The pseudomorphous replacement of gypsum by halite at the transition between bedded sulfate and chloride in cyclic evaporite sequences of Permian age is interpreted as a thermal disequilibrium feature. In solar salt ponds, the hot brines at halite saturation are undersaturated in gypsum and oversaturated in halite with respect to the cooler, previously accumulated gypsum substrate and will cause the replacement to occur when in contact with the substrate. Solar heating of the brines and production of porosity within the gypsum crystals, both of which are key features of the replacement mechanism, are materially aided by bacterial activity. The large-scale transition of gypsum to anhydrite cannot be directly involved in this process because the resulting diluted brines would corrode halite rather than precipitate it. Modern examples of this process appear to be rare, perhaps because the appropriate brines are often decanted for commercial collection of halite. The process is unlikely to occur in deep water masses that do not experience sufficient superheating during evaporation-concentration processes.
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