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Anhydrite can be precipitated from natural brine in the presence of gypsum in the temperature range of 60°-70°C. Below this temperature range and within the anhydrite stability field, the rate of growth is extremely slow. Growth of gypsum in brine in the presence of anhydrite within the gypsum stability field is rapid. Recent anhydrite in the Persian Gulf in general occurs above the free-water level in supratidal sediments. It is postulated that this anhydrite is formed in the dark sediments during the hottest summer days and is preserved throughout the year in the partly dry sediments because of lack of water. However, at temperatures below approximately 23°C. in the presence of sea water which has been evaporated to precipitate halite, or at higher temperatures in les concentrated brine, the anhydrite will be dissolved and gypsum will precipitate. Thus, with burial of a few feet below the free-water level, any surface anhydrite should be dissolved easily and gypsum precipitated, at least during winter, at any known mean annual temperature. Only gypsum would be carried into the subsurface, despite the fact that anhydrite may have formed at or near the surface. This gypsum, and all original gypsum, will be replaced by anhydrite with burial to a depth of 500-2,000 feet, depending on the salinity of the subsurface water and on the geothermal gradient. During this stage, there will be at least a 38-per cent volume reduction of the solid. It is unlikely that this volume will be compensated by addition of anhydrite from an outside source because of the gene ation of abnormal fluid pressure and, thus, outward water flow during the gypsum-anhydrite replacement. Anhydrite in the subsurface commonly is devoid of pore space, indicating additional compaction. Therefore, ancient anhydrite sections must represent approximately one-third of their original depositional thickness. Because much of the volume reduction is delayed, the compaction and any dissolution of evaporites provide a mechanism for increasing and perpetuating the subsidence of an established evaporite basin in addition to and after its tectonic history.

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