Zoned Dolomite Crystals

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Abstract
Dolomite crystals occurring in the Mahmal Formation (Middle Jurassic) in Makhtesh Ramon (southern Israel) display four types of zoning: (a) calcian dolomite zones; (b) polycrystalline calcite zones; (c) ferroan dolomite zones; (d) polycrystalline hematite zones. All four types are early diagenetic and were formed before the growth of the dolomite crystal was completed. Calcian dolomite zones were formed by dilution of the interstitial dolomitizing brines with normal sea water or fresh water, leading to an increase in the ratio in the solution. On occasions, this dilution and the consequent ratio increase was sufficient to transfer the water-sediment system into the calcite stability field, resulting in replacement of calcian dolomite zones by polycrystalline calcite (dedolomitization). Periodically, shallow and stagnant reducing conditions caused the ratio in the solution to increase leading to the formation of ferroan dolomite zones. At stormy dilution periods, the interstitial solutions were oxygenated and ferroan dolomite zones were oxidized to polycrystalline hematite zones.