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Abstract

Despite recent suggestions to consider dolomite a product involving freshwater, especially the reaction between freshwater and seawater, more recent work in the rock record and in sea-marginal ponds of the Red Sea indicates that dolomite is an evaporite mineral, and that most dolomites owe their origin to hypersaline brines. Although dolomite may form under a variety of depositional conditions, including freshwater, soils, caliche or deep sea, most dolomites in the rock record formed under conditions of hypersahnity. A close lateral, vertical, or temporal relationship commonly exists between dolomite and evaporite deposits, although this relationship is not universal nor everywhere demonstrable. In most situations in which evaporites fail to accompany dolomite, original evaporites have commonly been removed following their precipitation. However, the imprint of evaporite minerals and other evidence for hypersahnity may be preserved. Evidence for vanished evaporites has been inferred from mineralogic, sedimentary, and biotic characteristics, including pseudomorphs after sulfate nodules, pseudomorphs after halite, euhedral quartz crystals, quartzine and lutecine, saddle-shaped dolomite crystals, authigenic potassium feldspar, solution-co11apse breccias, abundance of oöids, half-moon oöids, presence of stromatolites, and paucity of fauna. In places evaporite minerals have been preserved as small inclusions.

Research in modern sea-marginal ponds of the Red Sea shows that dolomite forms only where gypsum and /or anhydrite is also present. Among submerged algal mats where gypsum is absent, the carbonate minerals are aragonite or high-magnesian calcite; by contrast, where gypsum is abundant in deeper parts of ponds, or among submerged algal mats, dolomite is present. Likewise, in a pond-marginal sabkha, not only haute, gypsum, and anhydrite, but also dolomite, form a cement between constituent particles. The high salinities at which gypsum precipitates (up to 330 × 103 mg/l in the summer) and the observation that dolomite prefers sulfate association suggest that both minerals owe their origin to hypersaline brines. Fresh or brackish waters are unavailable in these ponds; the waters are hypersaline at all times. Dolomite, almost by definition, must be considered an evaporite mineral in these hypersaline ponds.

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