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The subsurface distribution of gypsum and anhydrite, the lack of subsurface bassanite, and variations in trace-element ratios between mineral phases confirm experimentally determined stability relations and indicate that the replacement of one mineral phase by another proceeds by a dissolution-reprecipitation process. The earth surface occurrence of calcium sulfate minerals indicates that long-lived metastability is almost the rule and that in many situations kinetic factors dominate over equilibrium controls. In a standing body of brine, gypsum apparently is the only phase precipitated, either at the brine-air interface with elongate growth forms indicative of rapid rate of crystal growth, or at the brine-sediment interface, commonly as cemented layers of discs with thei c crystallographic axes almost horizontal. Significant anhydrite deposits are known today only from the warmer salt-flat or sabkha environments of the Persian Gulf and Baja California, precipitated interstitially within host sediments as an early diagenetic mineral. The anhydrite is present as contorted layers and nodules, commonly with a felted lath texture. These structures, textures, and other associations are characteristic of anhydrite occurrences.
Experimental studies and Holocene gypsum and anhydrite occurrences provide the information needed to determine which mineral was formed first in a particular situation. Where gypsum is the mineral buried, when and at what depth did it dehydrate, in which direction did the large volumes of dehydration water move, and what diagenetic effects did this dehydration water have on adjacent rocks and pore fluids? Where
anhydrite is the mineral buried, when did it lose its large primary porosity, and has it recrystallized since initial formation? By (1) defining the first-formed mineral and its structures, textures, and chemical composition and (2) making analogous studies of ancient subsurface calcium sulfate minerals, we can, hopefully, answer some of these questions.
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