where seawater is concentrated by solar heating (evaporation) versus brine freezing (cryogenesis) (After Herrero et al., 2015; Herut et al., 1990; Nelson and Thompson (1954).
Evaporative gypsum precipitation decreases the relative proportions of both Ca and SO4 in the brine, while cryogenic precipitation of mirabilite decreases the sulphate proportion and drives the inflexion of the Na cryogenic curve slightly earlier than Na inflexion created by the precipitation of evaporative halite. In both the freezing and the evaporation situations, the brine remains chloride dominant prior to bittern crystallisation. Freezing seawater becomes increasingly sulphate enriched to where sulphate levels exceed sodium around 20 times the initial concentration. Evaporating seawater remains a Na-Cl dominant brine until the bittern stage is reached around 60 times the initial concentration (Warren, 2016, Chapter 2). Mirabilite (NaSO4.10H2O) is one of several sodium sulphate salts and is stable in sulphate brines at temperatures lower than a few centigrade degrees.
Ionic evolution and main salts precipitated in a concentrating seawater brine upon its freezing or evaporation (data from Nelson and Thompson 1954 and McCaffrey et al., 1987).
The other sodium sulphate salt grouping is characterised by varying combinations of glauberite (CaSO4.Na2SO4)/bloedite-astrakanite (Na2SO4.MgSO4.4H2O) salts, which crystallised at higher temperatures via the evaporation of continental brines in saline groundwater sumps in warm to hot arid climates (as discussed in Warren, 2016, Chapter 12). The climatic dichotomy reflects the fact that sodium sulphate solubility in water changes as a nonlinear function of temperature. Below 1.2°C, ice and mirabilite tend to precipitate as seawater or a sodium sulphate brine freezes. As the temperature increases above 0°C, increasing amounts of hydrous sodium sulphate (as the decahydrate, mirabilite) become soluble, while the anhydrous form (thenardite- NaSO4) becomes the precipitative phase in brines saturated with respect to the sodium sulphate. At 32.4°C in pure water, a transition point on the solubility curve is reached, whereby mirabilite melts in its water of crystallisation and thenardite crystallises. Presence of other dissolved salts changes the transition temperature and solubility characteristics of sodium sulphate due to the double salt effect.
Temperature and phase stability. A) Temperature dependency of Na2SO4 minerals in an NaCl-saturated NaCl–Na2SO4–MgCl2–H2O system (after Braitsch, 1964). B) Temperature dependency of hydrohalite (after Warren, 2016).
Hence, mirabilite and hydrohalite are commonplace cold-climate lacustrine precipitates. Mirabilite beds are commercially exploited in colder climates; their latitudinal and altitudinal occurrences illustrate an interesting climatic dichotomy inherent to economic deposits of the various sodium sulphate salts. One sodium sulphate grouping of exploited deposits is characterised by mirabilite precipitated via brine freezing, as in the Great Salt Lake, Karabogazgol and Hedong (Yucheng) salt lake. Then there are the antarcticite deposits in the Don Juan ponds of Antarctica and the hydrohalite springs atop the Stolz diapir in the Canadian Archipelago (Salty Matters, May 18, 2019)
Examples of cryogenic lacustrine precipitates. A) Winter mirabilite in Great Salt Lake Utah. B) Winter mirabilite dunes on the edge of Karabogazgol. Turkmenistan. C) Winter mirabilite crusts in Hedong Lake, China. D) Mirabilite crust on pond sumps in glacial till at the edge of Lake Garwood, McMurdo Dry Valley, Antarctica. E) Hydrohalite created in the laboratory by brine freezing. F) Hydrohalite and mirabilite formed at a brine spring outflow near the edge of Stolz diapir, Canadian Archipelago. G and H) Antarcticite crusts forming in Don Juan Pond, Antarctica (For detail see Salty Matters, May 19, 2019)